The structure of ancient native pinewoods and other woodlands in the Highlands of Scotland

Forest Ecology and Management 119 (1999) 231±245

The structure of ancient native pinewoods and other woodlands in the Highlands of Scotland R.W. Summersa,*, R.A. Mavora, A.M. MacLennana, G.W. Rebeccab a

Royal Society for the Protection of Birds, Etive House, Beechwood Park, Inverness IV2 3BW, Scotland, UK b Royal Society for the Protection of Birds, 10 Albyn Terrace, Aberdeen AB10 1YP, Scotland, UK Received 8 December 1997; accepted 6 November 1998

Abstract A survey was carried out in the Highlands of Scotland of the tree species, tree density, stand structure, and ®eld or ground layer cover in ancient native pinewoods and other woodlands (primarily plantations). The trees in ancient native pinewoods were largely Scots pines but the western sites also had a high proportion of birch. Other woodlands largely comprised planted Scots pine, Sitka spruce, lodgepole pine, larches and Douglas ®r. The sizes of the trees were greater in ancient native pinewoods than other woodlands, but some ancient native pinewoods had small trees re¯ecting recent regeneration or planting. The density of live trees in other woodlands was about 10 times higher than in ancient native pinewoods. The density and percentage of dead trees was also higher in other woodlands. The low density of trees in the ancient native pinewoods has allowed trees to develop deep and broad crowns. The other woodlands were characterised by pre-thickets and thickets. Tree species diversity was greater in other woodlands than ancient native pinewoods. Also, other woodlands had a greater structural variety in terms of stand types, but less variation within stand types, compared with ancient native pinewood. Calluna vulgaris and Vaccinium spp. comprised most of the ®eld layer in the ancient native pinewoods whilst bryophytes, other plants and dead needles made up most of the ground cover in other woodlands. Ancient native pinewoods are generally regarded as biologically richer than plantations, largely due to the differences in stand age and the ®eld layer. However, comparative studies are lacking for many taxa. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Forest structure; Native pinewood; Plantation; Pinus sylvestris

1. Introduction Woodland was once the main vegetation type in Scotland, but declined from Neolithic times (Goodwin, 1975). A reconstruction of the distribution of these forests indicates that southern Scotland would *Corresponding author. Tel.: +44-1463-71-5000; fax: +44-146371-5315.

have had oak Quercus sp. and ash Fraxinus excelsior woodland whilst Highland of Scotland was largely covered by a forest of Scots pine Pinus sylvestris and birches Betula spp. ± the Caledonian forest (McVean and Ratcliffe, 1962; Bennett, 1988). However, most of the ancient native pinewoods have been cut, burned and over-grazed or have receded naturally (Steven and Carlisle, 1959; Tipping, 1994), leaving only 12 000± 16 000 ha (Bain, 1987; Forestry Authority, 1994).

0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00515-5

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Even the remaining fragments have been cut and under-planted to some extent (Bain and Bainbridge, 1988), but are believed to retain some characteristics of the original forest (Gimingham, 1977). Ancient native pinewoods have received much recent interest from ecologists, conservationists and foresters (Bunce and Jeffers, 1977; Aldhous, 1995), and there are schemes and legislation designed to protect the remnants (Bain and Bainbridge, 1988). Also, the Forestry Commission aims to create a similar type of woodland through the Woodland Grant Scheme (Forestry Authority, 1989). The conservation value of ancient native pine forest has been recognised by the British government in its response to the Convention on Biological Diversity (Anon., 1994, 1995a, b) and a target has been set for new native pinewood to be planted, or otherwise begun to be established, over an area of 25 000 ha of the former natural range of this habitat by the year 2005. Such habitat re-creation and regeneration will help conserve a variety of species which have also been identi®ed as UK priorities for action (Gibbons et al., 1995). Steven and Carlisle (1959) identi®ed 36 remnant ancient native pinewoods and described their geography, history, vegetation and faunal communities. Recently, the boundaries of these remnants have been re-mapped (Bain, 1987; Forestry Authority, 1994). The two surveys estimated different areas due differences in de®nition of boundaries rather than as a result of real changes in area between the two surveys. Bunce (1977) described the ecological variation between ancient native pinewoods from the plant communities, tree girths and soil pro®les. Also, there have been a few additional descriptions of individual pinewoods detailing the age and size distributions of the trees (Nixon and Clifford, 1995) or structure (Peterken and Stace, 1987; Summers et al., 1997). However, a systematic survey of the species composition and structure of ancient native pinewoods has never been undertaken. The management of the ancient native pinewoods, either for conservation, recreation or timber production requires a description of the resource if these woodlands are to be integrated into the socio-economic development of the Highlands (Callander, 1995). By the beginning of the 20th century, forest cover had fallen to about 5% of the total land area of Scotland (Gill, 1994). The Forestry Commission

