Effects of season and intensity of sheep grazing on tree regeneration in a British upland woodland

~ores~~;ology Management ForestEcologyand Management88 (19%) 99- 106

Effects of season and intensity of sheep grazing on tree regeneration in a British upland woodland A.J. Hester ‘, F.J.G. Mitchell “*“, K.J. Kirby ’ ’ English

a MLURI, Craigiebuckler, Aberdeen AR9 2QJ, UK Nature, Northminster House, Peterborough, PEI IUA,

l3K

Abstract

This paper reports some effects of experimental alteration of season(summer, winter) and intensity of sheep grazing on tree regeneration, growth and browsing damage in an upland broadleaved woodland in Cumbria, UK. ‘During the first 3 years of grazing treatments, seedling recruitment was primarily Betula pubescens, Sorbus aucuparia and Fraxinus excelsior. After 7 years of grazing treatments, seedlings and saplings of many species were numerous, with fewer seedlings in winter-grazed plots but a higher proportion of these were browsed. More seedlings had reached sapling stage in winter-grazed plots. Sapling numbers increasedwith decreasing grazing intensity, with lower proportions browsed at low grazing intensities. Within the first 3 years grazing treatments, recruitment of Betula pubescens seedlings was greater in grazed than ungrazed plots, and greater in winter-grazed than in summer-grazed plots. Browsing damage to these seedlings increased with grazing intensity in winter, and height increments were greatest in ungrazed plots. After 7 years, B. pubescens seedlingsand saplings showed no significant treatment effects. After 3 years, recruitment of Sorbus aucuparia seedlings was greater in grazed than ungrazed plots, and greater in summer-grazedthan winter-grazed plots. Fewer seedlingswere browsed in low grazing intensity plots, and seedling height incrementswere greatestin ungrazed plots. After 7 years, more S. aucuparia seedlingswere browsed in winter than summer, but saplings showed no treatment effects. After 3 years, recruitment of F. excelsior was greatest in low grazing intensity plots and smallest in medium grazing intensity plots. More seedlings were browsed at higher grazing intensities, but in summer-grazed plots height incmments tended to be greatesf in the most heavily grazed plots. After 7 years, F. excelsior seedlings were more numerous in summer-grazed plots but were not differentially browsed; sapling numbers were not affected by grazing treatments but browsing damage was very high in all except summer low grazing plots. The implications of the results are discussedin relation to woodland grazing management issues. Keywords:

Grazing; Sheep; Woodland; Regeneration

’ Cormspondingauthor.Tel: +44 1224318611;fax: +44 1224311556;e-mail:[email protected]. ’ Current address: Jkpartment of Botany, Trinity College, Dublin, Ireland. 0378-l 127/%/$lXMl Copyright 0 1996 Elsevier Science B.V. All rights reserved. PU SO378-1127(96)03815-7

100

AJ.

Hesrer

er al./Foresf

Ecology

1. In~uctiorl

Most semi-natural woodlands in the British uplands have developed in the presence of grazing animals. Semi-natural woodlands are important in terms of nature conservation, but they also play a significant role in upland agriculture in Britain by providing grazing and shelter for stock. However, heavy grazing by both domestic and wild herbivores can significantly reduce tree regeneration, and the long term detrimental effects of intense grazing are apparent in many upland woodland areas, with skewed age structures of existing trees, little or no successful regeneration and heavily-grazed ground vegetation (Taylor, 1978; Evans, 1984 Coed, 1985; Mackenzie, 1987). Efforts to promote regeneration of semi-natural woodland have generally involved the complete removal of stock and deer by fencing. This has generally promoted regeneration in the early stages after exclosure, but longer-term regeneration has generally been minimal due to (a) the removal of the disturbance effect of animals, which provides ‘niches’ for seedling establishment, and (b) increased competition from the more vigorous components of the ungrazed ground vegetation (Linhurt and Whelan, 1980; Mitchell and Kirby, 19!90). Control of grazing animals rather than complete removal is therefore considered to be a more desirable option for successful woodland management, yet little research has been undertaken to test this hypothesis (Mitchell and Kirby, 1990; Kirby et al., 1994 Hester and Miller, 1995). A comprehensive review of the literature (Mitchell and Kirby, 1990) revealed very little data that could be applied to the prescription of effective management strategies for the longer term conservation of upland semi-natural woods. The vast majority of data available related to either grazing exclosures or overgrazed sites with no quantification of the level of grazing. Since then, the reduction, rather than removal, of wild red deer (Ceruus eZu@zus)to encourage woodland regeneration is being explored in a few sites which will provide some much needed broad-scale information @tames et al., 1995) on the effects of reductions in densities of this species. For domestic stock, there is considerable potential for the control of both density and season of grazing, yet

and bfanagement

88 (1996)

