regulatory services delivered by hedges: the evidence base

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Regulatory services delivered by hedges:

The evidence base

For Defra

By

Robert Wolton, Katie Pollard, Amy Goodwin and Lisa Norton

LM0106 Report for Defra and Natural England

7 April 2014

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Contract title: Regulatory services delivered by hedges: the

evidence base Client: Department for Environment, Food and Rural

Affairs (Defra) contract number LM0106 Prepared by: Robert Wolton1, Katie Pollard2, Amy Goodwin3 and

Lisa Norton3

1Locks Park Farm, Hatherleigh, Okehampton, Devon, EX20 3LZ, UK 2KP Ecology Ltd, 5 Sherwood Avenue, Poole, Dorset, BH14 8DH, UK 3Centre for Ecology and Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancs, LA1 4AP, UK Recommended citation: Wolton, R.J., Pollard, K.A., Goodwin, A. & Norton, L. 2014. Regulatory services delivered by hedges: the evidence base. Report of Defra project LM0106. 99pp.

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Executive summary

1. This report presents an evidence based review of the regulatory ecosystem services delivered by hedges. These services are those that benefit people through the regulation of processes such as water purification, soil erosion and crop pollination, as opposed to provisioning services which generate products, or cultural services that benefit our recreation, health or learning. All these services are underpinned by biodiversity.

2. Nine regulatory ecosystem services are covered: water quality improvement, flood risk

reduction, soil loss reduction (erosion), crop water availability, crop pest reduction, crop pollination improvement, shelter provision (crops and livestock), climate change mitigation and urban air quality. These are the main regulatory services delivered by hedges, with the exception of their role as barriers regulating movement of livestock and people and as markers of land occupancy.

3. This is the first published detailed evidence based review of the regulatory services

specifically delivered by hedges. Much current UK action appears to be based on opinion and anecdotal information. The aim of this review is to make published research more accessible to policy makers, land managers and others, and to identify gaps in the evidence base. The report presents the evidence for the role played by hedges in delivery of each of the nine services in a UK context, including the mechanisms involved. The levels of service provided are quantified where possible.

4. Based on the evidence found, suggestions are provided to assist with the effective

delivery of management options within the successor scheme to Environmental Stewardship, the outgoing agri-environment scheme (AES) in England. The successor scheme, which has the working title of the New Environmental Land Management Scheme (NELMS), is expected to be fully developed by late 2014 with the first agreements beginning in early 2016. Principle objectives of this new scheme will be to maintain and improve biodiversity and water quality. It will encompass the England Woodland Grant Scheme (EWGS) and elements currently covered by Catchment Sensitive Farming (CSF). A summary is given of the main suggested NELMS management actions for each of the nine service covered (Table 1, p.9).

5. Suggestions are also made in the context of Greening measures proposed within

England as part of the current round of Common Agricultural Policy (CAP) reform (hedges within Ecological Focus Areas (EFA)). Likewise, advice is given on the likely impacts of existing Cross Compliance requirements of the Single Payment Scheme (SPS) for England, if carried forward. A summary of these suggestions is provided (Table 2, p.10).

6. To capture as much relevant evidence as possible, many individuals from across a wide

range of academic, policy and delivery organisations were contacted, in addition to searching related reviews, online scientific citation indexes and the internet for published papers and reports. Particular attention was paid to locating relevant work elsewhere in Europe and in other parts of the world with temperate climates. Much relevant work originates from Brittany, France. Where research evidence was not available, anecdotal information was sought, although little was obtained and far less reliance placed upon it.

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7. A broad definition of the word hedge is used. This definition encompasses lines of trees

as well as classic shrubby hedges. It also includes associated basal and marginal vegetation where clearly influenced by the existence of the shrubs and trees, and any associated earth banks and ditches. As such, hedges can be up to 9 metres wide or more. This definition encompasses those linear boundary features known as windbreaks, together with thin shelterbelts. It also encompasses many buffer strips where these have shrubs or trees in addition to permanent herbaceous (usually grassy) growth. Even where buffer strips do not support any woody growth, they have some similar functionality to hedges. This applies to beetle banks too. Consequently, relevant research into buffer strips, beetle banks, windbreaks and shelterbelts is included in this review.

8. There are many linkages between the nine regulatory services. For example, dense,

continuous, hedges that follow contours are likely to deliver water quality improvement, flood risk reduction and soil loss reduction. Likewise, hedges that are in favourable condition to enhance populations of the natural enemies (mainly predators and parasites) of crop pests are also likely to benefit crop pollinators like bees and hoverflies. The review draws attention to these linkages. Any conflicting effects are also identified, although these are rare, together with any potential harmful impacts on humans or biodiversity.

9. Many gaps in available evidence were found. These gaps are frequently large – there is

much need for further research. Priorities for further research are identified for each service and collated (Table 3, p.11). A common theme is the need for research at the farm, landscape or catchment level as opposed to the individual hedge level.

10. Summaries for each of the nine services are as follows: Water quality improvement Strong evidence exists to show that buffer strips and hedges can be effective at preventing nutrients and other pollutants from reaching water bodies, particularly if placed along contours or beside water bodies. Weaker evidence suggests that hedges which are more than c. 2 m wide and those with an underlying earth bank are most effective. Hedges act as a physical barrier to the movement of water and associated sediment, including pollutants, increasing rates of infiltration into the soil, and acting as a sink for nutrients. Limited evidence suggests the presence of trees and shrubs increases the effectiveness of grassy strips through increasing rates of infiltration and uptake of water and nutrients into biomass. Only one research study into the effectiveness of buffer strips or hedges at a catchment level was found (Benhamou et al. 2013). Here, modelling based on experiment results suggested that a banked hedge network in a 5 km2 catchment in Brittany would result in a decline in nitrogen at the outflow of 3.3%. The hedges were at a density of 48 m ha-1 and field drainage systems were in place. It is probable that the potential for hedges to intercept and absorb nutrients and pollutants at a landscape level has been underestimated because of lack of inclusion of features such as field drainage systems in models of nutrient loss from land cover. However, further research is required to confirm this.

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Flood risk reduction Strong evidence exists to show that individual hedges (and other forms of buffer strip) along contours or fringing water courses have the potential to reduce the volume of water reaching streams and rivers, and the speed with which it does so, following storms. Lines of shrubs or trees of species commonly found in British hedges can greatly increase infiltration of water into the soil even when only a metre or two wide – by a factor of 60 to 70 times compared to compacted upland sheep pasture. Hedges also reduce soil water levels in and beyond the hedge root zone during the summer, so it takes longer to become saturated during the autumn, providing a buffer against flooding events. Individual hedges can therefore, to some extent, reduce the risk of flooding lower down in the catchment. Evidence into the potential for hedges to reduce flood risk at a landscape scale is less strong but still reveals that hedges can play a significant role in this regard, with the key research coming from Brittany (NW France). A banked hedge network in a 32 ha catchment reduced peak and total flow within streams following storms by between a quarter and a half (Merot 1999). The hedges were at a density of 106 m ha-1, with 64 m ha-1 perpendicular to slopes. Further work in Brittany, based on modelling, suggested that even at the low hedge density of 27 m ha-1 annual stream flow would be reduced by 10%, and that in the particular landscape studied overall water interception by banked hedges was likely to reach its maximum at a density of 60 m h-1 (Viaud et al. 2005). (For comparison’s sake, the average density of hedges across lowland Devon is estimated at 107 m ha-1 (Devon Local Nature Partnership 2014).) Anecdotal information suggests that hedges in floodplains can slow down the movement of water across them, and so increase their storage capacity. Removal of these floodplain hedges may increase the risk of flooding of land or properties downstream: research is urgently required on this, and more generally to evaluate the full potential of hedges and other linear features to reduce flood risk at the catchment level under different topographical, geological, soil and farm management conditions. Soil loss reduction Strong evidence exists that hedges can reduce soil loss from fields through intercepting water-borne sediment and reducing surface flow rate. Little relevant research on water-borne soil erosion has been carried out in the UK, but evidence from Brittany (NW France) shows that hedges can be effective at both capturing soil from fields above them and decreasing the rate of water flow over fields below, so reducing soil loss. Hedges lying perpendicular to the slope increase the depth of the soil A-horizon (top soil) on their uphill side and can result in terrace formation over time. This accumulation of soil and sediment reflects the ability of hedges to act a physical barrier to surface water run-off and to increase water infiltration into the ground. The hedge species planted can exert a considerable influence over the effectiveness of a hedge to reduce run-off and erosion: dense and structurally strong species such as hawthorn, hazel and oak have been shown to be the most effective barriers. Evidence for the effectiveness of hedges in reducing wind-borne soil loss in the UK is less strong. However, it is clear that tall, moderately dense, hedges can serve as windbreaks to reduce soil erosion in flat, open, landscapes and those with light, for example sandy or peaty, soils. However, little if any evidence is available to quantify this effect (see Shelter Provision below for further information). Crop water availability Research shows that hedges have impacts on field level hydrology which vary according to rainfall conditions, slope, hedge type, hedge height, root extent, position of hedge in

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relation to nearby water bodies, etc... These impacts range from being highly beneficial (increasing crop yields) to highly damaging. In most cases, damaging impacts of hedges are confined, in temperate climates, to very dry places or to dry years. Hedges influence the availability of water to crops by a) reducing water loss through evaporation and transpiration as a result of increased shading and wind shelter, b) the retention of moisture in leaf litter, c) providing a water gradient which allows lateral transfer from soil beneath hedges to soil under crops, and d) facilitating the infiltration of surface water into the soil. Research is needed into the potential role of hedges in enhancing crop water availability and hence crop water use efficiency under UK conditions, particularly where potential exists for hedges to be used to improve crop water availability at either the field or the landscape scale.

Crop pest reduction Strong evidence exists to show that field margins and beetle banks, which have similar functional properties to the herbaceous vegetation found in the base and margins of hedges, attract predatory insects and spiders. In particular, they can provide suitable overwintering habitat, the predators moving out into the crops in the spring and summer. Hedges enhance populations of the natural enemies (predators and parasites) of crop pests by providing a wide range of microhabitats across the shrub layer, trees, banks, base, margins, ditches and soil. They also provide shelter (especially for overwintering), alternative prey and host species (particularly during the early spring and before hibernation), nectar and pollen resources, and larval development resources. Most research has been on ground dwelling arthropods such as carabid beetles, and to a lesser extent spiders, and their use of grassy, often tussocky, margins. Evidence relating to the aerial dispersal of predators such as hoverflies from hedges into crops is much more limited. Research shows that the greater the structural and floristic diversity of hedges, the greater their invertebrate diversity, but the influence of this relationship on crop pests remains unknown. The pattern of field margins in the landscape may influence predator densities and species richness, as opposed to habitat quality alone. Pest species themselves may utilise the additional resources managed or created to benefit their natural enemies, with unknown effects on biological control. No research appears to have been carried out in the UK or similar temperate conditions on the effects of predators associated with hedges on crop yields, let alone their impact on farm profitability: this is a major gap in the evidence base. Crop pollination improvement Strong evidence exists, based largely upon bees (especially bumblebees) and hoverflies, that hedges, together with other patches of non-cropped ground, are important in agricultural landscapes for the existence of healthy and diverse pollinator populations. Furthermore, there is good evidence that hedges attract pollinators into intensive farmland and export those pollinators into crops, increasing yield. Hedges provide breeding sites, food when crops are not in flower, shelter, protection and flight lines. They are of particular importance for nesting bumblebees. Their value can be enhanced by the cultivation of nearby strips or patches of flowers grown for nectar and pollen. However, practically no research has been carried out on the cost effectiveness of hedges in increasing crop yields through boosting pollination. Such research needs to encompass the activities of insects other than bees and hoverflies, since these other taxa may be as important - for example, several other families of flies are frequent visitors to flowers in hedges and presumably crops. There is an indication that hedges can influence the pollination of crops over a

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distance of at least 750 m, but this is also a matter which requires further research. A list of hedge plants considered to be of value to pollinators, focusing on flies, is included (Table 4, p58). Shelter provision (crops and livestock) Strong evidence exists to prove that hedges managed as windbreaks or shelterbelts can improve crop yields. In particular vegetables and fruits, along with broad-leaved crops like potatoes, sugar beet and beans, are likely to benefit, especially if grown on well-drained soils in the drier parts of Britain. Available information on likely net increases in yield suggests that for arable crops, such as cereals, this is likely to range from just a few percent to as much as 25% within the sheltered area. For vegetables and fruit, the yield increase may be much greater than this, perhaps as high as 75%. Crop type, local climate and soil, and hedge structure will all affect yields. Likewise, there is plentiful evidence to show that livestock such as sheep and cattle benefit from the protection from wind, driving rain and snow which can be provided by hedges. The role of hedges in providing shade may be particularly important in temperate areas such as the British Isles, especially with climate change. Windbreaks can reduce livestock mortality (particularly of young animals) and heat stress, and increase growth rates, milk yield, disease resistance and fertility. Although much research has been done to quantify these effects, most are location specific or from abroad and cannot be transferred directly across to Britain. Hedges acting as windbreaks, if properly designed, are likely to reduce wind speed significantly over a distance of 12 times the height of a hedge on the downwind side, and 4 times on the upwind side. Thus a hedge that has grown 6.25 m high may provide significant shelter over a width of 100 m. Moreover, at a wider landscape scale evidence exists to show that networks of hedges can exert a significant influence on local climate, decreasing average wind speed by up to 50%, increasing daytime and decreasing night time temperatures, and increasing humidity. Climate change mitigation Strong evidence exists to confirm that hedges store more carbon than cropped land: as a consequence, they have a role to play in climate change mitigation. Carbon is sequestered both in woody growth above ground and in roots, leaf litter and other soil organic matter at and below ground level. Tree lines store more carbon than shrubby hedges: mature trees have greater above ground biomass than shrubs and input more carbon into the soil through higher leaf and small branch litter fall. To have any significant impact upon greenhouse gas levels, carbon needs to be locked up within the hedges and soil over the long-term. Alternatively, hedges can be cropped for woodfuel, reducing demand for fossil fuel use. Through capturing eroding soil, hedges across slopes can increase soil organic carbon (SOC) for up to 60 m uphill, although cultivation will reduce such accumulation substantially through facilitating oxidisation. Little research has been carried out on rates of carbon sequestration by hedges in the British Isles or elsewhere in Europe. This applies to both above ground and below ground sequestration, with the best available estimates being derived from woodlands. Above ground, uncut shrubby hedges may accumulate around 0.5 tonne ha-1 yr- 1, while tree lines may accumulate more than 3 t ha-1 yr- 1. Below ground, both shrubby hedges and tree lines may sequester 0.5 t ha-1 yr- 1. While shrubs and trees will only continue to accumulate carbon if not trimmed, coppiced or laid until they are mature, below ground woodland research suggests soils may continue to accumulate carbon for more than 700 years. For established hedge networks, evidence from Britain, Germany and France, suggests that hedges may store roughly 100 t C ha-1 although this will vary considerably according to hedge structure, woody species and age. The hedges within a particular

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Brittany landscape (NW France) have been estimated to contain 13% of the carbon stored in the biomass and soils of that landscape. The hedge density within this landscape was 50 m ha-1, which roughly corresponds to that found across much of lowland England and Wales (the current density across the well-hedged county of Devon is estimated to be 107 m ha-11

(Devon Local Nature Partnership 2014)). Planting new hedgerow trees and allowing others to grow to maturity will substantially increase the carbon stored in hedges. Urban air quality improvement Research shows that mature trees in urban environments can reduce pollutant loads by between 7% and 26%, and indeed tree planting in urban areas is widely considered to be of worthwhile benefit due to ability of trees to remove health damaging particles from the air. While the majority of research towards improving urban air quality has been on individual trees rather than on hedges, a few studies show that hedges can perform the same function. The efficiency of different plant species to capture pollutant particles depends on a number of factors, including the dimensions and complexity of leaf shape, whether or not the cuticle is waxy or covered with fine hairs, and the spatial position and age of the individual plants. Choice of species and location is therefore important, with rough-leaved broadleaved trees like whitebeam (Sorbus species) being particularly effective. In UK cities, studies suggest that planting trees on one quarter of the available urban area will reduce the particulate concentrations by between 2% and 10% (McDonald et al. 2007). Some pollutants captured by trees are not fully removed from the system but instead accumulate in the soils beneath the trees, with implications for land use in the future. Overall, planting trees and shrubs, perhaps as hedges, provides the only practical way to remove pollutants from the air in open urban spaces: trees and shrubs provide effective and relatively cheap air purification (Dzierżanowski et al. 2011).

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Table 1. Key NELMS management and delivery advice (x = strong positive impact, x = weak positive impact)

Action Water quality

Flood risk

Soil loss

Crop water

Crop pest

Crop pollination

Shelter Climate change

Urban air

quality

1 Plant new hedges, and restore hedges, that contour slopes x x x x x x x

2 Plant new hedges, and restore hedges, beside water bodies x x x x x x x

3 Target hedges on compacted and arable farmland x x x

4 Fill in gaps in hedges, relocating gateways as appropriate x x x x

5 Encourage the development of earth banks under the shrub layer x x x x x

6 Encourage hedges, including permanent herbaceous (grassy) margins, that are at least 2m wide and preferably 5m or more wide

x x x x x x x

7 Manage field drainage systems to prevent rapid surface run-off to watercourses, enabling hedge uptake (blocking ditches and drains if necessary)

x x x x

8 Increase landscape hedge density by planting new hedges x x x x x x

9 Increase landscape hedge connectivity by planting new hedges and filling in gaps

x x x x x

10 In areas at high risk from water erosion, consider introduction of alley-cropping (agroforestry) systems, with hedges planted across slopes

x x x x x

11 Promote structurally diverse hedges, with emergent trees, banks, ditches and herbaceous margins as well as the shrub layer

x x

12 Encourage development of tussocky grass margins and basal vegetation x x x x x x x

13 Encourage flowering of shrubs, and within basal and marginal vegetation to provide nectar and pollen from spring to autumn

x x

14 Cut hedge shrubs no more frequently than once every three years to encourage plentiful flowers

x x

15 Encourage tall hedges that allow about 50% of the wind to pass through them: they will normally provide significant shelter over 12 x their height downwind and 4 x upwind

x x x

16 Choose hedge shrubs and trees that are effective at capturing particulates and able to withstand pollution

x

17 Encourage the growth and retention of mature hedgerow trees x x x x x

18 Reduce herbicide use near hedges to encourage development of perennial herbaceous vegetation

x x x x x x x

19 Rejuvenate hedges periodically by laying or coppicing, to keep them healthy in the long term

x x x x x x x x x

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Table 2. Summary of implications for CAP greening and cross-compliance measures (x = positive impact) Action Water quality

improvement Flood risk reduction

Soil loss reduction

Crop water

availability

Crop pest reduction

Crop pollination

increase

Shelter for crops

and livestock

Climate change

mitigation

Urban air quality

improvement

1 Retention of hedges within EFA and cross compliance (GAEC 15) requirements

x x x 1?

2? x x x

2 No cultivation of 2 m hedge protection zones (cross compliance GAEC 14)

x x x 1?

2? x x

3 Maintain green cover within 2 m hedge protection zone

x x x 1?

2? x x

4 No fertiliser use within 2 m hedge protection zone

x 1?

2? x x

5 No pesticide use within 2 m hedge protection zone

x 2? x

4x

6 No hedge cutting or trimming between 1 March and 31 July (cross compliance GAEC 15)

1?

3? x

4?

Notes: General: Retention of hedges and existing measures will maintain the status quo, not deliver any improvement to ecosystem service delivery. Hedges (both woody growth and herbaceous growth) need to be managed to remain in good structural health, highlighting the need for agri-environment schemes or similar mechanisms to promote active management. EFA = Ecological Focus Area. GAEC = Good Agricultural and Environmental Condition. 1Hedges can be either beneficial or harmful to crops through influencing water availability. They are more likely to be harmful in very dry years or in very dry places. Otherwise, the

combined effects of shelter and water availability are likely to be beneficial. 2No hard information is available on the impact of hedges on crop yields or profitability as a result of boosting populations of natural enemies of crop pests (or pollinators). No

accurate assessment can therefore be made as to whether it is more cost effective to retain hedges and therefore use fewer insecticides or to remove hedges and use more insecticides. On organic farms, however, it is more likely that the presence of hedges will increase yield and profitability. 3The impact that hedge cutting has on the abundance of crop pest predators is not known. On the one hand reduced cutting may lead to an increase in natural enemies through

assisting flowering; on the other hand it may reduce the availability of prey. 4The impact of hedge cutting on soil carbon storage has not yet been fully assessed. Preventing pesticide (i.e. herbicide) use will facilitate accumulation of carbon in soil.

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Table 3. Key requirements for further research

1. Water quality improvement

I. To understand better how hedges (as opposed to purely herbaceous (grassy) buffer

strips) impact on water quality at both field and landscape scales.

II. To establish a classification of hedges (e.g. type, location and management) on the basis of their effectiveness in relation to water quality provision, to make it possible to model the landscape impacts of hedges on water quality.

2. Flood risk reduction

I. To quantify the effect of hedge networks in reducing flood risk in areas with differing

topography, geology and soils, and in catchments of different sizes, at a catchment (or landscape) level, including the ability of hedges to reduce flood risk following extreme weather events.

II. To determine the conditions under which hedges across flood plains are beneficial

or detrimental.

III. To evaluate the comparative effectiveness of different hedge structures and dimensions (e.g. banked or unbanked, shrubby or lines of trees, thin or wide).

3. Soil loss reduction

I. To evaluate the role of UK hedges in reducing water-borne soil erosion. Research

should focus on hedges in areas at high risk from erosion, and preferably conducted at a landscape scale and over many years.

II. To compare the effectiveness of different UK woody species and hedge types.

Different hedge species are likely to have different effects upon erosion losses due to the hedge canopy structure and rooting systems. Hedges with banks may be better at preventing soil loss.

4. Crop water availability

I. To understand the extent to which hedges may either enhance or negatively impact

on yield, and which types of hedges have the potential to be most beneficial. Research should focus on hedges in areas that either particularly dry or which experience either unusually low and or unusually high rainfall.

