estimating the age of groups of trees in historic landscapes

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ESTIMATING THE AGEOF GROUPS OF TREES INHISTORIC LANDSCAPESBen LennonPublished online: 27 Mar 2012.

To cite this article: Ben Lennon (2009) ESTIMATING THE AGE OFGROUPS OF TREES IN HISTORIC LANDSCAPES, Arboricultural Journal:The International Journal of Urban Forestry, 32:3, 167-188, DOI:10.1080/03071375.2009.9747572

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Arboricultural Journal 2009, Vol. 32, pp. 167-188 © AB Academic Publishers 2009 Printed in Great Britain

ESTIMATING THE AGE OF GROUPS OF TREES IN HISTORIC LANDSCAPES

Ben Lennon

Summary

John White's method of the non-intrusive dating of large and veteran trees provides a useful framework to the understanding of the historic development of treed landscapes in Britain. The method is based on the dating of individual veteran trees and can return immensely variable results among trees of a common establishment date. It is proposed that the method for single trees may be extended to clumps, avenues and other features of historic landscapes through the analysis of the frequency distribution of girth. Although individually very variable, trees of a common species, planting age, site type and management regime conform to a natural pattern of variability with a predictable frequency distribution of girth. Analysis of the distribution of girth within a given population can reveal certain characteristics in relation to its establishment period and development over time. These models can then be applied to other known populations of trees and individual specimens within the same landscape in order to establish a historical pattern of development. Differences in growths patterns between different species can also be exploited to refine models. This method is best applied to the level of the landscape where there is an abundance of relevant data and supporting documentary evidence. Where establishment dates cannot be accurately derived a chronological pattern of establishment may be predicted. It is not reliable where there is a paucity of data or where populations of trees are small.

Outline of the Basic Method

In 1998 John White proposed a non-intrusive method for the aging of ancient individual trees. The method relies upon comparisons with lists of trees established at a known date. From these data the likely performance of various trees in different locations could be evaluated through crossreferencing with trees in similar conditions (WHITE, 1998). The method relies on a measurement of girth, or diameter at breast height (dbh), as this is the single non-reversible and measurable parameter of the tree. 1

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168 ARBORICULTURAL JOURNAL

White notes that different species of tree have a range of years over which they may grow actively. The lifespan within the range is largely governed by the conditions under which any individual is subject during its lifetime. Pioneer species such as birch or rowan rarely live beyond 150 years of age. Beech may start to decline after 150 but may live much longer. Oak may live for many centuries and native limes are virtually indestructible, as they will tend to propagate themselves through layering (RACKHAM, 1990, 14-15). The key element in growth patterns of trees is the amount of sunlight that a tree can photosynthesise. This is primarily dependent on the area of leaves containing chlorophyll that the tree can produce within any given growing season. Hence, the larger the crown that can contain leaves, the greater the amount of cellulose in the annual ring of the stem and the branches that can be produced (RACKHAM, 1990, 11).

This pattern of growth is not evenly distributed throughout the life of the tree. White has described three phases of development in the growth pattern of open-grown trees in Great Britain: a formative period, middle age (or mature state), and senescence (WHITE, 1998). During the formative period the tree progressively develops its crown and consequently lays down increasing amounts of woody material. Providing that competition for light from adjacent crowns does not start to suppress the development of leaves then the tree will continue to increase in crown and stem diameter. During this phase the annual increment of woody material laid down will gradually increase although, due to the constant increase in stem diameter, the ring width will appear relatively even.

At some point the canopy will achieve an optimum state for growth and the tree will enter into its mature phase. During this phase the level of production of annual cellulose will become stable but this will be spread evermore thinly over a larger area as the stem diameter of the tree continues to increase. The tree rings thus comprise of the same cross sectional area per ring but will decline in thickness being spread around an expanding circumference.

Eventually the tree will enter into the final stage of senescence. 2 The actual point of decline can be influenced by a number of factors including competition from surrounding vegetation, disturbance to roots, drought, flooding, or attack from biotic agents. During this period the large crown of the previous stage can no longer be supported by the limited uptake of nutrients and this may become manifest as die back in the crown. The production of woody material may decline and thus the annual rings will become narrower. Most species will be barely able to survive when rings are reduced to 0.5mm (20 rings to 1cm). It is quite common for old trees to pass into this state of retrenchment through crown reduction but to still carry on living for very many years with an annual increment proportional to the new crown index (RACKHAM, 1990, 12). This provides trees with the visual characteristic known as stag-headedness (Figure 1).

