Seasonal habitat use and browsing by deer in Caledonian pinewoods

Forest Ecology and Management 174 (2003) 149–166

Seasonal habitat use and browsing by deer in Caledonian pinewoods S.C.F. Palmer*, A.-M. Truscott Centre for Ecology and Hydrology, Natural Environmental Research Council, Hill of Brathens, Banchory AB31 4BW, UK Received 3 July 2001; received in revised form 15 November 2001; accepted 18 December 2001

Abstract The effects of browsing by deer on the regeneration of native Caledonian pinewoods were studied at two contrasting sites in the Highlands of Scotland: one with high winter use by red deer (Cervus elaphus), where regeneration of Scots pine (Pinus sylvestris) had been prevented by browsing, and one with lower use by red deer all year and roe deer (Capreolus capreolus) also present, where regeneration had been suppressed. The principal aims were to examine the effects of habitat characteristics on the level of use by deer, and to test whether browsing of pine saplings and ground vegetation was related to habitat use by deer. Use of the sites by deer was estimated from dung pellet group density. A novel method for estimating the accumulation period of dung was developed, in which pellet group counts on paired plots, one of which was cleared, were compared. Habitat use by deer during winter and summer was related to topographical characteristics and ground vegetation cover. During winter at the high use site, red deer made greater use of lower altitude, more sheltered plots with a higher cover of heather (Calluna vulgaris) and blaeberry (Vaccinium myrtillus), and less use of open, higher altitude plots dominated by wet heath. Habitat use by both deer species in summer and winter was much more evenly distributed across the lower use site, although there was an indication that roe deer might have favoured areas with greatest availability of pine sapling browse and blaeberry during summer. Seasonal differences in the incidence of browsing to pine saplings were observed at both sites. At the high use site, browsing incidence during winter could not be related to herbivore use at the plot scale. Browsing of the much larger pine saplings at the lower use site was heavier in summer than in winter. Leading shoots of saplings about 1 m tall were likely to be damaged in summer. Spatial variation in the annual utilisation of heather was related to deer use at both sites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Browsing; Capreolus capreolus; Cervus elaphus; Faeces; Natural regeneration; Pinus sylvestris

1. Introduction Heavy browsing by red deer (Cervus elaphus L.) has been identified as an important factor in preventing the natural regeneration of native Caledonian pinewoods in the Highlands of Scotland (Scottish Natural Heritage, 1994; Staines, 1995). In some areas, *

Corresponding author. Tel.: þ44-1330-826311; fax: þ44-1330-823303. E-mail address: [email protected] (S.C.F. Palmer).

regeneration has been suppressed for centuries (Watson, 1983), and the age structure of many woods is severely degraded (Nixon and Cameron, 1994; Goucher and Nixon, 1996). Although exclusion of deer by fences has led to prolific regeneration in some areas, an enclosed wood without its natural herbivores is likely to develop a different structure from an open wood which is in balance with its natural herbivores (Watson, 1993). Furthermore, fencing has been shown to be a major cause of mortality in the two woodland grouse species, the capercaillie (Tetrao urogallus L.)