II. To evaluate the role of hedges in intercepting rainfall at a landscape level.

5. Crop pest reduction

I. The cost effectiveness to farmers of retaining and managing hedges (and related features such as beetle banks) to boost populations of natural enemies.

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II. The distance from hedges into crops over which crop pests are significantly reduced as a result of the presence of hedges.

6. Crop pollination improvement

I. The degree to which pollinators other than bees (especially other than bumblebees)

are dependent upon hedges.

II. The distance from hedges into crops over which crop yields are significantly increased as a result of pollinator abundance and diversity being increased through the presence of hedges.

III. The economic value of hedges for crop pollination.

7. Shelter provision (crops and livestock)

I. The cost effectiveness of hedges as windbreaks and shade providers in the UK, in different geographical locations, weather conditions and soil type.

8. Climate change mitigation

I. To understand better the rates of carbon accumulation both above and below

ground specifically in hedges, and how different factors such as soil depth, cultivation practices, litter decomposition rates and different tree/shrub species influence the ability of hedges to accumulate and store soil organic carbon (SOC).

II. To understand better the impact of various management practices on the ability of hedges to sequester soil organic carbon. This should focus on the impact of hedge trimming, and on the management to hedges to produce a woodfuel crop.

III. To understand better how long hedges continue to accumulate further carbon

before they reach equilibrium, and how they can best be managed to store carbon over the long-term as well as which trees and shrub species would be the most effective for this purpose.

IV. To quantify the amount of carbon currently and potentially stored in UK hedges in

comparison to that stored in woodlands and other woody landscape features.

9. Urban air quality improvement

I. To understand better the effectiveness of hedges, and of native tree and shrub species, at removing air pollutants in urban UK environments.

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Contents Executive summary ................................................................................................................................. 3

Table 1. Key NELMS management and delivery advice ..................................................................... 9

Table 2. Summary of implications for CAP greening and cross-compliance measures ................... 10

Table 3. Key requirements for further research .............................................................................. 11

Contents ................................................................................................................................................ 13

Acknowledgements ............................................................................................................................... 17

Introduction .......................................................................................................................................... 18

Scope of the review .............................................................................................................................. 19

Definition of hedges, and relationship with buffer strips, beetle banks, windbreaks and shelterbelts ................................................................................................................................... 19

Regulatory ecosystem services covered ....................................................................................... 20

Relationship between services covered ....................................................................................... 21

Evidence sources examined .................................................................................................................. 21

Approach to providing NELMS and CAP advice .................................................................................... 23

Review of individual services ................................................................................................................ 24

1. Water quality improvement ........................................................................................................ 24

1.1 Overview of information sources...................................................................................... 24

1.2 Anecdotes ......................................................................................................................... 24

1.3 How hedges deliver the service ........................................................................................ 24

1.4 Quantification of effects ................................................................................................... 25

1.5 Disadvantages of service ................................................................................................... 25

1.6 Relationship with other services ....................................................................................... 26

1.7 Recommendations for NELMS .......................................................................................... 26

1.8 Value of evidence to CAP greening and cross compliance measures ............................... 27

1.9 Priorities for further research ........................................................................................... 27

1.10 Conclusions ....................................................................................................................... 27

2. Flood risk reduction ..................................................................................................................... 29

2.1 Overview of information sources...................................................................................... 29

2.2 Anecdotes ......................................................................................................................... 29

2.3. How hedges deliver the service ............................................................................................. 30

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2.4. Quantification of effects ........................................................................................................ 30

2.5. Disadvantages of service ........................................................................................................ 32

2.6. Relationship with other services ............................................................................................ 32

2.7. Recommendations for NELMS ............................................................................................... 33

2.8. Value of evidence to CAP greening and cross compliance measures .................................... 33

2.9. Priorities for further research ................................................................................................ 33

2.10. Conclusions .......................................................................................................................... 34

3. Soil loss reduction ........................................................................................................................ 35

3.1. Overview of information sources........................................................................................... 35

3.2. Anecdotal information ........................................................................................................... 35








3.10. Conclusions .......................................................................................................................... 40

4. Crop water availability ................................................................................................................. 41










4.10. Conclusions .......................................................................................................................... 44

5. Crop pest reduction ..................................................................................................................... 45






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5.10. Conclusions .......................................................................................................................... 51

6. Crop pollination improvement ..................................................................................................... 53


6.2. Anecdotal information .......................................................................................................... 53








6.10. Conclusions .......................................................................................................................... 57

Table 4. Plants native to the British Isles which are considered to be of value to pollinators. ... 58

7. Shelter provision (crops and livestock) ........................................................................................ 59

7.1. Overview of information sources .......................................................................................... 59








7.9. Priorities for further research ............................................................................................... 63

7.10. Conclusions .......................................................................................................................... 64

8. Climate change mitigation ........................................................................................................... 65







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8.10. Conclusions .......................................................................................................................... 71

9. Urban air quality improvement.................................................................................................... 73










9.10. Conclusions .......................................................................................................................... 78

References ............................................................................................................................................ 79

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Acknowledgements The review was undertaken in the space of just two months from start to completion. Meeting such a tight timescale would not have been possible without the rapid and helpful responses we received from many individuals to our requests for information. We would like to express our particular thanks to the following Catherine Grimaldi (INRA, Rennes) Emily Ledder (Natural England Project Officer) Georg Müller (Germany) Heather Robertson Jacques Baudry (INRA, Rennes) James Grischeff (CSF, Natural England) Oliver Edmonds (Defra) Philippe Merot (INRA, Rennes) Sally Westaway (ORC) We are indebted and most grateful to Mary Crossland, Sally Westaway and Jo Smith at The Organic Research Centre for making available to us an unpublished manuscript on the ecosystem services delivered by hedges in northern Europe. This has been a valuable source of information. Many others have willingly helped us, and we are grateful to them all. They are Adrian Collins, Alain Coic, Benedikt Brink, John Dover, Bérengère Ize, Bryn Thomas, David Hogan, John Holland, Dirk Cuvelier, Donald Gabriëls, Françoise Burel, George Pidgeon, Gilles Pinay, Helen Pontier, Jamie Letts, Jonathan Bailey, Judy Webb, Leanne Sargeant, Lex Roeleveld, Lindsey Stewart, Liz Dixon, Martin Turner, Merijn Bos, Philippe Pointereau, Rob Dixon, Rosemary Teverson, Sally Hope Johnson, Sarah Guest and Susie Willows.

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Introduction Ecosystems are communities of plants, animals and other life forms associated with particular environments, and the ways that ecosystems benefit people are known as ecosystem services. These services can broadly be divided into four categories:

1. Regulating services – those that regulate processes such as water purification, crop pollination and pest control.

2. Provisioning services – those generating products such as food and energy. 3. Cultural services – those that assist our recreation and well-being, or provide aesthetic

experiences and spiritual enrichment or learning opportunities. 4. Supporting services – those that are necessary for the production of all other ecosystem

services including soil formation, photosynthesis, primary production and nutrient cycling.

Biodiversity is not itself an ecosystem service, but underpins all ecosystem services. This report examines the regulatory ecosystem services delivered by hedges in Britain. In comparison to the biodiversity of hedges and the cultural services they deliver, these regulatory services are poorly understood by many policy shapers, advisers and land managers. There have been no previous evidence-based reviews of the regulatory services delivered specifically by hedges in Britain or indeed in temperate Europe, with the exception of an unpublished manuscript prepared by The Organic Research Centre (Crossland et al. 2013) generously made available to the authors of this report. There have been two general reviews of ecosystem services delivered by Environmental Stewardship which encompass hedges along with other habitats and landscape features (Food and Environment Research Agency 2012, Land Use Consultants 2000). This review has been commissioned primarily to help inform the delivery of the successor scheme to Environmental Stewardship, the current agri-environment scheme in England, under which farmers and landowners agree to manage their land to achieve environmental objectives in return for public funding. The successor scheme is currently known as the New Environmental Land Management Scheme (NELMS), although it will have a new name when it becomes operational in 2015. Primary objectives of this new scheme will be to enhance biodiversity and water quality in line with Biodiversity 2020 (Defra 2011) and the European Union’s Water Framework Directive (WFD). Consequently, this report pays particular attention to the role of hedges in relation to water quality, together with their potential role in reducing both soil loss and flood risk. The authors were also asked to highlight implications for the development of greening and cross-compliance measures within the current round of Common Agricultural Practice (CAP) reform. The full range of regulatory services covered by this report is:

1. Water quality improvement 2. Flood risk reduction 3. Soil loss reduction 4. Crop water availability 5. Crop pest reduction 6. Crop pollination improvement 7. Shelter provision (crops and livestock) 8. Climate change mitigation 9. Urban air quality improvement

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The aim of the research reported here is to find and review available evidence for all these services, and to make the information more accessible to those working towards the environmental improvement of our farms, towns and landscapes.

Scope of the review

Definition of hedges, and relationship with buffer strips, beetle banks, windbreaks and shelterbelts Understanding what is meant by the term ‘hedge’ is critical to the aims of this report. We use an expanded version of the definition used in the Hedgerow Survey Handbook (Defra 2007). This definition reads: ‘A hedgerow is defined as a boundary line of trees or shrubs over 20m long and less than 5m wide at the base between major woody stems, provided that at one time the trees or shrubs were more or less continuous. This includes ‘classic’ shrubby hedgerows, lines of trees, shrubby hedgerows with trees and very gappy hedgerows.’ We expand this definition to make it clear that a hedge encompasses not just the line of trees or shrubs, but also the base of the hedge (which may be an earth bank), any associated ditch, and the immediate herbaceous margins. All these structural components together form the hedge. It is for this reason that we prefer to use the word hedge, rather than hedgerow. (The term herbaceous includes all leafy non-woody plants, so in Britain includes all grasses and ferns.) To be in favourable condition for biodiversity, a hedge should have at least 1m of herbaceous vegetation between the centreline and any adjacent cultivated ground (Defra 2007). Since the area under the shrub or tree layer is often so densely shaded that it cannot support herbs, the requisite 1m herbaceous margin usually lies beyond the edge of the main woody stems. It follows that a hedge may be 7m wide (5m between major woody stems plus 1m herbaceous margins on either side). In fact, we do not define a minimum width for a herbaceous margin, so a hedge could be even wider – provided the distance between major woody stems is not more than 5m and the herbaceous vegetation is permanent and not subject to regular cultivation. It will, however, be exceptional to call a linear feature over 9m wide a hedge. The inclusion of herbaceous field layers within the definition of hedge, whether at the base of the hedge, below the shrub or tree canopy, or in the immediate margins, means that wide hedges are often effectively synonymous with buffer strips. These are normally taken to be linear strips of permanent grass and other herbs, often with bushes or trees growing in them. Beetle banks, which are low earth banks across fields with permanent, usually tussocky, herbaceous vegetation, likewise have similar functions to the basal and marginal herbaceous vegetation associated with hedges. Consequently, this report considers relevant evidence from buffer strips and beetle banks. We recognise, however, that buffer strips planted with trees and wider than about 9m are more appropriately called linear woodlands than hedges, so place less emphasis on evidence arising from such wide features. As recognised in the Hedgerow Survey Handbook definition, hedges that have grown into lines of trees are still hedges. As a consequence the distinction between hedges and windbreaks or shelterbelts becomes blurred, and they may practically be regarded as synonymous where there is 5m or less distance between major woody stems. Even where wider than this, evidence from windbreaks and shelterbelts is nevertheless considered in this report because they have similar functional properties to tall hedges. The definition of a hedge used in this report may therefore be taken as:

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‘A hedge is a boundary line of trees or shrubs over 20m long and less than 5m wide at the base between major woody stems. This includes ‘classic’ shrubby hedges, lines of trees, shrubby hedges with trees and very gappy hedges. The hedge encompasses the ground beneath the trees and shrubs including any earth bank, as well as associated ditches and herbaceous margins. To qualify as part of the hedge, the herbaceous margins must be permanent and their management must be influenced by the presence of the shrubs and trees. Many buffer strips, windbreaks and shelterbelts include hedge characteristics and effectively fall within this definition, but linear features wider than 9m will not normally be considered to be hedges.’

Regulatory ecosystem services covered The services covered in this report are:

1. Water quality improvement – in particular the role of hedges in preventing diffuse pollution from reaching water courses.

2. Flood risk reduction – the role hedges can play in reducing the risk of flooding to downstream properties and land.

3. Soil loss reduction – the capture by hedges of both water-borne and wind-borne soil. 4. Crop water availability – the effects hedges can have on the growth of nearby crops through

influencing the availability of water in the soil and rates of transpiration and evaporation. 5. Crop pest reduction – the ways in which hedges can help maintain or increase populations of

the natural enemies (predators and parasites) of crop pests. 6. Crop pollination improvement - the ways in which hedges can help maintain or increase

populations of crop insect pollinators. 7. Shelter provision – the protection of both crops and livestock from wind, driving rain or

snow, and protection of livestock from hot sun. 8. Climate change mitigation – the role of hedges in carbon storage and sequestration, and as a

source of renewable energy offsetting fossil fuel use. 9. Air quality improvement – the way that hedges, particularly in towns and cities, can remove

pollutants from the air, improving its quality and lessening risks to human health.

The regulatory services not covered or only weakly covered are:

1. Boundary provision – the role hedges can play in regulating the movement of livestock and humans.

2. Pesticide drift reduction – the role hedges can play in capturing air-borne pesticides in rural situations.

3. Climate change adaptation – the ways in which hedges can assist biodiversity and people cope with climate change.

4. Soil fauna improvement – the soil associated with hedges may act as a reservoir for beneficial soil macro fauna which have the potential to re-colonise and improve the soil in adjacent fields (Smith et al. 2008).

5. Odour reduction - windbreaks of low porosity are effective at reducing the size of odour plumes, for example the odours emanating from housed livestock. Conifers are more effective in this regard than poplars (Lin et al. 2006).

6. Ammonia movement restriction – hedges (shelterbelts) can reduce the movement of gaseous ammonia away from intensive livestock areas, reducing nitrogen deposition on sensitive nearby sites (James Grischeff, pers. comm.).

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Relationship between services covered The nine services covered in this report are far from independent of one another. For example, the same mechanisms that deliver improved water quality through preventing diffuse pollution from reaching water courses also act to reduce water-borne soil loss and flood risk, and to regulate the water available to crops. Not all services will, however, be synergistic. For example, hedges near watercourses may play a very valuable role in absorbing nutrients, but this in turn may encourage plants such as nettle, cleavers and couch, which would make the hedge less suitable in terms of provision of food sources for crop pollinators. Where such interactions occur, whether positive or negative, these are referred to in the separate accounts of each service. Different hedges may be important for different services in different places and according to their condition. In view of this, it will be rare for any one hedge actually to, or even to have the potential to, deliver all of the nine regulatory services at the same time.

Evidence sources examined To locate relevant published papers and other documents, including ‘grey’ literature, we undertook the following steps:

A. Examined existing reviews, in particular:

Defra report BD2305, an assessment of the impacts of Environmental Stewardship on greenhouse gas emissions (Jarvis & Unwin 2008).

Defra report BD5214, an assessment of the combined biodiversity benefits of the component features of hedges (Wolton et al. 2013).

Defra research contract NR0121, an assessment of the ecosystem services provided by the Environmental Stewardship scheme (Land Use Consultants 2009).

Natural England report on the ecosystem services from Environmental Stewardship that benefit agricultural production (Food and Environment Research Agency 2012).

Unpublished manuscript of a review prepared by The Organic Research Centre of ecosystem provision by hedgerows in northern Europe (Crossland et al. 2013).

B. Contacted key people within the following organisations and schemes both nationally and locally (South-West England and South-East England):

Catchment Sensitive Farming (CSF)

Centre for Ecology and Hydrology (CEH)

Department for Environment, Food and Rural Affairs (Defra)

Environment Agency (EA)

Environmental Stewardship (ES)

Farming and Wildlife Advisory Groups (FWAG)

Game and Wildlife Conservation Trust (GWCT)

Natural England (NE)

Organic Research Centre (ORC)

South West Water (SWW) C. Consulted members of key networks:

Devon Hedge Group

Hedgelink

International Association for Landscape Ecology (IALE)

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Participants at the European hedge conferences held at Stoke-on-Trent (England) in 2012 and Bemmel (Netherlands) in 2013.

D. Searched the internet, including online scientific citation indexes, for papers using a wide range of key words. This included using key terms in all main European languages.

For most services evidence directly relevant to British hedges was scarce. The strength, quality and relevance of each piece of evidence were assessed according to the following main criteria:

The source: most reliance was placed on evidence from peer-reviewed journals, followed by that from other published documents and reports (grey literature) and information provided by acknowledged experts in the topic concerned. Less reliance was placed on anecdotal information.

The hedge type: most reliance was placed on hedges (or equivalent features) of similar structure and species composition to those found in the British Isles.

The geographic location: most reliance was placed on studies from temperate latitudes, especially from the northern hemisphere and from regions with similar climates to the British Isles (e.g. north-western continental Europe).

The specificity of the evidence: more reliance was placed on studies where the research was replicated across several hedges, or preferably across several landscapes, rather than research based that on just one hedge or one small area.

The contextual information provided: more reliance was placed on studies where information was provided on matters such as soil type, tree species, catchment size and slope gradient.

Where the evidence for any particular service or delivery mechanism appears weak, this is stated in the report. Where the evidence is judged to prove delivery of a service or mechanism beyond reasonable doubt, this is referred to as ‘strong evidence’.

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Approach to providing NELMS and CAP advice As noted in the introduction, a primary objective of this review was to synthesise current evidence in the context of the New Environmental Land Management Scheme (NELMS), the working title for the successor scheme to Environmental Stewardship. Since one of the main aims of NELMS is to improve water quality, the authors were asked to pay particular attention to these three closely-related regulatory services: water quality improvement, flood alleviation and soil loss reduction. NELMS management options had already been drafted at the time of writing this review, and no further opportunity existed to add further options or to change option scope. However, we were asked to provide advice relevant to the successful delivery of options, including hedge management to improve option delivery and placement within the local landscape context. Proposed NELMS options and capital items of specific relevance to hedges include:

Hedge management – covering cutting frequency and times, hedge dimensions, and management of existing trees

Hedge laying

Hedge coppicing

Hedge planting

Hedge gapping-up

Hedgerow tree planting

Earth bank restoration

Earth bank creation

Stone-faced bank restoration

Stone-faced bank creation In addition to providing management advice for the three priority services referred to above relevant to the above options and capital items, and to field margin and ditch management, we have also made suggestions on how hedges can be managed to deliver the remaining six regulatory services covered by this report. Across all nine services, the advice given in this report is at times not fully supported by direct research evidence because such evidence is not yet available. However all advice given is in our view sensible, logical and practical, reflecting both indirect research-based evidence and the experience of practitioners. In addition to synthesising the evidence in the context of NELMS delivery, we were also asked to provide some advice on the ways in which greening measures proposed as part of the current round of Current Agricultural Policy (CAP) reform might assist or hinder the delivery of the regulatory ecosystem services provided by hedges. At the same time we were asked to consider relevant cross-compliance measures in the same way. Where greening is concerned, we have focused on Ecological Focus Areas (EFAs), and with cross-compliance on those requirements currently in place in England (i.e. Good Agricultural and Environmental Conditions (GAEC) 14 and 15 – see Defra & Rural Payments Agency (2013)).

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Review of individual services

1. Water quality improvement

1.1 Overview of information sources Information on the potential role of hedges in ensuring the delivery of clean water was gathered from a range of sources including formal academic reviews, technical booklets, academic papers and personal communication with experts. Although there are a number of papers from France supported by a technical booklet based on that work which focuses specifically on the role of hedges (and one hedge in particular), much of the rest of the work is more generally focused around various forms of buffer strip or shelterbelt. Relatively few papers focus specifically on how the presence of hedges in a landscape affected water quality. Crossland et al. (2013) point out that numerous studies focused on the hydrological impact of hedges have been conducted by the Institut National de la Recherche Agronomique (National Institute of Agronomic Research) (INRA) in Brittany, western France, where hedges are a common feature within the rural landscape (Ryszkowski 1993, Caubel 2003, Viaud 2005, Ghazavi 2011, Benhamou 2013, Ghazavi 2008). A number of studies investigate or review the role of vegetated buffer strips in enhancing water quality (Barling & Moore 1994, Blandco-Canqui et al. 2004, Borin et al. 2010, Dorioz et al. 2011, Muscutt et al. 1993, Yuan et al. 2009). In most cases these strips do not incorporate trees but it is important to note that hedges consist of more than just lines of trees/shrubs and the vegetation at the base of the hedge will play a role in nutrient and soil retention, thereby enhancing water quality.

1.2 Anecdotes

None.

1.3 How hedges deliver the service

The provision of clean water within catchments depends on the process by which water, falling as rain, travels across the land into water bodies and on the extent to which it picks up nutrients present on land in that process. Some nutrients may dissolve directly in water, for example sprayed pesticides, but the majority of pollutants remain attached to soil particles or sediment which may become eroded by water and carried from land into water bodies. Hedges act as a physical barrier to the movement of water and associated sediment, including sediment-clinging pollutants like particulate phosphate and some pesticides (e.g. Cypermethrin) (Borin 2010).

By increasing water infiltration into the ground. The physical structure of hedges means that not only can they act as barriers to surface run-off, but they can also increase percolation of nutrients through the deep roots of the shrubs and trees (Grimaldi 2012). The multi-species community constituting a hedge means that those nutrients can then be exploited

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effectively, for example by different groups of plant species at different times of year (Solagro 2000).

By acting as a sink for nutrients carried in solution, particularly in arable landscapes. Once trapped by the hedge, reductions in nutrients have been attributed to multiple factors such as microbial immobilisation and plant uptake (Vought 2005).