was established in 1919 in order to re-establish woodland. Initially, its primary aim was to grow wood to provide a strategic reserve for industry, so fast growing conifers, principally Sitka spruce Picea sitchensis, were increasingly used in preference to broadleaf species and pines. By 1992, 15% of Scotland had forest cover (Forestry Commission, 1992). Censuses document the areas of different tree species and planting dates for these woods (Locke, 1987). However, concern was expressed that the composition and structure of these new forests was so dissimilar to the ancient native woods that they lacked aesthetic appeal and would be of limited value to wildlife (Tompkins, 1986; Avery and Leslie, 1990). Given the lack of direct comparisons between ancient native pinewoods and plantations, this study set out to describe the tree species, ®eld layer composition and structure of the ancient native pinewoods, and demonstrate how these compare and contrast with other woodlands, primarily conifer plantations, in the Highlands of Scotland. This sets a scene against which the wildlife conservation value of the two types of woodland can be discussed. 2. Study area and methods All woods in the Dee and Moray faunal areas (Harvie-Brown and Bartholomew, 1893) were classed as ancient native pinewood (de®ned as native pinewood by Steven and Carlisle, 1959 and mapped by Bain, 1987) and other woodland as the basis for the selection of a strati®ed sample (Fig. 1). We use the term `ancient native pinewood' to distinguish these woods from plantations of native species and the new native pinewoods as being developed through the Woodland Grant Scheme (Gill, 1995 and see Peterken, 1996 p. 17 for de®nitions). The other woodlands were primarily conifer plantations. Approximately 2500 km2 of woodlands occur in this region (Catt et al., 1998) of which 124.5 km2 are ancient native pinewoods (Forestry Authority, 1994). A further 35.5 km2 of ancient native pinewoods occur outside these faunal areas and were also sampled. Sixty-®ve 1 km squares within all ancient native pinewoods were selected systematically, like the white squares on a chess board. The square was chosen if the six-number grid reference xx8yy5 fell within woodland. The

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Fig. 1. The study area in the Highlands. Ancient native pinewoods are shown as black areas or are arrowed: 1 Wester Ross-shire, 2 Beauly catchment, 3 Strathspey, 4 Deeside, 5 Lochaber, and 6 Perthshire. Dots show the locations of transects surveyed in other woodlands in the Dee and Moray faunal areas.

selection of this six-number grid reference allowed central placing of a transect within the 1 km square. This sampling procedure meant that many of the smaller ancient native pinewoods were omitted. The distribution of other woodlands was obtained from Bartholomew maps and 100 random six-®gure grid reference numbers were chosen from this habitat. Transects were drawn on maps such that the chosen grid reference fell mid-way along one side (the primary side) of an equilateral triangle whose total length for all three sides was 2 km. The angle of the primary side ran north±south in ancient native pinewoods but at random directions in other woodlands. Although the chosen grid reference points had, by de®nition, to fall on the prescribed habitat (ancient native pinewoods and other woodlands), the entire transects laid out from these points did not always fall within the required habitat. Therefore, in order to obtain sample units of 2 km, extensions were made along the length of one of the arms of the transect so that 2 km of woodland were always surveyed. This did not bias the sample towards woodland edge; rather it compensated for those triangles lying partially in woodland that