99-106

there is still almost no experimental data avail&Ie on this in relation to semi-natural woodland dynamics. This scarcity of data prompted the estabhshment in 1986 of the long-term experiment described in this paper, which was designed to investigate me impact of control of season and intensity of grazing by sheep on the vegetation dynamics of a broadleaved woodland in Cumbria in north-west England, UK. Recruitment, growth and browsing damage of the three most abundant species of naturally regenerated seedlings over the first three years of grazing treatments are presented here, i.e. birch (Beth pubescens), rowan (&IJ&.u ~c~puria) and ash (Fruxinus excelsior). After seven years of the grazing treatments many seedlings had grown above 30 cm in height (i.e. defined as saplings in this paper). and data on recruitment and damage to both seedlings and saplings is presented for these three species.

2. Site

Naddle Low Forest is situated on the north and north-west facing slopes above Haweswater Reservoir in eastern Cumbria, UK (National Grid Reference NY495145). The altitudinal range is from 200 to 400 m OD, and the slopes are steep with some precipitous crags. The prevaihng winds are westerly. with annual rainfall between 1600 and 1800 mm, The principal soil parent materials are stony colluvium derived from Ordovician rocks of the Borrowdale Volcanic Group (Mitchell, 1991). Brown Forest soils predominate at the base of the wood and these grade into brown podzohc soils with increasmg altiOthers

&&la

Sorbus

pubescens

auctmaria Fraxinus

Fig. 1. Species composition experiment ( 1986).

of mature

excelsior

trees

at the start of the

A J. Hester

et al. / Forest

Ecology

tude, with soil pH ranging from 3.8 to 5.2 (Mitchell, 1991). The vegetation is typical of British upland sessile oakwoods (Peterken, 1981; Rodwell, 199la). The main tree species present are B. pubescens, Quercus petraea and Corylus avellana with S. aucuparia and F. excelsior and smaller quantities of other tree species (Fig. 1). The field layer is diverse, with four main vegetation commumties present, three defined by Rodwell, 199la; Rodwell, 199lb as Quercus-Betula-Dicranwn (Wl7), Quercus-Betula-Oxalis (WI I) and Fraxinus-Sorbus-Mercurialis (W9), and the fourth comprises areas of boulder scree dominated by ferns (primarily Dryopteris species). More detailed vegetation descriptions of the site are given in Mitchell (1991). The 36 ha block of woodland used was fenced to exclude sheep and deer in 1986. Sheep and deer previously had free access to the wood and, with the exception of a few scattered birch saplings, there was no tree regeneration greater than 30 cm in height at the start of the experiment. Nomenclature of vascular plants follows Clapham et al., 1987.

3. Methods 3.1. Experimental design and grazing treatments

The experimental design incorporated two grazing seasons, summer and winter, and three levels of grazing intensity, representing high, medium and low vegetation utilization. A 2 X 3 factorial design was used, with two replicates of each treatment combination. The twelve plots were varied in size to achieve different stocking densities with a minimum of two sheep per plot (Table 1). Internal fencing ran upslope, so that each grazing plot covered the full Table 1 Akcation

of sheep grazing

treatments

and Management

88 (1996)

99-106

101

altitudinal range of the woodland area. Six small (10 X 25 m) exclosure plots were fenced as ungrazed controls. Each grazing season ran for 5 months. The summer season was from May to October and the winter season was from October to May, with the sheep removed from the site during January and February when snow cover was frequent. No supplementary food was given to the sheep at any time during the experiment. The grazing intensity categories (Table I) encompassed sheep densities of 2-l-3.8 sheep/ha (high), 1.2-2.0 sheep ha-’ (medium) and 0.6-1.2 sheep ha-’ (low). The sheep were a local genotype, Swaledale x Herdwick. Yearling wether (castrated male) sheep were used and were replaced in May each year. The grazing treatments started in June 1986. 3.2- Seedling measurements 1986-1989

