development of a nitrophobe classification for sand dunes
TRANSCRIPT
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C O M M I S S I O N E D R E P O R T
Commissioned Report No. DK1605b
Development of a nitrophobe/nitrophile
classification for sand dunes.
For further information on this report please contact
Claire Campbell
SEPA Corporate Office
Strathallan House
Castle Business Park
STIRLING
FK9 4TZ
Telephone: 01786 452448
E-mail: [email protected]>
This report should be quoted as:
Jones, L. and Stevens, C. (2017). Development of a nitrophobe/nitrophile
classification for sand dunes
Scottish Environment Protection Agency Commissioned Report No. DK1605b.
This report, or any part of it, should not be reproduced without the permission of Scottish Environment
Protection Agency. This permission will not be withheld unreasonably. The views expressed by the author(s)
of this report should not be taken as the views and policies of Scottish Environment Protection Agency.
© Scottish Environment Protection Agency [2017].
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COMMISSIONED REPORT
Summary Development of a nitrophobe/nitrophile
classification for sand dunes
Commissioned Report No. DK1605b
Contractor: Jones, L. and Stevens, C.
Year of publication: [2017]
Executive Summary Both dry dune habitats and wet dune slack communities of sand dunes are known to be sensitive to
N deposition. However, despite their biological importance, they are not currently covered within
the nitrophobe/nitrophile assessment methodology. This study aimed to develop indicators for sand
dune communities.
We developed lists of nitrophile indicator species, focusing on those above a threshold Ellenberg N
value, defined in relation to the 80th percentile Ellenberg N of component NVC communities. Species
lists were extracted from the NVC manual for dune habitats.
The method differs slightly from that in Pitcairn et al. (2006) who developed both nitrophile and
nitrophobe indicators. In the approach taken here it was decided not to include nitrophobe species,
but to focus on nitrophiles expected to increase with eutrophication. The index for any quadrat of
plant data is calculated as the sum of percentage cover of listed nitrophile indicator species for the
relevant dune habitat type.
UK-wide data were used to develop and test the indicators. This approach is generalizable to any
situation in which the dune species have Ellenberg indicator values defined (i.e. primarily temperate
Europe). Local benchmarking of indicator values against N deposition would be required to develop
absolute values for the metrics in a new situation.
Testing of the indicators against survey data showed a high degree of scatter in individual quadrats,
but site means showed broadly positive relationships with N deposition. Testing of the dune slack
indicator against an N-gradient in groundwater at a single site also showed some promise. However,
this initial testing is based on limited datasets.
We recommend more extensive testing of the indicators against wider datasets, and to consider
including terricolous lichens as a component of the indicator in the acidic dune grasslands habitat.
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Contents 1 Introduction .................................................................................................................................... 5
2 Methods .......................................................................................................................................... 5
2.1 Categorising dune communities ............................................................................................. 5
2.2 Selecting indicator species ...................................................................................................... 8
3 Results – selected nitrophile indicator species ............................................................................... 9
3.1 Semi-fixed and blow-out communities ................................................................................... 9
3.2 Calcareous fixed dune grassland ........................................................................................... 10
3.3 Acidic dune grassland & dune annuals ................................................................................. 11
3.4 Dune slacks............................................................................................................................ 12
4 Testing the indicators .................................................................................................................... 13
5 Discussion of indicators and testing ............................................................................................. 13
6 Conclusions ................................................................................................................................... 18
7 References .................................................................................................................................... 19