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ESTIMATING THE AGE OF GROUPS OF TREES IN HISTORIC LANDSCAPES 169

FIGURE 1. A Stag headed tree.

In conjunction with Alan Mitchell, White measured a huge number of veteran trees over a long period of time and compiled models for use in estimating the age of large and veteran trees. It is recognised that assessments based on measurements from a single tree are fraught with difficulties in ascribing an accurate age and the paper recommends comparisons with a number of similar trees preferably in the same locality or in similar conditions (wHITE, 1998).

The basic and standard parameter used in the estimation of age using White's system is that of girth.3 Girth is the single parameter which sums the infinite number of diameters in an irregular cross-section (MITCHELL, scHILLING and WHITE, 1994, 1 ). Girth is measured using standard procedures at 1.3m above ground level (Figure 2). This is commonly known as Diameter at Breast Height (dbh). This measurement is then used to calculate the cross sectional area of annual increment through each of the developmental stages described above and may be expressed in terms of square centimetres (cm2).

This measurement is known as basal area in standard forestry parlance.

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170 ARBORJCULTURAL JOURNAL

FIGURE 2. The correct positions at which to take measurements (from WHITE

1998).

Potential problems in the application of the method

Being based on a large number of measurements and observations over a long period of time White's methodology is highly valuable to the dendrologist and the landscape historian alike. The accurate dating of establishment periods has a wide range of uses, particularly in the reconstructing histories of parks, gardens and the dating of some landscape features. However, WHITE recognises that there are a number of difficulties associated with the method, and interpretation of the data needs to be approached with a degree of caution. The following factors need to be taken into account when making observations in the field:

Identical species that grow on identical sites in identical conditions will tend to display a range of variability in terms of their growth characteristics and girth. Therefore, two trees of the same species standing next to each other may have a certain degree of variation in their girths but are exactly the same age. Using White's single diameter measurement different planting years would be derived for each tree that may well vary enough to be confusing. However, the range of variation among even-aged populations is predictable and can be used to arrive at a realistic average (see below).

Site type can be highly variable and can have a marked effect on the growth patterns of individual trees. All tree species have optimal growing conditions in terms of soil acidity, altitude, soil moisture regime, soil nutrient regime and level of exposure. These can produce highly variable results depending on individual site type and tree species. White recognises this and has produced a range of models to span the variability of site type. Provenance can add an extra level of complexity to this problem. Trees of different provenances or origins can grow at

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variable rates depending on the site type. For example, some species grown from the seed of an inland provenance may produce misleading results when grown in a coastal situation and vice versa.

The management regime of the tree or group of trees may produce significant variation in growth rates. Trees grown as individuals will tend to achieve the mature state much more quickly than those grown as groups, clumps, and avenues or in plantations due the lack of crown competition in the early stage of development. Trees grown with surrounding competition will tend to grow apically in search of more light. Where thinning is regular and adequate this will allow the crowns to expand, produce a higher density of leaves, and consequently add to the stem diameter (see below). Historical management at the individual tree level may also affect girth measurements. Trees that have been subjected to intermittent removal of branches through pollarding, shredding or lopping may have wide variations in ring width depending on the cycle of intervention (see below). These activities and other forms of disturbances may instigate the growth of disorganised cambium appearing in the form of burring and bottle-butting which is a common feature among many old trees. These features produce difficulties in the practical aspects of measuring, as well as potentially exaggerating the real girth.

White has produced a provisional series of tables for individual species commonly found in the British landscape (Table 1 ). It is stressed that these tables are incomplete and represent a synthesis of the information currently available. The tables provide a solid starting point but a degree of interpolation and refinement is essential if anything other than a very crude indication of age is required (see Appendix 2 for a worked example). More refined local models can be constructed with an in depth study of known data and site analysis. Ring width intervals of lmm can make significant differences to the results of the calculations. For the landscape historian, this level of detail may offer insights into the chronological order of establishment but would be a poor indicator of absolute dating taken without refinement (Table 2).