1.4 Quantification of effects

Caubel et al. (2001) compared concentrations of nitrate in soils between two sites, one with and the other without a hedge. They showed that nitrate concentrations were strongly affected by the presence of the hedge, up to distances of 10 m from the hedge. Nitrate in groundwater was three times lower with the hedge, with removal rates around 90% compared to 53% for the site without hedge. Borin et al. (2010) in Italy found that even a newly established 4 m wide buffer strip containing a line of trees and a grass strip reduced total run-off by 33%, losses of nitrogen (N) by 44% and phosphorus (P) by 50% compared to sites without buffer strips. A mature buffer strip was able to abate both nitrates (NO3–N) and dissolved P concentrations by almost 100%. In most cases it also proved a useful barrier for herbicides, with concentrations abated by 60% and 90%, depending on the chemical and the time elapsed since application. Duchemin et al. (2009) investigated agricultural non-point source pollution associated with liquid swine manure spread in corn plots adjacent to buffer strips. They found that in their first year newly established 5 m wide grass buffer strips reduced combined run-off and subsurface drainage water volumes by about 15%, total suspended solids (TSS) by 85%, total P by 75%, dissolved P by 30%, ammonia (NH4) by 50%, NO3 by 60% and E. coli by 25%. The addition of two year old poplars to the grass strips did not bring about a significant increase in the filtering capacity of the buffer strips during that first year. Hedge location, orientation and network density are often cited as important factors in the hydrological effects of hedge systems (e.g. Crossland et al. 2013). Hedges reduce the lateral movement of both surface and subsurface water most effectively when parallel to the contours of a slope and when planted at high densities (Benhamou 2013). The presence of embankments impacts on sediment movement in water (technical paper http://wwww.synagri.com, Adrian Collins pers. comm.). Also, nitrate removal will vary seasonally with higher levels of nitrate uptake by plants during warmer conditions (Ryszkowski 2007) and leafed periods (Vought 1995). A catchment level model (Benhamou et al. 2013) at the 5 km2 scale provided estimates of how the presence of hedges may impact on water flow and associated nitrogen flow at the catchment outlet. Results showed that the hedge network decreased predicted water flow at the outlet by 4.5% and N by 3.3%, compared to those flows when the watershed had no hedges.

1.5 Disadvantages of service The ability of buffer strips to absorb excess nutrients during growth periods has been shown

to be counteracted during the dormant season when they can release accumulated nutrients such as P, perhaps due to leaching following the decomposition of organic matter (leaves) (Osborne & Kovacic 1993). Stutter et al. (2012) highlight the need to understand ways of managing buffers to ensure that they can continue to provide an effective barrier

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against the transfer of nutrients to watercourses after, for example, complete saturation with P (which unlike N cannot be lost to the atmosphere). Periodic removal of woody growth, for example following coppicing, may be one way of retaining the capacity of hedges to continue to accumulate P.

One study highlighted the possible negative effects of shelterbelts in an agricultural landscape (Ryszkowski & Kedziora 2007) in terms of their potential to turn from a nitrogen sink to a nitrogen source due to the release of ammonium ions following the decomposition of leaves and other organic debris.

1.6 Relationship with other services

Flood risk reduction. Through acting as a physical barrier to surface water run-off and increasing the rate at which water soaks into the ground, hedges not only remove pollutants but also reduce the rate and volume of water reaching streams and rivers following heavy rainfall, so reducing the risk of flooding downstream. Soil erosion reduction. Water quality is directly linked with erosion reduction since water-borne sediment is the primary source of nutrients in water. Physical interception of eroded material by hedges is likely to have significant impacts on water quality (Adrian Collins pers. comm.). Crop water availability. The interception of water by hedges affects water availability for crops both negatively and positively, the roots of trees and shrubs either increasing soil water holding capacity or out-competing crops for water. Climate change mitigation. Linked to the capture of sediment in water moving downhill, hedges that fulfil a role in improving water quality may also play a role in climate change mitigation through increasing the store of carbon in the soil. The uptake of soluble nitrogen by tree roots from sub-surface water may reduce de-nitrification by soil microbes and hence the rate at which nitrous oxide (N2O), a powerful greenhouse gas, is emitted.

1.7 Recommendations for NELMS

1. To maximise the effectiveness of hedges at reducing diffuse pollution, the creation and restoration of hedges that contour slopes should be a priority target. Banked hedges are likely to be more effective than unbanked ones. While hedges that fringe valley bottoms or watercourses are best placed to intercept pollution running off slopes above, they can become saturated with nutrients and will lose phosphate (P) to water once this happens. For this reason, contouring hedges should also be encouraged higher up slopes, away from water courses. It may also be appropriate to remove woody growth off site following hedge laying or coppicing, to remove P from the system and prevent soil P saturation.

2. Although the evidence suggests that hedges just 1-2 m wide can be effective at

enhancing water infiltration even without a bank, the wider the hedge (shrub layer plus non-compacted herbaceous margins) the better.

3. Hedges will be most effective at reducing diffuse pollution in landscapes where soils are

compacted (e.g. many sheep-grazed uplands) or prone to rapid water run-off while

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receiving high inputs (e.g. arable soils with low porosity or where crops have not yet established).

4. Even small gaps in contouring hedges or those that border watercourses can greatly

reduce the effectiveness of such hedges in removing diffuse pollution, so priority should be given to restoring these. Gaps channel water and provide a focus point for intensified run-off and gully formation. This is particularly true for gateways that are heavily trafficked.

5. Ditches running alongside hedges, and drains cutting across them, will greatly reduce

the effectiveness of hedges at intercepting and impeding surface water flow. Consequently new hedges created to reduce diffuse pollution should normally not have ditches or associated drainage, and drains running under existing hedges should be blocked.

1.8 Value of evidence to CAP greening and cross compliance measures

Retention of hedges together with 2 m hedge protection zones will help to prevent deterioration in the water quality of rivers and streams, where the hedges contour slopes and are not associated with an effective field drainage system. However, these hedges are likely to offer few or no benefits if they are not maintained in good structural condition, in terms of the continuity of both herbaceous (grassy) bases and margins and woody growth.

1.9 Priorities for further research

Research studies needed to address major gaps in the evidence base, so that resources and management effort can be efficiently targeted and applied, are:

I. To understand how hedges (as opposed to purely herbaceous (grassy) buffer strips) impact on water quality at both field and landscape scale. While there is some evidence provided here, the research relevant to hedges is very sparse and largely focused around just one mid-slope hedge in Brittany, where nutrient measures in water are in-field rather than as delivered to a water course. Hedges of different types in different contexts (soil type, crop type, slope, hedge type, associated features/vegetation, field input levels, distance from watercourses, etc.) will have differing effects on water quality.

II. To establish a classification of hedges on the basis of their effectiveness in relation to

water quality provision, to make it possible to model the landscape impacts of hedges on water quality as done for a catchment in Brittany (Benhamou et al. 2013). This classification should include hedge position in the landscape, topographical factors, soil types, field management practices, woody species and management.

1.10 Conclusions

1. There is a large amount of published evidence that experimental buffer strips of varying widths can have significant positive impacts on water quality. Some evidence specific to the role of hedges (effectively grassy buffer strips with shrubs and trees) shows that they too can

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have significant positive impacts on water quality of water courses when running across slopes.

2. Results from research in Brittany (France) have been used to model the impacts of the

hedges on the quality of water discharged from a small catchment (5km2) (Benhamou et al. 2013). The catchment’s hedges were estimated to result in a decline in nitrogen (N) in the outflow of 3.3%. It should be noted that this model was based on a real life catchment and not one designed or managed to improve water quality. The hedges were at a density of 48 m per ha and field drainage systems were in place.

3. Lack of evidence about the potential for hedges to intercept and absorb nutrients and other

pollutants is likely to mean that their role in water quality improvement at a landscape level has been underestimated. Most water quality models fail to take account of woody linear features.

4. More research is required on the extent to which the position and location of hedged buffer

strips, together with their management and that of adjacent fields and their proximity to water bodies, are likely to affect their role in improving water quality.

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2. Flood risk reduction

2.1 Overview of information sources

Prior to 1995 very little information existed for the hydrological role of hedges in temperate regions (Merot & Reyne 1995). Even now relevant published information is scant, especially where quantified effects at a local landscape or catchment scale are sought. This applies to buffer strips in general (Viaud et al. 2004). Much of the most relevant published research comes from the INRA research institute at Rennes, Brittany (Benhamou et al. 2013, Caubel et al. 2001 & 2003, Ghazavi et al. 2008 & 2011, Merot & Bruneau 1993, Merot & Reyne 1999, Thomas et al. 2008, Viaud et al. 2004 & 2005). This information relates to banked hedges occurring in a climate and topography similar to south-west England and south Wales. It covers both individual hedges and networks of hedges. Otherwise, valuable evidence comes from the Pontbren experimental area in mid-Wales where the effects of tree planting on downstream flooding have been investigated (Carroll et al. 2004, Marshall et al. 2009 & 2013). However, to date no information on effects at a catchment scale has been published.

2.2 Anecdotes

1. The flood plain of the river La Sée at Tirepied in Normandy (France) was formerly crossed by some 40 parallel hedges which served to slow down the flow of water so that it acted as a slow-release reservoir (Jean Colette pers.comm.). The removal of these hedges is believed to have resulted in increased flooding of a town further down the catchment. The thick hedges had a low earth bank, the water being slowed down by this and by the numerous stems and low branches of hawthorn, hazel, willow, bramble and other shrubs and by trapped debris. During flood episodes water could be heard trickling through the hedges.

2. The village of Hillfarrance, near Taunton in Somerset (England), was regularly flooded by the Hillfarrance Brook, a tributary of the River Tone. Three or four years ago, the Environment Agency installed a sluice gate across the brook upriver of the village, to divert water during periods of high flow onto adjacent farmland. At the same time as the sluice was installed, new hedges, some with banks some without, were planted across the farmland to both divert the water and slow its movement. Subsequently, the village has not flooded, even during periods of exceptional rainfall such as the winter of 2013/2104. Hedges in the flood plain of the River Yarty in East Devon (England) have been observed to slow down the movement of flood waters, even those without banks due to the resistance of the thick growth (George Pidgeon pers. comm.).

3. Removal of many hedges in the mid to late 1970s from valley slopes above the village of Kenton, Teignbridge, Devon, to create large fields resulted in regular flooding of downstream settlements. The Environment Agency is now working with the landowners to create a terraced landscape to increase water infiltration and decrease surface water run-off (Martin Turner pers. comm.).

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2.3. How hedges deliver the service By increasing the rate of water infiltration, reflecting both the lack of the soil compaction

commonly found in adjacent fields and the greater root penetration of trees and shrubs compared to grasses and arable crops (Borin et al. 2010, Carroll et al. 2004, Kovar et al. 2011, Lenka et al. 2012, Marshall et al. 2009, Marshall et al. 2013).

By reducing soil water levels in and beyond the hedge root zone during the summer, so it takes longer to become saturated during the autumn, providing a buffer against flooding events. This drying effect of adjacent soil is due to the much greater loss of water in the hedge zone than from cropped areas due to the higher transpiration rates of hedgerow trees and shrubs, and the evaporation of rainfall intercepted by the hedge canopy before it reaches the ground (Caubel et al. 2003, Ghazavi et al. 2008, Ghazavi et al. 2011, Thomas et al. 2008, Thomas et al. 2012)).

By capturing soil from fields above, the upslope sides of banked hedges can build up to effectively form mini-terraces. This increases soil infiltration and prevents the upslope spread of saturated soils from valley bottom land. These effects can greatly reduce storm discharge to water courses, and highlight the particular benefits of banked hedges encircling saturated valley bottom fields (Merot & Bruneau 1993, Thomas et al. 2008).

By increasing the storage capacity of flood plains and slowing their release of water. Hedges, particularly banked ones but also unbanked thick shrubby hedges, can slow the movement of water across flood plains though increasing resistance to water flow. This helps to protect downstream settlements. This is based on anecdotal information only (see section 2 above): no relevant publications have been found.

2.4. Quantification of effects

Philippe Merot in his PhD thesis compared the hydrology of two topographically, geologically and edaphically similar 32 ha catchments in Brittany, one with and the other without a hedge network (bocage). The hedges were on banks and at a density of 106 m per ha, including 64 m per ha perpendicular to the slope and with a continuous bank enclosing the valley bottom. He found that following a typical storm, run-off volume and peak flow were 1.5 to 2 times lower in streams draining the bocage catchment, than in the catchment where there were no hedges. He found that within the bocage catchment about 5% of a storm’s precipitation reached the watercourse as storm flow regardless of the size of the storm, while the equivalent figure (i.e. the run-off coefficient) varied between a few percent and 15% for the catchment without hedges. The evacuation of the storm run-off took longer in the bocage catchment and the discharge peak was reduced. He noted, however, that this effect may not be so pronounced in large catchments due to the smoothing effect of large river flows. Banks must contour slopes to impede surface run-off effectively. It is these contouring hedges, and not the total hedge density, which is critical. Contouring banked hedges can prevent up to 40% of catchment run-off. Hedges enclosing the bottom of valleys play a critical role, because they can also limit the extension of saturated areas connected to the watercourses (Merot, 1999). Based on modelling and 17 years’ weather data, Benhamou et al. (2013) predict that an actual low density hedge network in Brittany would reduce overall water flow at the outlet from a 5km2 catchment by 4.5% (and N by 3%). It should be noted that the hedge network

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density was low at 48 m ha-1 covering 1.5% of land surface, and that neither it nor presumably any drainage system was designed or managed to hold back water. Modelling and simulation carried out by Viaud et al. (2005) in Brittany confirms that hedge networks have the potential to have a greater effect than individual hedges on stream discharge at a catchment scale. They estimate that even at the very low hedge density of 27 m ha-1, annual stream flow will be reduced by 10%. Their models suggest that water interception by the banked hedges studied reached its maximum at a density of 60 m ha-1 which will result in 51 mm of water being removed by the hedges in a year. By comparison, a density of 38 m ha-1 will result in a loss of 34 mm yr-1. In Italy, Borin et al. (2010) found that a 6m wide buffer strip on a 1.8˚ slope above a ditch reduced total surface and subsurface run-off by 78% over a five year period compared to a stretch with no buffer strip (where the total run off over the five years was 231 mm). The buffer strip consisted of two mature rows of plane Platanus hybrida trees and guelder-rose Viburnum opulus shrubs. At another site with a 4 m buffer strip planted at the beginning of the monitoring period with one line of trees (species not specified) and a strip of grass, total run-off over six years was reduced from 97 mm to 61 mm, a 33% reduction. No information is given on annual rainfall in the study site or on the crops grown in the fields above the buffer strip. Nevertheless, this research shows that hedges can at times be highly effective at reducing run-off even when freshly planted with trees and grass. In the Czech Republic, Kovar et al. (2011) estimated that contouring banked hedges will reduce extreme (one in a hundred year events) storm discharges to water courses by a factor of 2.5. (Even with less extreme events (one in ten year), these hedges will reduce soil loss.) At Pontbren, Carroll et al. (2004) report that strips of native trees (mainly birch and alder but with some blackthorn, oak and ash) can increase water infiltration compared to adjacent sheep grazed upland pasture by 60 times, when the trees are only six or seven years old. Rates of infiltration were nearly as great just 1 m into the trees as 5 m away from the field edge, although clearly the wider the tree belt the greater the overall infiltration. The authors conclude that farm trees can have a significant impact on run-off even when present as a small proportion of land cover. Further research at Pontbren has revealed that 12 m x 12 m plots of native trees typical of hedges (alder, ash, birch, blackthorn, hawthorn, Prunus spp., hazel and rowan) planted in upland sheep pasture reduce average run-off volumes by 78% and increased infiltration rates by 67 times. However, there was considerable variation between plots (Marshall et al. 2013). No figures are as yet available for any reduction in stream flows at a catchment level. In Brittany, Ghazavi et al. (2008) found that in a dry year the soil drying effect of a 15 m high oak hedge extended 9 m upslope and 6 m down slope. Re-wetting was delayed by three months within this zone, so reducing the risk of surface water discharge (and therefore flooding downstream) by three months. In a wet year, however, this effect was reduced to one month (Caubel et al. 2003, Ghazavi et al. 2011). Investigating the rainfall interception properties of hedges, Herbst et al. (2006) found that rainfall reaching the ground was reduced by 57% when the hedge they were studying was in leaf, and 49% when not in leaf. (They also showed that hedges create a rainfall shadow downwind of a width similar to the height of the hedge.)

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Thomas et al. (2012) found that the water removed from the root zone of hedges by transpiration (i.e. root uptake) was twice that of the combined effects of transpiration and evaporation from an adjacent short green crop. He concluded that hedges can remove excess water effectively under wet conditions. Previous research had shown that hedgerow oaks transpire nearly twice as fast as trees in woodland because the air around them is more turbulent and their root zone larger (Thomas et al. 2008). This demonstrates that hedges which have developed into lines of trees can play a major role in landscape water balance, movement and quality.

2.5. Disadvantages of service Hedges in floodplains can potentially increase flooding risk to downstream settlements by

reducing storage capacity. This is likely to occur in floodplains adjacent to major rivers where the priority is to remove water from the system quickly, to restore storage capacity for future storm events. For example, in the Netherlands trees and hedges are being removed from the floodplain next to the Meuse (Maas) upstream of Amsterdam and Rotterdam to reduce the risk of flooding to those cities (Wilfred Vruggink, Rijkswaterstaat, NL, pers. comm.). However, it is not known to the author of this report whether any similar situations occur in Britain. In the Netherlands they recognise that while it is desirable to remove hedges in the low part of the Meuse catchment, conversely further up the catchment, in France and Germany, it is desirable to have more hedges to stabilise and reduce peak flows.

2.6. Relationship with other services

Water quality. The reduction in the amount of water entering watercourses will clearly also reduce the amounts of pollutants (e.g. fertilisers and pesticides) and sediment reaching those water courses (Benhamou et al. 2013, Caubel et al. 2003, Merot 1999, Viaud et al. 2004, Viaud et al. 2005). The rapid removal of water from the root zone associated with hedge trees through transpiration has the effect of reducing the amount of soluble pollutants (e.g. N) reaching watercourses (Caubel et al. 2003). Erosion reduction. Where hedges impede surface water flow they will facilitate the deposition of sediment on the upslope side of hedges, especially those that are banked (Kovar et al. 2011, Lenka et al. 2012, Merot 1999). This process may result in terrace formation. Crop growth. Since hedges dry out the soil beneath and adjacent to them faster than crops do, hedges may reduce crop growth through limiting water supply. The effected zone can extend 9 m down slope and 6 m upslope (Ghazavi et al. 2008, Herbst et al. 2006). Conversely during wet periods, hedges may remove excess water from the soil in this zone and facilitate crop growth (Thomas et al. 2012). Climate change mitigation. Linked to the capture of sediment in water moving downhill, hedges that fulfil a flood alleviation role may also play a role in climate change mitigation through increasing the store of carbon in the soil. The uptake of soluble nitrogen by tree roots from sub-surface water may reduce de-nitrification by soil microbes and hence the rate at which nitrous oxide (N2O), a powerful greenhouse gas, is emitted.

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2.7. Recommendations for NELMS

1. To maximise the effectiveness of hedges at reducing flood risk, priority should be given to creating and restoring hedges that contour slopes and especially to those that fringe valley bottoms or watercourses. Banked hedges will be more effective than unbanked ones.

2. Where floodplain managers wish to prolong the duration over which they store water,

consideration should be given to planting hedges perpendicular to the flow, across the floodplains.

3. Although the evidence suggests that hedges just 1-2 m wide can be effective at

enhancing water infiltration even without a bank, the wider the hedge (shrub layer plus non-compacted herbaceous margins) the better.

4. Hedges will also be most effective in landscapes where soils are compacted (e.g. many

sheep-grazed uplands) or prone to rapid water run-off (e.g. arable soils with low porosity).

5. Even small gaps in contouring hedges or those that border watercourses can greatly

reduce the effectiveness of such hedges in hindering storm water, so priority should be given to restoring these. Gaps can channel water, enhancing erosion and soil loss.

6. Ditches running alongside hedges, and drains cutting across them, will greatly reduce

the effectiveness of hedges at intercepting and impeding surface water flow. Consequently new hedges created to reduce flooding should normally not have ditches or associated drainage, and drains running under existing hedges should be blocked.

2.8. Value of evidence to CAP greening and cross compliance measures

Retaining hedges together with 2 m hedge protection zones can reduce flood risk lower down catchments, but will offer few or no benefits in this respect if the hedges are not maintained in good structural condition, or if field drainage systems bypass them.

2.9. Priorities for further research


I. To quantify the effect of hedge networks in reducing flood risk in areas with differing topography, geology and soils, and in catchments of different sizes, at a catchment (or landscape) level. The ability of hedges to reduce flood risk following extreme weather events (i.e. exceptionally high rainfall) also requires research.

II. To determine the conditions under which hedges across floodplains are beneficial or detrimental.

III. To evaluate the comparative effectiveness of different hedge structures and dimensions (e.g. banked or unbanked, shrubby or lines of trees, thin or wide).

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2.10. Conclusions

1. Strong evidence exists to show that individual hedges (and other forms of buffer strip) contouring slopes or fringing water courses have the potential to reduce the volume of water reaching water bodies and the speed with which it does so, following storms. Lines of shrubs or trees of species commonly found in British hedges can greatly increase infiltration of water into the soil even when only a metre or two wide – by a factor of 60 to 70 times compared to compacted upland sheep pasture. Hedges also reduce soil water levels in and beyond the hedge root zone during the summer, so it takes longer to become saturated during the autumn, providing a buffer against flooding events. Individual hedges can therefore, to some extent, reduce the risk of flooding lower down in the catchment.

2. The real gains are likely to be achieved through networks of hedges. However little

published information is available to assess the potential here. The only relevant research comes from Brittany where it has been shown that a moderately dense banked hedge network in a 32 ha catchment with 64 m ha-1 following contours (108 m ha-1 in total) reduced peak and total flow within streams following storms by between a quarter and a half. In other words, contouring hedges in a real life example were able to halve the amount of surface run-off reaching streams (Merot 1999).

3. Further work in Brittany, based on models, suggests that networks that are half as dense

as those referred to in the preceding paragraph will reduce overall outflow over the course of year by between about 5% and 10%. Overall, water interception by banked hedges in the particular landscape studied was estimated to reach its maximum at a hedge density of 60 m ha-1 under average annual rainfall conditions. (The average density of hedges across lowland Devon is estimated at 107 m ha-1 (Devon Local Nature Partnership 2014)).