0378-1127/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 0 3 2 - 4

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and black grouse (T. tetrix L.) (Catt et al., 1994), both of which have been declining throughout Scotland in recent years. Natural regeneration in the presence of low numbers of deer is therefore the preferred option, and in a few locations red deer numbers have been reduced sufficiently over a large enough area to allow regeneration to occur (e.g. Beaumont et al., 1995; Staines et al., 1995). In order to be able to predict browsing rates, one needs to be able to estimate levels of use by deer, and this is itself problematic for such wide-ranging animals. Vantage point counts (Ratcliffe, 1987) or driven counts yield estimates of deer numbers over a short time period, but they do not necessarily reflect the average seasonal or annual density, and hence the browsing pressure on vegetation. Faecal pellet group counts (Neff, 1968) can give estimates integrated over time, but require certain assumptions and additional data. One particular issue, of critical importance in estimating density from standing crop faecal counts, concerns the decay rate of dung (Putman, 1984; Mayle, 1996; Mayle and Staines, 1998). Although it is recognised that regular clearance of dung plots circumvents the need to estimate decay rate, this is not always practical. Attempts have been made to estimate decay rate by laying out freshly collected pellet groups and subsequently monitoring their decomposition (e.g. Latham et al., 1996), but there are several possible biases involved with this method (location and timing of dung collection and placement, selection of pellet groups to mark in situ, etc.). When converting a standing crop dung count to density, the required parameter is actually the mean period over which the dung has accumulated (which depends on the decay rate which may vary between microhabitats and over time). A possible way to estimate the accumulation period directly for a given site is by comparison of contemporary standing crop and faecal accumulation (cleared) dung counts, which offers the advantage that all dung is deposited naturally over time and space by the population of deer under study. Previous research has suggested that regeneration of Scots pine (Pinus sylvestris L.) can occur at around 5 deer/km2 but not at 25 deer/km2 (Holloway, 1967), and reduction of red deer densities from around 12 to 5 deer/km2 at Abernethy Forest Reserve coincided with increased seedling and sapling densities of Scots pine, rowan (Sorbus aucuparia L.) and juniper

(Juniperus communis L.) (Beaumont et al., 1995). However, it is not only the overall density of deer in an area which is important, but also the use which they make of their range. If deer concentrate in an area of potential pine regeneration, perhaps because it offers good shelter (Staines, 1976), then the overall deer density which will allow regeneration in that part of the range must be lower than if their habitat use is more evenly distributed. A preliminary study of the relationships between deer use and browsing of pine saplings at 22 sites throughout the Highlands of Scotland emphasised the importance of accounting for habitat use by deer over sufficiently large areas, as much of the variation in browsing incidence recorded occurred between sites, and could not be explained by local dung density and vegetation characteristics (Palmer et al., 1998). The principal aims of this work were (1) to test whether the dung accumulation period could be estimated directly from dung counts, (2) to determine to what extent the use by deer of areas of potential pinewood regeneration was related to habitat characteristics (shelter and available forage), and (3) to test whether browsing of pine saplings and ground vegetation within those areas were related to differential habitat use within the area.

2. Methods 2.1. Study sites Two sites, each covering an area of roughly 1 km2, were established in April–May 1997, chosen to be representative of relatively high and low densities of red deer. The higher density site was at Glen Affric in Inverness-shire (578150 N, 5800 W; O.S. ref. NH1822) and the lower density site at Glen Tanar in Aberdeenshire (57820 N, 28520 W; O.S. ref. NO4795). Roe deer (Capreolus capreolus L.) were also present at each site, and small numbers of stray sheep from a neighbouring estate occurred at the Glen Affric site. Subsequent visits were made each autumn (September–November) and spring (April–May), to estimate occupancy and browsing during the summer and winter months, respectively, until spring 2000. The Glen Affric site lay on the south-eastern slopes of the glen between two large fenced exclosures to the

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east and west. The site was also fenced along its northern boundary, but was open to the hill to the south. A stock fence was erected across the site during summer 1997, restricting access for the few sheep present to the higher elevation half of the site to the south-east of the fence. The lower northern end of the site was characterised by mature Scots pine trees with an understorey dominated by heather (Calluna vulgaris (L.) Hull.) and blaeberry (Vaccinium myrtillus L.), but with increasing elevation to the south the density of pines decreased (trees being confined to locally drier areas), and the ground vegetation was wet heath with heather, cotton-grasses (Eriophorum spp.), purple moor-grass (Molinia caerulea (L.) Moench.), deer-grass (Trichophorum cespitosum (L.) Hartm.), bog myrtle (Myrica gale L.) and cross-leaved heath (Erica tetralix L.). Regeneration of pine on the site had been completely prevented by browsing. Pine saplings1 on the site had mostly been suppressed by browsing to form a ‘sapling bank’ generally at about the same height as the surrounding ground vegetation (Miller et al., 1998). In contrast, in the exclosure immediately to the east, which was about 40 years old (Fenton, 1985), there were many pine saplings which had reached heights of over 2 m. In contrast, the Glen Tanar site, which lay mostly on the southern flank of a hill, was wooded throughout. Tree densities varied considerably, however, as part of the site comprised a band mostly of planted pine, which had been recently thinned. Although the soils were dry throughout most of the site, with the ground vegetation dominated by heather and blaeberry, the ground vegetation varied considerably across the site. In some parts there were high densities of pine saplings, but regeneration had been greatly suppressed, with many saplings having multiple leaders and being generally less than 1.5 m tall at the start of the study.