The Effects of Pollarding on Tree Ring Width

Pollarding is the removal of the limbs of a tree at a height above which any new growth resulting from cutting can be eaten by grazing animals. Traditionally this practice is periodically repeated in order to produce young growth for fodder, firewood and other uses (Figure 3). Pollarding is a practice that has been common for many thousands of years possibly dating back to the Neolithic (PETIT and WATKINS, 2003). It was common and well recorded in the Domesday Book as wood pasture but during the medieval

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TABLE 2. Example of the variability of results with relatively small fluctuations in ring width.

Avenue Name Serpentine ride

Species Beech

Location Modell Model 2 Model 3 Core Age 70 Core Age 70 Core Age 70

Core Ring Core Ring Core Ring width 4mm width 4.5mm width 5mm

Dbh range 59-169 59-169 59-169 Average dbh 114 114 114 Number of Trees 43 43 43 Stem radius (r )/em 57 57 57 Total basal area (BA)/cm2 10208 10208 10208 Core age 70 70 70 Core ring width/mm 4 0.45 5 Core basal area/cm2 2463 3118 3848 Area (CAl) of outer core ring/cm2 69.9 88 109.2 BA exc. Core (total BA-core BA) 7745 7091 6360 Age of outer section of stem! years Ill 80 58 Add core age/ years 181 150 128 Add years of decline 0 0 0 Estimate tree age (total last 3 entries) 292 230 186 Estimated planting year 1714 1776 1820 Documented Planting year 1769 1769 1769 Date measured 2006 2006 2006

period the practice of wood pasture and associated pollarding declined in favour of coppicing and later high forest (JONES, 1998). It was retained as a common practice among hedgerow trees but even this has declined sharply over the last 200 years. The vast majority of the old pollards that we see today have not been actively managed in the traditional method for a long period of time, in some cases centuries. Pollarding is usually started quite early in the life cycle of a tree. Some trees will respond to pollarding at a late stage of maturity (Sweet chestnut, most species of lime) whereas others need to be first brought into the pollarding cycle under fifty years of age (oak, beech). With these species the cycle needs to be repeated regularly or abandoned completely if the tree is to survive.

Based on observations in Savemake and the Forest of Dean, the resumption of severe pollarding of old beech and oak will result in death in the majority of cases. Because the tree is regularly being cut back and the crown is constantly having to reform, pollarding can delay the emergence of the tree from the formative growth period. Where trees are continually pollarded the ring width will remain trapped in the formative cycle (Figure 4). This can extend the natural lifespan of the tree significantly and some of the oldest and largest trees in the country have been managed under this system for many centuries. If pollarding is abandoned as a practice then

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FIGURE 3. A working oak pollard that has been regularly cut over several centuries.

the crown will start to develop more fully and ring width will follow the pattern described by White. However, it is not always clear what period the cycle of pollarding might have taken place over and when the practice ceased. In some cases this can be estimated by the dimensions of the limbs which may well act as a surrogate measurement for the main trunk during this unmanaged period. In situations where the pollarding of an old tree has lapsed and the tree is allowed to develop a full crown, the ring width is likely to remain small over the new core development phase due to the large diameter bole about which the new woody increment is distributed. Consequently, White's models for pollards are sparse. Occasionally, crossreferencing with historical records may give and indication as to when pollarding as a practice may have ceased. This may well coincide with emparkment or the cessation of grazing and commoning practices.

Patterns of growth among groups, avenues and clumps

Trees grown in high forest or plantation conditions adopt a different growth characteristic to those described above. These have been well studied in

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FIGURE 4. Typical ring width development in regularly pollarded trees.

the British context since the early part of the Twentieth Century and much data has been collected to produce a series of yield models (EDWARDS

and CHRISTIE, 1981). The establishment of plantations of trees has been a common practice since the Seventeenth Century and is now the most widespread form of woodland management in Britain. Essentially trees are planted relatively closely together in order to create lateral crown competition and force apical growth. Trees are periodically and selectively removed, thereby allowing crowns of the remaining trees to expand into the gaps created by those that are removed. In this way a balance of good tree height, light branching, and relatively even ring width is produced. Within clumps and avenues the same basic model applies although there may be a higher degree of variation, and effect on the development of ring width (and consequently girth) depending on group size, spacing, edge effect and subsequent thinning. The core development period of the trees within a plantation is controlled and extended through a combination of lateral competition followed by periodic release through thinning. Regularly thinned plantations, groups, clumps and avenues are thus largely characterised by an even ring width and stable girth development which is in contrast to that of the open grown tree. This ring width is subject to less variation over long periods of time than that of the open grown tree (Figure 5). However, trees in avenues and belts that are planted as single lines will experience competition only from the adjacent trees to each side so there will be less competition for light than is found in closed canopy conditions. Therefore the trees in belts or single avenues may well adopt the growth characteristics of relatively open grown specimens.