233

were not sampled because the reference point fell outside the woodland. In order to describe the habitat, the nearest tree at every 50 m along each transect was selected, identi®ed and diameter at breast height (DBH) recorded. The density of trees taller than 1 m was determined within a 4±25 m radius of each selected tree. Different radii were chosen so that a minimum of ten trees were counted unless, even at a radius of 25 m, there were less than ten. The selected tree (either conifer or broadleaf) and surrounding trees of a similar structure type (see below) were classed as the primary trees. If the stand was composed of trees of different types, trees dissimilar to the selected tree were classed as secondary and counted separately (see below). Broadleaf trees and dead trees and stumps (if over 1 m tall) were also counted. A visual estimate was made of percentage cover of heather Calluna vulgaris, blaeberry Vaccinium myrtillus, cowberry V. vitis-idaea, grasses, bryophytes, other plants and dead needles within c.10 m of each selected tree. Ground layer plants were ignored if covered by a ®eld layer. Descriptions of forest structure generally refer to the different layers; ground layer, ®eld layer, shrub layer and tree layer (Elton and Miller, 1954). However, in this paper we also considered features associated with tree size and spacing. We used a key developed by Picozzi et al. (1992) to allot a given stand to a type, based on two principal component scores. Their key was developed for describing conifer woods in Scotland. Stands with very old, deep-crowned trees with thick trunks had high PRIN1 scores. Tall, thin trees growing close together with interlocking branches had high PRIN2 scores. A bivariate plot of the two scores was gridded into boxes by Picozzi et al. (1992) so that stand types could be categorised into types ranging from 2 to 27 (Fig. 2; see Picozzi et al., 1992 Appendix 2 for mean values describing each type). In order to retain a link between this quantitative description and qualitative categories used by foresters, we give examples where forest types relate to terms such as `thicket' and `pole' stages (Rose, 1979). Ombrotrophic wooded bogs were not studied by Picozzi et al. (1992). However, by extrapolating the mean values for other forest types, we predicted that pines on bogs would fall within type 8. The coef®cients of variation in the PRIN1 and PRIN2 scores of the primary trees of stands were

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Arkaig and Cona Glen) and the Perthshire pinewoods (Black Wood of Rannoch and Meggernie, Glen Lyon). The other woodlands were grouped within the Dee and Moray faunal areas (Fig. 1). 3. Results 3.1. Ancient native pinewoods

Fig. 2. Diagrammatic representations of trees in forest types according to two principal component scores. Based on data in Picozzi et al. (1992). Stipple-crown, black-crown overlap with neighbours, white-dead branches. Blank boxes represent forest types not found in the Picozzi et al. (1992) study.

taken as measures of the structural diversity for each transect. This gave a measure of the variety of stand types found along transects but did not measure heterogeneity within a stand. To measure this, the percentage of the density of secondary conifers in relation to total conifer density was calculated. An index of tree species diversity was obtained for each transect using the Shannon±Wiener function (H) (Krebs, 1972, p. 506): X Hˆÿ …pi †…log2 pi † where pi is the proportion of the ith species. For purposes of description, the following ancient native pinewoods were grouped together: the Deeside pinewoods (Glen Tanar, Ballochbuie, Glen Quoich, Glen Lui and Glen Derry), the Strathspey pinewoods (Abernethy Forest, Rothiemurchus and Glen Feshie), the Beauly catchment pinewoods (Glen Affric and Strathfarrar), the Wester Ross-shire pinewoods (Beinn Eighe and Coulin), the Lochaber pinewoods (Glen

The most abundant tree in all ancient native pinewoods was Scots pine (Fig. 3(a)). The next most common trees were birches, which were relatively more abundant in the western and southern pinewoods than the eastern woods. Other conifer species included juniper Juniperus communis, particularly in the Strathspey woods. The Black Wood of Rannoch had the greatest number of non-native species, including lodgepole pine Pinus contorta and Sitka spruce. Other broadleaf trees included willow Salix spp., aspen Populus tremula, alder Alnus glutinosa, hazel Corylus avellana, oak, holly Ilex aquifolium and bird cherry Prunus padus. Holly was most common in the Lochaber pinewoods. The DBHs of all trees were positively skewed and had a broad range (Fig. 4). The median was 39 cm. On average, the broadest trees were in Deeside and narrowest in Wester Ross-shire (Table 1). The proportions of small trees varied from a few in the Lochaber and Deeside woods to a high proportion in Wester Ross-shire and Perthshire where there has been more regeneration or planting (Fig. 4). The median densities of primary conifers ranged from 5 haÿ1 in the Lochaber woods to 109 haÿ1 in the Strathspey woods (Table 1). There was a high density of secondary conifers in the Perthshire and Wester Ross-shire woods. The highest density of broadleaf trees was in Lochaber and Wester Ross-shire. Combining all the live trees, the lowest densities were in the Lochaber woods and highest in Perthshire. The density of dead trees and percentage dead was highest in Lochaber, the result of a ®re in the Arkaig woods in 1942 (Steven and Carlisle, 1959). The overall percentage of dead trees in all ancient native pinewoods was 3.0% (Table 1). Heather comprised most of the ®eld layer, with median values ranging from 20 to 60% (Table 1). The next most important plants were blaeberry and

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235

Fig. 3. The tree species composition of (A) ancient native pinewoods and (B) other woodlands.

cowberry which formed 10±20% cover each. Grass cover was highest in the west coast pinewoods. The median densities of Scots pine and birch were 132 and 71 trees haÿ1, respectively. Also, the densities of all live trees were greater around Scots pines than around birch (Table 2). Heather comprised most of the ®eld layer under Scots pines followed by cowberry and blaeberry in similar percentages. Ground cover under birch was dominated by heather and bryophytes. The median DBH of Scots pines was 43 cm, and 26 cm for the birches (Table 2).