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placed on small hillocks with high cover of blaeberry at the lower altitude part of the site, as such local conditions were otherwise not represented) and 21 plots at Glen Tanar. The altitude of each plot was recorded. The shelter afforded to deer by the local topography and by mature trees was estimated using three different methods: (i) at the largest scale (typically upwards of 100 m) a topex score was calculated (the sum of positive angles to the horizon towards the eight principal compass points); (ii) on the plot itself, the slope and aspect at the centre of the plot and at the mid-point of each edge were measured, from which mean and standard deviation northerly and easterly slope components were calculated; (iii) an index of the shelter given by mature trees was estimated subjectively within 50 m of the centre of the plot towards each of the four principal compass points on a four-point scale (0: none, 1: light, 2: moderate, 3: high), and the scores summed. 2.3. Deer use On each plot, a permanently marked sub-plot of 25 m  4 m (from the centre to the N edge of the plot) was set out in spring 1997 for counting pellet groups of red deer, roe deer and sheep (and other herbivore dung where present). A pellet group was defined as having at least ten pellets of similar size, shape and colour. Pellet groups lying on animal paths were differentiated, as were ‘strings’ left by animals moving through the plot. From autumn 1997, the permanent sub-plots were cleared of all dung at each visit; thus from spring 1998 counts on the permanent sub-plots were of deposits made over a known period, i.e. by the faecal accumulation (FA) method (Putman, 1984). From spring 1998, an additional standing crop (SC) dung count was made on a second 25 m  4 m subplot on each plot. The SC plot was positioned sequentially towards each of the remaining plot edges in turn on a seasonal basis.

2.2. Plots 2.4. Saplings Each site comprised an array of permanently marked 50 m  50 m square plots set out in a grid pattern at 200 m spacing. There were 22 plots at Glen Affric (of which 20 were on the grid, and two were 1

Defined as young Scots pine trees up to 2 m tall.

All pine saplings equal to or above the height of the surrounding ground vegetation canopy within each plot were counted and measured at Glen Tanar. There were no pine saplings above the ground vegetation height at Glen Affric other than on one plot (along the

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edge of a vehicle track where the ground was disturbed), and they were treated together with the sapling bank on that site (see below). At Glen Tanar, a sub-set of saplings on each plot was measured on each visit such that all saplings on each plot were counted after two or more visits. The height, basal diameter and leader length of each sapling was measured, and the sapling classed as having a single or multiple leader and as equal to or above the ground vegetation canopy. If the sapling was browsed, the percentage of shoots browsed was estimated by eye and the location of browsing recorded (leading shoots, lateral shoots or both). At Glen Affric, an estimate of the sapling bank was made within each dung sub-plot, and each tree was classed as never browsed, browsed during the previous season only (recent browsing), browsed prior to the previous season only (old browsing), or having both old and recent browsing. Additional observations were made on saplings elsewhere in the plot on an ad hoc basis as they were encountered whilst measuring the ground vegetation, and in autumn 1999 and spring 2000, the percentage of shoots browsed was also estimated. As most saplings at Glen Affric were small, any sapling which was browsed had its leading shoots browsed, and no distinction was made between leader and lateral browsing. 2.5. Ground vegetation The cover and utilisation of the ground vegetation were estimated from 16 or 20 quadrats of size 50 cm  50 cm on each plot from spring 1997 to 1999. The plots were sub-divided into four quarters (NW, NE, SE, SW), and the quadrats apportioned equally between them. The cover of heather shoots, blaeberry, bog myrtle, graminoids, Sphagnum spp., other mosses and bare ground was estimated within each quadrat to the nearest 5% by eye (with a trace, a few shoots or very limited presence recorded as 1, 2 or 3%, respectively). The utilisation of heather (sensu Grant, 1971) was estimated by the method of Grant et al. (1981). Up to five long shoots (where present, those nearest to each corner and to the centre of the quadrat) were examined and classed as not browsed, browsed to less than half of the most recent year’s growth, browsed to more than half, or browsed into the previous year’s growth. The