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List of Figures Page Figure 1. Frequency distribution of Ellenberg N values, showing mean and 80%ile Ellenberg N for
each NVC sand dune community. From Jones et al. 2002. 7
Figure 2. Nitrophile index for quadrats of semi-fixed and blow-out communities. Open circles are
individual quadrats, filled circles represent the mean of quadrats at each site. 14
Figure 3. Nitrophile index for quadrats of calcareous fixed dune grassland communities. Open circles
are individual quadrats, filled circles represent the mean of quadrats at each site. 14
Figure 4. Nitrophile index for quadrats of acidic dune grassland & dune annuals communities. Open
circles are individual quadrats, filled circles represent the mean of quadrats at each site. 15
Figure 5. Nitrophile index for quadrats of dune slacks. Open circles are individual quadrats, filled
circles represent the mean of quadrats at each site. 15
Figure 6. Nitrophile index for quadrats of dune slacks, tested against a gradient of nitrate
concentration in groundwater at Aberffraw dunes in North Wales. Data from Rhymes et al. (2014;
2015) 16
List of Tables Page Table 1. Groupings of dune NVC communities, 80th percentile Ellenberg N score for each, and
threshold Ellenberg N value applied to determine which species were used as indicators for each
group. 6
Table 2. Nitrophile indicator species for semi-fixed and blow-out communities (SD7, SD10) 9
Table 3. Nitrophile indicator species for calcareous fixed dune grassland (SD8, SD9) 10
Table 4. Nitrophile indicator species for Acidic dune grassland & dune annuals (SD11, SD12, SD19) 11
Table 5. Nitrophile indicator species for Dune slacks (SD13 – SD17) 12
Table 6. Species positively associated with high N deposition in the Scottish dune repeat survey
together with the habitats groups for which they are eutrophic indicators. 16
5
1 Introduction Both dry dune habitats and wet dune slack communities of sand dunes are known to be sensitive to
N deposition (Jones et al. 2004; Jones et al. 2013; Remke et al. 2009a; Rhymes et al. 2014).
Demonstrated impacts include shifts in species composition and reductions in plant diversity (Field
et al. 2014). Despite their biological importance and high sensitivity to nitrogen deposition, they are
not currently covered within the nitrophobe/nitrophile assessment methodology.
Therefore, this project will develop a field-based classification of the response to atmospheric
nitrogen of higher plant species and bryophytes typically found in sand dunes in Scotland and the
wider UK. It will develop a nitrophobe/nitrophile classification using species with an extreme
Ellenberg response, broadly following the methodology used by Pitcairn et al. (2006).
2 Methods
2.1 Categorising dune communities Sand dune communities represent a complex continuum of habitats from dry to wet, high pH to low
pH and young to old. This is reflected in the NVC classification which comprises 18 dune
communities, including strandline communities (Rodwell, 2000). In order to simplify the
development and application of an indicator system, these communities have been grouped
according to their likely sensitivity to N. This built on previous work to assess the N sensitivity of
dune habitats (Jones et al. 2002), which characterised the distribution profile of Ellenberg N values in
each sand dune NVC community, based on their vascular plant composition (Figure 1), calculated for
each community based on the species lists provided in Rodwell (2000) and using the Hill-modified
Ellenberg N value from PlantAtt (Hill et al. 1999). Using this information as a starting point,
communities were grouped according to four main principles:
similar sensitivity to N as indicated by the Ellenberg N distribution of their component
species
similar degree of soil development, mainly separating the younger or more mobile
communities from the more stable fixed dune grasslands
soil pH, separating calcareous communities from those which have started to decalcify or
those which have acidic sand as parent material, and
any hydrological influence, separating dune slacks from the dry dune communities
When deciding on communities where the group membership was not immediately clear, we looked
in more detail at the species composition of the groups, and at the community average Ellenberg R
values (as a proxy for soil pH), to make final allocation to groups. The resulting groupings are shown
in Table 1.
Strandline communities were excluded as they are exposed to an open N cycle with regular inputs of
N from saltspray, seaweed and other marine biological material. The most mobile dune communities
were also excluded as they are highly dynamic and the role of N as a factor governing species
composition plays a much smaller role than physical disturbance and proximity to the sea. Lastly,
infrequent communities such as dune heath and dunes with Juniper were also excluded.
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Table 1. Groupings of dune NVC communities, 80th percentile Ellenberg N score for each, and threshold Ellenberg N value applied to determine which species were used as indicators for each group.