4. If an effective field drainage system has been installed, most water will find its way to

stream and rivers through this, regardless of how well-designed and managed any network of hedges or buffers may be.

5. Anecdotal information suggests that hedges in floodplains can slow down the movement

of water across them, and so increase their storage capacity. Removal of these hedges may increase the risk of flooding of land, or properties downstream.

6. More research is urgently needed to evaluate the full potential of hedges, buffer strips

and other linear features, to reduce flood risk at the catchment level under different conditions.

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3. Soil loss reduction

3.1. Overview of information sources

The majority of the literature comes in the form of peer-reviewed journals alongside a small number of published reports. However no studies have been carried out in UK; the most relevant studies are those carried out in hedge network (bocage) landscapes in France. However, even these studies are limited in number and hedge plant species are not identified. Instead they largely focus on changes in A-horizon (top soil) thickness and soil distribution. Soil erosion is a major problem in many countries outside Europe, particularly in Central Africa, China, India, Thailand and the Philippines, where agricultural land is located on steep hill slopes, reflecting the nature of the terrain and local farming practices. In these countries, soil conservation measures have been introduced in the form of contour hedges, an agroforestry system in which lines of trees, shrubs and stiff grasses (mainly napier and vetiver grass) are planted along the contours of sloping land at regular intervals. The strips between the hedges are planted with lines of crops, a system known as alley-cropping (a form of agroforestry). Research in these areas has either quantified the effectiveness of grass species already known to be efficient at controlling soil erosion, or tested a range of shrubs and grasses planted together to identify new ones to use. Since this research has largely taken place in the tropics or involves tall grass hedges with species that do not grow well in temperate regions, it is of minor relevance to the UK. However, agroforestry systems, including alley-cropping, are of increasing interest here as a more sustainable manner of food production than conventional large scale monocultures (Smith et al. 2013, Wolfe & Smith 2013). Contour hedges control soil erosion by acting as a permeable barrier to slow water run-off and trap and accumulate eroded sediment (Mutegi et al. 2008). For example, calliandra shrub and napier grass hedges have been shown to enhance soil deposition upslope of the hedge, so gradually forming a terrace, which not only reduces the amount of water moving through the hedge, but also significantly reduces the erosive force of water, so that less soil erodes downhill from the hedge (Angima et al. 2002).

3.2. Anecdotal information

None.

3.3. How hedges deliver the service Hedges act as physical barriers to reduce the movement and distribution of soil particles

carried down slope by water run-off or mechanical erosion (Follain et al. 2009, Mutegi et al. 2008). As a result of this barrier effect, eroded soil and sediment particles accumulate uphill from the hedge which causes a thickening of the A-horizon (top soil) (Follain et al. 2009, Follain et al. 2007). On sloping land this accumulation of soil uphill from the hedges gradually acts to reduce the slope of the land which results in the formation of natural terraces over a period of decades if not centuries (Chaowen et al. 2007, Salvador-Blanes et al. 2006). Over time the terraces themselves enhance the obstruction of soil movement (Mutegi et al. 2008).

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The ability of a hedge to reduce soil erosion is dependent upon its position in relation to the

slope gradient, the hedge type and agricultural practices (Walter et al. 2003). Hedges perpendicular to the slope are particularly important in the control of surface run-off and soil erosion (Pointereau & Colon-Solagro 2008), whilst earth and stone banks associated with hedges further aid the interception of soil and run-off (Walter et al. 2003).

Below ground, the hedge root system (particularly in shallow soils) can help to stabilise soil

as well as increase infiltration, thus diminishing run-off and the risk of erosion (Pointereau & Colon-Solagro 2008). Species such as napier grass Pennisetum purpureum grass with spreading, dense root systems near to the soil surface can be particularly effective at reinforcing soil and bringing about an increase in cohesion (Andreau et al. 1998, Mutegi et al. 2008). Above ground, leaf litter from hedges adds to the organic matter of the soil and this in turn improves soil structure and stability over time (Bu et al. 2008).

The greater the speed and vigour at which hedge species grow, the quicker a hedge is likely

to be effective at reducing water erosion (Andreau et al. 1998). As hedges mature, they create a more intact a barrier, so enhancing their capability to block soil eroded from land above and to increase the deposition of sediment (Mutegi et al. 2008).

Hedges can also be effective at reducing wind-borne soil erosion through acting as

windbreaks. They are likely to be particularly useful in this respect in flat open landscapes and those with light, typically sandy or peaty, soils (Farmer et al. 2008, Pointereau & Colon-Solagro 2008, Pollard et al. 1974).


Follain et al. (2006) developed a model to simulate the change in soil thickness over time depending on the effects of water erosion and diffusive transport processes. This model was based on a high resolution digital elevation model and a soil thickness map of an ancient 8 ha agricultural area in Brittany, France, with a high density of hedges. It was used to investigate the effect of hedges (type not specified) on soil redistribution. Using an initial uniform soil depth of 1.19 m within the landscape, the eroded and accumulated soil cells were shown to change depending on the presence of hedges within the landscape. After 1,200 years of simulation, the percentage of accumulated cells was 10.3% higher in the hedged landscape (48.5%) than in the landscape without hedges (38.2%), showing that hedges increase the capacity of landscapes to retain and accumulate soil. In a landscape without hedges it was shown that soil can be redistributed over long distances, whereas in a hedged landscape soil redistribution is limited to the area within fields. Overall, simulated soil thickness after 1,200 years was shown to decrease by 74% in the landscape without hedges, but increase by 62% in that with hedges. Soil thickness increased near hedges, particularly on their uphill sides. As already mentioned, hedges perpendicular to field slopes can result in the formation of soil terraces immediately uphill of the hedges. Using a digital elevation model Salvador-Blanes et al. (2006) observed three obvious field terraces within an 11 ha site made up of seven fields in the Massif Central, France. Of the three terraces, the hedge with the shortest length (85 m) accumulated more than twice the amount of soil material per unit length (7.4 m3 m-1) as the other two, longer, hedges. However, rather than this being a direct effect of hedge length, it was thought likely to be due to more intensive ploughing and/or longer use of the field above for cultivation. Furthermore, the distribution of coarse fragments, particle

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size distribution and soil depth in the upslope part of the fields showed that the formation of the terraces was more likely to be due to redistribution of soil through tillage, than to hedges acting as barriers. Walter et al. (2003) also studied hedges perpendicular to the slope under a range of topographical and geological conditions in Brittany. From seven study sites, the soil A-horizon thickness was seen to increase at all sites within the near vicinity of the hedge, particularly in the uphill position. Soil thickness ranged from 30-60 cm near the top of the slopes, whereas down slope thickness increased to more than 1 m under the hedge. Andreau et al. (1998) showed that the greater the intensity of a rainfall event, the greater the rate of erosion. The study based in North Valencia, Spain, showed that shrub barriers parallel to the slope were able to reduce run-off during 56 natural rain events over seven years. On bare sloping ground, the total run-off collected over seven years was 23,722 L, whereas total run-off collected on plots with shrub barriers was 18,938 L and 15,807 L. The shrub barriers were clearly able to reduce the effects water erosion. Hedges have been shown to reduce wind speed by 60% and have a wind break effect over a zone sixteen times their height thereby reducing wind-borne soil erosion (Pointereau & Colon-Solagro 2008). (Please see chapter 7 ‘Shelter’ for details of further research on the effectiveness of hedges as windbreaks.) Whilst the above studies address woody hedges, the majority of studies conducted on soil erosion have been focused on hedges composed of tall (1.5 m or more) clumped grasses such as vetiver or napier (also known as elephant grass) unlike any found in the British Isles. While these studies are of less relevance to the UK situation, nevertheless some of the principles emerging from them are applicable here. In a study in Kenya, Mutegi et al. (2008) assessed the effectiveness of two shrub species and napier grass planted along contours, either on their own or in shrub-grass combinations over a period of 17 months. At numerous slope gradients (5% to >30%), the stiff grass hedge was found to be the most effective at reducing soil losses. Whilst up to 11.9 t ha-1 of soil was lost from land with the stiff grass hedges, shrub hedges lost as much as 29.7 t ha-1 and 28.9 t ha-1

respectively. The effectiveness of the napier grass hedge was attributed to the spreading, fibrous roots which act to reinforce the soil in the first 30 cm. In India, Lenka et al. (2012) also studied stiff grass and shrub hedges, but over a period of three years. In contrast to Mutegi et al. (2008), a combination of stiff grass, in this case a Saccharum species, and shrubs planted as hedges were found to be the most effective at controlling both run-off and soil loss. Average run-off and soil loss over three years was 13.8% and 7.48 t ha-1

respectively from grass strips, but as low as 8.88% and 5.04 t ha-1 from a grass and shrub hedge. In a study by Fujisaka (1992), leguminous trees and napier grass were planted as contour hedges on steep sloping land in the Philippines. Across nine fields, the mean height of hedge embankments created as a result of soil accumulation was 36 cm after just one year, 44 cm after two years and 49 cm after three years. As a result, the mean slope of the strips between hedges (planted so the average field size was 0.8 ha) decreased from 16% to 9% after one year, 8% after two years and 7% after three years. Furthermore, 75% of the embankment was formed after just one year, suggesting that alleys formed relatively stable slopes within the first year after hedge planting.

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Bu et al. (2008) showed that a range of single-species shrub hedges in China were able to reduce run-off on sloping land by 17.2 - 70.8% and soil loss by 18.4 - 70%. In addition, the distribution and size of sediment particles within the experimental hedge plots were changed as a result of the hedges. Sediment particles within the size ranges of 2-0.2 mm and 0.2-0.02 mm increased, but sediment particles of the 0.02-0.002 mm and smaller than 0.002 mm groups decreased. Soil structure therefore became more stable as a result of selective erosion. Another study in China, specifically the upper reaches of the Huajiao River, Chaowen et al. (2007) observed that a shrub (false indigo) and vetiver grass hedge planted at widths of 0.5 m and spaced at regular intervals of 6.16 m was able to greatly reduce run-off and sediment just one year after planting. In comparison to fields without hedges, shrub hedges were able to reduce run-off by 63.0 - 70.8% and sediment loss by 81.9 - 85.7%, while the grass hedges were 25.2% more effective than the shrub hedges at reducing run-off and 39.7% better at preventing sediment loss. As the hedges matured, sediment deposition uphill from the hedges further increased over the 8 year study period. As a result of soil accumulation, stiff grass hedges were able to reduce the slope from 20° to almost 16°, whilst a shrub hedge reduced the slope to just over 17°, so aiding the formation of natural terraces. Spatial distribution of soil particles was also modified; average clay content on the uphill side of the stiff grass hedge was 6.5% higher than on the downhill side. In contrast, average sand content on the uphill side of the hedge was 22.8% lower than on the downhill side. The results show that the vetiver stiff grass hedges were more effective at obstructing the movement of small soil particles than larger ones. In Thailand, Donjadee et al. (2010) also observed the considerable merits of vetiver grass hedges to reduce soil erosion. Simulated rainfall was performed with three rainfall intensities: 60, 85 and 100 mm h-1. The magnitude of erosion increased within an increase in rainfall intensity, with erosion rates peaking before declining to an almost steady/constant rate. Average steady erosion rates of bare ground ranged from 4.73 – 37.80 gm-2 min-1, whereas erosion rates in the vetiver hedge plots ranged from just 0.99 –7.45 gm_2 min_1. Soil loss in the vetiver hedge plots ranged from just 1.02–5.85 t ha_1, whereas bare soil plots lost 4.56 to 29.90 t ha_1. Overall, vetiver hedges reduced runoff volume by 38.7 - 68.6% and total soil loss by 56.2 - 87.9%. These results were similar to those of Chaowen et al. (2007).

3.5. Disadvantages of service

Hedges planted or managed to control soil erosion may have an adverse effect on crop growth due to competition for water, nutrients and light. For example, in an agroforestry system above ground biomass of the food crop was found to be greater in the middle of alleys, away from the hedges (Agus et al. 1997). (See chapter on Crop Water Availability for further information on the effects of hedges on crop growth).


Climate change mitigation. Within the near vicinity of the hedges, soil organic carbon (SOC) increases as a result of the thickening A-horizon uphill from the hedge. The barrier effect of hedges therefore can have a local effect on carbon storage (Walter et al. 2003, Follain et al. 2007). An increased depth of the A-horizon was seen as the single most important factor for increase in SOC (Walter et al. 2003). Furthermore the leaf litter and root systems of hedges

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not only act to stabilise soil, but the accumulation of leaf litter has also been shown to increase the storage of soil organic carbon (Crossland et al. 2012). Crop growth. While hedges may adversely affect crop growth through competition for water, light and nutrients as noted above, they may also benefit crops through capturing nutrients that would otherwise be lost from fields and returning them to the soil through leaf fall, or through hedge trimmings. Hedges can therefore fertilise field soil (Angima et al. 2002, Lenka et al. 2012, Pointereau & Colon-Solagro 2008), although evidence for this occurring under temperate climates such as in the UK is weak. Water quality improvement. The capture of soil particles by hedges reduces not just soil loss from field systems, but also prevents nutrients and other pollutants attached to soil particles from entering water bodies.


1. Plant and restore hedges along contours to impede surface run-off and facilitate sediment deposition. Hedges that have both shrub and well-developed herbaceous (e.g. grassy) growth at their base and in their margins are most effective at preventing soil loss.

2. Hedges cannot stop erosion occurring in the fields above them, except in their immediate vicinity (Follain et al. 2006). On steeply sloping land, to prevent loss of soil to water courses, preference should therefore be given to planting or restoring hedges at the foot of the slopes.

3. Increase overall hedge density within landscapes. A reduction in field size will reduce the speed and erosive power of water (Pointereau & Colon-Solagro 2008) and is also likely to reduce wind-borne soil loss.

4. The establishment of alley cropping agroforestry systems, with hedges planted at regular

intervals between strips of crops, is likely to be effective at preventing soil erosion in high risk areas (Smith et al. 2013). The hedges can be used to provide woodfuel or perhaps fruits and nuts, and so generate an income for the farmer (Westaway et al. 2013).


Retention of hedges together with 2m hedge protection zones will help to prevent soil erosion and loss. Hedges that run perpendicular to slopes can be effective at preventing water-borne soil loss, where those which are perpendicular to the wind will be more effective at preventing wind-borne loss. However hedges are likely to offer little such benefit in the long-term if they are not maintained in good structural condition.



I. To evaluate the role of UK hedges in reducing water-borne soil erosion. Whilst

studies carried out in north-western France are of considerable relevance to the

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western parts of the British Isles, due to similarities in climate, hedge structure and farming landscape, the potential for hedges to control soil erosion in the UK can best be quantified if studies are carried out in this county. Research should be focused on hedges in areas at high risk from erosion rates, and preferably be conducted at a landscape scale and over many years.

II. To compare the effectiveness of different UK woody species. Different hedge species are likely to have different effects upon erosion losses due to the hedge canopy structure and rooting systems.

Research is also desirable on the following subject, but less of a priority:

III. Tall tussocky grass hedges and agroforestry systems have been shown to be effective at impeding soil erosion on sloping agricultural land in other parts of the world. The suitability of such approaches to the UK should be explored, especially in the context of climate change.

3.10. Conclusions

1. Little relevant research on water-borne soil erosion has been carried out in the UK, but evidence from France shows that hedges can be effective at both capturing soil from fields above, and lessening the rate of water flow over fields below, so reducing soil loss. Hedges lying perpendicular to the slope increase the depth of the soil A-horizon (top soil) and can result in terrace formation over time, particularly on their uphill side. This accumulation of soil and sediment reflects the ability of hedges to act a physical barrier to surface water run-off and to increase water infiltration into the ground. These effects are likely to be enhanced as hedges mature and form more intact barriers.

2. The species planted as a hedge can exert a considerable influence over the effectiveness of that hedge to reduce run-off and erosion. Hedges species with dense, strong woody and herbaceous growth close to the ground, and dense root systems, have been shown to be the most effective barriers. Tall stiff grasses, as widely used in tropical situations, meet these requirements and use of similar species adapted to temperate conditions should be explored.

3. Tall moderately dense hedges can serve as windbreaks to reduce wind borne soil erosion, particularly in flat open, landscapes and those with light, for example sandy or peaty, soils. (For further information on this, see chapter 7 ‘Shelter’).

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4. Crop water availability


Information on the role of hedges and their impact on crop water use was primarily taken from two modelling/review papers from the USA (Dickey 1988, Davis & Norman 1998) and a number of global academic papers (Alegre & Rao 1996, Campi et al. 2009, Caubel-Forget 2001, Costa & Surenthran 2005, Ghazavi et al. 2008, Ghazavi et al. 2011, Herbst et al. 2006, Huxley et al. 1994, Jose et al. 2000, Kang et al. 2008, Kowalchuk & Jong 1995, McIntyre et al. 1996, McIntyre et al. 1997, Nurberg 1998, Onyewotu 1994, Smith et al. 1997, Smith et al. 2013, Steppuhn & Waddington 1996, Thomas et al. 2008, Thomas et al. 2012, Viaud et al. 2005). As with information about other water-related ecosystem services there are a number of papers (6) from INRA in France focused on understanding the hydrological impacts of hedges and a particular model hedge in Brittany (Caubel-Forget 2001, Ghazavi et al. 2008, Ghazavi et al. 2011, Thomas et al. 2008, Thomas et al. 2012, Viaud et al. 2005). There was a further paper on the hydrological impacts of two hedges in southern Britain (Herbst et al. 2006). The remaining papers focused on a range of hedge-like features which included shelterbelts, windbreaks, contour hedging and agroforestry hedges and their various impacts on water availability to crops.


None.

4.3. How hedges deliver the service

The impact of hedges on the availability of water to crops nearby may be either negative or positive. The shrubs and trees and their associated root structures may result in net up-take and transpiration of water, making it unavailable to the crop. Conversely, the shrubs and trees can benefit crops through encouraging the retention of moisture by intercepting rainfall and the control of water movement through the landscape. As with other ecosystem services, the type of hedge and its condition and position in relation to the crop are all likely to impact upon the delivery of this service, as is rainfall. Hedges influence the availability of water to crops in the following ways:

Reducing water loss through evaporation and transpiration by shading and decreasing wind speed across fields.

Moisture retention in leaf litter.

Increasing lateral transfer of water – through creating high soil water potential gradients in their vicinity.

Providing pathways for water to be conducted through the soil column to depth.


Few studies have quantified the effects of hedges on crop water balance. Caubel et al. (2001) found that a hedge in Brittany running across a slope had negligible effects on the hydrological dynamics of the adjacent soil in winter. However, from late spring to late

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autumn the soil become increasingly dry upslope from the hedge (in the area covered by hedge roots), which led to a delayed re-wetting of soils in autumn. A further study (Ghazavi et al. 2008) indicated that hedges influence (1) spatial rainfall distribution, (2) soil re-wetting and (3) ground water recharge at distances well beyond the drip line of the hedge in dry years, with interception of rainfall by the hedge comprising 28% of local rainfall. Ghazavi et al. (2011) also observed that hedges control water availability by increasing lateral transfer, reflecting high soil water potential gradients in the vicinity of the hedge, a response which varies according to rainfall. Herbst et al. (2006) found that the hedges in their research site in southern Britain intercepted and prevented 24% of rainfall from reaching the ground when in full leaf, reducing to 19% when the hedges were leafless. They also found that during many small rainstorms, often associated with high wind speeds, the hedges intercepted rainwater that would otherwise have fallen on adjacent crops. Research on contour hedging in Peru showed that intercropping conserved, on average, 287 mm water each year (Alegre & Rao 1996). This represented 83% of the amount that would have been lost if all the land had been under crops. Other work, in Sri Lanka, showed negative impacts for five out of six hedging species on crop (tea) yields of between 22% and 40% (Costa & Surenthran 2005). Only one species resulted in increased yields (23%) although the addition of mulch from all species was beneficial to crop production. Studies in Kenya, Nigeria and Niger revealed that competition between woody hedge species and crops resulted in reduced crop yields, except where there was clear resource differentiation between hedge species and crop, either in use of groundwater or through a temporal difference in water requirements (Huxley et al. 1994, McIntyre et al. 1996, McIntyre et al. 1997, Onyewotu 1994, Smith et al. 1997). These papers show that in dry agricultural systems the presence of hedges can be detrimental to crop water availability. Studies on shelterbelts and windbreaks are also relevant to the impact of hedges on crop Water Use Efficiency (WUE) – the ability of plants to use water to grow. In Italy, windbreaks were found to reduce water loss from evapotranspiration for a distance of 12.7 times the windbreak height, WUE within this zone being 1.15 compared to 0.70 outside of it (Campi et al. 2009). In Canada, in low rainfall years, crop yields fell immediately adjacent to a shelterbelt due to competition for water but were slightly higher in the next band outwards into the crop (Kowalchuk & Jong 1995). Models of water use efficiency developed in the USA indicate that in some cases shelterbelts could contribute towards greater water use efficiency in crops, but that the exact mechanism by which they do so would be difficult to determine (Davis et al. 1988).


Since hedges dry out the soil beneath and adjacent to them faster than crops do, and produce a rain shadow downwind as wide as the hedge is high, hedges may induce a summer soil water deficit reducing crop growth. The effected zone can extend 9 m down slope and 6 m upslope (Ghazavi et al. 2008, Herbst et al. 2006). Conversely during wet periods, hedges may remove excess water from the soil in this zone and facilitate crop growth (Thomas et al. 2012). Whether hedges (or shelterbelts or windbreaks) will have a net positive or negative effect on crop growth through hydrological effects will vary with crop type, hedge type, climate, topography and soils.