The structure of the ancient native woodlands, based on primary trees, comprised mainly stands of types 2 and 3, with all other types occurring at less than 10% (Table 3, Fig. 5(a)). Stand type 3 was the most frequently found type in ®ve of the six ancient native pinewood areas. The Wester Rossshire woods, which had the lowest percentage of type 3 stands, had proportionately more young trees and bog pines (types 22, 15 and 8) (Table 3). The ancient native pinewoods of Strathspey had the greatest diversity of structure including a high

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Fig. 4. The frequency distribution of diameters at breast height (DBH) for trees in ancient native pinewoods and other woodlands.

proportion of bog pines (type 8). In contrast, the ancient native pinewoods of Deeside had the least structural diversity and the smallest percentage of bog pines. The overall median coef®cients of variation of the PRIN1 and PRIN2 scores for primary trees were 0.323 (inter-quartile range 0.221±0.449) and 0.346 (0.262±0.466), respectively. The median percentage of secondary conifers was 20.0 (inter-quartile range 5.7±43.3).

The median index of tree species diversity was 0.392 for all the ancient native pinewood transects (inter-quartile range 0.000±0.890). 3.2. Other woodlands Only one of the other woodlands was entirely broadleaf woodland and in ®ve others the density of broadleaf trees exceeded the density of conifers. Thus,

Deeside

Strathspey

Beauly

Median

Median

Median

IQR

Median

IQR

5 0 10 23 41 5 (13.4) 39

(0±14) 50 (0±5) 79 (0±23) 211 (8±59) 0 (15±79) 317 (0±15) 0 ± (1.8) (24±54) 30

(5±290) 55 (0±318) 0 (20±715) 99 (0±44) 0 (72±975) 142 (0±5) 0 ± (3.0) (12±52) 39

(11±223) (0±49) (25±350) (0±7) (46±393) (0±11) ± (21±59)

(0±5) (0±10) (10±55) (10±40) (10±30) (0±10)

(0±10) (0±30) (10±40) (0±20) (10±20) (0±20)

(0±10) (0±20) (20±70) (0±20) (0±10) (0±10)

IQR

Density of primary conifers Density of secondary conifers Density of all conifers Density of broadleaf trees Density of all live trees Density of dead trees (Percent dead) DBH (cm)

56 0 61 0 74 0 (2.9) 53

(20±221) 109 (0±0) 14 (25±221) 205 (0±0) 0 (31±221) 212 (0±10) 0 ± (1.4) (33±72) 35

(31±350) 19 (0±110) 0 (79±478) 46 (0±0) 0 (87±498) 127 (0±14) 0 ± (3.6) (18±52) 38

(0±88) 36 (0±31) 20 (5±158) 120 (0±100) 3 (51±287) 240 (0±15) 0 ± (4.8) (19±56) 26

(10±318) (0±96) (31±414) (0±96) (92±498) (0±25) ± (9±46)

Ground cover Vaccinium vitis-idaea Vaccinium myrtillus Calluna vulgaris Gramineae Bryophytes Others

0 20 60 0 0 0

(0±10) (10±30) (30±80) (0±0) (0±10) (0±10)

(0±20) (0±20) (30±80) (0±20) (0±10) (0±0)

(0±20) (0±20) (20±60) (0±30) (0±20) (0±10)

(0±0) (0±20) (20±70) (0±50) (0±10) (0±0)

Inter-quartile ranges (IQR) are given in brackets. The percentages of dead trees are based on mean values.