NVC
code NVC Community (80th percentile Ellenberg N score) Group name
Ellenberg
N
threshold
SD2-
SD3
Honkenya peploides - Cakile maritima strandline community;
Matricaria maritima - Galium aparine strandline community
Strandline
communities Exclude
SD4-
SD6
Elymus farctus ssp. boreali-atlanticus foredune community;
Leymus arenarius mobile dune community;
Ammophila arenaria mobile dune community
Foredune and
mobile dune
communities Exclude
SD7
Ammophila arenaria - Festuca rubra semi-fixed dune
community (6.0)
Semi-fixed and
blow-out
communities
(SD7, SD10)
6
SD10 Carex arenaria dune community (6.0)
SD8 Festuca rubra - Galium verum fixed dune grassland (5.2) Calcareous
fixed dune
grassland
(SD8, SD9)
6
SD9
Ammophila arenaria - Arrhenatherum elatius dune grassland
(6.0)
SD11 Carex arenaria - Cornicularia aculeata dune community (3.0) Acidic dune
grassland &
dune annuals
(SD11, SD12,
SD19)
4 SD12
Carex arenaria - Festuca ovina - Agrostis capillaris dune
grassland (5.0)
SD19
Phleum arenarium - Arenaria serpyllifolia dune annual
community Tortulo-Phleetum arenariae (Massart 1908) Br.-
Bl. & de Leeuw 1936 (4.0)
SD13
Sagina nodosa - Bryum pseudotriquetrum dune-slack
community (5.0)
Dune slacks
(SD13-SD17) 6 SD14
Salix repens - Campylium stellatum dune-slack community
(5.0)
SD15
Salix repens - Calliergon cuspidatum dune-slack community
(5.4)
SD16 Salix repens - Holcus lanatus dune-slack community (5.0)
SD17 Potentilla anserina - Carex nigra dune-slack community (6.0)
SD18 Hippophae rhamnoides dune scrub
Hippophae
scrub Exclude
- Other communities: Dune heath, Dunes with Juniper Other Exclude
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Figure 1. Frequency distribution of Ellenberg N values, showing mean and 80%ile Ellenberg N for each NVC sand dune
community. From Jones et al. 2002.
SD 2 Honkenya-Cakile (Southern) SD 3 Matricaria-Galium (Northern)
SD 4 Elymus farctus SD 5 Leymus arenarius SD 6 Ammophila arenaria
SD 7 Ammophila-Festuca SD 10 Carex arenaria
SD 8 Festuca-Galium (grazed) SD 9 Ammophila-Arrhenatherum (ungrazed)
SD 11 Carex-Cornicularia SD 12 Carex, Festuca, Agrostis SD 19 Phleum arenaria
Dune slacks SD 13 Sagina-Bryum (immature slack)SD 14 Salix-Campylium SD 15 Salix-Calliergon
SD 16 Salix-Holcus (drier, ungrazed) SD 17 Potentilla-Carex nigra (wetter, grazed) SD 18 Hippophae scrub
Foredune
and mobile
dune
communities
Strand line
communities
Semi-fixed
and blow-out
communities
Calcareous
fixed dune
grassland
Acidic dune
grassland &
dune annuals
Hippophae scrub
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 5.94, 80%ile = 7.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 6.08, 80%ile = 7.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean =3.99, 80%ile = 5.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 3.89, 80%ile = 5.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 4.10, 80%ile = 6.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 4.08, 80%ile = 6.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
SD 18, mean = 5.00
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 4.14, 80%ile = 6.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 3.16, 80%ile = 4.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 4.34, 80%ile = 6.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 3.98, 80%ile = 5.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 4.31, 80%ile = 5.4
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 3.82, 80%ile = 5.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 3.37, 80%ile = 5.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 2.53, 80%ile = 3.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 5.18, 80%ile = 6.4
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 5.44, 80%ile = 7.0
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
mean = 5.93, 80%ile = 7.0
8
The first grouping for which indicators were developed is mobile and semi-fixed dunes. This includes
SD7 and SD10, both largely calcareous communities. The second group comprises fixed dune
grasslands which are also calcareous SD8 and SD9, the former grazed, the latter ungrazed. The third
group comprises the more acidic dune communities SD11 which is strongly acidic and SD12 where
surface soils are starting to acidify. It also includes the dune annuals community SD19. A number of
combinations were trialled, with preliminary testing against observed data. The testing revealed that
although SD12 is often only partly decalcified, the species complement and Ellenberg score profile of
the community (Figure 1) was closer to that of the acidic SD11 than the more calcareous SD8 and
SD9. Similarly, the species complement of the dune annual community in the NVC has closer
affinities to the acidic sand communities than to the calcareous communities. Although the slack
communities differed slightly in terms of Ellenberg N profile, they were not considered sufficiently
different to further subdivide this group.