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Water quality improvement. Where hedges increase crop water availability to crops through either decreasing evapotranspiration or improving soil water storage or accessibility, they are likely to have a positive impact on the water quality of water bodies, through increasing nutrient uptake. Flood risk reduction. Likewise, where hedges have a positive impact on crop water availability through the same mechanisms they are likely to reduce flood risk downstream through the retention and storage of water. Soil erosion reduction. Crop water availability is indirectly linked with the potential for hedges to reduce water-borne soil erosion, since both depend heavily on the ability of hedges to intercept and retain water. Reduction of wind speed across fields due to hedge presence will reduce erosion and enhance crop water availability. Shelter. Hedges grown as windbreaks reduce water loss in adjacent crops through reducing evapotranspiration, but on the other hand, the hedge plants themselves can compete with the crops for water.


The enhancement of crop growth is not a NELMS objective. Nevertheless, it is desirable that hedges should be planted and managed in ways that, while delivering NELMS objectives, prevent any adverse effects on crops.

Hedges are most likely to have a net detrimental effect on the water available to crops in the immediate vicinity either during dry periods when the shrubs and trees will out-compete the crops for water or during periods of high rainfall when hedges can prolong soil saturation on uphill slopes, producing waterlogged conditions.

The promotion of wide field margins may reduce the risk of such adverse conditions arising, although productive land would be lost.


Since hedges can be either beneficial or detrimental to adjacent crop growth through altering water availability, the retention of hedges as landscape features either under greening or cross-compliance measures may be regarded as largely neutral in terms of crop yield. As noted under Shelter (see Chapter 7), overall the retention of hedges may be expected to maintain yields in some areas such as sandy or peaty soils in East Anglia, and in those specialising in horticulture or fruit, as well as benefiting livestock.



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I. To understand the extent to which hedges may either enhance or negatively impact on crop yield and which types of hedges have the potential to be most beneficial. This research should take place at both field and landscape scales, and consider type of hedge, the crop type, hedge location, weather conditions, etc... The effects of hedges on yield in places that are either atypically dry or which experience unusually low or unusually high rainfall (or both), are likely to be particularly useful.

II. To evaluate the role of hedges in intercepting rainfall at a landscape level. Hedges may be of particular importance in this respect where few other deep-rooted plants are present. Research investigating how field scale impacts scale up across landscapes, similar to that carried out by Benhamou et al. (2013), would be valuable.

4.10. Conclusions

1. Published evidence shows that hedges have impacts on field level hydrology which vary according to rainfall conditions, slope, hedge height, position of nearby water bodies, etc…

2. These impacts range from being highly beneficial (increase in productivity) to highly

damaging to crops, dependent on crop, hedge species, local weather conditions, hedge position, root extent, etc... In most cases, damaging impacts of hedges are confined in temperate climates to very dry places or to dry years.

3. There is a need for research to determine the potential role of hedges in enhancing

crop water availability and hence crop water use efficiency under UK conditions. In particular evidence is needed about where there is potential for hedges to be used to improve crop water availability, at either the field or the landscape scale.

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5. Crop pest reduction


Information on the potential role of hedges in crop pest reduction was gathered mostly from peer-reviewed academic papers, together with some reviews and reports. Hedges provide all of the four key resources (pollen and nectar, alternative prey, shelter and larval development microhabitats) for predatory and parasitic arthropods to thrive and to have a significant impact on crop pest reduction in agricultural landscapes (Holland & Oakley 2007). The complexity of the habitat structure and the plant species composition will determine the range of niches and hosts available and consequently the diversity and abundance of associated beneficial arthropods (Burel & Baudry 1995). Within intensively managed cropping systems polyphagous predators, predators that prey on several different species of animal, that have overwintered in field boundary habitats are more likely to be present and even dominate the arthropod fauna in the spring (Wissinger 1997, Ribera et al. 2001). This is the time of year when predators have greatest potential to control pest population outbreaks (Chambers et al. 1986). In particular, generalist predators within the beetles (Carabidae and Staphylinidae) and spiders (Lycosidae and Linyphiidae) are numerically dominant in temperate fields and a large proportion overwinter as adults or juveniles in non-crop areas (Sunderland et al. 1987). Many of these species show seasonal migration between crop and shelter habitats at different stages in their life cycle (Coombes & Sotherton 1986, Duelli et al. 1990). Certain species of polyphagous predators will also use shelter habitats during the summer as sites for refuge from unsuitable microclimatic conditions, oviposition and larval development (Wallin & Ekbom 1988). Optimal shelter habitat can improve overwintering survival and subsequent reproductive success of polyphagous predators. This is achieved by ensuring availability of pre-overwintering food for the build-up of fat reserves, and of drier microhabitats with stable but relatively low temperatures to limit direct mortality and the depletion of fat reserves (Sotherton 1985, Van Dijk 1994, Dennis et al. 1994, Zhou et al. 1995, Petersen et al. 1996, Petersen 1999). Prey availability in shelter habitats in early spring can also influence post-overwintering mortality, fecundity of surviving individuals and timing of dispersal into the crop (Thomas et al. 1992, Bommarco 1998, Peters 1999). Although generalist predators vary in their degree of polyphagy, several studies have indicated (Toft 1995, Jorgensen &Toft 1997, Toft & Wise 1999) that this behaviour is necessary to enable predator populations to be maintained when the pest is absent or present at only low levels (Settle et al. 1996). Therefore, a source of alternative prey or hosts is needed to ensure survival and to maximise reproductive potential. These may be present within non-crop habitats or the crop. For soil living natural enemies a source of diverse organic matter is needed on which alternative prey (e.g. bacterial and fungal feeding nematodes) can thrive (Food and Environment Research Agency 2012). Field margins, particularly those that are grass dominated, offer overwintering sites for beetles and spiders. Tussock-forming grasses (e.g. Dactylis glomerata and Holcus lanatus) provide appropriate and relatively stable conditions during the winter (Luff 1966) and result

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in better invertebrate survival compared to other plants structures (D'Hulster & Desender 1982). In autumn and spring grass margins that were either sown or established through natural regeneration were found to contain predatory beetles (Coleoptera: Carabidae, Cantharidae, Coccinelidae and Staphylinidae), harvestman (Opiliones), spiders (Araneae: Lycosidae and Linyphiidae) and bugs (Heteroptera: Anthochoridae) (Meek et al. 2002). In winter a diverse range of carabid and staphylinid species and spiders from the families Lycosidae, Linyphiidae, Tetragnathidae and Clubionidae were found (Pywell et al. 2005) and other beneficial species including woodlice (Isopoda) and earthworms (Lumbricidae) (Smith et al. 2008). Grass margins also support a diverse range of alternative prey including phytophagous invertebrates (Woodcock et al. 2008) and the hosts of parasitic wasps (Powell & Pickett 2003). Plant diversity, density of herbaceous vegetation and leaf litter can all influence the distribution of soil-dwelling species that are prey items for polyphagous predators in the hedge base and margin (Altieri & Letourneau 1982, Dennis et al. 1994, Hovermeyer 1999). In addition to provision of food, certain attributes of shelter habitats have been associated with preferential microclimatic conditions that benefit overwintering arthropods. For example, a raised bank can improve drainage (Sotherton 1985). Different vegetation types and cover will affect fluctuations in temperature, with tussocky grasses appearing to provide favourable stable temperatures for overwintering polyphagous predators (Thomas et al. 1992, Collins et al. 2003). Grass mixtures also increase the structural diversity of the vegetation and consequently the diversity of beneficial arthropods (Baines et al. 1998). Web-building spiders have been shown to prefer a closed structure (Robinson 1981), and litter depth is an important determinant of spider community composition (Bultman & Uetz 1982). However, some relatively simple habitats (e.g. fence lines) can support a high number but low diversity of beneficial arthropods that may contribute to pest control (Griffiths et al. 2007). The value of the shrubby component of the hedge as an overwintering habitat is not well documented. In winter hedges were found to contain spiders (Maudsley et al. 2002) and ladybirds (Coccinelidae) (van Emden 1965). When the emergence of overwintering invertebrates was measured using pitfalls located at the hedge base in completely enclosed hedge sections, a greater range of taxa was collected, suggesting that invertebrates overwintered in the hedge base (Griffiths 2003, Griffiths et al. 2007). The presence of a well-established hedge base or ditch can, however, lead to a greater invertebrate diversity within the hedge (Pollard 1968, Maudsley et al. 1997). Similarly, the presence of mature trees along a hedge adds structural diversity and provides a specialised habitat for some invertebrates, for example, true flies (Diptera) (Peng et al. 1992). No information could be found on the relationship between shrub layer species diversity and invertebrate populations within adjacent fields, and no studies investigated whether changes in hedge management had an impact on either the predators or levels of pest control in adjacent fields for the UK (Food and Environment Research Agency 2012).


At a Hedgelink visit to East Lothian in Scotland in July 2010, Michael Williams of Eaglescairnie Mains and Hugh Broad of Woodhead Farm both reported that since planting new hedges and creating beetle banks neither had needed to use invertebrate pesticides on their cereal crops. They attributed this in part at least to increased populations of pest predators. The

47

average size of fields at Woodhead Farm was 50 ha (Robert Wolton, unpublished Hedgelink report).

5.3. How hedges deliver the service The main way that hedges deliver this service is by supporting a wide range of invertebrates

within their five main structural components – shrub layer, tree, base, bank and ditch. The diversity of arthropod species in a hedge is determined to a large extent by the structural complexity of the hedge, that is the more complex the structure, the more varied the habitats and niches available.

Floral diversity in hedges is linked to a diverse fauna, complementing the range of

microhabitats, food, shelter and alternative prey present (Pollard et al. 1974). In the shrub and hedge tree components of a hedge the number of arthropod species associated with individual plant species found can vary enormously. Kennedy & Southwood (1984) found that hawthorn Crataegus monogyna supported 209 invertebrate species, whilst holly Ilex aquifolium only supported ten species. Predators from five orders (Araneae, Coleoptera, Diptera, Hemiptera and Hymenoptera) were found to be the dominant functional group associated with the shrub component of hedges in southern Britain, accounting for 90% of all arthropods captured (Pollard & Holland 2006). The study confirms the role of hedges as a habitat for predators, as suggested previously (Lipkow 1966, D’Hulster & Desender 1982, Sotherton 1985).

Field margins increase general arthropod diversity on farmland: in summer by providing a

stable, complex habitat for species that would not survive in the farm landscape with the presence only of crop habitats, and in winter by providing refuges for many arthropod species including those predators active in arable fields during the summer. Field margins influence the species richness and densities of predators of crop pests in adjacent crops in spring, when they have the potential to suppress pest populations.

The presence of polyphagous predators within the hedge base is determined by plant

diversity, density of herbaceous vegetation and leaf litter, and by the distribution of soil-dwelling species that are prey items for these predators (Altieri & Letourneau 1982, Dennis et al. 1994, Hovermeyer 1999).

Insect feeding bats have been shown to reduce crop pests in the USA, with considerable

economic value to the cotton industry in particular (Boyles et al. 2011, Cleveland et al. 2012). Bats use hedges as flight lines, to navigate and to feed. Whether their diet consists of crop pests in large enough numbers to have an impact on crop pest reduction in the UK is not known.


If farmers are to be convinced of the pest regulation services provided by hedges then they will also need information on the financial implications. However, no research appears to have been carried out on this important point. One study did assess the economic benefits of beetle banks, but based only on the cost of establishment and income foregone for the land occupied by the bank (Collins et al. 2002). The study did not include any measure of reductions in insecticide use or any subsequent yield gain. In 2002, the establishment costs were £975 ha-1 with subsequent costs of £2 ha-1 for income foregone from the land occupied. Thus the agri-environment scheme payments of £600 ha-1 would cover these

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costs within two years. The cost of an insecticide was £3 - £12 ha-1 without application costs, but aphicides are typically added to a fungicide programme. Without the AES payments, beetle banks are not therefore economically advantageous (Food and Environment Research Agency 2012). Beetle banks may be more cost effective in organic systems. (Beetle banks typically comprise low earth banks created across arable fields and sown with a high proportion of tussocky grasses. As such, they resemble the base and margins of hedges where these features have a permanent grassy cover.) Two studies were conducted to determine whether beetle banks lead to more even predation across fields. When artificial prey were located across fields up to 60 m from the beetle bank, predation rates were even at all locations, although predation was highest on the bank itself (Thomas 1990). The impact on naturally occurring aphid infestations was evaluated within confined plots that excluded ground-dispersing predators. These were established at 8, 33, 58 and 83 m from a beetle bank (Collins et al. 2002). The mean number of aphids and aphid peak was reduced up to 58 m from the beetle banks, but reductions were greatest at 8 m. The use of habitats (including hedges) to improve pest control has been examined across the world in a variety of cropping systems. The approaches taken and the success of 51 studies published between 1990 and 1999 were reviewed by Gurr et al. (2000), as reported in Food and Environment Research Agency (2012). Nineteen studies investigated the impact on natural enemies in the target crop with most reporting an increase. Pest levels were recorded in 22 studies with 19 reporting a reduction of the pest. Of the 22 studies, 15 demonstrated that the higher levels of natural enemies were responsible for improved pest control. Only one showed that the habitat was acting as a sink, attracting natural enemies away from the crop. Ten studies looked at damage levels, with six showing reduced damage. The effect of hedges on natural enemies in orchards has been investigated in several European countries and was reviewed by Simon et al. (2010). Natural enemies were more abundant in trees nearest the hedge and a gradient of density from the hedge towards the orchard was found for some species, for example lacewings (Neuroptera). However, such a gradient was not always found nor was there a correlated impact on pests and some pests were able to utilise the resources provided by the hedge. Debras et al. (2008) demonstrated that there was a relationship between the structural complexity of hedges and the diversity of arthropod populations in pear orchards (Fournier & Loreau 2002, Aviron et al. 2003). This relationship strengthened when floral species were chosen for their ability to attract beneficial organisms in the autumn, shelter them over winter, and provide them with alternative prey in early spring when pest prey populations are low, favouring their dissemination into the orchard (Jervis & Kidd 1996, Kreiter et al. 2002). Certain floral species were shown to harbour specific insects which feed on them and which are harmless to pear trees, for example elder Sambucus nigra and its aphid Aphis sambucci (Hemiptera: Aphididae). In return, these insects attract a broad range of general and specific predators and parasitoids, such as the predatory bug Anthocoris nemoralis (Hemiptera: Anthocoridae), and generalists including many other bugs (Heteroptera), lacewings (Neuroptera), etc., which can control general pest populations and particularly those of Cacopsylla pyri (a pear psyllid) (Erler 2004). Bianchi & van der Werf (2003) examined the impact of shape, area and fragmentation of non-crop landscape elements on overwintering of the ladybird Coccinella septempunctata

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and aphid control in arable crops. Landscapes with 9% and 16% non-crop habitat supported enough C. septempunctata to control aphid infestations, particularly when small hedge elements were evenly distributed across the landscape. Potts & Vickerman (1974) found that diversity of other arthropods in the field increased in relation to higher numbers of predatory arthropods and a corresponding increase in predator density towards field margins. They concluded that on the single field scale, variation in the density of overwintering predators cannot be explained only by the quality of adjacent field margins. The landscape scale matrix of field margins may be vital for the effective dispersal of predators into crops and for achieving conservation goals (Mader 1988 & 1990). In other words, the pattern of field margins may contribute to differences in predator densities and arthropod species richness found within farmland, as opposed to the habitat quality alone. The possibility that bats may have a beneficial impact through reducing crop pests has not been explored in this country, but research carried out in north and central America (see below) suggests that they could play a role in this respect in the UK. Here bats are known preferentially to use hedges for feeding and for flight lines (e.g. Downs and Racey 2006), so it is possible that hedges may exert some influence on crop pest numbers through encouraging bats onto farmland. In Indiana, a single colony of 150 big brown bats Eptesicus fuscus was estimated to eat nearly 1.3 million pest insects each year, possibly contributing to the disruption of population cycles of agricultural pests (Boyles et al. 2011). Estimating the economic importance of bats in agricultural systems is challenging, but published estimates of the value of pest suppression services provided by bats ranges from about $12 to $173 acre-1 (with a most likely scenario of $74 acre-1) in a cotton dominated agricultural landscape in south-central Texas. Boyles et al. (2011) estimated the value of bats to the agricultural industry as between $3.7 and $53 billion yr-1 (with a median value of $22.9 billion yr-1). These estimates include the reduced costs of pesticide applications that are not needed to suppress the insects consumed by bats. However, they do not include the ‘downstream’ impacts of pesticides on ecosystems which can be substantial, or other secondary effects of predation, such as reducing the potential for evolved resistance of insects to pesticides and genetically modified crops. Boyles et al. (2011) concluded that bats have enormous potential to influence the economics of agriculture and forestry. Cleveland et al. (2012) examined how Brazilian free-tailed bats Tadarida brasiliensis could contribute to crop pest reduction in south-central Texas and northern Mexico. Brazilian free-tailed bats form enormous breeding colonies, mostly in caves and under bridges, in the summer. Their prey includes several species of adult insects whose larvae are known to be important agricultural pests, including the corn earworm or cotton bollworm Helicoverpa zea. The bats' value for pest control to cotton production in an eight county region in south-central Texas was estimated as $741,000 per year, with a range of $121,000 to $725,000. The total annual value of the cotton harvest was $4.6 to $6.4 million per year.

5.5. Disadvantages of service Hedges may disadvantage crops by harbouring pests (Jeanneret 2000), in addition to being a

source of natural enemies (Debras et al. 2008). For example, in one study grain aphids in arable crops were found to originate from adjacent hedges in some years (Vialatte et al. 2007). Hedges may also depress yield because they harbour pests such as carrot root fly Psila rosae (Pollard et al. 1974). Generally though, benefits are considered to outweigh the disadvantages.

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Pollination. Some crop pest predators are also pollinators and may increase crop yield through improved pollination services. For example, the larvae of hoverflies of the subfamily Syrphinae are thought to be important predators of cereal aphids, while the adults are considered effective pollinators. Water quality improvement. The provision of favourable habitats within hedges for crop pest predators, in particular the presence of tussocky grass margins, has the potential to increase the effectiveness of hedges to reduce diffuse pollution of water bodies (Schlosser & Karr 1981). Also, if pesticide use is reduced as a result of the presence of hedges, then this may bring benefits for water quality (and human health). Soil erosion. Similarly hedges that are in favourable condition for crop pest predators are likely to be effective at reducing soil erosion (Forman & Baudry 1984).


The enhancement of natural enemies of crop pests, or biological control, is not an objective of NELMS. Nevertheless, it can bring environmental benefits through reductions in the use of pesticides, so some management advice is offered here: 1. Management to promote structurally diverse hedges is likely to be beneficial to crop

predators, through increasing the range of available microhabitats and resources.

2. The creation or maintenance of tussocky grass margins and basal vegetation will be of particular benefit to overwintering predators, as has been shown for beetle banks.

3. Likewise, the encouragement of flowers both in shrub and tree layers and in basal and

marginal vegetation is likely to be beneficial to at least some pest predators and parasitoids. The availability of flowers from spring through to autumn is desirable, although those plants which flower early in the year may be of particular value.

The evidence on whether woody species should be cut every year or less frequently is equivocal. While hedges cut once every three years will on average produce more flowers (Staley et al. 2012), annual cutting may encourage more vegetative growth and benefit herbivorous insects (Maudsley et al. 2000) that may then serve as alternative prey for natural enemies.


The retention of hedges under greening and cross-compliance measures is likely to offer some advantages to farmers through enhancing populations of crop pest predators in the landscape. It may also reduce the need for insecticides and increase profitability, although evidence for this is weak.

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I. The cost effectiveness to farmers of retaining and managing hedges (and related

features such as beetle banks) to boost populations of natural enemies. Surprisingly, there appears to be practically no research on this topic relevant to the UK.

II. The distance into fields over which crop pests are significantly reduced as a result of the

presence of hedges. Existing research on this is very limited.

Research is also desirable on the following:

III. Optimal means of managing hedges for natural enemies focused on taxa other than beetles and spiders, such as on hoverflies, empidid and dolichopid flies, and Parasitica (Hymenoptera). Research to date has been overwhelmingly focused on beetles, and to a lesser extent on spiders.

IV. The influence of bats on crop pest abundance. The studies on this reported here were

carried out in the USA. Bats are known to preferentially use hedges for feeding and flight lines, and potentially could have an impact on some crop pests (for example some beetles and flies).

5.10. Conclusions

1. Evidence conclusively shows that field margins and beetle banks, both of which are very similar to the ground flora components of hedges, attract predatory arthropods. In particular, they can provide suitable overwintering habitat, the insects and spiders moving out into the crops in the spring and summer. Most research has been on ground dwelling arthropods such as carabid beetles and to a lesser extent spiders and their use of grassy, often tussocky, margins. Evidence relating to the aerial dispersal of predators such as hoverflies from hedges into crops is much more limited (Food and Environment Research Agency 2012). Research shows that the greater the structural and floristic diversity of hedges the greater their invertebrate diversity, but the influence of this relationship on crop pests remains unknown.

2. A favourable landscape scale matrix of field margins may be vital for the effective

dispersal of predators into crops (Mader 1988 & 1990). In other words, the pattern of field margins may contribute to the differences in predator densities and species richness of arthropods on farmland, as opposed to the habitat quality alone.

3. No research appears to have been carried out in the UK or similar temperate conditions

on the effects of predators associated with hedges, field margins or beetle banks on crop yields, let alone the cost effectiveness of these linear habitats in terms of crop yield or pesticide use. Available evidence is anecdotal only.

4. If natural enemies are to provide a significant level of biological control, it is necessary to

ensure that pest predators have sufficient resources to ensure their survival and

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reproductive capacity throughout the year. Hedges together with crops may not be sufficient to meet these needs for all species. Additional non-cropped habitats such as extended flower-rich field margins, fallow land, bare ground and woodland are likely to be important for the survival of many predators and parasitoids (Bianchi et al. 2006).

5. Conservation biological control theory suggests that creating habitats with greater floral

diversity will increase the diversity of resources for natural enemies leading to an increase in biological control (Tscharntke et al. 2005). However, even in more complex habitats, plant species that provide key natural enemies with necessary floral resources are not always present (Olson & Wäckers 2007); instead adding plants that have known uses for natural enemies may be more beneficial (Baggen et al. 1999, Wäckers 1996). Examples of such plants may be hogweed Heracleum sphondylium, wild carrot Daucus carrota are other umbellifers which are highly valued as a nectar source by adult hoverflies and parasitoids (Robert Wolton, pers. obs., see also Pocock et al. 2012).