10 10 40 0 10 0

IQR

Lochaber

Median

10 10 60 0 0 0

IQR

Wester Ross-shire

0 10 50 10 10 0

10 10 30 20 20 0

Perthshire Median

10 20 20 0 10 10

All IQR

Median

10 10 50 0 0 0

IQR

R.W. Summers et al. / Forest Ecology and Management 119 (1999) 231±245

Table 1 Median tree densities (no. haÿ1), DBHs and ground cover (%) in ancient native pinewoods

237

238

Other woodlands Scots pine

Ancient native pinewoods Lodgepole pine

Sitka spruce

Birch

IQR

Median

IQR

Median

Median

Density of primary conifers 1146 Density of secondary conifers 0 Density of all conifers 1401 Density of broadleaf trees 0 Density of all live trees 1415 Density of dead trees 0 (Percent dead) (7.9) DBH (cm) 18

(597±2045) (0±15) (715±2389) (0±0) (764±2389) (0±199) ± (13±23)

1990 0 2189 0 2189 0 (4.1) 12

(1393±2674) 2189 (0±199) 0 (1730±2831) 2359 (0±0) 0 (1730±2831) 2389 (0±0) 0 ± (5.5) (9±16) 14

Ground cover Vaccinium vitis-idaea Vaccinium myrtillus Calluna vulgaris Gramineae Bryophytes Others

(0±0) (0±0) (0±40) (0±40) (10±40) (0±30)

(0±0) (0±0) (0±20) (0±30) (0±30) (0±80)

Median

0 0 0 0 20 10

Inter-quartile ranges (IQR) are given in brackets. The percentages of dead trees are based on mean values.

0 0 0 0 10 50

0 0 0 0 10 75

IQR

Scots pine

Birch

IQR

Median

IQR

Median

IQR

(1274±2986) ± (0±0) 0 (1592±2986) 0 (0±0) 354 (1592±3185) 511 (0±199) 0 ± (2.3) (7±19) 18

(±) (0±133) (0±133) (138±1216) (178±1529) (0±0) ± (12±28)

79 0 132 0 142 0 (3.3) 43

(25±263) (0±52) (41±382) (0±0) (46±393) (0±13) ± (25±62)

± 0 ± 71 93 0 (7.9) 26

(-) (0±0) (±) (31±211) (36±269) (0±10) ± (18±33)

(0±0) (0±0) (0±10) (0±20) (0±20) (10±100)

(0±0) (0±0) (0±25) (0±50) (10±50) (0±20)

10 10 50 0 0 0

(0±10) (0±20) (30±80) (0±10) (0±10) (0±0)

10 10 20 10 20 0

(0±10) (10±30) (10±40) (0±30) (10±30) (0±10)

0 0 0 20 30 10

R.W. Summers et al. / Forest Ecology and Management 119 (1999) 231±245

Table 2 Median tree densities (no. haÿ1), DBHs and ground cover (%) for principle tree species in ancient native pinewoods and other woodlands

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239

Table 3 Percentages of structure types of primary trees in ancient native pinewoods and other woodlands Structure type

2 3 4 8 9 10 11 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Total

Ancient native pinewoods

Other woodlands

Wester Ross

Beauly

Lochaber

Strathspey

Deeside

Perthshire

All

Moray

Deeside

All

11.5 7.7 5.1 16.7 7.7 1.3 5.1 0.0 11.5 1.3 7.7 0.0 0.0 0.0 0.0 23.1 0.0 1.3 0.0 0.0 0.0

13.5 21.2 7.0 5.4 11.9 3.5 2.3 0.0 3.3 6.3 10.5 2.1 0.2 0.0 0.0 7.9 1.2 3.0 0.2 0.5 0.0

7.1 30.9 2.6 8.3 11.5 2.6 5.8 0.0 1.9 5.1 17.3 1.9 0.0 0.0 0.0 1.9 0.6 0.6 0.6 1.3 0.0

11.7 14.2 3.9 12.9 13.1 9.1 5.6 0.1 6.4 5.6 4.1 3.5 1.5 0.3 0.2 6.4 0.2 0.3 0.4 0.4 0.1

26.8 40.2 11.9 0.5 0.0 0.0 0.0 0.0 5.4 9.8 2.2 1.9 1.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

8.1 14.9 7.3 3.0 11.9 5.6 5.6 0.0 6.0 8.5 3.0 1.7 1.3 2.6 0 10.7 0.4 1.7 2.6 3.8 1.3

15.0 22.6 6.6 7.4 9.2 5.0 3.7 0.0 5.5 6.9 5.6 2.5 1.1 0.4 0.1 5.7 0.4 0.9 0.5 0.7 0.2

0.7 1.0 0.5 1.2 1.0 0.7 5.2 1.1 5.8 8.7 3.7 6.5 9.2 5.8 2.1 9.3 2.6 12.3 6.0 13.6 3.0

2.2 1.3 3.6 0.0 0.8 0.6 0.0 4.0 9.4 12.4 8.2 6.0 12.3 11.4 1.5 1.4 1.3 9.5 2.2 10.1 1.8