2.2 Selecting indicator species The approach of selecting indicator species within each community group differs slightly from earlier
work by Pitcairn et al. (2006). It was felt that reliance only on species in the NVC lists which had a
constancy value of III or higher (i.e. frequency > 40%) might miss species present at low cover or
frequency which may nonetheless become dominant in future due to eutrophication.
In addition, rather than selecting particular species as either nitrophobes or nitrophiles, it was
decided to focus solely on the nitrophiles which might expand in cover under more eutrophic
conditions. Therefore, the 80th percentile of Ellenberg N value was calculated for each community.
This threshold was chosen to include sufficient of the most nitrophilic species from the NVC lists for
each community, without creating impractically long lists of species. For consistency, a single
representative Ellenberg N value was then chosen across all NVC communities in each group, and all
species with an Ellenberg N value equal to or greater than that threshold were selected as nitrophile
indicator species for that group. The lists contain both vascular plants and bryophytes, but not
lichens. Terricolous lichens were not included as they make up a relatively small proportion of the
species complement, with the exception of acidic dune grasslands where they do form an important
component of the vegetation. For consistency of approach across communities they were not
included in this assessment, but the approach could be modified in future.
9
3 Results – selected nitrophile indicator species Nitrophile indicator species for each community group are listed below.
3.1 Semi-fixed and blow-out communities Table 2. Nitrophile indicator species for semi-fixed and blow-out communities (SD7, SD10)
Species Ellenberg
N
Vascular & other plants Arrhenatherum elatius 7
Cirsium arvense 6
Cirsium vulgare 6
Dactylis glomerata 6
Elytrigia atherica 6
Elytrigia juncea 6
Elytrigia repens 7
Heracleum sphondylium 7
Honckenya peploides 6
Leymus arenarius 6
Potentilla anserina 6
Ranunculus repens 7
Rubus caesius 6
Rubus fruticosus agg. 6
Rumex crispus 6
Senecio vulgaris 7
Sonchus arvensis 6
Sonchus asper 6
Trifolium repens 6 Tripleurospermum maritimum
sens.lat. 6
Tussilago farfara 6
Bryophytes Brachythecium rutabulum 8
Bryum argenteum 8
Eurhynchium praelongum 6
10
3.2 Calcareous fixed dune grassland Table 3. Nitrophile indicator species for calcareous fixed dune grassland (SD8, SD9)
Species Ellenberg
N
Vascular & other plants Agrostis stolonifera 6
Arrhenatherum elatius 7
Cirsium arvense 6
Cirsium vulgare 6
Dactylis glomerata 6
Elytrigia atherica 6
Elytrigia juncea 6
Elytrigia repens 7
Equisetum arvense 6
Heracleum sphondylium 7
Leymus arenarius 6
Lolium perenne 6
Myosotis arvensis 6
Plantago major 7
Poa trivialis 6
Potentilla anserina 6
Ranunculus repens 7
Silene latifolia 6
Sonchus oleraceus 7
Torilis japonica 7
Trifolium repens 6
Bryophytes Brachythecium rutabulum 8
Eurhynchium praelongum 6
Plagiomnium rostratum 6
Plagiomnium undulatum 6
11
3.3 Acidic dune grassland & dune annuals Table 4. Nitrophile indicator species for Acidic dune grassland & dune annuals (SD11, SD12, SD19)
Species Ellenberg
N
Vascular & other plants Agrostis capillaris 4
Arenaria serpyllifolia 5
Cirsium arvense 6
Crepis capillaris 4
Erodium cicutarium agg. 4
Euphorbia paralias 5
Festuca rubra agg. 5
Geranium molle 5
Holcus lanatus 5
Lathyrus pratensis 5
Plantago coronopus 4
Plantago lanceolata 4
Poa pratensis sens.lat. 5
Ranunculus repens 7
Senecio jacobaea 4
Trifolium campestre 4
Trifolium dubium 5
Trifolium repens 6
Urtica dioica 8
Viola tricolor 4
Bryophytes Brachythecium albicans 4
12
3.4 Dune slacks Table 5. Nitrophile indicator species for Dune slacks (SD13 – SD17)
Species Ellenberg
N
Vascular & other plants Agrostis stolonifera 6
Alopecurus geniculatus 6
Arrhenatherum elatius 7
Bolboschoenus maritimus 7
Carex hirta 6
Cirsium arvense 6
Elytrigia repens 7
Equisetum arvense 6
Eupatorium cannabinum 7
Glechoma hederacea 7
Iris pseudacorus 6
Juncus gerardii 6