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FIGURE 5. A comparison of typical growth patterns in plantation and open grown trees.

Application of the method to groups of trees

Given the difficulties and caveats described above, White's method is suitable for extension to distributions of individual trees and groups that are commonly found in historic landscapes such as clumps, avenues, groves, and plantations. The mechanism for identifying even-aged populations of trees will be familiar to foresters but has received little attention from landscape historians.

As has been mentioned above, trees of a given species grown in identical conditions will tend to exhibit a degree of variation in their growth patterns. In forestry practice girth is the parameter wherein this phenomenon is most commonly encountered. The frequency distribution describes the relative frequency with which different values of a variable occur within a given population. Of the different types of frequency distribution, the one that is most commonly applied to forestry practice in Great Britain is known as the normal distribution (HAMILTON, 1988, 120). This typically takes the pattern of a bell shaped curve in which the highest frequency of the variable is centred around the mean and the lowest frequencies are at each end of the range. The pattern of distribution rarely conforms to a perfect model but nevertheless a line of best fit assigned to an even aged population will describe this characteristic bell-shaped form.4 This pattern of distribution

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FIGURE 6. A comparison of the distribution of girth in even-aged and naturalistic woodlands (values are indicative).

is in sharp contrast to the frequency distribution of complex mixtures that can characterise more naturalistic stands. These tend to display a much greater variation of size classes with higher proportion of smaller trees and a decline in the number of trees inversely proportional to increasing girth (Figure 6). This is commonly referred to as the reverse-J curve (KERR et a/., 2002, 7).

There are two important aspects to this normal pattern of distribution. The first is that a group of trees on a known site with a common management history and with a single establishment date will display these characteristics in terms of the frequency distribution of their girth (Figure 7). Woodlands that have a single period of establishment will have a different pattern of distribution to those that have been planted or added to over a number of distinct periods. The second point is that a mean value for the even-aged population can be calculated. This takes into account variation in growth rates and acts as a normalising factor. The greater the sample of trees taken or the greater the overall population, the more accurate and representative this figure will become. This in tum provides a more reliable single girth measurement that can be used to calculate the age of the entire population. A minimum of eight trees will normally be required to constitute a valid mean. The distribution of girth measurements in stands of younger trees is inherently constricted and thus the spread of the girth remains relatively

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FIGURE 7 A Typical Distribution of dbh within an even-aged stand (example taken from Little Frith Wood, Wiltshire. 58 yr old beech).

narrow. As age increases the spread becomes much wider and a greater number of measurements will be required in order to make the pattern of distribution more pronounced. This is particularly accentuated in populations of very old trees where the distribution may be very wide indeed.

Avenues of trees have been a popular device of landscape gardeners for many centuries both in Britain and elsewhere. They are highly formal features which rely on a uniform aesthetic for full effect. This effect is most commonly expressed through regular spacing (or repeating intervals) and even-aged uniformity of growth. In other words they exhibit many of the same characteristics as plantations. Figure 8 shows the frequency distribution of girth among trees measured on a single avenue in Savernake Forest. The distribution pattern is almost identical to that of Figure 7 although the trees are almost 200 years older.

Avenues and clumps of a single tree species are commonly found in historic landscapes and where these are encountered the collation and initial processing of data may well be superficially simplified. However, this does not imply that an establishment date is any more accurate than that of a mixed species. Avenues and groups of mixed species are also regularly encountered in historic landscapes and the differences in growth rates and patterns of distribution may well reveal clues and opportunities for the cross-referencing of results not afforded by a single species.

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>.. c . = .,. I!! ...


DBH Range

FIGURE 8. Distribution of girth found in beech on the Long Harry Ride, Savemake Forest, Wiltshire.