1.0 1.1 1.1 1.0 1.0 0.7 4.2 1.6 6.5 9.5 4.6 6.4 9.8 6.9 2.0 7.7 2.3 11.7 5.3 12.8 2.8

100.0

100.0

100.0

100.0

100.0

100.0

100.0

100.0

100.0

100.0

See Fig. 2 for diagrams of types.

the majority of the other woodlands were conifer plantations. The other woodlands in the Moray and Dee faunal areas had similar proportions of Scots pines and Sitka spruces, though the Moray area had proportionately more lodgepole pines and the Dee area had more Norway spruces Picea abies, larches Larix spp. and birches (Fig. 3(b)). The DBHs were positively skewed but with a single mode, and were smaller than in ancient native pinewoods (Fig. 4). There was little difference in the DBHs of trees in the Dee and Moray areas and the overall median size was 16 cm, less than half that for ancient native pinewoods (Table 4). There were similar densities of primary conifers in the Moray and Dee areas and, given the same planting dates for trees within stands, there were few secondary trees. The density of all live trees was more than 10 times the tree density in ancient native pinewoods. The percentage of dead trees was 6.0%, twice that of

ancient native pinewoods (Tables 1 and 4). The mean density of dead trees was 124 haÿ1. By combining the tree sizes (DBH) and densities in a bivariate plot, the differences between ancient native pinewoods and other woodlands are shown (Fig. 6). The relative range of sizes and densities are also seen, with ancient native pinewoods showing the greater ranges. The Dee faunal area had some outlying values referring to small trees growing at a high density within the ancient native pinewoods. Ericaceous shrubs were largely absent from other woodlands. Rather, bryophytes, other plants (e.g. bracken Pteridium aquilinum) and dead needles made up the ground cover (Table 4). Scots pine, lodgepole pine, Sitka spruce and birch were the principal tree species in other woodlands. The three conifer species occurred at 4±7 times the density of birch, with Sitka spruce having the highest density (Table 2). The percentage of dead trees was highest around Scots pine and lowest around birch.

240

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Fig. 5. The percentages of different forest structure types, based on PRIN1 and PRIN2 scores, found in ancient native pinewoods (A) and other woodlands (B). The areas of the spots are proportional to the percentage of records for given types.

The density of dead trees around Scots pines in other woodlands was more than twice that of the ancient native areas (Table 2). The range of median DBHs for the four species was between 12 and 18 cm, with Scots pines and birches having the highest values, though these were far less than the DBHs recorded for these species in ancient native pinewoods (Table 2). Under Scots pines and birches, bryophytes comprised the dominant ground vegetation while under lodgepole pine and Sitka spruce it was mainly dead needles that covered the ground (Table 2).

The structure types recorded most frequently in the other woodlands were thickets (types 24 and 26). Stands with high PRIN1 scores and bog pines were rare (Table 3, Fig. 5(b)). The Moray area had higher proportions of types 24 and 26, whereas in Deeside, structure types 16, 19, 20 and 26 were more frequent (Table 3). The median coef®cients of variation of the PRIN1 and PRIN2 scores were 0.555 (inter-quartile range 0.417±0.641) and 0.332 (0.228±0.432), respectively. In terms of the PRIN1 scores, this was a signi®cantly

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241

Table 4 Median tree densities (no. haÿ1), DBHs and ground cover (%) in other woodlands

Density of primary conifers Density of secondary conifers Density of all conifers Density of broadleaf trees Density of all live trees Density of dead trees (Percent dead) DBH (cm) Ground cover Vaccinium vitis-idaea Vaccinium myrtillus Calluna vulgaris Gramineae Bryophytes Others

Moray faunal area

Dee faunal area

Median

IQR

Median

IQR

Median

IQR

1401 0 1783 0 1791 0 (6.0) 16

(597±2359) (0±199) (892±2516) (0±0) (1019±2674) (0±157) ± (11±21)

1529 0 1592 0 1990 0 (6.0) 18

(240±2986) (0±0) (398±2986) (0±88) (763±3185) (0±199) ± (13±23)

1415 0 1730 0 1791 0 (6.0) 16

(531±2389) (0±39) (796±2588) (0±0) (973±2787) (0±157) ± (11±21)

0 0 0 0 20 20

(0±0) (0±0) (0±20) (0±35) (0±40) (0±70)

0 0 0 0 5 30

All

(0±0) (0±0) (0±20) (0±40) (0±20) (10±90)

0 0 0 0 10 20

(0±0) (0±0) (0±20) (0±40) (0±40) (0±70)

Inter-quartile ranges (IQR) are given in brackets. The percentages of dead trees are based on mean values.