Over the first 3 years of grazing treatments, naturally regenerated tree seedlings were located and measured along one-metre wide permanent transects in each plot in May and October each year. The transects were 400 to 600 m in length per plot and zig-zagged upslope, using random coordinates, to cover the range of vertical and horizontal variation within each plot. In the context of this paper ‘seedlings’ are defined as tree regeneration at least one year old and < 30 cm in height. Seedlings in their first year were ignored because of the very high mortality rates in the first year of growth (cf. Miles and Kimraird, 1979). For each seedling the following measurements were made: species, precise location, height, evidence of grazing (loss of leaf or shoot material) and surrounding vegetation. On successive visits, each previously recorded seedling was relocated and re-measured and new seedlings (> 1 year

to plots

Plot

1

2

3

4

5

6

7

8

9

10

11

12

Vegetated plot area (ha) ’ Grazing treatment Sheep per veget. ha a

0.97 SH 2.1

2.09 WL 1.0

I .30 WH 3.8

2.03 SM 2.0

2.43 WM 1.6

6.28 SL 0.6

1.59 WH 2.5

3.01 SM 2.0

5.15 WM 1.2

4.57 SL 0.9

4.14 WL 1.2

0.59 SH 3.4

a Plot area minus medium; L, low.

area of slopes

steeper

than 5O’.Treatments:

Season - S, summer;

W, winter.

Grazing

intensity

categories

- H, high;

M,

102

AJ.

Hester

et ul. /Forest

Ecology

old) were added to the data base. The data were analysed using Analysis of Variance in the GENSTAT 5 package (Genstat 5 Committee, 1987). Proportional data were angular-transformed for analysis. All data presented have been backtransformed.

and Management

88 Cl9961 99-106 Quercus Cotylus

pstraea

avellana

Others

Betula

3.3. Seedling and sapiing measurements 1993

In June 1993, after 7 years of grazing treatments, seedlings and saplings present in each plot were recorded within twenty 10 X 10 m quadrats located at stratified (by the four main vegetation types present) random points within each plot. As the previously used transects were no longer relocatable, quadrat sampling was selected in preference to transects to facilitate stratification of sampling by vegetation type. The species, height and evidence of browsing damage were recorded as before. Analysis was carried out using Reduced Maximum Likelihood (REML) with vegetation type as a source of covariation accounted for in the analysis (Genstat 5 Committee, 1993). Data on seedling and sapling numbers were log-transformed and proportional data were angular-transformed before analysis.

4. Results 4, I. Recruitment

At the start of the experiment B. pubescens, S. aucuparia and F. excelsior accounted for 96% of the tree seedlings recorded (Fig. 2) and were widely distributed across the plots. Recruitment of seedlings of these three species over the first 3 years of the experiment was defined as the total increase in seedling numbers over that time, calculated on an accumulative basis with the addition of new seedlings to the data set following each recording period, expressed as a proportion of the number of seedlings present at the start of the experiment (Table 2). B. pubescens showed significantly less seedling recruitment in ungrazed than grazed plots (P < 0.05), and more recruitment in winter grazed than summer grazed plots (P < 0.05). Seedling recruitment of S. aucuparia increased with increasing grazing intensity (P < 0.05) and was greater in summer grazed than winter grazed plots (P < 0.05). Recruitment of

Fig. 2. Species composition of seedlings at the start of the experiment (1986).

F. excelsior seedlings was highest in the low grazing intensity plots and lowest in the medium grazing plots (P < 0.05). After 7 years grazing treatments, total numbers of both seedlings and saplings showed treatment effects (Table 3). Significantly fewer seedlings were present in winter grazed plots than summer grazed plots (P < O.OOl), and overall, fewer seedlings were present at medium grazing intensity than at high or low grazing intensity plots (P < 0.01). Sapling numbers were much lower than seedlings, were more numerous in winter grazed plots than summer grazed plots (P < 0,051 and increased with decreasing grazing intensity (P C 0.05). By this stage, F. excelsior, S. aucuparia and B. pubescens were present in relatively large numbers as both seedlings and saplings (Table 4). Numbers of S. aucuparia and B, pubescens seedlings and saplings showed no effects of the 7 years of grazing treatments, but more F. excelsior seedlings were present in summer grazed than winter grazed plots (P -C 0.001~. 4.2. Browsing