6. Pest species themselves may utilise the additional resources managed or created to

benefit their natural enemies, within unknown effects on biological control (Baggen & Gurr 1998).

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6. Crop pollination improvement


There is much evidence that in areas of intensive farming hedges, together with other patches of non-cropped land such as headlands, are important to the survival of many pollinators (Nicholls & Altieri 2013). Indeed, appropriate management of non-cropped areas to encourage wild pollinators is considered likely to be a cost effective means of maximising crop yield. Hedges, with their shrubs and trees, basal and marginal herbaceous flora, can provide essential resources for pollinators that are otherwise lacking in the landscape (Hannon & Sick 2009). Furthermore they attract pollinators into the farmed landscape, and into nearby crops (Haenke et al. 2014, Morandin & Kremen 2013) including apple orchards (Miñarro & Prida 2013). Farms with large field sizes necessarily have a low proportion of hedges or other field margins. Since these features provide important resources for wild pollinators, including nest sites and floral resources when crops are not flowering, then farms with large fields will have relatively few pollinators, regardless of the pesticide regime adopted (Belfrage et al. 2005). Mass flowering crops such as oilseed rape and field beans provide food resources for adult pollinators but only for a short time and are of little overall conservation benefit (Hanley et al. 2011). The majority of research into the effects of hedges and other field boundary habitats on crop pollinators has been on bees, especially on bumblebees. Several authors have noted that bumblebee food resources could be improved by managing and extending field margins and other uncultivated areas on the farm (e.g. Fussell & Corbett 1991). However, although bees are known to be of considerable economic importance for the pollination of food crops (Delaplane & Mayer 2000), other insect orders are also likely to be important, for example, hoverflies (Haenke et al. 2014). A study of flower-visitors in an English meadow found that true flies (Diptera) were more abundant as flower visitors than all other orders combined (Memmott 1999). Indeed, although most British plants have a number of pollinators and most pollinators visit a number of plants (Memmott 1999), the diversity of pollinators is considered more important for crops than their abundance (Christmann & Aw-Hassan 2012). High pollinator diversity is desirable to allow adaptation to climate change - for example, honey bees cannot operate in cold or cloudy conditions (Christmann & Aw-Hassan 2012). British crops that are dependent on insect pollination include oilseed rape, field beans, top fruit (apples, pears, etc.) and soft fruit (strawberries, etc.). Clovers and other legumes which are commonly sown and encouraged within grass fields for their forage value or to fix atmospheric nitrogen are also insect-pollinated.


None.

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Hedges provide a range of essential resources, in particular food (when not available from crops), shelter from unfavourable weather and predators, and nesting sites (Kells et al. 2001, Kells & Goulson 2003, Pywell et al. 2005, Svensson et al. 2000). For example, hedges can provide a succession of flowers throughout the spring and summer (e.g. Miñarro & Prida 2013). Indeed uncultivated, naturally regenerated, field margins consisting of native plants provide better food resources and nesting sites for bumblebees than crops (including grass crops) (Kells et al. 2001, Kells & Goulson 2003, Pywell et al. 2005, Svensson et al. 2000). More bumblebee nests are found in linear features like hedges than in arable fields, grasslands and woodlands (Kells et al. 2001, Osborne et al. 2008a).

Hedges benefit pollinators and other beneficial insects through increasing landscape connectivity in intensive agricultural landscapes. Hedges connected to woodlands (Haenke et al. 2014) or semi-natural grasslands (Öckinger & Smith 2007) may be particularly useful in this context.

Hedges provide flight lines which facilitate pollinator movement between food resources and breeding sites. Bumblebees, for example, prefer to commute along hedges rather than fly across open ground, and patches of flowering plants connected by hedges set more seed (Cranmer et al. 2011). The same behaviour has been reported for solitary wasps (Heard et al. 2012). The reasons are not known, but may be related to navigation, microclimate, protection or shelter.

Hedges provide a microclimate which is frequently warmer and less windy that that in

open fields and so attractive to potential crop pollinators (Croxton et al. 2002, Lewis & Smith 1969). Green lanes, between the hedges that enclose them on either side, support more bumblebee activity than the sides of hedges facing open fields (Croxton et al. 2002).

The combined result of the above factors is that hedges attract pollinators into farmed landscapes, with a resulting increase in the abundance of these pollinators and crop pest predators in nearby crops (Haenke et al. 2014, Morandin & Kremen 2013), including apple orchards (Miñarro & Prida 2013).


Work in Alberta, Canada, found a strong correlation between oilseed rape seed set and wild bee abundance. Bee abundance and seed set was greatest in fields which had uncultivated land at their edges, up to a distance of 750m in to the fields. Modelling suggested that maximum economic profit would be achieved within a closed 576ha area divided into nine equal fields when 33% of the area was uncultivated and within 750m of field edges. Above this proportion of uncultivated land, the declining area of cultivated rape outweighed the benefits of increased pollination from more uncultivated land (Morandin & Winston 2006). The bumblebee Bombus terrestris is capable of foraging at least 1.5km from nest sites, and probably much further. So, provided there are suitable forage sources within the landscape in a circle of this radius, colonies can survive and nest site position is not critical. While this species can operate at quite a large landscape scale, other bumblebees such as B.

55

pascuorum, are believed to fly only a few hundred metres and may need smaller scale landscapes (Osborne et al. 2008b). In Saxony (Germany), solitary wild bee species richness and abundance correlated with amount of semi-natural habitat for landscape scales up to 750 m. Bumblebees and honey bees operated at larger landscape scales. In other words, solitary wild bees, bumblebees and honey bees respond to landscape context at different spatial scales, and local landscape impoverishment will affect solitary wild bees more than social bees (Steffan-Dewenter et al. 2002).


While naturally regenerated crop margins may provide more nest sites and food resources for pollinators such as bumblebees than intensively farmed field margins, they may also be sources of agricultural weeds like thistles and docks. Consequently, cultivating nectar and pollen margins may be a better option (Pywell et al. 2005).

Although linear features may increase pollinator nesting opportunities, this may not

necessarily result in increased reproductive success due to risk of food shortages, parasitism or predation (Kells and Goulson 2003, Osborne et al. 2008a).


Improving the pollinator resources provided by hedges can enhance overall biodiversity and deliver a range of other ecosystem services including crop pest reduction, soil protection, and improved water quality (Davidson & Howlett 2010, Haenke et al. 2014, Morandin & Kremen 2013, Pollard & Holland 2006, Wratten et al. 2012).


To promote healthy and diverse populations of crop pollinators in the farmed landscape, land managers are advised to:

1. Plant new hedges and restore existing ones to increase available pollinator habitat and

connectivity. Hedges connected to larger areas of semi-natural habitat are likely to be especially valuable. Examples of shrubs and trees that are likely to provide particularly useful nectar and pollen resources for pollinators are given in Table 4 below.

2. To provide suitable conditions for a wide range of pollinators (e.g. solitary bees, bumblebees, hoverflies and other flies) hedges should as far as possible contain all major structural components – shrub layer, trees, base, bank, ditch and margins. Diversity in structure and plants should be the aim. Clearly the attractiveness of hedges to pollinators as a foraging resource will depend on the plants they contain (whether as shrubs, trees or herbs) (Davidson & Howlett 2010, Moradin & Kremen 2012), whereas their attractiveness as a nesting or larval development site may depend more on physical structure (e.g. banks, tussocks). For example, different species of bumblebee have different nest site preferences, so conservation of the “big six” (the six most common species) requires the maintenance of a variety of different field boundary types (Svensson et al. 2000). Banks are especially useful for queens that emerge early in the

56

year, and tussocky grasses for later species. Thus the three early-emerging species (Bombus terrestris, B. lapidarius, B. lucorum) prefer to search banks for their subterranean nest sites, while the three later-emerging species (B. pascuorum, B. hortorum, B. ruderarius) prefer tussocks. The latter group are long-tongued and pollinate crops like beans and red clover, while the former group are short-tongued and prefer crops like rape and apples (Kells &Goulson 2003).

3. Hedges should be planted or managed to provide favourable pollinator breeding (e.g. nesting or larval development) microhabitat, as well as pollen and nectar sources. Early in the year the ease with which bumblebee queens can find nest sites (e.g. in holes in banks or grass tussocks) may be more limiting to colony survival than the availability for food resources later in the year (Lye et al. 2009). The creation or maintenance of banks where early spring queens can find suitable holes in which to nest, and of tussocky grass growth for later queens, is particularly important.

4. Hedge bottom, ditch sides and margins should be managed to promote a diversity of herbs and to allow their flowering, to provide pollen and nectar supplies for pollinators. Early spring and late summer flowers are likely to be especially important since these may be absent from the surrounding landscape. See Table 4 for examples of herbs associated with hedges that are particularly attractive to pollinators. Members of the umbellifer family (Apiaceae) are especially valuable, being highly attractive to hoverflies, other flies and parasitoids (Dipterists Forum committee members pers.comm., Pocock et al. 2012). Long-tongued bumblebees favour margins sown with wildflowers or with pollen and nectar seed mix (Pywell et al. 2006).

5. Hedge shrubs should not be cut more often than once every three years to ensure a reasonable amount of flowers (Staley et al. 2012).


If hedges are among the features which arable farmers are required to retain under greening and cross-compliance measures, then this will help to maintain the yield of some crops such as oilseed rape and field beans through wild pollinator services. However, hedges are likely to lose much of their value in this respect if they are not managed appropriately – retaining them is not enough on its own. The hedgerow protection zones currently required under cross-compliance (GAEC14), under which land within 2m of the centreline of hedges cannot be cultivated or have fertilisers or pesticides applied to it, are of benefit to pollinators including bumblebees through helping to retain favourable breeding and food resources. However, these zones, or margins, also require management to realise their potential in this respect.



I. The distance from hedges into crops over which crop yields are significantly increased as a result of the presence of hedges.

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II. The extent to which pollinators other than bees, and especially other than bumblebees, are dependent upon hedges, and in what ways.

III. The economic value of hedges for crop pollination.

6.10. Conclusions

1. Strong evidence exists, based largely upon bees, especially bumblebees, and hoverflies, that hedges together with other patches of non-cropped ground are important in agricultural landscapes, for the existence of healthy and diverse pollinator populations. Furthermore, there is good evidence that hedges attract pollinators onto intensive farmland and export those pollinators to crops, increasing yield.

2. Hedges provide breeding sites, food when crops are not in flower, shelter, protection and flight lines. They are of particular importance as nesting sites, at least for bumblebees. Their value can be enhanced by the cultivation of nearby strips or patches of flowers grown for nectar and pollen.

3. However, practically no research has been carried out on the cost effectiveness of hedges in increasing crop yields through boosting pollination. Such research needs to encompass the activities of insects other than bees and hoverflies, since it likely that these other taxa are also important - for example, several other families of flies are frequent visitors to flowers in hedges and presumably crops. There is an indication that hedges can influence the pollination of crops over a distance of at least 750 m, but this is also a matter which requires further research.

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Table 4. Plants native to the British Isles which are considered to be of value to pollinators, focussed on flies (Diptera) (Dipterists Forum committee members, pers.comm, April 2014). Species Soil

preferences Comments

Shrubs and trees

Blackthorn Prunus spinosa None Early spring

Box Buxus sempevirens Lime-rich

Dogwood Cornus sanguinea Lime-rich

Field maple Acer campestris Not acid

Guelder-rose Viburnum opulus Acidic

Hawthorn Crataegus monogyna None

Holly Ilex aquifolius All

Wild privet Ligustrum vulgare Lime-rich

Willows Salix spp Wet Very valuable in early spring

Ramblers and creepers

Bramble Rubus fruticosus All Very valuable in summer

Ivy Hedera helix All Very valuable in autumn

Roses Rosa spp All

Herbs

Bird’s-foot trefoil Lotus corniculatus Not acid

Borage Borago officinalis Not acid

Burnet saxifrage Pimpinella saxifraga Lime-rich

Butterbur Petasites hybridus Not acid Good for early hoverflies

Buttercups Ranunculus spp All R. acris not so good

Campions Silene spp All

Colt’s-foot Tussilago farfara Not acid Good for early hoverflies

Cow parsley Anthriscus sylvestris All Very good on rare occasions, but now often too dominant

Dandelions Taraxacum sp All Valuable in early spring

Dog’s mercury Mercurialis perennis All Male plants preferred

Enchanter's nightshade Circaea lutetiana All

Fleabane Pulicaria dysenterica Acidic

Foxglove Digitalis purpurea All

Garlic mustard Alliaria petiolata All

Ground elder Aegopodium podagraria All Invasive

Ground ivy Glechoma hederacea All

Hemlock water-dropwort Oenanthe crocata Wet

Hemp agrimony Eupatorium cannabinum All

Hogweed Heracleum sphondylium All Very valuable in summer

Knapweed Centaurea nigra All

Mayweeds Matricaria spp All

Meadowsweet Filipendula ulmaria Wet

Mints Mentha spp. All + wet

Nipplewort Lapsana communis All

Pepper-saxifrage Silaum silaus Lime-rich

Pignut Conopodium major All

Ragworts Senecio spp All Invasive

Raspberry Rubus idaeus Acidic Especially in Scotland

Red clover Trifolium pratense All

Ribwort plantain Plantago lanceolata All

Rough chervil Chaerophyllum temulum All

Stitchworts Cerastium spp All

Stone parsley Sison amomum Lime-rich

Teasel Dipsacus fullonum All

Thistles Cirsium and Carduus spp All Some species invasive

Upright hedge-parsley Torilis japonica All

White dead-nettle Lamium album All

Wild angelica Angelica sylvestris Wet

Wild carrot Daucus carrota Lime-rich

Wild parsnip Pastinaca sativa Lime-rich

Woundworts Stachys spp All

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7. Shelter provision (crops and livestock)


A considerable body of evidence exists both confirming and quantifying the benefits of shelter provided by hedges, to both livestock and crops (Baldwin 1988, Biber 1988, Bird 1998, Forman & Baudry 1984, Kort 1988, Van Laer et al. 2014). Much of this evidence relates to windbreaks or shelterbelts, but these features are to a large extent synonymous with hedges – the term windbreak is normally used to describe in effect thin hedges (1-3 rows wide) that have developed into lines of trees, while the term shelterbelt is usually used to describe wider lines of tall trees. Although shelterbelts can be wider than the maximum 5m between major woody stems used to define a hedge, their influence on crops and livestock is much the same. Indeed, wider is not necessarily better, as explained below. Hedges provide wind protection over a much greater distance than solid barriers because they are porous, allowing some wind to pass through them. This permeating wind prevents eddies and turbulence forming in the lee of the hedge, resulting in a reduction in wind speed across fields for a distance many times the height of the hedge. Blocks of woodland by contrast provide poor shelter to crops and livestock in adjacent fields (e.g. Pollard et al. 1974). Although the focus of research is on the wind speed reduction properties of hedges, their sun shading effects are also addressed – in terms of both benefits to livestock and reduction in crop growth. Evidence also demonstrates that hedge networks can affect climate at a landscape level, influencing overall wind speed, temperature and humidity (Jensen reported in Pollard et al. 1974, Guyot & Seguin 1976).


None.


The wind shelter provided by hedges has the following benefits (Biber 1988, Bird 1988, Brandle et al. 2004, Pollard et al. 1974): Reduction in physical damage to crops, increasing yields. Such damage includes cereal

lodging (when the stems are beaten down). Generally fruits and vegetables are more sensitive to wind stress than arable crops.

Reduction of water loss from soils and crop leaves through evaporation and

transpiration, together with the retention of night dews and generally keeping plants more humid. This reduction, a result of decreased wind speed, increases crop growth rates. The crops which are likely to benefit most from this effect are those growing on

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well-drained soils in dry areas, and broad-leaved water-demanding ones like sugar beet (Pollard et al. 1974).

Increase in daytime temperatures – the reduction in wind speed and turbulent mixing

near hedges can result in these areas being several degrees warmer than in the open. This too will usually increase growth rates.

Protection for livestock from the wind and driving rain – evidence shows this can reduce

mortality especially of young animals and increase growth rates. Preventing snow or sand drifting onto access routes, providing refuges for livestock in

the lee of hedges. Reduction in heat stress for livestock (and people) – the shade available under and

beside hedges may be even more important in temperate latitudes for stock than protection from cold winds, an effect which is likely to increase with climate change (Van Laer et al. 2014). Access to shade improves summer-cattle performance under hot conditions, at least until they are acclimatised (Mader et al. 1999). While hardy breeds of cattle may be able to withstand extremes of climate in temperate latitudes, faster growing and more productive commercial beef and dairy breeds such as the Holstein, Jersey, Charolais, Limousin, Blonde d'Aquitaine and Belgian Blue may not be able to cope without loss of productivity. Belgian climatic data suggests that these more sensitive breeds are likely to suffer not so much from harsh winter weather as heat stress in the summer, a situation likely to increase with global warming (Van Laer et al. 2014). Heat stress affects disease resistance, fertility, birth weights and milk yield, and depresses weight gain (Gregory 1995).

Provision of shelter for game.

Reduction in salt spray.

Improved sprinkling irrigation.

Reduction in soil wind erosion.


A hedge of 40% permeability will reduce wind speed by more than 20% over 8-12 times the height of the hedge (h), in addition to a smaller area windward of the hedge extending to 4 h (Pollard et al. 1974). On occasion, the sheltered zone can extend as far as 20 h for single rows of trees, or even 30 h for multiple rows (Biber 1988). However, it is normally reckoned that crop yields and animal well-being will be significantly affected within 8-12 h downwind of a hedge and 2-4 h upwind (Dronen 1988, Forman & Baudry 1984, Gregory 1995, Pollard et al. 1974). Thus a hedge 7 m high will affect crop yields and animal well-being within a distance of 14-28 m upwind and 56-96 m downwind – a substantial area. Biber (1988), covering Europe generally, presents figures for increases in yield in different crops that may be expected from windbreak use. These include sugar beet up by 11-12%, wheat 6-26%, maize 10-15%, grass 27-67%, potatoes 9-17%, apples 16-75% and pears 121%. It is not clear whether or not these figures take account of land lost to the shelterbelts and yield reductions due to shading and competition between crops and trees. We assume that

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they do not. Likewise, we assume that they do not consider the natural depression of yields along field margins or headlands due to compaction, machinery turning, pests like rabbits and poor nutrient status (Kuemmel 2003). Reviewing data from well-replicated studies from temperate zones across the northern hemisphere, Kort (1988) concluded that shelterbelts resulted in a net increase in spring wheat yield of 3.5% within 15 h on each side of shelterbelts. Contrastingly, the mean increase of winter wheat yield reported was 24%. Winter wheat, barley and hay (mixed grasses and legumes) are, Kort asserted, most responsive to protection, spring wheat, oats and maize less so. In the absence of any information to the contrary in the source studies, Kort assumed these figures to take account of the land lost to the shelterbelts, and yield reductions due to shading and competition between crops and trees. He noted that the benefits of shelter on yields vary not just with crop but also with geographical location, weather conditions, soil type and shelterbelt design. In Jutland, Denmark, shelterbelts have been widely planted in flat areas with sandy podzolic soils both to conserve soil and to enhance crop growth. These shelterbelts, typically 5 m wide, are reported to increase overall crop yields across fields by 7-8% (Jan Svejgaard Jensen, Plantning & Landskab, Billund, pers.comm.). Baldwin (1988) stated that there was overwhelming evidence that windbreaks benefitted vegetable and speciality crops, including potatoes, beans, sugar beet and strawberries. He reported that yield increased within 10 h, with maximum responses within 3-6 h. Overall yield among these crops increased between 5% and 50%. Windbreaks also led to earlier maturity, higher quality and greater economic gain from crops (Baldwin 1988). Kort (1988) presented information suggesting that the payback period for new shelterbelts ranged from 15 to 40 years. Clearly the width of the shelterbelt, or hedge, will have a considerable impact on this, along with the locality, local climate and soil. As an example of the effects of shelter on livestock, research in a flat treeless area in Australia with periods of rain, sleet or light snow and frosty nights showed that hedges 1-1.5 m high and 20 m apart increased survival of single lambs by 10% and multiples by 32% (Alexander et al. 1980). To investigate the effects on local climate of hedge networks, Guyot & Seguin (1976) compared landscapes with and without hedge networks in Brittany. They found that well hedged landscapes can affect local climate, decreasing wind speed over the more or less level landscape by 30-50%, increasing daytime and reducing night time temperatures, and increasing air saturation deficit (humidity) and therefore presumably precipitation. Previously, Jensen (reported in Pollard et al. 1974) had investigated the effect of hedges on wind speed across Jutland, Denmark. They recorded wind speed along two transects across the peninsula, one across countryside with few hedges, the other across countryside with many hedges. Wind speed was measured at points along the transects at a height of 2 m and well away from any hedges or other windbreaks. With a westerly wind speed averaging 64 km h-1 at the west coast, wind speed along the transect with few hedges was reduced by a third, but along that with many hedges by a half. This represented a difference of nearly 10 km h-1. It is clear that hedge networks can have a considerable impact on local climate at a landscape scale.

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The shade cast by hedges coupled with competition for moisture is likely to reduce crop yields within 1-3 h (Biber 1988, Forman & Baudry 1984). However, field margins or headlands typically produce lower yields than the centres of hedges for other reasons (Kuemmel 2003). Research on commercial arable farms in East Anglia has shown that headlands without any adjacent trees produce much reduced yields of sugar beet (down by 26%) and barley (down by 7%). This decrease was attributed to soil compaction and physical damage to plants caused by machinery turning. Increased pest damage (e.g. from rabbits) may be an additional cause, along with poor nutrient status. Nevertheless, yields of winter wheat within a headland bordered by 15 m high trees were nearly 50% less than those in the centre of the field, suggesting most of the loss was due to the shading effects of the trees; when harvested the grain was also 6% wetter. These effects extended over a width of 20m (1.3 h) (Sparkes et al. 1988). On balance, however, increases in crop yield arising from wind protection outweigh losses due to shade: hedges result in a net increase in yield (Biber 1988, Kort 1988).