Figure 9 shows the distribution of girth of two species planted within a single avenue at the same period of time. There are a higher number of beech trees in the avenue than oaks which accounts for the difference in heights of the curves. The mean diameter of the beech is slightly higher than that of oaks. (98cm and 92cm respectively) revealed here as a slight offset in the curves. This is accounted for by the general pattern of growth which tends to be greater in beech than in oak on this site type. Nevertheless the shape described by the curves are identifiable as having a common characteristic and this suggests that both oak and beech were planted as a single avenue feature within a common time period. Moreover, the observable offset between the two species demonstrated here may well be useful in fitting other local avenues into the landscape chronology. That is to say that if the establishment date of this avenue was recorded and either of these species was found as a mixture with a third species, then a growth rate model for the third species could be calculated without having a recorded establishment date (see Appendix 3).

Figure 10 shows a slightly different scenario. The ride from which this data is collected appears as an even-aged avenue predominantly planted with beech, intermittent oak and ash. However, when analysed in terms of the frequency distribution of girth it can be seen that although the beech bears the characteristic bell shaped curve the ash shows a much greater diversity more akin to an uneven-aged population.5 This discrepancy between the two curves marks them out as having little or no connection in terms of their period of establishment or purpose. Essentially, this is a beech avenue and the ash having apparently invaded later.

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FIGURE 9. Distribution of girth between beech and oak planted during the same period within a single avenue. Grand Avenue, Savemake Forest.

FIGURE 10. Comparison of beech and ash in a single avenue with different establishment periods. Ashlade Firs, Savemake Forest.

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Procedures for collecting and analysing data

To derive the maximum amount of information from this methodology it is essential that that data is collected and handled systematically. The use of use of technologies such as global positioning systems (GPS), Geographic Information Systems (GIS) and computer spreadsheets are useful tools in capture and processing of the data but are by no means essential.

As many measurements as is practicable should be taken. Depending on the extent of the landscape involved a degree of stratification will be required in the processing of data (e.g. initially by species, distinct avenues, or groups sharing other visual characteristics). Good quality mapping and an understanding of the spatial layout of the site is essential in deciding how the data might be interpreted. Measurement of girth should be carried out following the standard convention as shown in Figure 2.

Unless the population or sample is very large it will be necessary to group the girths into diameter classes. These are commonly grouped into fives (61-65, 66--70, 71-75, 76--80, etc), or into tens (61-70, 71-80, etc) recording the number of trees which fall into each category. There is no inherent reason why these groupings are used other than pure convenience. The critical factor is to use grouping that allows for the emergence of patterns of variation. In the majority of circumstances populations are likely to be so small that the use of single integers as the basis of classification will render the data unintelligible.

Occasionally anomalies will occur where the distribution of diameter classes generally suggests a normal age distribution. These are commonly represented by trees that appear to be massively outside the acceptable range compared to the rest of the population. This can occur where an already extant tree is incorporated into an avenue or group or trees have been added at a much later date. It may indicate the remnant of an older avenue or group. Where these anomalies are evident they should be removed from the overall calculations of the mean but noted, as their presence may well yield vital clues to an earlier landscape phase. Once adequate ring growth models have been derived for the site these trees can be dated as individuals using the basic White single tree methodology.

By far the most complex and sensitive part of this process is creating a suitable tree ring model for individual circumstances. As can be seen in Table 2 small shifts in the assignment of ring width can create wide variations in projected timescale. However, creating a model that is realistic and credible is in many ways the most important part of the process. This can only realistically be achieved by either acquiring ring count data for the particular circumstance or finding documentary evidence to confirm a planting date (or range of dates). It is also important to reiterate that trees that have grown in competition with other trees (such as in clumps,

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avenues or plantations) will tend to put on girth much slower than open grown specimens. The planting of "working" trees such as pollards are also unlikely to be recorded. These are more commonly landscape features which have been partially or wholly derived from naturally arising trees.

Tree count data can be acquired in several different ways. Opportunities for ring counts can occasionally occur when trees are felled (for safety reasons, for example) or are blown down. Alternatively, ring count data can be gathered through core drilling. However, very large veterans are likely to be hollow and will thus only reveal a certain amount of data from the outer part of the tree. For a full description of the calculation method see White ( 1998) Estimating the Age of Large & Veteran Trees in Britain. Forestry Commission Information Note. HMSO.

Other sources of data are historical records, maps, forestry inventories and tariffs. It must be noted that any data gained from measured sources such as volume tariffs will only apply to similar circumstances, and extrapolation will be required to deviate from the site conditions under which the data was gathered. Models can be calculated using the basic White method (see Appendices 1 & 2) but tailored to local conditions to coincide with a known planting period (see Appendix 3).