Fig. 6. The relationship between tree size (log DBH) and log tree density for ancient native pinewoods and other woodland in the Moray and Dee faunal areas. Contours representing the density of points were drawn using the Kernel method (Silverman, 1986).

greater variation than in ancient native pinewood (Mann±Whitney U ˆ 4941, p < 0.001). Generally, the ancient native pinewoods had old, large, broadly-spaced trees, so the stands had a high degree of uniformity at the upper level for the PRIN1 scores. In contrast, the coef®cients of variation in the PRIN2

scores for the other woodlands were not signi®cantly different compared with ancient native pinewoods (Mann±Whitney U ˆ 2730.5, p ˆ 0.07). The median percentage of secondary conifers was 1.9 (inter-quartile range 0±26.1) in relation to total conifer density. This value was signi®cantly less than

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for ancient native pinewoods (Mann±Whitney U ˆ 1948, p < 0.001) indicating that ancient native pinewoods were structurally more diverse within a stand, compared with other woodland. The median index of tree species diversity was 1.130 (inter-quartile range 0.732±1.634, range 0± 2.484) which was signi®cantly greater than in ancient native pinewood (Mann±Whitney U ˆ 1336.5, p < 0.001). 4. Discussion The results show a number of differences between ancient native pinewoods and other Highland woods, which might in¯uence their conservation value. These include tree density, tree age and hence size, tree species diversity, the dead wood component and ground vegetation. The low tree density in ancient native pinewoods results in one of their characteristic features; `granny' Scots pine trees which have high aesthetic value (Thom, 1977). The great breadths and depths of their crowns show that they have grown for most of their life in the absence of close neighbours. Where there has been recent regeneration or planting around `granny' trees, the young trees shade out the lower branches on the `granny' trees and the foliage dies (Peterken et al., 1992). Although these giants are regarded as an integral part of the ancient native pinewoods, it is likely that they are a product of past management. A possible origin for `granny' trees is that, in the past, people selectively felled the straight-trunked trees, leaving poorer quality trees (e.g. with many lateral branches) to act as seed trees for the next generation (Steven and Carlisle, 1959). However, continual use by farmers who grazed cattle and sheep, and latterly sportsmen who encouraged deer in the forests, prevented natural tree regeneration (Penistan, 1942; Grant, 1994). Also, a change to a wetter climate may have reduced the potential for regeneration (Tipping, 1994), particularly on the wetter west side of the country. Thus, the remaining scattered trees grew in the absence of competitors and many ancient native pinewoods developed a park-like landscape. As well as the above general features of ancient native pinewoods, several have unique histories which have given rise to their current structure and diversity

of tree species. One of the most striking features is the scarcity of broadleaf trees in the eastern pinewoods. This may be due to the less oceanic climate in the east favouring pine (Steven and Carlisle, 1959, p. 68) but may also be due to the selective removal of broadleaved trees (Peterken and Stace, 1987; Summers et al., 1997). Analyses of pollen from cores show that eastern ancient native pinewoods, like Abernethy Forest, once had a higher percentage of broadleaf trees, particularly birch (O'Sullivan, 1977). Only the Lochaber, Beauly catchment and Perthshire pinewoods currently have a high proportion of birch (Fig. 3(a)). All the other likely species are present though some, like aspen, are rare. A higher percentage of broadleaf trees would increase the diversity of lepidoptera, birds and mammals (Young, 1986; Staines, 1986; Petty and Avery, 1990). The ancient native pinewoods which spread over ombrotrophic bogs give a landscape of stunted `bogpines'. The largest areas of wooded bogs are in Strathspey, perhaps because the drier climate of the east allows some drying out of the bogs and stimulates some microbial activity and tree growth (MacKenzie and Worrell, 1995). Topography is likely to be a major feature determining the distribution of bog pines, and their scarcity in Deeside and Perthshire is probably because most of the surviving pinewoods are on slopes. One of the main differences between ancient native pinewoods and plantations is in the ages, and hence sizes, of the trees. Plantations are relatively young and most are still in their ®rst rotation (Locke, 1987). Also, by felling plantations at around 50±80 years, plantations will not have very large trees and consequently no large dead trees. The wildlife value of plantations could be greatly enhanced by having longer felling rotations or areas that are never cut, especially for species with poor dispersal powers (Peterken et al., 1992). Standing and fallen dead wood is an important habitat for fungi (Boddy, 1994) and insects, particularly beetles (Hunter, 1977). Also, many birds and mammals use standing dead wood for foraging on or nesting in, either creating their own nest cavities or using existing cavities. In ancient native pinewoods, there are often many large dead trees which provide suitable sites for wildlife. However, there are fewer dead trees than in natural forests because dead trees were often felled by foresters in order to `tidy up' the