heights of the shoots were also measured. The utilisation of blaeberry, bog myrtle and graminoids was estimated by eye as the percentage of shoots browsed within a four-point classification in 1997 (0: 0%, 1: <33%, 2: 33–67%, 3: >67%), which was superseded by a seven-point classification in 1998 (0: 0%, 1: 1– 5%, 2: 5–15%, 3: 15–33%, 4: 33–67%, 5: 67–90%, 6: >90%). 2.6. Data analysis 2.6.1. Ordination of ground vegetation The vegetation cover and heather height data for each site were ordinated by principal components analysis (PCA) at the individual quadrat scale, and plot means were then calculated for the retained axes. Although there were significant differences in cover of some species between visits, they were much less than the differences between plots, and as spatial rather than temporal variation was of principal interest, data from all visits were grouped together. 2.6.2. Ordination of plot characteristics As topographical parameters and vegetation cover of plots were inter-correlated, a set of orthogonal axes was calculated for each site by PCA. Twelve original variables were input for each plot (mean and standard deviation of northerly and easterly slope components, altitude, topex score, tree shelter index, heather, blaeberry, bog myrtle and graminoid cover, and pine sapling index). For Glen Tanar, the pine sapling index for each plot was the natural logarithm2 of the estimated total number of pine shoots available on pine saplings less than 2 m tall. This was derived from counts of saplings on each plot and estimated shoot numbers derived from relationships to height, basal diameter and other biometric data (S.C.F. Palmer and A.-M. Truscott, unpublished data). For Glen Affric, where most pines were suppressed and had few shoots, a simpler index, the natural logarithm of the estimated number of pine saplings in the sapling bank, was used. 2.6.3. Dung counts Winter dung counts (as recorded in spring 1998– 2000) were fitted to a generalised linear mixed model 2

To normalise a strong positive skew between plots.

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(GLMM; SAS macro Glimmix) with sub-plot type (SC or FA), year and the interaction of sub-plot type and year as fixed effects. Pellet groups falling on animal paths were omitted. The model assumed a Poisson error distribution, and incorporated a logarithmic link function and random plot effect. The logarithm of the number of days during which the dung had accumulated was also included as an offset (i.e. an observation-dependent adjustment to the intercept); this allowed modelling of the deposition rate whilst retaining a Poisson distribution for count data about the predicted means. For FA plots, the offset represented the accumulation period since the previous dung clearance. For SC plots, the accumulation period was unknown, and instead was set to 1. Thus the model provided a pairwise comparison between dung count methods across years, and the least squares mean difference in the coefficients for sub-plot type gave an estimate of the accumulation period for SC plots, which could be compared with the known accumulation period for FA plots. To allow for possible habitat-related variation between sub-plots, optional habitat variables were also included in the model. These comprised either (i) none, (ii) a set of orthogonal vegetation indices for the sub-plot (derived from the mean vegetation PCA scores of all quadrats falling in the half of the plot in which the sub-plot lay), (iii) the aspect of the sub-plot (as orthogonal northerly and easterly components), (iv) aspect and vegetation indices, (v) the set of orthogonal plot indices combining topography, vegetation and pine sapling indices. Habitat variables were selected by backwards elimination. 2.6.4. Habitat use by deer The influence of topographical and vegetation characteristics of plots on their seasonal use by deer (as indexed by dung counts) was examined by fitting logtransformed FA counts to year, plot PCA scores and the interactions of year with plot PCA scores by the method of residual maximum likelihood (REML), with plot entered as a random effect. To attempt to assess the relative importance of the inter-correlated shelter indices and vegetation characteristics of plots, dung counts were fitted to models incorporating only year and the original variables which contributed most to the first plot axis (the only ordinated axis which was significant for either site). The vegetation variables