Pooled data on browsing damage in the first 3 years are presented in Table 2 as the percentages of seedlings displaying evidence of browsing. in winter, browsing of B. pubescens seedlings increased with increasing grazing intensity (P < 0.05), and fewer X aucuparia seedlings were browsed in low grazing intensity plots (P < 0.05). A greater proportion of F. excelsior seedlings were browsed at the highest grazing intensities in both seasons (P < 0.051. By 1993, significantly higher proportions of

A J. Hester

et al. /Forest

Ecology

and Management

88 (1996)

99-l

103

06

Table 2 Mean seedfmg densities in 1986, recruitment, browsing damage and height increments of Beth pubescens, Sorbus aucuparia and Fraxinus excelsior between 1986 and 1989. Treatment definitions as per Table I, plus tutgrazed control plots. For each rme, numbers with the same superscripts are not significantly different at P < 0.05 Grazing treatments Summer Winter Not grazed High

Med.

Low

High

Med.

LOW

211 ztb 19 abc

233 a’ 34 bc 1.4 .xb

586 ’ 43 c

;:7;

C2.4

96 =b 21 abc 1.9 ab (8.6)

328 =’ 10 ab 3.2 a’ (5.6)

387 a’ 0= 3.4 ab (7.4)

272 b 32 ’

133 a

135 a 27 ’ 0.6 a’ (16.7)

96 = 24 ’ 1.5 c (15.3)

61 a 15 ab

(23.2)

127 a 17 Glb I.3 bc (21.6)

67 a Oa 2.7 ’ (26.5)

159 ab 8 .a’

291 a’ 6a

161 a’ 18 ’

131 a 4a

342 ’ 9 ab 1.5 a (7.9)

239 a’ 0=

Betula pubescens

Recruitment (% new seedlings) % Browsed Height increment (cm per year) (No. seedlings per 100 m* 1986) Sorbus

4.6 ’ (4.4)

aucuparia

Recruitment (%I % Browsed Height increment (cm per year) (No. seedlings per 100 m* 1986) Fraxinus

4oa 0=

b oftbc

excelsior

Recruitment (%) % Browsed Height increment (cm per year) (No. seedlings per 100 m* 1986)

179 ab 33 c ;:3;

k32;

seedlings (all species combined) had been browsed in the winter-grazed than the summer-grazed plots (P < 0.001) (Table 3) but there were no significant effects of grazing intensity. Much higher proportions of saplings wem browsed than seedlings in all treatments. In summer, significantly fewer saplings were browsed at the low grazing intensities than at medium or high intensities (grazing: P < 0.05, grazing X season interaction: P < 0.05). A higher proportion of S. uucuparia seedlings were browsed in winter grazed than summer grazed plots (P < 0.05) (Table 4) but there were no signifi-

cant effects on the proportions of B. pubescens and F. excelsior seedlings browsed. Almost all F. excelsior saplings were browsed in all but the low summer grazing intensity plots (P < 0.051, but the proportions of S. aucuparia and B. pubescens saplings browsed were not affecmd by the grazing treatments. 4.3. Seedling growth

Growth of seedlings was measured over the first 3 years of the experiment, and the data are presented as mean annual height increments over that time

Table 3 Total seedling and saprmg densities and percentages browsed after 7 years’ grazing treatments (1993). REML adjusted means. Treatments as per Table 1. Superscripts used as per Table 2 Grazing treatments Summer High No. seedlings per 100 m2 % of seedlings browsed No. saplings per 109 m* % of saplings browsed

17 c 6a 0.02 = lOOk

Winter Med. 8b 13 ab 0.18 a’ 100 bc

Low 15 bc 10 ab 0.54 b 24 =

High 2fb:

0.34 .lb 60 .ab

Med. 4= 30 c 0.36 a’ 61 =’

Low 6’ 30 c 0.66 b 51 a

104

AJ.