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wood or cut by crofters for ®rewood (NethersoleThompson and Watson, 1974, p. 191). Surprisingly, the density and percentage of dead trees in the ancient native pinewoods was less than in plantations which are clear-felled at the end of a rotation (Avery and Leslie, 1990). The dead trees in plantations were mainly the result of self-thinning and are generally short thin trees so the volume of dead wood in plantations is less than in ancient native pinewoods (Reid et al., 1996). Although they may be suitable for some fungi and insects, and provide feeding sites for woodpeckers, most would be too narrow to be used as nest sites; e.g. for crested tits (Denny and Summers, 1996). Retention of dead wood, both in ancient native pinewoods and plantations, would have bene®ts to wildlife. The differing tree densities in ancient native pinewoods and other woodlands have led to different ground ¯oras. The commonest plant of the ®eld layer of ancient native pinewoods is heather (Table 1). Analyses of pollen from cores at Abernethy Forest show that there was an increase in the amount of heather in and around ancient native pinewoods from as early as 3600 BP and particularly since the Dark Ages (500±1000 AD) (O'Sullivan, 1973a, 1974). The increase in heather was probably due to the opening up of the forest as well as total loss of woodland and the development of heathland (O'Sullivan, 1973b, 1977). Therefore, the original forest may have had a higher proportion of shade-tolerant species. The other extreme is seen in plantations. At the thicket stage of plantations of spruces and ®rs, most vascular plants are lost. Even after thinning, there is slow recovery due to the heavy shade. In contrast, pine and larch plantations, particularly mature ones, can have a rich ¯ora (Hill, 1986). However, the mature stage in the rotation of a plantation is relatively short-lived compared to the period that old trees stand in ancient native pinewoods. The differences in the ground vegetation and ®eld layer composition in ancient native pinewoods and other woodlands has a major bearing on wildlife. In particular, the ancient native pinewoods will have a more diverse invertebrate fauna because many species are associated with grasses, herbs and the ericaceous shrub layer (Young, 1986; Baines et al., 1994). In some respects, pine plantations compensate wildlife for the scarcity of ancient native pinewoods in Scotland, whilst good forest management (Forestry

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Commission, 1990; Petty and Avery, 1990) and Woodland Improvement Grants (Forestry Authority, 1996) help to enhance plantations for wildlife. There is now a far greater area (810 km2) of Scots pine plantation in the Dee and Moray faunal areas than of ancient native pinewood (124.5 km2). Thus, although the density and diversity of pinewood birds is generally lower in pine plantations by virtue of their simpler vertical structure (Newton and Moss, 1977), the total number of individuals in plantations in Scotland is greater than in ancient native pinewoods. This even applies to those birds which prefer ancient native pinewoods: e.g. crested tit and capercaillie (Cook, 1982; Catt et al., 1998). Many mammals have also bene®ted from afforestation by providing cover and food (Staines, 1986; Staines et al., 1987). However, there are few comparative densities for mammals in conifer plantations and ancient native pinewoods, so it is not possible to say whether they would have bene®ted more from provision of ancient native pinewood rather than plantation woodland. Forest cover and luxuriant ground vegetation in the early stages of plantation development lead to high populations of small mammals which in turn bene®t carnivores. Roe deer Capreolus capreolus densities are also highest in young plantations. Even after canopy closure, which reduces the ®eld layer vegetation, red deer Cervus elaphus densities can be as high as on open hill ground, whilst red squirrels bene®t once some species start coning (Staines, 1986). Given the current commitment to improving the wildlife value of forests in Britain (Forestry Commission, 1990), there is a need to have a better understanding of the abundances and habitat requirements of associated plants and animals, so that both ancient native pinewoods and plantations can be improved. Initial steps at describing forest biodiversity are under way (Ferris-Kaan, 1995). Acknowledgements We thank the many landowners and organisations, particularly the Forestry Commission, who gave access permission to their forests. The following helped with ®eld work: Dr. I. Francis, Dr. P. Mayhew and R. Proctor. The drafts were commented on by Dr.

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