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were added individually after having included all the shelter variables, and likewise each shelter variable after including all the vegetation variables. 2.6.5. Browsing of pine saplings The probability of a pine sapling being browsed can depend on its height (S.C.F. Palmer and A.-M. Truscott, unpublished data). Browsing and leader browsing incidence at Glen Tanar, where saplings ranged in height from equal with the heather canopy to 2 m tall, were therefore analysed seasonally at the individual tree level by GLMM with a binomial error distribution and logit link function. The possible explanatory variables were sapling height, height squared, a normalised index of pine shoot availability on the plot (as above), seasonal FA dung counts, the interactions of dung counts with the three pine variables, year and the ordinated ground vegetation indices. Non-significant variables were discarded by backwards elimination. Plot and plot by year interaction were included as random variables. The models were also fitted with random effects only, in order to examine the contribution of each stratum to the variation in the data. At Glen Affric, almost all the pine saplings had been browsed to the heather canopy height, and winter browsing incidence was analysed by GLMM at the plot level, omitting the sapling height variables and the plot by year interaction. 2.6.6. Utilisation of ground vegetation Estimates of the annual utilisation of heather and the proportion of shoots grazed of blaeberry and graminoids on each plot were derived as the mean of individual quadrat estimates weighted by the estimated cover of the component within the quadrat. The relationships of utilisation of heather with deer use and vegetation cover were investigated by fitting annual utilisation estimates to winter FA dung counts (as summer utilisation was negligible), vegetation cover estimates and year by REML. Plot was included as a random effect, and observations were weighted by the number of quadrats (maximum 20) within the plot which had some heather cover. An index of heather biomass removed by browsing, derived as the product of utilisation and cover, was also fitted to the same model. Similar analyses were also conducted for the proportion of shoots grazed of blaeberry and graminoids.

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3. Results 3.1. Ground vegetation quadrats At both sites, the first PCA axis of ground vegetation quadrat cover (explaining 31% of the variance at Glen Affric and 34% at Glen Tanar) separated plots dominated by heather from those with higher cover of graminoids (Table 1). The wetter character of the Glen Affric site was reflected in the contrast between Sphagnum and other mosses on the second axis, whereas at Glen Tanar the second axis contrasted other mosses with bare ground. 3.2. Plot characteristics and ground vegetation cover The first four axes resulting from the PCA ordination of plot topography, pine indices and ground vegetation cover (explaining 76% of the variance) were retained for Glen Affric, and the first three (73%) for Glen Tanar (Table 2). For both sites, the first axis contrasted plots dominated by heather and blaeberry with those having greater cover of graminoids. However, the sites differed in that at Glen Affric it was the dwarf shrub dominated plots at lower altitude which offered the greatest shelter by mature pine trees, whereas at Glen Tanar the best shelter was on plots falling in dense pine stands with sparse ground vegetation, typically dominated by wavy hairgrass (Deschampsia flexuosa (L.) Trin.). Moreover, at

Glen Tanar the highest availability of pine browse occurred on the more open plots with dwarf shrubs, but at Glen Affric the pine sapling index was greatest on the open mid-altitude plots and associated with bog myrtle cover. 3.3. Dung accumulation period In no model was there a significant interaction between sub-plot type (FA or SC) and year, and this term was therefore dropped. For red deer dung at Glen Affric, there was also no significant year effect, and the model with sub-plot type only estimated the accumulation period on SC plots to be 203 days (Table 3), which was not significantly different from the mean known period of 186 days on FA plots. Addition of optional habitat variables improved the model in each case, with estimated mean accumulation periods ranging from 183 to 203 days, which likewise did not differ significantly from the known period. Roe deer dung at Glen Affric was too infrequent for the model to converge. For red deer dung at Glen Tanar, there was a significant difference between years (with lower counts in 2000 than in 1998 or 1999), but no habitat variables could improve the fit of the model. The estimated accumulation period on SC plots was 238 days, which was significantly greater than the mean known period of 175 days on FA plots. For roe deer dung, there were no significant effects of year or