Lolium perenne 6
Lycopus europaeus 6
Persicaria maculosa 7
Phragmites australis 6
Plantago major 7
Poa annua 7
Poa trivialis 6
Potentilla anserina 6
Ranunculus repens 7
Rubus caesius 6
Rumex crispus 6
Salix caprea 7
Solanum dulcamara 7
Sonchus arvensis 6
Trifolium fragiferum 6
Trifolium repens 6
Bryophytes Amblystegium serpens 7
Brachythecium rutabulum 8
Eurhynchium praelongum 6
Plagiomnium rostratum 6
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4 Testing the indicators Available data from two targeted gradient surveys and a site-specific study (for dune slacks only)
were used to test and refine the indicators. These included a survey in 2006 focusing on dune slack
species (Jones 2007) and a survey focusing primarily on decalcified dune grasslands (Field et al.
2014), although incorporating a number of other communities. In these available datasets, there
was incomplete representation across all the communities, but they served the purpose of initial
testing of the indicators, pending more thorough testing in the companion project RAD031-Botanical
Benchmarks (Jones et al. 2017), with more extensive dune datasets. In each figure below, the open
circles represent individual quadrats, the black filled circles represent the mean nitrophile score for
the site.
Data for the semi-fixed and disturbance communities group was taken from the 2009 survey (Field
et al. 2014), only for quadrats recorded as being in the SD7 community. Results from this fairly
limited dataset are shown in Figure 2. There is a fairly poor fit.
Data for the calcareous fixed dune grassland group was also taken from the 2009 survey (Field et al.
2014), covering quadrats recorded as matching SD8 and SD9. Figure 3 shows a large scatter among
individual quadrats, with the site means showing a trend towards increasing nitrophile index with
increasing N deposition. However, the three sites with the largest N deposition show very low index
values.
Data for the acidic dune grassland & dune annuals group was also taken from the 2009 survey (Field
et al. 2014), covering quadrats recorded as matching SD12. Although individual quadrats show a fair
degree of scatter (Figure 4), the site means appear to show a positive relationship with N deposition.
Data for the dune slacks were taken from the 2006 survey (Jones 2007). Although there were fewer
sites in this survey, there were far more quadrats taken at each site. Overall, there is a positive trend
to the data (Figure 5), with a suggestion that the relationship reaches a plateau above 10 kg N/ha/yr.
An additional site-specific dataset from dune slacks at Aberffraw dunes in North Wales was tested,
where there is a strong gradient of nitrate concentrations in the groundwater underlying the site,
which has been shown to have deleterious effects on the dune slack ecology (Rhymes et al. 2014;
2015). This dataset shows a good relationship with the nitrophile indicator, with a steep increase at
low nitrate levels, but reaching a plateau at higher concentrations (Figure 6).
5 Discussion of indicators and testing A number of the eutrophic indicators species identified in this study have previously been identified
as responding positively to increased levels of nitrogen, either through experimental N additions or
spatial gradients. A repeat survey of 89 coastal dunes in Scotland 34 years after the initial survey
revealed a number of species that were positively associated with high N deposition (Pakeman et al.
2016). All of the species most strongly associated with high N deposition in the repeat survey have
been identified as eutrophic indicator species in this study. Species correlated with high N deposition
in the repeat survey are listed in Table 6.
14
Figure 2. Nitrophile index for quadrats of semi-fixed and blow-out communities. Open circles are individual quadrats, filled circles represent the mean of quadrats at each site.
Figure 3. Nitrophile index for quadrats of calcareous fixed dune grassland communities. Open circles are individual quadrats, filled circles represent the mean of quadrats at each site.