Limits of the application

The method and process described has a wide range of applications in the field of landscape history and archaeology. Clumps, avenues and distributions of individual trees can be compared across a common landscape. One avenue can be compared to another to determine the chronology of establishment. The basic method of assessment is simple and straightforward. The initial data is easily processed and obvious patterns may emerge quickly. However, where absolute dates are required this can only be achieved through detailed examination of trees in similar conditions and careful cross checking with known records or maps.

Patterns of development may emerge merely by analysis of mean girths of groups, although the lack of documentary evidence may prevent assignment to a particular period. Where documentary evidence is not available a chronology of development may still be achievable based on stylistic applications, species choice, and nuances in the planting methods. However, occasionally these can be deceptive and lead one to erroneous conclusions.

Notes

All measurement conventions used in this paper are as prescribed in White (1998).

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2 White recognises that some trees do not follow this pattern. Some pioneer species such as birch miss out the mature phase going from the formative phase straight into decline.

3 There are a number of features of a tree which can be measured. However, many are an unrealistic guide to age. Girth, as described above, is the single non-reversible parameter that is easily accessible and non-intrusive (White, 1998).

4 The trend line used in all examples in this study is a polynomial to the degree of 3.

5 Ash is not a tree that is commonly found in avenues. The species is not particularly long lived and trees and individuals with a girth in excess of 150cm are uncommon. The likelihood is that the majority of these trees have seeded in naturally and were incorporated into the avenue where the planted beech had failed.

References

EDWARDS, P.N. and CHRISTIE, J.M. (1981) Yield Models for Forest Management. FC Booklet 48. HMSO. JoNES, M. (1998) The rise, decline and extinction of spring wood management in south west Yorkshire. In: WATKINS, C. (Ed.) European Woods and Forests: Studies in Cultural History. CAB International, Oxon. KERR, G., MASON, B., BoswELL, R. and PoMMERENING, A. (2002) Monitoring the Transformation of Even-aged Stands to Continuous Cover Management. Forestry Commission Information Note 45. HAMILTON, J.G. (1988) Forest Mensuration Handbook. Forestry Commission Booklet 39. HMSO. MITCHELL, A.F., ScHILLING, V.E. and WHITE, J.E.J. (1994) Champion Trees of the British Isles. Forestry Commission Technical Paper 7. Forestry Commission, Edinburgh. PETIT, S. and WATKINS, C. (2003) Pollarding Trees: Changing Attitudes to a Traditional Land Management Practice in Britain 1600-1900. Rural History 2003, 14(2), 157-176. RACKHAM, 0. (1990) Trees & Woodland in the British Landscape. Phoenix Giant. WHITE, J. (1998) Estimating the Age of Large & Veteran Trees in Britain. Forestry Commission Information Note. HMSO.

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Appendix 1

Measurements and calculations of individual Trees (see White, 1998 for cross referencing and associated tables) 1) On the site: a. Identify the tree. b. Take situation notes (with reference to the core development category. c. Measure girth. d. If several trees occur together, measure all of them. e. If crown decline or damage is found, estimate how long ago. f. Enquire about the history of the location.

2) Using a calculator: a. Determine the age when optimum crown development occurred and

possible average annual ring width up to that point. b. Calculate core radius (age x ring width). c. Calculate basal area of this central core of wood:

[dbh/2]2 X 3.14159. d. Calculate CAl of the outer annual ring on the core (mature state CAl).

Subtract one ring width from the core radius (2b), calculate a new basal area (as in 2c). Subtract this from basal area 2b.

3) Using a calculator and following the assessment sequence determine the age of the tree as follows:

a. Calculate basal area of the whole tree: [dbh/2]2 X 3.14159.

b. Subtract basal area of core (2c from 3a). c. Divide remaining basal area by the mature state CAl (2d), to give the

age of this outer section. d. Add 2a (age of core) to 3c to obtain total age of tree (subject to 4

below).

4) After crown decline, annual rings on most species can be presumed to be in the region of 0.05 em (20 rings per em). For the estimated duration of decline a separate calculation is required and diameter at 3a reduced accordingly. If a tree is dead the time since death has to be estimated (or determined) and simply added to the calculated age.