Table 1 Principal components ordination of ground vegetation quadrats at Glen Affric and Glen Tanar (variables with eigenvector coefficients greater than 0.3 are shown on the axes retained for comparing dung counting methods) Axis

Glen Affric ðn ¼ 1107Þ Positive loading

Negative loading Eigenvalue Variation explained (%) Glen Tanar ðn ¼ 1116Þ Positive loading Negative loading Eigenvalue Variation explained (%)

1

2

3

4

5

Heather cover and height, Other mosses, Blaeberry Graminoids, Sphagnum 2.8 31

Heather height, Sphagnum

Bare ground

Bog myrtle, Bare ground

Bog myrtle, Blaeberry

Other mosses 1.8 20

Graminoids 1.0 12

Sphagnum 0.95 11

Graminoids 0.90 10

Heather cover and height Graminoids 2.7 34

Bare ground Other mosses 1.4 17

Sphagnum, graminoids Bare ground 1.1 14

Blaeberry 0.97 12

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Table 2 Principal components ordination of plot topography, pine indices and vegetation cover for Glen Affric and Glen Tanar (variables with eigenvector coefficients greater than 0.3 are shown on the axes retained for input to linear mixed models) Axis

Glen Affric ðn ¼ 22Þ Positive loading Negative loading Eigenvalue Variation explained (%) Glen Tanar ðn ¼ 21Þ Positive loading Negative loading Eigenvalue Variation explained (%)

1

2

3

4

Heather, blaeberry, Tree shelter Graminoids, altitude 4.3 36

Pine saplings, Topex score, Bog myrtle Altitude 2.1 18

N aspect, Topex score E aspect 1.5 12

N aspect, Topex score, E–W undulation Bog myrtle 1.2 10

Tree shelter, E aspect, Graminoids Blaeberry, heather, Pine shoot index 4.1 37

N aspect, altitude, Graminoids Topex score

E aspect, N–S Undulation, Topex score

2.6 24

1.3 12

any habitat variables, and the estimated accumulation period on SC plots, although very low at 149 days, did not differ significantly from the known period.

0:001); winter counts were significantly higher in 1999 than in 1998 and 2000 (F2;42 ¼ 6:7; P < 0:005). In contrast, sheep occupancy of the site (above the stock fence) was significantly higher in summer ðF1;39 ¼ 12:5; P < 0:005Þ, but summer counts did not differ between 1998 and 1999 (F1;9 ¼ 0:2, n.s.). Roe deer dung counts were low throughout the study, and did not differ between seasons (F1;87 ¼ 1:0, n.s.). Mountain hares (Lepus timidus L.) were present at very low density (mean count on FA plots in spring 1998 was 0.73 pellets/100 m2).

3.4. Seasonal occupancy A high degree of seasonal variation in occupancy by red deer at Glen Affric was apparent from the dung counts (Fig. 1a). The mean red deer FA dung count over winter was significantly higher than over summer (GLMM with random plot effect: F1;87 ¼ 102; P <

Table 3 Comparison of SC and FA dung count methods at Glen Affric and Glen Tanar, winter 1998–2000 Habitat variables included in the model

Estimated accumulation period for SC plots L.S. diff.

a

Known accumulation period for FA plots

S.E.

Days

Days

logN (days)

Sig.b

Glen Affric, winter, red deer None (sub-plot type only) Vegetation PCA axis 1 Aspect (E) þ vegetation PCA axis 1 Plot indices 1, 3 and 4

5.31 5.21 5.21 5.31

0.083 0.085 0.084 0.083

203 184 183 203

186

5.22

n.s.c n.s. n.s. n.s.

Glen Tanar, winter, red deer None (sub-plot type þ year only)

5.47

0.133

238

175

5.16

*

Glen Tanar, winter, roe deer None (sub-plot type only)

5.00

0.102

149

175

5.16

n.s.

a

L.S. diff. is the least squares mean difference between sub-plot types, and provides an estimate of the accumulation period on SC plots. Sig. indicates whether the known period on FA plots (as logN(days)) falls within two standard errors of the estimate for SC plots. c Not significant. * P < 0:05. b

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Fig. 1. Mean seasonal counts of red deer, roe deer and sheep dung at (a) Glen Affric and (b) Glen Tanar, 1997–2000. Error bars show 1 standard error for counts made on plots cleared at the previous visit; data for 1997, shown without error bars, are from standing crop counts, and are not included in the overall means. From 1998, data for sheep at Glen Affric were restricted to the 10 plots above a stock fence.