In wet years, hedges may depress yield by delaying autumn ripening (Pollard et al.

1974).


Pollination. Windbreaks such as hedges can greatly increased the number of insects, including some potential pollinators, in the sheltered zones on either side (Lewis & Smith 1969). It is likely that this will result in increased pollination of some crops such as top fruit (apples, pears, etc.). Soil erosion. Windbreaks such as hedges can reduce soil loss through capturing wind-blown particles. Crop water availability. Reduction of crop water loss through evapotranspiration results in the wind shelter provided by hedges increasing crop yields. On the other hand, hedge plants can compete with crops for water, reducing yields. Climate change. Hedges that serve as windbreaks can sequester carbon just like other hedges. In addition research in the United States and Canada suggests that well-designed windbreaks can cut the average energy use of a typical farmstead in northern areas by 10-20% through reducing local heat loss (Brandle et al. 2004). Furthermore, windbreaks may play a significant role in allowing producers to adapt to climate change, through sheltering crops (e.g. maize) and livestock from the detrimental effects of extreme weather events such as heat waves, heavy rain and strong winds (Brandle et al. 2004).


Neither the enhancement of crop growth nor the sheltering or shading of livestock is a NELMS objective. As a consequence, hedges grown and managed as windbreaks solely for these purposes are not relevant in the context of the scheme. However, since windbreak hedges can play a role in reducing soil loss from fields and in pollination, as well as in climate change mitigation and adaptation, relevant management advice is given here:

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1. Hedges grown as windbreaks will be most effective, both economically and environmentally, in areas where the soils are light and drain freely, such as sandy or peaty areas in East Anglia, and where sugar beet, potatoes, vegetables and top or soft fruit are being grown.

2. Clearly the higher hedges are allowed to grow, the greater the distance over which they

can shelter crops and stock from the wind. However there is little point in having windbreak hedges closer together than about 12 times their height, a distance apart of 100 m for an 8 m high hedge.

3. Windbreaks that permit about 40% of the wind to pass through their branches are most

effective for both crops and animals. If they are any denser, the width of the zone they shelter is likely to be reduced. For snow protection, rather less dense (50% permeability) hedges are optimal. For livestock, ideal windbreak hedges are likely to be denser close to the ground than towards the top.

4. Good windbreak hedges can be created by planting one or two rows of shrubs which

have a dense structure (e.g. hawthorn, blackthorn and hazel) together with one or two rows of tall-growing trees with a medium density (e.g. oak, beech and sycamore). Rows should be about 1.25 m apart.

5. Like all other hedge types, windbreak and shade-giving hedges need to be rejuvenated

periodically. For taller hedges, this will normally entail coppicing. These hedges may also need side trimming, and perhaps even root pruning, to maintain the desired density or to prevent excessive shading of crops or grass, or excessive root competition for moisture.


The retention of hedges as landscape features contributing towards Ecological Focus Areas may be expected to maintain net crop yields in some areas, such as sandy or peaty soils in East Anglia, through acting as windbreaks and reducing soil loss. They may also be expected to maintain yields in horticultural and fruit farms, and more generally to maintain livestock welfare and profitability, especially in flat landscapes that would otherwise be windswept. Hedges provide shelter from harsh conditions, including heavy snow, and shade from summer sun. The benefits to both crops and livestock may be expected to increase with climate change, providing a buffering effect. However, like all hedge types, retention is not enough on its own – windbreak and shade-providing hedges need to be managed to retain their function.



I. The cost effectiveness of hedges as windbreaks, shelterbelts and shade providers in the UK, in different geographical locations, weather conditions and soil type. This research should focus on effects on farm profitability. It should cover the provision of shade for livestock, reducing heat stress.

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7.10. Conclusions

1. Strong evidence exists to prove that hedges managed as windbreaks or shelterbelts can improve crop yields. In particular vegetables and fruits along with broad leaved crops like potatoes, sugar beet and beans are likely to benefit, especially if grown on well-drained soils in the drier parts of Britain. Available information on the likely net increase in yield suggests that for arable crops such as cereals this is likely to range from just a few percent to as much as 25% within the sheltered area. For vegetables and fruit the yield increase may be much greater than this, perhaps as high as 75%. Crop type, local climate and soil and hedge structure will all affect yields, leading to high levels of uncertainty around expected yield increase.

2. Likewise there is plentiful evidence to show that livestock such as sheep and cattle benefit from the protection from wind, driving rain and snow which can be provided by hedges. The role of hedges in providing shade may be especially important in temperate climates. Hedges can reduce mortality (particularly of young animals) and heat stress, and increase growth rates, milk yield, disease resistance and fertility. Although much research has been done to quantify theses effects, most are location specific or from abroad and cannot be transferred directly across to Britain, especially given our considerable diversity of climates and landscapes.

3. Hedges acting as windbreaks, if properly designed, are likely to reduce wind speed significantly over a distance of 12 times the height of a hedge on the downwind side, and 4 times on the upwind side. Thus a hedge that has grown 6.25 m high may well provide significant shelter over a width of 100 m.

4. At a landscape scale, evidence exists to show that networks of hedges can exert a significant influence on local climate, decreasing average wind speed by up to 50%, increasing daytime and decreasing night time temperatures, and increasing humidity (and probably precipitation).

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8. Climate change mitigation


The majority of the information has been gathered from peer-reviewed scientific papers (Axe et al. 2012, Bailey et al. 2009, D’Acunto et al. 2014, Falloon et al. 2004, Follain et al. 2007, Gratini & Varone 2006, Gratini & Varone 2013, Gupta et al. 2009, Kort & Turnock 1998, Lenka et al. 2012, Mungai et al. 2006, Paulsen & Bauer 2009, Peichl et al. 2006, Schoeneberger 2009, Walter et al. 2003) and a small number of published reports (Pointereau & Colon-Solagro 2008, Robertson et al. 2012, Taylor et al. 2010). Numerous relevant studies within agricultural landscapes have been carried out in Europe, whilst a small number of studies (most notably Falloon et al. 2004) have used the limited data available for the UK. Consequently quantity assessments and conclusions should largely be treated as provisional. A number of studies were based on agroforestry systems such as planting lines of trees across cropped fields (alley cropping) (Smith et al. 2013). Whilst the focus was on poplar trees, the studies were particularly useful in highlighting the carbon sequestration effects of trees alone, particularly in relation to tree age (Gupta et al. 2009, Kort & Turnock 1998, Peichl et al. 2006).


None.

8.3. How hedges deliver the service Hedges and tree lines are able to sequester large amounts of carbon both in above-ground

biomass and in soil organic carbon (SOC) as organic matter (Falloon et al. 2004). Above ground, hedges are able to contribute to the reduction of greenhouse gasses (mainly

carbon dioxide) by sequestering and storing carbon in the branches and stems (and doubtfully leaves) of woody vegetation (Pointereau & Colon-Solagro et al. 2008). Reflecting their large amounts of above-ground biomass, trees offer the greatest potential to fix large amounts of carbon dioxide as they mature (Kort & Turnock 1998, Piechl et al. 2006).

Below ground, in the absence of cultivation, organic matter is able to accumulate in the soil

beneath and beside hedges (Borin et al. 2010, D’Acunto et al. 2014). Carbon is stored both in root systems (Piechl et al. 2006) and as SOC resulting from branches, twigs and leaves falling from trees, shrubs and other plants (Piechl et al. 2006). The highest amounts of SOC are found near the soil surface (0-30 cm). Hedge banks may increase the amount of soil carbon sequestration (Paulsen & Bauer 2008). At a landscape scale, hedges can play only a local role in SOC accumulation and storage as their effects in this respect are confined to the immediate vicinity, especially in intensively cultivated farmland (D’Acunto et al. 2014, Follain et al. 2007, Walter et al. 2003).

On sloping land Follain et al. (2007) and Walter et al. (2003) attributed the thickening of the

topsoil (A-horizon) uphill from hedges to increased amounts of SOC following soil erosion from the fields above, in addition to the accumulation of leaf litter.

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The capacity of a hedge to sequester carbon is likely to increase with vegetation height

(Robertson et al. 2012), hedge width (Falloon et al. 2004) and hedge density within a landscape (Walter et al. 2003).

Whilst the ways in which hedges can mitigate climate change are important, for hedges to

have any significant impacts upon greenhouse gas emissions, carbon in hedges needs to be locked-up over the long term (Taylor et al. 2010).

However, hedges can be cost–effectively managed and cropped for woodfuel, so potentially

reducing demand on fossil fuels and overall greenhouse gas emissions (Wolton 2012b, Westaway et al. 2013).


Carbon storage Based on figures from 118 year old woodland at Rothamstead (UK) (Poulton et al. 2003), and assuming the hedges are at least 100 years old and only including soil carbon in the top 30 cm of soil, Robertson et al. (2012) conservatively estimated that hedges store about 85 t/ha of carbon below ground. Also based on Rothamstead figures, which cover hawthorn and hazel for shrubby hedges, and oak, ash and sycamore tree lines, Robertson conservatively estimated that lines of trees (>6 m high) store 166 t ha-1 of C above ground, tall hedges (3-6 m) 45 t ha-1, medium hedges (2-3 m high) 22.5 t ha-1, and short hedges (2 m or less) 11.25 t ha-1. Based on data from seven hedges running across hill slopes and sampling SOC above, within and below the hedges, Walter et al. (2003) estimated the relationship between hedge density and soil carbon storage in Brittany (France). Three of the hedges were below arable crops and the other four below pasture, and all but one had wet meadowland below. At a high hedge density, corresponding to the historic regional density of 200 m ha-1, the mean total SOC stock across the landscape was estimated to be 177 t C ha-1. At a hedge density of 100 m ha-1, mean total SOC across the landscape was estimated to be 94.9 t C ha-1, while at a hedge density of just 50 m ha-1, mean total stock fell to 83.9 t C ha-1. At the highest density as much as 38% of the total carbon stock could be attributed to the hedge network, while at the lowest density (more typical of the modern Brittany landscape) this figure fell to 13%. Walter et al. (2003) showed that hedges perpendicular to the slope have a local and gradual effect on SOC storage within soils up to 60 m from the hedges. At 60, 40, 20 and 10 m uphill from the hedge, due to increasing thickness, mean SOC storage in the A horizon was respectively 1.40, 1.45, 1.52 and 1.85 kg m-2. Thickening of the soil A-horizon was therefore seen as the most important process contributing to carbon storage as SOC increased nearer to the hedge. However, since the first 30 cm of the soil corresponds to the ploughed layer, the SOC stock in this surface layer will be fairly independent of hedges’ influence. Also in Brittany, a bocage landscape of 8.4 ha grassland with a hedge density of 106 m ha-1 was studied by Follain et al. (2007). Total SOC stock within the 8.4 ha landscape was estimated to be 1185 t C. Land within 20 m either side of the hedges covered 43% of the total surface area and contributed to 50% of the total SOC stock. However, SOC stocks were variable within the landscape; highest SOC stocks were found within thick soil A-horizons (top 30 cm) which accumulated in the uphill position from the hedges. As a result, hedges were found to always yield higher stocks in relation the soil surface when compared to

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stocks calculated at the landscape scale. Within 20 m either side of the hedges, cumulative SOC stocks (within the A horizon) were equal to 9.1 kg C m-2 (91 t C ha-1) and represented 65% of the total stock in the A-horizon at the landscape scale. Paulsen & Bauer (2008) found that in Schleswig-Holstein (Germany) hedge banks play a role in sequestering carbon in the soil. A hedge bank of approximately 28 km in length, with an average width of 3.9 m and height of 0.6 m was found to have an average SOC content of 2.1%, which decreased with increasing depth. Along the hedge, 49.6 t km-1 (equivalent to 127 t ha-1) of SOC could be stored. They estimate that 45,000 km of hedges could capture 2.2 Mt of SOC, an amount equivalent to the methane emission potential of 360,000 dairy cows over 6.5 years. Methods used to obtain the height and width of the hedge-banks may have led to underestimations and stones within the soil were not considered. In Argentina, soil carbon stocks were shown to be larger in woody hedges (3.21%) than herbaceous margins (2.08%) which bordered arable soybean fields: soil organic matter and carbon was 50% greater in woody hedges than herbaceous margins (D’Acunto et al. 2014). The effect of woody margins was spatially limited: whilst soil total carbon and leaf litter was higher within the near vicinity (approximately 0-5 m) of woody hedges, total soil carbon and litter mass decreased with increasing distance from the hedge. SOC in the middle of the field (average field size 50 ha) was not affected by the woody hedge. Herbaceous margins also had no effect on SOC within the neighbouring fields. Lignin content accounted for 54% of the variation in litter decomposition rates – the more lignin the slower the rate of decomposition rate - suggesting that soil carbon accumulation was affected by slower rates of decomposition of woody leaf litter. In Canada, Piechl et al. (2006) studies poplar trees Populus deltoides x Populus nigra (clone DN-177) in an agroforestry system along with Norway spruce Picea abies in a 13 year old intercropping system. In poplars, 85% of total tree carbon was to found to be stored as above ground biomass while 15% was stored in the roots. Similarly in Norway spruce, 82% of the total tree carbon in stored was within above ground biomass, whilst below ground 18% was stored in the roots. Of the above ground biomass, 63% of the carbon was stored in the needles and branches for spruce, compared to just 44% in the leaves and branches of poplar. Litter fall and pruning can therefore input large amounts of carbon into the soil. After 13 years, total tree carbon was found to be more than twice as big in poplar trees that in spruce trees. Total soil carbon in the poplar system was on average 4% higher than that of spruce. In both poplar and spruce intercropping systems, total soil carbon concentration barely fluctuated with distance from the tree lines (1-12 m). Even though more carbon is stored in the needles and branches of spruce, the height of poplar trees (18 m) was thought to facilitate a more even distribution of litter fall across the soil, as well a higher litter fall overall in comparison to the spruce. Finally, after 13 years, the carbon pool of the poplar system was estimated to be 30% higher than that of the spruce system and 41% higher than a barley crop with no tree lines. Carbon accumulation Net soil carbon accumulation may not cease until hedges are very old indeed, based on Rothamstead research which suggests woodland soils may take at least 766 years to reach equilibrium between accumulation and loss although accumulation rates decrease over time. Below ground, Robertson et al. (2012) estimated that new hedges may accumulate 0.542 t SOC ha-1 yr-1, whilst old hedges may accumulate 0.456 t SOC ha-1 yr-1 in the top 30 cm of soil.

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These measurements do not take into account the size of the hedge - larger hedges are likely to accumulate C in the soil faster. The figures also do not take account of management but rather assume that if a hedge is flailed the trimmings drop to the ground and contribute to soil carbon regardless. Above ground, Robertson et al. (2012) assumed that trimming management (e.g. with a flail mower) will prevent annual accumulation of C (although noting that stems may thicken). Again based on Rothamstead figures, for unmanaged hedges, it was calculated that lines of trees will accumulate 1.38 t C ha-1 yr-1, tall hedges (3-6 m) will accumulate 0.51 t C ha-1 yr-1, medium ones (2-3 m) 0.26 t C ha-1 yr-1 and short ones (<2 m) 0.13 t C ha-1 yr-1. Biomass yields from hedges harvested for wood fuel in Devon suggest that a tall (6 – 8 m) hedge may accumulate 3.4 t C ha-1 yr-1 above ground (based on figures within Wolton 2012a). This estimate is considerably higher than that given by Robertson et al (2012), but while those figures came from what the researchers at Rothamstead considered to be ‘a very wide hedgerow’, the method adopted by Robertson et al (2012) to calculate carbon storage and accumulation by woody plant species means that they consider their estimates likely to be closer to those for gappy hedges than to dense continuous hedges. By comparison, the average carbon accumulation rate for open-grown poplar (Yield Class 12) grown in the UK across 26 years is given as 7 t C ha-1 yr-1, while open-grown oak trees (Yield Class 6) across 90 years accumulate approximately 2 t C ha-1 yr-1 (Cannell 1999). Carbon sequestration rates of non-flailed hedges from 20 Welsh lamb and beef farms were modelled by Taylor et al. (2010). They assumed that hedges sequester C at the same rate as short-rotation poplar. A minimum sequestration rate was estimated to be 2.20 t C ha-1 yr-1, a mid-range estimate was 6.37 t C ha-1 yr-1 and a maximum rate was estimated to be 11.40 t C ha-1 yr-1. Of the 20 farms studied, the largest farm was 6,308 ha and the smallest farm was 722 ha; overall the average farm was size was 2,454 ha. On average it was estimated farms sequestered 1 tonne of CO2 e/ha/year. Of this, hedges contributed an average of 5% of the carbon sequestration (range 1-14%), and isolated trees contributed an average of 8% (range 1-33%), however the size and ages of trees was unknown. To further increase carbon sequestration on farms it is was estimated that planting just 50 isolated trees per farm could increase sequestration rates up to 5% depending on the soils and growth rates of trees. (The largest carbon sink is grassland which sequesters on average 83% of the annual carbon). There is, however, uncertainty surrounding the sequestration rates of hedges as hedge lengths and widths were measured by the farmers and hedges were assumed to have the same sequestration rates as coppiced poplar. It was also assumed that without a hedge on the slope, the A-horizon thickness would be less or equal to 30 cm and soil depth would remain constant along the slope. Based on datasets from 2,000 arable farms across England and Wales and published figures for carbon storage within set-aside, Falloon et al. (2004), considering both above and below ground C storage, estimated that shrubby hedges would accumulate carbon at 1.0 t C ha-1 yr-

1, while those that have developed into lines of unmanaged trees could accumulate as much as 2.8 t C ha-1 yr-1. The shrubby hedge figure is broadly similar to that derived from Robertson et al. (2012) of 0.7 t C ha-1 yr-1, while the line of trees figure is higher than the 1.8 t C ha-1 yr-1 derived from Robertson et al. (2012) but less than the figures of 3.4 t C ha-1 yr-1 estimated from Wolton et al (2013) ). Assuming that the hedges were planted on previously cropped field margins, Falloon et al (2004) noted that there would be a corresponding reduction in N2O (a powerful greenhouse gas) emissions. They estimated

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these to be equivalent to a further 1.3 t C ha-1 yr-1 stored by both shrubby hedges and lines of trees. Plane trees Platanus hybrida within buffer strips (some with additional guelder-rose Viburnum opulus shrubs) were harvested and analysed for their carbon content in Italy (Borin et al. 2010). Carbon immobilised per tree was found to be 104 kg in ’young’ trees and three times higher in trees that were 20 years older. Assuming a 6 m wide buffer strip is composed of two rows of alternating trees and shrubs, carbon sequestration rate of the young) buffer strip was found to be 20 t yr-1, whilst the older hedge could sequester 50 t yr-1 (it is unclear over what length or area, but the strips may have been 35 m long). The authors estimated that the buffer strip’s wood and soil together could store up to 80 t CO2 ha−1 yr−1, but it is unclear how this figure was calculated. Trees act as a CO2 sink by fixing carbon during photosynthesis and storing the surplus carbon as tree biomass (Varone et al. 2006). In Rome (Italy), two tree species, Quercus ilex and Quercus pubescens, were studied to estimate the capacity of evergreen and deciduous tree species to sequester carbon. Trees with a DBH of 20-50 cm were classed as small trees and tree with a DBH of 50-80 cm were classed as large trees. Larger trees were shown to have a total carbon sequestration rate 67-80% higher than smaller trees. In a later study by (Gratini & Varone 2013) total yearly carbon sequestration capacity ranged from 142-379 kg C02 yr-1 for four evergreen hedge species studied. On average, hedges had a 77% lower carbon sequestration rate in comparison to the trees studied by Varone et al. (2006). However, their contribution to total CO2 sequestration was still considered to be important particularly as there was a large amount of hedges in the city centre. Gupta et al. (2009) analysed soil organic carbon in surface (0-15 cm) and subsurface (15-30 cm) soils over 1, 3 and 6 years in a poplar Populus deltoides agroforestry system (lines of trees) in India. Over a number of years, surface SOC (61%) was significantly higher than subsurface SOC (41%). Average litter fall of 2 and 4 year old poplars was found to be 684.1 and 831.9 kg ha-1 respectively. Total SOC was shown to increase with tree age as it was highest (88%) in the 6 year-old plantation, an effect attributed to leaf litter fall. However, in the 0-30 cm soil depth, carbon sequestration rates decreased from 3.37 t ha-1 yr-1 after the first year of growth to 2.63 and 1.95 t ha-1 yr-1 in the third and sixth year of growth. This was thought to be due to an increase in decomposition over time even though older trees produce more leaf litter.

8.5. Disadvantages of service Whilst there are no obvious negative effects of hedges on climate change, the use of

tractors for hedge management usually results in the consumption of fossil fuels and subsequent emission of greenhouse gases, mainly carbon dioxide and nitrous oxide. To trim a well-established hedge of 2 m high and 1.5 m wide, about 6 passes of the machine are needed to cut both sides and the top. Based on these figures, it has been estimated that on average one litre of fuel would be needed to trim 55 m length of hedge (Robertson et al. 2012). Nevertheless, accumulation of carbon was considered greater than carbon lost through management leading to net carbon accumulation by hedges.

The capture of soluble nitrogen from sub-surface water moving across hedges may facilitate de-nitrification by soil microbes, releasing nitrous oxide (NO2), a powerful greenhouse gas, although the uptake of N by tree roots will counteract this: net effects on NO2 emissions as a result of hedge presence are not clear.

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8.6. Relationship with other services Soil erosion reduction. On sloping land, accumulation of soil along and just above hedges serves both to increase soil carbon storage and prevent soil loss. Water quality improvement. Hedge root systems and leaf litter can not only increase soil carbon, but increase infiltration rates. They also stabilise the soil structure mainly due to the input of organic matter into the soil from leaf litter. Consequently, surface runoff and subsurface water moving across the hedge are captured, reducing the risk of diffuse pollution. Wood fuel provision. Hedges can be harvested for their wood to provide a renewable source of energy and so reduce fossil fuel emissions (Wolton & Vergette 2012, Westaway et al. 2013). The wider and taller a hedges is, the more fuel it is likely to produce and this is will in turn increase soil carbon sequestration.