Hester

et d/Forest

Ecology

and Managemwzt

88 (i996199-106

Table 4 Seedling and sapling densities and browsing damage to Bet& pubescens, .%rbus aucu~ria and Fruxinus excelsior grazing treatments (1993). REML adjusted means. Treatment codes as on Table 1. Superscripts u#d as per Table 2 Grazing tmatments Summer High

afler

seva

years’

Winter Med.

Low

High

Med.

L-Jw-~-

Betula pubescens

No. of seedlings per 100 m* % of seedlings browsed No. of saplings per 100 m2 % of saplings browsed Sorbus

2a

Oa -

0.5 a I2 a 0.061 a 100 a

0.4 a

0.7 a

9a

14a

0.428 48

zl a

0.082

a 0 a

0.5 a 2.7 23 0.267 cl 66 a

I.1 J 0.470 ’ wa

36'

aucupuria

No. of seedlings per 100 m’ % of seedlings browsed No. of saplings per 100 m2 % of saplings browsed Frahus

0.3 a

I.5

I.8 a 17 ab 0.104 a 100 a

il Ii a 0.251 a 39 a

1.8 2 17 Llll 0.406 a s2 a

;;lb: 0.154 a sa

3.0 cl 28 ab 0.232 a y a

9.? c 0.4 a Oa -

a’ 2.9 a 0.085 a loo b

5.0 bc 0.3 z3 0.023 a Oa

1.5 a 2.8 a 0.211 a 100 tl

1.2 zl 0.3 a 0.019 = loo b

0.7 it.7 0. I24 9s

.3 IO tl Oa

I.7

excelsior

No. of seedlings per 100 m* % of seedlings browsed No. of saplings per 100 m* % of saplings browsed

2.5

period (Table 21. Seedling growth of s. u~czqztiu was significantly greater on the ungrazed plots than grazed plots (P < 0.011 and was greater overall in winter-grazed plots than in summer-grazed plots (P < 0.05). Increasing grazing intensity reduced seedling growth of both B. pubescem and S. aucupwiu (P < 0.05). However, F. exe&w showed the opposite trend in summer, with poorest growth in low and ungrazed plots, although growth was extremely variable and this trend was not statistically significant.

5. Discussion

Comparison of seedling densities of the three main species at the start of the experiment (Table 21 and after 7 years (Table 41 indicates reductions in recruitment of all three species, which presumably reflects the reductions in sheep densities, as even the highest grazing treatments had lower sheep densities than before the experiment started in 1986. Soil disturbance and reductions in above-ground competition from established vegetation, due to grazing and trampling by large herbivores, is considered particu-

‘I ,I a b

larly important for regeneration of B. ptibescens (Kinnaird, 1974 Piggott, 1983). However, seedling numbers of S. amqxzt-ia, which has larger seed reserves and is less dependent on bare soil and light for establishment @ester, 1995), had also declined greatly. Greater seedling establishment rates in grazed rather than ungrazed woods have also been documented in several upland sites in Britain (Miles and Kinnaird, 1979). However, seedling recruitment was still relatively widespread in all plots and does not appear to & limiting regeneration in this wood, even at the lowest grazing intensities. After 7 years of the grazing treatments. reduced seedling numbers and increased browsing damage in winter-grazed plots is presumably a result of increased seedling visibility in winter, and a corresponding scarcity of alternative food? when much of the herbaceous vegetation has died back. However, increased sapling numbers in winter-grazed pIots suggest that a higher proportion of the seedlings are reaching sapling height in winter-grazed than summer-grazed plots. This suggests that winter browsing may have a less detrimentaleffect oneseedling growth than summer browsing. There is some evidence in the literature to support this suggestion: Miller et ai., 1982 found greater mortality of both pine and birch