15
Figure 4. Nitrophile index for quadrats of acidic dune grassland & dune annuals communities. Open circles are individual quadrats, filled circles represent the mean of quadrats at each site.
Figure 5. Nitrophile index for quadrats of dune slacks. Open circles are individual quadrats, filled circles represent the mean of quadrats at each site.
16
Figure 6. Nitrophile index for quadrats of dune slacks, tested against a gradient of nitrate concentration in groundwater at Aberffraw dunes in North Wales. Data from Rhymes et al. (2014; 2015)
Table 6. Species positively associated with high N deposition in the Scottish dune repeat survey together with the habitats groups for which they are eutrophic indicators.
Species
SD7 + disturbance
SD10 SD8 + SD9 Fixed
Acid SD11 + SD12 +
disturbance SD19
Agrostis capillaris
Agrostis stolonifera
Arrhenatherum elatius
Cirsium arvense
Cirsium vulgare
Dactylis glomerata
Heracleum sphondylium
Holcus lanatus
Potentilla anserina
Ranunculus repens
17
There have been relatively few studies of changes in species composition in response to N
deposition in sand dunes, compared with other habitats. In an experimental N addition study on a
calcareous fixed dune grassland, Plassmann et al. 2009 found no significant effects of N addition on
species composition after two years of N addition at rates of 0, 7.5, 15 kg N ha-1 yr-1, but after eight
years of N addition there was an increase in the biomass of bryophytes (Phoenix et al., 2011),
although species richness of higher plants was not impacted. Subsequent research suggests that the
fixed dune grassland at this site is N and P co-limited (Ford et al. 2016), which may explain the lack of
treatment effects. In a modelling study at the same site, Rowe et al. (2011) showed that it might
take many decades to achieve species change in response to increased N deposition. In an ammonia
exposure experiment conducted using sand dune mesocosms of a calcareous fixed dune grassland
vegetation in a gradient away from a poultry unit Dactylis glomerata, Festuca rubra and Plantago
lanceolata all responded positively. F. rubra showed significant increases in biomass as did D.
glomerata and P. lanceolata which came to dominate mesocosms (Jones et al. 2013). In a different
component of the same study, Mohd-Said (1999) also reported increases in these species with N
applied at 10 kg N ha-1 yr-1. Other less eutrophic species had lower biomass (Jones et al., 2013).
In dune slacks, there is also some evidence of N effects, although primarily from groundwater
nutrients rather than atmospheric N deposition. Although not an atmospheric-focused study,
Rhymes et al. (2014) found Trifolium repens was positively associated with elevated N in ground
water at Aberffraw dune system in North Wales. Lotus corniculatus, which was not identified as an
indicator using this method, was also positively associated with elevated N in ground water
suggesting it may also be a eutrophic indicator in dunes. In older studies, Elytrigia juncea has been
reported to increase with fertiliser (NPK) addition (Adriani and Terwindt, 1974, Willis, 1965; Pavlik,
1983; Fay and Jeffrey, 1992) together with Sonchus asper, F. rubra, Poa pratensis (Willis, 1965),
Crepis capillaris, Geranium molle and Senecio jacobaea (Willis and Yemm, 1961). Changes in the
seedbank of dune slacks were observed in a germination study treated with elevated N (Plassmann
et al. 2008). Anagallis tenella, Carex viridula, Centaurium littorale, Juncus articulatus, Parnassia
palustris and the glaucous sedges (Carex flacca, Carex panacea) all increased their germination
success in response to N addition which suggests there could potentially be future changes in their
abundance but none of these species have been identified as eutrophic indicators. Plassmann et al.
(2008) suggested that germination was enhanced by N additions in these species, but N impacts on
subsequent growth of the plants was not studied.
The relative importance of different sources of N to dune slacks has been little studied, but is highly
relevant to wetland habitats which could be receiving N from multiple sources in addition to
atmospheric N deposition (Rhymes et al. 2015). Further work is required to establish whether the
source of N causes differences in ecological impact. A few commissioned studies have started to
explore these issues (e.g. Farr & Hall 2014; Farr et al. 2017).