At Glen Tanar, both red and roe deer FA dung counts were higher over winter than over summer (F1;83 ¼ 16:2; P < 0:001 and F1;83 ¼ 49:8; P < 0:001 , respectively; Fig. 1b). Red deer winter dung counts increased between 1998 and 2000 ðF2;40 ¼ 5:5; P < 0:01Þ, and

in 2000 were significantly higher than in 1998, but the contemporary slight decrease in roe deer winter dung counts was not significant (F2;40 ¼ 0:2, n.s.). Red deer summer dung counts did not differ between years (F1;20 ¼ 4:2, n.s.), but roe deer summer dung

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counts were higher in 1999 than in 1998 ðF1;20 ¼ 7:9; P < 0:05Þ. Rabbits (Oryctolagus cuniculus L.) were patchily distributed around the site at low density (mean count on FA plots in spring was 39 pellets/ 100 m2). 3.5. Habitat use by deer At Glen Affric, there was a significant effect of PCA axis 1 on the counts of both red deer and roe deer dung over winter, with the highest dung counts on plots offering the best shelter by mature trees and high cover of heather and blaeberry (F1;17 ¼ 20:0; P < 0:01 and F1;17 ¼ 9:2; P < 0:01 for red deer and roe deer dung counts, respectively). For both deer species, there was no significant difference in winter dung count between years, nor were there any significant interactions of year with plot indices. There were no significant effects of any of the explanatory variables on summer dung counts for either deer species. The patterns at Glen Tanar were different. There was no significant relationship between the winter dung count and any of the plot axes for either deer species, although there was a difference between years for red deer, with significantly higher counts in 2000 than in the previous 2 years ðF2;34 ¼ 4:5; P < 0:05Þ. Likewise, there were no significant effects for red deer dung over summer. Roe deer dung over summer was significantly more plentiful in 1999 than in 1998 ðF1;17 ¼ 11:1; P < 0:01Þ, and in both years was significantly related to the first plot axis ðF1;17 ¼ 5:1; P < 0:05Þ, with higher dung counts on plots having good heather and blaeberry cover and high availability of pine browse than on the more easterly facing, well sheltered plots with high graminoid cover. For red deer winter dung counts at Glen Affric, heather, blaeberry and graminoid cover were each significant when added after altitude and tree shelter index (F1;18 ¼ 7:4, 13.8, 5.4, P < 0:05, 0.01, 0.05, respectively), but neither of the two shelter variables was significant when added after the three cover variables. For roe deer winter dung counts, however, only blaeberry cover was significant after including altitude and tree shelter index ðF1;18 ¼ 9:3; P < 0:05Þ, and again the shelter variables were not significant when added after the cover estimates. For roe deer summer dung counts at Glen Tanar, none of the vegetation variables (heather, blaeberry