To increase the carbon storage potential of hedges, farmers could be encouraged to:

1. Plant further hedges.

2. Encourage hedges to expand in width. Permanent herbaceous margins are of value for carbon sequestration as well as trees and shrubs, although less so.

3. Retain existing mature hedgerow trees, allow others to grow to maturity or actively plant trees within existing field boundaries.

4. Consider the use of alley cropping agroforestry systems, the cropping strips being separated by hedges (or equivalent linear features) (Smith et al. 2013).

5. Restrict herbicide use near hedges, to allow perennial vegetation to dominate and increase soil carbon accumulation (D’Acunto et al. 2014).


The retention of hedges as landscape features and a component of Ecological Focus Areas under CAP greening proposals will help to maintain their value as carbon stores and prevent the stored carbon being released. The retention of hedges and of existing 2 m hedge protection zones under cross compliance measures will act in the same way. However without any positive management, current greening and cross-compliance proposals will not themselves increase carbon storage, or indeed have any direct effect towards climate change mitigation. Although in the short to medium term the tendency for increasing numbers of hedges to be neglected (Countryside Survey 2009) and therefore to get larger may result in increased carbon sequestration, in the long term if hedges are not managed their ability to sequester carbon will be reduced. Falloon et al. (2004) calculated how much land would be taken out of production if all arable field margins were planted with trees or grass to reduce atmospheric carbon. If a network of 2 m or 6 m field margins was planted in a 260 ha farm, 5.9 ha & 17.3 ha respectively

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would need to be taken out of production. At a British level, preliminary calculations suggested that 2.3%, 6.7% and 21.3% of the total arable land would be taken out of production if field margins of 2 m, 6 m and 12 m respectively were created around all fields. 6 m margins (trees 2 m + grass 4 m) would offset about 1% of 1990 UK CO2 emissions.



I. To understand better the rates of carbon accumulation both above and below ground specifically in hedges, and how different factors such as soil depth (most studies have considered only the surface layers while carbon stored in deeper layers may be most resistant to decay), cultivation practices, litter decomposition rates and different tree/shrub species influence the ability of hedges to accumulate and store soil organic carbon (SOC).

II. To understand better the impact of various hedge management practices on carbon

sequestration. In particular, the impact of hedge trimming on SOC needs to be explored (see Axe et al. 2012), as does the management of hedges to produce a woodfuel crop.

III. To understand better how long hedges continue to accumulate further carbon

before they reach equilibrium, and how they can best be managed to store carbon over the long-term as well as which species (trees and shrubs) would be the most effective for this purpose. For hedges to have a significant impact up climate change mitigation long term carbon storage is essential (Taylor et al. 2010). For example, Piechl et al. (2008) have demonstrated that fast growing poplars may result in significant short-term carbon storage, whereas as slower growing conifers may contribute to long-term carbon storage).

IV. To quantify the amount of carbon currently and potentially stored in UK hedges in

comparison to that stored in woodlands and other woody landscape features.

8.10. Conclusions

1. Hedges sequester carbon both in woody growth above ground and in roots and other soil organic matter below ground, and as a consequence play a role in climate change mitigation. Tree lines have been shown to store more carbon than shrubby hedges: mature trees store more carbon in their stems and branches reflecting their greater above ground biomass and input more carbon into the soil through higher woody litter fall and accumulation.

2. To have any significant impact upon greenhouse gas levels, carbon needs to be locked up within the hedges and soil over the long term. Alternatively, hedges can be cropped for woodfuel, reducing demand for fossil fuel use.

3. Through capturing eroding soil, hedges across slopes can increase soil organic carbon (SOC) for up to 60 m uphill from them. However, intensive cultivation within adjacent fields prevents the accumulation of organic matter.

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4. Above ground, based on woodland research, Robertson et al. (2012) conservatively

estimated that unmanaged British hawthorn and hazel hedges (2 m high or less) could store 0.13 t ha-1 yr-1 of carbon above ground, whilst taller oak, ash and sycamore hedges (>6 m) could store as much as 1.38 t ha-1 yr-1. Biomass yields from hedges harvested for wood fuel in Devon suggest that a tall hedge may accumulate 3.4 t C ha-1 yr-1 above ground (derived from Wolton 2012a). The difference between these estimates is probably because the Robertson figures reflect gappy hedges while the Wolton figures are from continuous hedges. Both sets of figures are for wide hedges.

5. Below ground, Robertson et al. (2012) conservatively estimated that new hedges could accumulate 0.54 t C ha-1 yr-1 as soil organic carbon (SOC) and old hedges 0.46 t C ha-1 yr-1. Based on data from British woodlands, they suggested hedges might continue to accumulate carbon for 700 or more years, before reaching a state of equilibrium.

6. Bases on all information available, it appears likely that unmanaged shrubby hedges may sequester about 1-2 t C ha-1 yr-1, rising to 4-5 t C ha-1 yr-1 for tree lines, although more research is required to confirm this. While above ground levels of carbon storage may reach equilibrium within a few decades even in uncut hedges, below ground this process may take many centuries: the above figures assume that soil carbon levels are not yet at equilibrium.

7. Estimates of the total amount of carbon stored in the soil beneath hedges are rather consistent. Robertson et al. (2012) estimated 85 t ha-1 for British hedges based on Rothamstead figures; Paulsen & Bauer (2008) estimated that hedge banks 3.9 m wide and 0.6 m high in Schleswig-Holstein (Germany) stored 127 t ha-1; and Follain et al. (2007) estimated that the banked hedges in a bocage landscape in Brittany, with a hedge density of 106 m ha-1, was 91 t ha-1 (over strips 20m wide on either side of the hedges). (The current density of hedges across lowland Devon is estimated at 107 m ha-

1.)

8. Studying a mixed farming landscape in Brittany (France), Walter et al. (2003) estimated that at a high hedge density of 200 m ha -1 as much as 38% of the total carbon stock across the local farmed landscape could be attributed to the hedge network, while at 50 m ha-1 this proportion fell to 13%. The latter figure is probably more typical of British landscapes. Based on 20 lamb and beef farms in Wales, Taylor et al. (2010) estimated that uncut hedges accounted for between 1% and 14% of on-farm carbon sequestration. In contrast, they reckoned that planting just 50 isolated trees on farmland could increase carbon sequestration rates by up to 5% per farm, although this is dependent on the soils and tree growth rates. However, it should be noted that they based all their calculations on data from poplar trees, one of the fastest accumulators of biomass known in temperate climates. This probably accounts for their estimated median hedge sequestration rate of 6.37 t C ha-1 yr-1 being considerably higher than the figures estimated by Robertson et al. (2012) and deduced from Wolton (2012a).

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9. Urban air quality improvement


Information on the potential role of hedges in improving urban air quality was gathered mostly from peer reviewed academic papers, together with some other published reviews and reports. Research has focused on the benefits of trees as particulate sinks, particularly the differences between broad-leaved and conifer species (Freer-Smith et al. 2005). This has meant that other forms of vegetation such as shrubs, hedges and more complex biogenic regulators such as green walls have been relatively overlooked (Shakleton et al. 2012). This seems to reflect the difficulties of analysing data at a habitat level compared to leaves from a single tree. Size is the main characteristic which determines the behaviour of particles in the atmosphere and it is expressed as aerodynamic diameter (Dp). The size fraction regularly monitored is called PM10 (particulate matter less than 10 µm in Dp). Below PM10 the range of particle sizes found in the atmosphere is usually considered in three groups. Ultrafine particles are most commonly formed by the condensation of hot vapours (from incinerators and vehicle exhausts) or by the chemical conversion of gases to particles (e.g. sulphuric acid particles from oxidation of SO2). Such particles have a short lifetime in the atmosphere quickly coagulating into larger particles. Particles in between 0.2 and 2 µm in diameter are more stable in the atmosphere, having a lifetime of 7-30 days because they are not deposited through gravitational settling and scavenging by rain. Most particles greater than 2µm in diameter are formed by mechanical attrition processes (e.g. soil dust, sea-spray and industrial dusts), and are rapidly deposited by sedimentation. Finer particles are more likely to be deposited deep in the alveoli of the human lung causing adverse health effects (Freer-Smith et al. 2005). The size distribution of suspended particulate material has altered over the years as sources changed from being mainly domestic coal burning to vehicles and industrial premises. As well as influencing deposition rates to vegetation, particle size distribution also has a major influence on the risk to human health since smaller particles penetrate further into the respiratory system. Stenberg et al. (2011) review proposals to reduce ambient PM10 concentrations in urban areas through tree establishment (Bealey et al. 2007, Nowak et al. 2006, McDonald et al. 2007). PM10 deposition to vegetation has been the subject of many investigations (Beckett et al. 1998, Gupta et al. 2004, Dammgen et al. 2005, Tiwary et al. 2006). However, the complexities involved in understanding the removal mechanisms for PM10 on different vegetation types, species, planting design and age class has resulted in a large degree of uncertainty regarding the level of reduction that could practically be achieved and how this would relate to human health. This uncertainty is exacerbated by the inherent assumptions and uncertainties in deposition models, where the density, terrain, and meteorological conditions can differ (Ruijgrok et al. 1995, He et al. 2002). The research shows that there are many ways to measure and analyse pollution on vegetation. However, it does seem that bio-monitoring of pollutants, conducted by analysing tree leaves and bark, is one of the preferred methods because of the relatively cheap analysis and because it is easy to collect the plant materials for analysis (Gratani et al. 2008, Aničič et al. 2011, Sawidis et al. 2011). If urban vegetation is to be employed as a

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measure to decrease air pollution, then the most efficient species and planting systems should be used. However, studies comparing PM deposition among species are few, especially for the smallest PM fractions (<2.5 μm).


None.


The main way that hedges deliver this service is by acting as a sink for pollutants. Particles are removed from the atmosphere by deposition and leaves on trees capture these particles. Trees and woodlands can be significant sinks for gaseous, particulate, aerosol and rain-borne pollutants (Fowler et al. 1989, Broadmeadow & Freer-Smith 1996). Hedges offer settling surfaces throughout the day and night. Moreover, they are self-renewing and not merely one-time traps. Regularly pruned hedges become dense and bushy, and more effective in reducing particulate pollution (Varshney & Mitra 1993).

Trees absorb gaseous pollutants or intercept particulate pollutants in three ways: by dry,

wet or occult deposition (Fowler et al. 2001, Scott et al. 2008). Wet deposition is the removal of pollutants by precipitation and is independent of land cover. Occult deposition occurs when mist or cloud covers the vegetation. Dry deposition can occur by gravitational settling, impaction, interception or diffusion, depending on particle size. Trees absorb gaseous pollutants through leaf stomata and bind pollutant particles onto leaf surfaces. Deposited pollutant gases and particles can be chemically altered by plant tissues and may be metabolised or cause foliar injury (Smith 1978 & 1981). Particles can be re-suspended by turbulence or other mechanical action. Absorbed pollutants can be deposited to the ground surface as litter or leaf fall (Scott et al. 2008). Work on forest canopies found them to have high capturing efficiencies for airborne particles (Peters & Eiden 1992, Erisman et al. 1997, Freer-Smith et al. 1997, Decker et al. 2000, Urbat et al. 2004). The structure of trees and the rough surfaces that they provide increase the incidence of particle impaction and interception by disrupting the flow of air (Beckett et al. 1998), mainly at canopy height (Erisman et al. 1997). As a result of their large leaf areas and the turbulent air movement created by their structure, trees take up more pollution, including PM10 than shorter vegetation (Fowler et al. 1989 & 2004).

Differences between tree species play an important role in capture rates: leaves with

complex shapes, large circumference-to-area ratios, waxy cuticles or fine hairs on their surfaces collect particles most efficiently (Freer-Smith et al. 2005). Conifers, which are also in leaf all year round, may be more effective than deciduous species (Freer-Smith et al. 2005).

Single trees and edge trees collect particles more efficiently than canopy trees.

Turbulence and transport in wind canopies depend on many factors such as street canyon geometry and the wind direction relative to street direction, which are still too complex and too poorly understood to be simulated in a generalised spatial model. The edge effect becomes important where large numbers of trees are planted in a small area (Branford et al. 2004), with more particles deposited on trees at the edge of the plantation than in its middle.

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Shrubs and perennials, while not as large as trees, present several advantages over trees

as tools to mitigate particulate emissions including monetary cost, logistics of planting, and a greater variety of species from which to choose (Smith, 2011). Shakleton et al. (2012) quantified the PM2.5-10 capture of a range of shrub species. They looked at the efficiency and efficacy of different species and quantify the characteristics which allow certain species to outperform others. Hair density was a primary factor in determining particulate capture.

The ability of hedges to capture aerosols (which include haze, dust, particulate air

pollutants and smoke) has been attributed to both surface roughness and to the creation of tortuous air flows (Tiwary et al. 2008). Tortuous airflows lead to higher turbulence and increased mixing of PM, and surface roughness increases the likelihood of particles impacting on their foliage surfaces. Vegetative barriers surrounding farmland, animal housing, and urban dwellings tend to reduce dispersal of particulate matter through these near-ground particle deposition mechanisms.

Trees are not only effective at collecting particulate matter PM10, but also at scavenging

other pollutants through the uptake of ozone (O3), sulphur dioxide (SO2) and nitrogen oxides (NOX) (Broadmeadow & Freer-Smith 1996, Scott et al. 1998).

The Natural Resources Conservation Service of the United States recommends the use of

filter strips such as shelterbelts to reduce the movement of pathogens, particulate organic sediment and adsorbed nutrients, pesticides and other possibly toxic particulate materials into human microenvironments (NRCS 2002). However, the physical and chemical dynamics involved in capturing ambient PM with foliage surfaces is complex and currently not well characterised (Tiwary et al. 2006).


The layered canopy structure of large trees provides a surface area for particulate deposition of between 2 and 12 times that of the area of land they cover (Broadmeadow & Freer-Smith 1996). Fowler et al. (2004) found that woodlands in the West Midlands, England, collected three times more PM10 than grassland. Varshney & Mitra (1993) showed that roadside hedges in New Delhi, India, trapped nearly 40% of particulate matter, most of which arose from traffic movement. The hedges in question were composed of three evergreen shrubs or climbing shrubs, Bougainvillea spectabilis, Duranta plumieri and Nerium indicum. They concluded that hedges can play a role in improving air quality along main traffic car routes in urban areas. Urban trees can decrease concentrations of SO2 and O3 by 20% (Beckett et al. 1998), and it has been estimated that existing urban forests in Chicago have removed 212 t of PM10 each year, which is equivalent to a 0.4% hourly average improvement in air quality (McPherson et al. 1994). Research conducted in Beijing (China), where air pollution is very high, showed that trees in the city centre removed 772 t of PM10 during one year (Yang et al. 2005). In similar studies in Chicago (USA) urban trees, which occupy 11% of city area, removed about 234 t of PM10 (Nowak 1994). In the whole USA, urban trees and shrubs remove about 215 kilotonnes of PM10 every year (Nowak et al. 2006). In studies in UK cities, estimates for the West Midlands (19% of primary PM10 removed for an increase in tree cover from 3.7% to

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16.5%) were significantly larger than Glasgow (6% of primary PM10 removed for an increase in tree cover from 3.6% to 8%) (McDonald et al. 2007). Depending on the modelling approach, the urban canopy of the Greater London Authority (GLA) is currently estimated to remove between 852 and 2,121 tonnes of PM10 annually, representing between 0.7% and 1.4% of PM10 from the urban boundary layer. Estimates of PM10 removal which take into account a planned increased in tree cover, from the current 20%-30% of the GLA land area, suggest deposition of 1109–2379 tonnes (1.1–2.6% removal) by the year 2050. The evidence provided here suggests that the targeting of tree planting in the most polluted areas of the GLA, and particularly the use of street trees which have the greatest exposure to PM10, would have the greatest benefit to future air quality. The increased deposition would be greatest if a larger proportion of coniferous than of broadleaved trees were used at such sites (Tallis et al. 2011). Efforts to assess the role of hedges in the deposition of ambient aerosols have mainly been restricted to a small number of empirical studies with pollens and pesticide droplets. Stands of vegetation over 1m high have been shown to be efficient pollen collectors (Treu & Emberlin 2000). In addition, experimental evidence of drift filtration of pesticides sprayed either under field conditions (Davis et al. 1994, Miller et al. 2000) or in a wind tunnel (Ucar et al. 2003) indicates that hedge barriers can reduce drift by up to 90%. A recent study in Britain has estimated that woodland reduced pollution prevents about 65-89 early deaths a year and around 45-62 hospital admissions (Powe & Willis 2004). This conclusion is important and topical since work on human toxicology shows that ultra-fine particles have greater adverse health effects than larger particles which penetrate the respiratory system less well.


Depending on tree species, varying amounts and types of volatile organic compounds (VOCs) are emitted by trees which pay pose a problem if the species involved are high emitters and can have a negative impact at the local scale on air quality (Owen et al. 2003, Simpson & McPherson 2011, Street et al. 1996). However, of the species commonly used in cities, only a few seem to be a problem in this respect (Baraldi et al. 2010).

Trees also release pollen in the spring, which, depending on the species, can have

implications for hay-fever sufferers, as well as increasing particle concentrations to the atmosphere (Garcia-Mozo et al. 2006).

In addition over time toxic pollutants accumulate in the soil below trees (Fowler et al.

2004), which will result in contaminated soil and may have possible consequences for possible future land use and ground water quality.


Shelter. Trees in urban spaces are frequently beneficial through the provision of shade and through providing shelter from wind and rain. They may also, through increasing evapotranspiration, reduce the heat island effects of cities (McPherson & Rowntree 1993).

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Through acting as a windbreak and buffering temperature changes they may also reduce energy loss from buildings as found in a Swedish study (Nord 1991). Climate change mitigation. Gratani & Varone (2011) characterised structural traits of the hedge types traditionally used for green infrastructure in Rome (Italy) in order to quantify their carbon dioxide (CO2) sequestration. CO2 sequestration capabilities were assessed for the following hedge types: Laurus nobilis, Nerium oleander, Pittosporum tobira and Pyracantha coccinea, largely used as green infrastructure in Rome (Italy). All the considered species, being evergreens, were active all year long, including winter, when CO2 emissions from road transport peaked. Nevertheless, among the considered hedge types, P. tobira and L. nobilis were the most efficient species in carbon sequestration and also had greater tolerance to severe pruning and a large ability to maintain high shoot density and basal foliage.


Since NELMS agreements are likely to be restricted to agricultural holding, this service is not relevant here. Nevertheless, it is opportune to offer some observations on appropriate species to plant within urban hedges if the aim is to help improve air quality: Choice of species to plant is important: ability to capture particulates and tolerance of stress are both important traits that should be considered when choosing optimal vegetation for urban and suburban areas (Sæbø et al. 2005). Pollutants affect the survival of plants (Chappelka & Freer-Smith 1995), and this will affect the potential benefits that can be expected from planting in urban areas (Morani et al. 2011).

Leaves of broad-leaved species, which have rough surfaces, are more effective in capturing PM than those with smooth surfaces (Beckett et al. 2000) (e.g. whitebeam Sorbus aria is better at capturing particles than poplars Populus spp. (Beckett et al. 2008)). Needles of coniferous trees, which produce a thick epicuticular wax layer, are more effective in PM accumulation than broad-leaved species. For example, pines Pinus spp., are better than cypresses (Cupressus spp.) at capturing particles (Beckett et al. 1998). Evergreen conifers also have the potential for accumulating toxic pollutants throughout the year. On the other hand, since they typically keep their needles for several years there is no annual recycling of PM, as occurs with deciduous species. In addition, conifers are in general less tolerant to high traffic-related pollution, especially if salt is used for road de-icing during winter, and they are often not recommended for roadside plantings. Evergreen conifers may not therefore be as useful overall as deciduous leafy species, in spite of their high efficiency in PM scavenging (Beckett et al. 2000). Ivy leaves can effectively absorb airborne particulate matter in urban environments. Stenberg et al. (2011) examined the potential bio-protective role of ivy Hedera helix and how it may interact with airborne particulates in urban environments. Using Scanning Electron Microscopy ivy leaves collected along on traffic corridors were examined to determine dust absorption rates. Results showed that ivy acts as a ‘particle sink’ and was effective in capturing fine (1-2.5 µm) and ultrafine (<1 µm) particles and pollutants from man-made sources.

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9.8. Value of evidence to CAP greening and cross compliance measures None.



I. To understand better the effectiveness of hedges, and of native tree and shrub species, at removing air pollutants in urban UK environments.

II. To quantify the extent of hedges in urban environments and their contribution to air quality (as well as other ecosystem services).

9.10. Conclusions

1. The majority of the research on improving urban air quality has been on individual trees (Freer-Smith et al. 2005, Beckett et al. 2000, Fowler et al. 1989 & 2004), rather than on hedges. This is likely to reflect practical difficulties.

2. The efficiency of different plant species to capture pollutant particles depends on a

number of factors, including the dimensions and complexity of leaf shape, whether or not the cuticle is waxy or covered with fine hairs, and the spatial position and age of the individual plants. Choice of species and location is therefore important.

3. Tree planting in urban areas is widely considered to be a worthwhile benefit due to their

ability to remove health damaging particles from the air (Tiwary et al. 2009). A few studies have shown that hedges can perform the same function (Vashney & Mitra 1993, Tiwary et al. 2008, Gratani & Varone 2011), but more research is needed on the optimal position, structure and choice of species.

4. Mature trees in urban environments can reduce pollutant loads by between 7% and 26%

(Tiwary et al. 2009). In studies in UK cities, McDonald et al. (2007) found that planting trees on one quarter of the available urban area was able to reduce the PM10 concentration by between 2% and 10%.

5. Some pollutants are not fully removed from the system by the trees but instead

accumulate in the soils beneath the trees, with implications for land use in the future (Tiwary et al. 2009).

6. Overall, planting trees and shrubs, perhaps as hedges, provides the only practical way to

remove pollutants from the air in open spaces – plants provide effective and relatively cheap air purification (Dzierżanowski et al. 2011).

79

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regulatory services delivered by hedges: the evidence base

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