AJ,

Hester

et aL/Forest

Ecology

saplings if clipped in summer as compared with winter. Growth of B. @esce?zr to sapling stage was apparently much more common than for the other two species (higher sapling:seedling ratio for B. pnbercenr), even though the proportions of seedlings browsed of each species were very similar in the first 3 years of the experiment. This suggests that the severity of browsing per seedling (rather than the proportion of individual seedlings browsed) may have been less for B. &%escerrr, and/or the resilience of B. pubescens to damage may have been greater than that of the other two species. B. pubescens is generally less preferred by herbivores than F. excelsior or S aucupuriu (Kullman, 1986; Evans, 1984 Mitchell et al., 1977; Hester and Miller, 1995). B. pubescens generally produces relatively high concentrations of secondary plant compounds which can reduce herbivore damage (Bryant and Kuropat, 1980; Haukioja and Neuvonen, 1985). Data on browsing severity of planted saplings in this wood (Mitchell, 1991 and unpublished data) showed that damage was less severe for B. pubescens than for F. excelsior or S. uucupuria, except under the heaviest grazing. B. pubescens is not considered to be more resilient to browsing damage than the other two species (e.g. Miller et al., 1982; Mitchell, 1991), therefore the apparent better survival and growth to sapling stage of B. pubescens seems more likely to reflect a lower severity of browsing of this species compared with the other two. Annual height increments of both B. pubescens and S. uucupuriu seedlings were depressed at higher grazing intensities, as one would expect, but the opposite trend for F. excehior in summer-grazed plots suggests that seedling growth of F. excehior may benefit more than the other two tree species from the presence of sheep, perhaps through a faster growth response to reduced above-ground competition from the surrounding (herbaceous) vegetation, which will be more heavily grazed under higher grazing intensities. Fruxinus excelsior saplings showed substantial compensatory growth following defoliation (replacing lost leaf material up to four times in one season (Mitchell, 1991) which would give this species an advantage over less resilient species (cf. McNaughton, 1983). F. excehior has a higher requirement for phosphorus than the seedlings

and Management

88 (1996)

99-106

105

of most other species found in Naddle wood (Barker, 1985) and was generally restricted to the more nutrient-rich flushed areas (Mitchell, 1991). Several studies suggest that plant nutrition can influence herbivore preferences as well as plant recovery after defoliation (McNaughton, 1983; Dane11 et al., 1991; Tahvanainen et al., 1991); it is possible that higher nutrient contents of the F. excelsior saplings increased their preference by sheep, but also led to increased recovery following browsing damage. To conclude, seedling recruitment was found even in the lowest grazing intensities used, but growth and survival to sapling stage was much greater in winter-grazed than summer-grazed plots and was negatively correlated with grazing intensity. These findings, combined with the high proportions of saplings browsed in all except the lowest grazing intensity plots, indicate that very few saplings will have the potential to attain canopy height except at the lowest grazing intensities. After 7 years of the grazing treatments, only about 2%~ of all saplings recorded had attained heights of more than 1 m (Hester and Kirby, unpublished data). At the individual species level, there were differences both between and within species in recruitment, browsing and growth, but the between-species differences in responses to browsing illustrate the need to consider not only the effects of herbivore damage per se but also the interactions of the different tree species with the surrounding vegetation. Very few studies have experimentally examined the effects of differential herbivory on growth and competition between saplings and herbaceous species and this is the subject of further research.

Acknowledgements

We are grateful to many colleagues for their help with initial planning and fieldwork (acknowledged fully in Mitchell, 1991), and to Rachel Thomas for help with fieldwork in 1993. We are particularly grateful to Gordon Baillie, Hunter Smith and Jim Smith for their invaluable help with management of the sheep. We thank John Day and North West Water for access to and maintenance of the site, and Hugh Handley and the Easthams for the use of their farm facilities. The work was funded by English

106

A.J. Hester

et al./Forest

Ecology

Nature (formerly Natme Conservancy Council) and the Scottish Qffice Agriculture and Fisheries Department.