Many of the species identified as eutrophic indicators for sand dune communities in this study have
been identified as responding positively to N addition or deposition in other communities. For
example, Agrostis capillaris, Festuca rubra and Holcus lanatus were among species identified as
increasing with higher N deposition over time in acid grasslands (Dupre et al. 2010). The bryophyte
Brachythecium rutabulum has been identified as increasing over time in a period of high N
18
deposition in chalk grassland in The Netherlands (During and Willems, 1986), and was identified as a
positive responder to N by Field et al. (2014) in a gradient study on heathlands.
There are also responses that have been observed which are not picked up by these indicators. In a
survey of 11 dune systems along a gradient of deposition Jones et al. (2004) found an increase in the
height and cover of Ammophila arenaria in mobile and semi-fixed dunes and an increase in cover of
Carex arenaria and Hypochaeris radicata. Increases in C. arenaria have also been observed in field
and mesocosm studies in dune systems in the Netherlands (e.g. Remke et al. 2009a; Remke et al.
2009b; van den Berg et al., 2005; Adriani and Terwindt, 1974). It is not currently planned to add
these species to the indicator lists, but there may be scope to revise them and include other species
known to respond to N, similar to the approach followed by Pitcairn et al. (2006).
Testing of the indicators for sand dunes against data from gradient surveys has shown they have
some promise in revealing eutrophication signals in the data. However, there is considerable scatter
in the relationships and the datasets used to test them so far are quite small when broken down to
smaller groups of communities. The factors underlying scatter in the relationships are unclear.
Within and across sites, this may include variation in soil pH or organic matter content. It may also
include additional drivers of vegetation composition such as climate, hydrology, and site
management practices such as grazing intensity. Figure 5 in particular illustrates the high variability
in indicator values within a site. This would be expected to be greater for dune slacks than other
habitats due to the high variability in hydrological regime, groundwater chemistry, degree of soil
development and soil pH in dune slacks of different ages and typologies (Curreli et al. 2013; Davy et
al. 2010).
The different habitat types appear to have different sensitivities to nitrogen, reflected in the
maximum observed indicator values on the y axis. The relatively low values for the semi-fixed and
blow-out communities partly reflect the low total vegetation cover in these habitats where there is
still considerable bare-sand present. However, the reason for differences among the other habitats
is unclear, and may simply be a result of the relatively small datasets used so far in testing the
indicators.
6 Conclusions From the testing so far, the approach seems to work best for acidic dune grasslands, with some
indication of a useful relationship at a site level for dune slacks. Until recently, there had been no
comprehensive testing of the Pitcairn et al. (2006) indicators approach, apart from the single
application to a highly eutrophic gradient away from a poultry unit in their report. This study
therefore is one of the first applications against observed N gradients at large scale and in the low-
to-middle range of N deposition.
The companion project (RAD031-Botanical Benchmarks) (Jones et al. 2017) also explored
benchmarking of a nitrophile index and a suite of other indicators derived from botanical data across
a range of habitats, including dune habitats. In the botanical benchmarks study the only dune
habitat tested was the acidic dune grassland and annuals, using a wider dataset including sites from
the Scottish sand dune survey and CEH data. The results for the nitrophile index showed
19
considerable scatter, with no clear pattern, contrasting to that reported here. The number of acidic
dune sites in the UK is rather limited, and full testing of this habitat may need to consider including
other sites from the continent to incorporate areas with both lower and higher N deposition. By
comparison, the nitrophile-nitrophobe index proved to be a useful metric in other habitats (acid
grassland, dry heaths, and wet heaths and bogs).
The reason for this is unclear. It may be that a more nuanced indicator which takes account of
known responses to nitrogen, as developed in Pitcairn et al. (2006), would work better than the
more objective approach taken here. However, other factors may also play a part including the much
greater heterogeneity in dune habitats within and across sites than is typically found in other habitat
types. For example, the site-specific data using a nitrogen-gradient within Aberffraw dune slacks in
Wales shows that the dune slack metric shows some promise.
Therefore, further testing of the nitrophile indicator for the other dune habitats against larger
datasets, and in other situations would help evaluate their usefulness as an indicator of
eutrophication. Modification of the acidic dune grasslands species list to include lichens may add
additional sensitivity to N responses in this habitat and should be considered.
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