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Appendix 2

Assessment Sheet for individual Specimens (Worked example after White, 1998)

Tree Name: Shaden Tuft Oak Location: Forest of Dean Stem (;irthJcm: Stem Diameter/em: Stem radius (r)/cm: Total basal area (BA)/cm2:

Core Category: Core age/years: Core ring widthJmm: Core basal arealcm2:

Area (CAl) of outer core ringlcm2:

BA Excluding the core (total BA minus core BA)/cm2:

Age of outer section of the stem/years:

Add core age/years: Add years of decline/years: Total of last 3 entries

(estimated age of tree/years): Planting year (date measured

minus estimated age):

Appendix 3

Scientific name: Quercus sp. Grid Ref: SO 630 092 575 183 91.5 26302.176 Woodland boundary pollard 100 3.5 3848 76.6

22454.176 (26302.176-3848)

293 100 Nil

393

1599

Case study: Savernake Forest & Tottenham Park, Wiltshire Savemake Forest & Tottenham Park, Marlborough, Wiltshire comprise a Grade II* listed landscape and SSSI. There are a number of avenues, tree lined rides and clumps throughout the woodland and parkland as well as a high proportion of individual pollards of different species including oak, beech, sweet chestnut and common lime. This case study focuses on the calculation of a model for beech found in avenues and groups based on observations, tree measurements and documentary cross-referencing.

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186 ARBORJCULTURAL JOURNAL

Much of the designed landscape was laid out during the Eighteenth century. However, there appear to have been several stages of development. The first was during the 1730s under the influence of Lord Burlington, the second being in the late 1760s, a period in which "Capability Brown" was known to have worked intermittently on the property. Further work on the house and gardens and parklands was carried out during the early to midNineteenth Century.

The earliest maps of the entire estate date from 1786 when the majority of the layout was already complete. A contrasting and misleading county map of Wiltshire by Andrew's and Dury is also available dating from 1773. This shows some of the features and omits others that appear on the 1786 map. Many of the clumps are problematic as they appear on both the 1773 and 1786 maps but are stylistically uncharacteristic of the work of Brown. These include a highly geometric layout of intersecting roads and platoons of beech trees at regular intervals.

The creation of a number of features are clearly referred to in the correspondence between Lord Bruce and his steward, Charles Bill (Wiltshire and Swindon Archive 1300/2006-2058). One particular example refers to the creation of a "Serpentine" ride in the southern portion of Savernake Forest in 1769. The letter is accompanied by a sketch map (WSA 1300/2010-11) which can be identified both on later OS maps (1885) and through the line of veteran trees on the ground (although the line of the ride is now lost). From the same record another serpentine ride, now known as the Charcoal Burner's Ride, appears to already have been established at this time.

On the ground the "Serpentine" consists almost entirely of beech. Analysis of the distribution of girth shows a normal distribution indicative of a common establishment date returning a mean girth of 114cm (Figure 11). This compares favourably to other even aged avenues in across the forest. Figure 12 shows the comparison of this avenue with a number of others throughout Savernake. Whilst the Serpentine contains only beech many of the others contain populations of oak exhibiting the same characteristics of a normal distribution.

The mean values of these beech avenues lie mostly between 105-115cm. This is a relatively small deviation given their age and is indicative of a common planting period. The oak would appear to have been planted as part of the overall mixture of certain avenues (i.e. Charcoal Burner's, Pigstye and Marie Louise). The relationship between the mean girths of the beech and oak are virtually identical in all (in the range of 85-91% of the girth of the beech in each avenue) with oak being the slower growing species on this calcareous site. An alternative hypothesis could be that the oak was planted into these avenues at a later date. However in this illustration real examples are given that show the comparison between the oak and beech in 60 and 150 year old plantations. In these younger populations a there

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FIGURE 12. Comparison of mean girths of oak and beech in Savemake Forest.

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is a ratio of between 82-91% on similar site types (clay with flints). This suggests that the ratio between beech and oak on these site types is constant over the life cycle of the trees.

By this method of analysis it can be seen that there is a strong likelihood that that all of these avenues share a common establishment period (circa 1760s). By extension a mean girth for oak of this period can also be calculated and the relationship between the growth patterns of beech and oak established. These data can now be used as a benchmark for the calculation of models and compared with other features in this landscape. This short case study also appears to suggest that at the time that Brown was working at Tottenham Park and Savemake, a number of features appear to have been established that were not characteristic of his usual methods and may not be attributable to him.

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estimating the age of groups of trees in historic landscapes

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