References Barker, S., 1985. The woodhmds and soils of the Coniston Basin, Cumbria. PhD Thesis, University of Lancaster. Bryant, J.P. and Kuropat, P.J., 1980. Selection of winter forage by subarctic browsing vertebrates: the role of plant chemistry. Ann. Rev. Ecol. System., 11: 261-285. Clapham, A.R., Tutin, R.G. and MOOIE, D.M., 1987. Flora of the British Isles, 3rd edn. Cambridge University Press, Cambridge. Coed Cymru, 1985. Broadleaved Woodlands in Wales. The Core Report. Comtryside Commission, Newtown. Danell, K., Niemela, P., Varvikko, T. and Vuorisala, T., 1991. Moose browsing on Scats pine under a gradient of plant prcductivity. Ecology, 72: 1624- 1633. Evans, J., 1984. Silvicultute of broadleaved woodland. Forestry Commission Bull., 62, HMSO, London. Genstat 5 Committee, 1987. GENSTAT 5 Reference Manual. Clarendon Press, Oxford. Genstat 5 Committee, 1993. GENSTAT 5 Release 3 Reference Manual. Clarendon Press, Oxford. Haukioja, K. and Neuvonen, S., 1985. Induced long term resistance of birch foliage against defoliators; defence or incidental? Ecology, 66: 1303- 1308. Hester, A.J., 1995. Scrub in the Scottish uplands. Scottish Natural Heritage Review No 24. Scottish Natural Heritage, Battleby, pp. l-37. Hester, A.J. and Miller, G.R., 1995. Scrub and woodland regeneration: prospects for the future. in: D.B.A. Thompson, A.J. Hester and M.B. Usher (Editors), Heaths and Moorlands, Cultural Landscapes. HMSO, Edmburgh, pp. 14&153. Kmnaird, J.W., 1974. Effect of site conditions on the regeneration of birch (B. per&la Roth and 8. pubescow, Ehrhk J. Ecol., 62: 467-472. Kirby, K.J., Mitchell, F.J.G. and Hester, A.J., 1994. A role for large herbivores (deer and domestic stock) in nature conservation management in British semi-natural woods. Arboricult. J., 18: 381-399.

and Management

88 (1996)

99-106

Kullman, L., 1986. Temporal and spatial aspects of subalpine popmations of Sorbus aucupariu in Sweden. Ann. Bat. Fenn., 23: 267-275. Linhurt, Y.B. and Whelan, R.J., 1980. Woodland regeneration in relation to grazing and fencing in Coed Gorswen, Narth Wales. J. Appl. Ecol., 17: 827-840. Mackenzie, N., 1987. Native woodlands in the Scottish Highlands. Friends of the Earth, Edinburgh. McNaughton, S.J., 1983. Compensatory plant growth as a rcsponse to herbivory. Oikos, 40: 329-336. Miles, J. and Kinnaird, J.W., 1979. The establishment and regeneration of birch, juniper and Scats pine in the Scottish Highlands. Scottish Forestry, 33: 102- 119. Miller, G.R., KiMaird, J.W. and Cummins, R.P., 1982. Liability of saplings to browsing on a ted deer range in the Scottish Highlands. J. Appl. Ecol., 19: 941-951. Mitchell, B., Staines, B.W. and Welch, D., 1977. Ecology of Red Deer: a Research Review Relevant to their Management in Scotland. institute of Terrestrial Ecology, Cambridge. Mitchell, F.J.G., 1991. Grazing in upland woods. NCC/MLURl Contract No. HF3-03-3 19. English Nature, Peterborough” Mitchell, F.J.G. and Kirby, K.J., 1990. The impact of large herbivores and the conservation of semi-natural woods in the British uplands. Forestry, 63: 333-353. Peterken, G.F.. 1981. Woodland Conservation and Management. Chapman and Hall, London. Piggott, C.D., 1983. Regeneration of oak-birch woodland following exclusion of sheep. J. Ecol., 71: 629-646. Rodwell, J. (Editor), 199la. British Plant Communities. I. Woodlands and Scrub. Cambridge University Press, Cambridge. Rodwell, J. (Editor), 199lb. British Plant Communities. 2. Mires and Heaths. Cambridge University Press, Cambridge. Staines, B.W., Balharry, R. and Welch, D., 1995. The impacts of red deer and their management on the natural heritage in the uplands. In: D.B.A. Thompson, A.J. Hester and M.B. Usher @ditorsl, Heaths and Moorlands, CuituraI Landscapes. HMSO. Edinburgh. Tahvanainen, J., Niemela, P. and Henttonen, H.,. 1991. Chemical aspects of herbivory in boreal forest feeding by small rodents, hare and cervids. ln: R.T. Palo and C.T. Robbins &litorsl, Plant Defenses against Mammalian Herbivory, CRC Press, FL, pp. 115-131. Taylor, M., 1978. The Broadleaved Wodlands of the Engiish Lake District. Lake District Special Planning Board Publication.