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and graminoid cover and the index of pine shoot availability) was significant after including shelter variables, and neither was either of the shelter indices (easterly aspect and tree shelter index) after including vegetation. However, when included as the second term of the model (after year), the highest F ratios were shown by blaeberry cover and pine shoot index (F1;15 ¼ 7:0, 6.8 respectively, P < 0:05 in each case). 3.6. Browsing of pine saplings 3.6.1. Browsing incidence At Glen Affric, the mean browsing incidence was significantly higher during winter than during summer by a factor of 20 (GLMM with random plot effect: F1;116 ¼ 41:0; P < 0:001; Fig. 2a). The mean winter incidence differed significantly between years (GLMM: F3;53 ¼ 8:0; P < 0:001), with 1998–1999 being higher (at 19.2%) than the other three winters (Fig. 2a). There was no significant effect of the stock fence. Winter FA dung counts could not explain any additional plot scale variation in winter browsing incidence, but there was a positive relationship with the fourth ground vegetation axis ðF1;16 ¼ 10:8; P < 0:005Þ and a negative relationship with the fifth ðF1;11 ¼ 9:7; P < 0:01Þ. However, the proportion of pine saplings showing signs of having been browsed during previous years was high, e.g. 77% in spring 1999. The browsing incidence in summer was very low (mean 0.7%), and could not be analysed by GLMM, but by treating all plots equally and analysing the proportion of saplings browsed on each plot, there was no effect of the stock fence. There was also a significant seasonal difference in browsing incidence at Glen Tanar (GLMM: F1;71 ¼ 20:7; P < 0:001), but, in contrast to Glen Affric, the higher rate was during the summer (Fig. 2b). Moreover, 70% of saplings browsed during summer had leading shoots eaten, whereas the corresponding proportion during winter was only 38% (GLMM: F2;12 ¼ 7:2; P < 0:01). During summer at Glen Tanar, the greatest variation in browsing and leader browsing incidence occurred at the plot by year interaction level, i.e. plot scale variation in browsing was high and varied between years (Table 4). However, summer dung counts had no significant effect in explaining browsing or leader browsing incidence, and neither did any plot scale

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Fig. 2. Seasonal overall browsing and leader browsing incidence to Scots pine saplings at (a) Glen Affric and (b) Glen Tanar, 1997–2000. Error bars show 1 standard error.

Table 4 Effects of plot vegetation cover (PCA scores) and pine shoot index, FA dung counts and pine sapling height on seasonal browsing incidence and leader browsing incidence to Scots pine saplings at Glen Tanar, 1998–2000 Summer

Winter

Browsing incidence F Random strata only Plot Plot  year Sapling

Cov. est.

Leader browsing incidence a

F

1.49 (0.37–131) 2.78 (1.25–10.4) 0.67 (0.59–0.76)

þ Fixed effects (no interactions across strata) retained by backwards elimination PCA axis 1 Plot 1.51 (0.38–103) Plot  year 2.60 (1.16–10.2) Sapling height 11.9*** 26.6*** 2 ** Sapling height 8.5 25.0*** Sapling 0.71 (0.63–0.82) þ Fixed effects (with interactions across PCA axis 1 Plot Roe deer dung Red deer dung  pine shoot index Plot  year Sapling height Sapling height2 Roe dung  ht Roe dung  ht2 Sapling

a

Cov. Est.

Browsing incidence F

Leader browsing incidence a

Cov. Est.

0.77 (0.14–2992) 2.20 (0.94–9.70) 0.62 (0.55–0.72)

0.53 (0.17–7.41) 0.62 (0.28–2.33) 0.77 (0.70–0.85)

0.53 ð0:078  106 Þ 2.36 (1.01–10.6)

0.56 (0.18–7.44) 0.66 (0.30–2.47)

F

Cov. est. 1.03 (0.35–11.2) 0.87 (0.35–4.78) 0.60 (0.55–0.66)

4.7*

4.3* 7.1** 0.66 (0.58–0.76)

0.55 (0.13–68.2) 1.10 (0.47–4.95) 11.3***

0.75 (0.69–0.82)

0.61 (0.56–0.67)

strata) retained by backwards elimination 6.1* 0.66 ð0:093  107 Þ

1.90 (0.49–111) 10.2**

4.9* 0.83 (0.30–6.62)

0.76 (0.20–35.3)

3.4 6.2* 2.43 (1.01–11.6)

17.5*** 13.6*** 12.0*** 11.0***

2.70 (1.13–12.6) 23.6*** 21.4*** 5.1* 5.6*

0.81 (0.71–0.93)

0.31 (0.11–3.06)

1.16 (0.50–5.00) 10.4**

10.0** 15.6*** 0.57 (0.50–0.66)

3.7 5.6* 0.77 (0.71–0.85)

0.56 (0.51–0.61)

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Effect

F ¼ (F type III F-value for fixed effects). a Covariance parameter estimates for random strata (with 95% confidence limits in the parentheses). * P < 0:05. ** P < 0:01. *** P < 0:001.

159

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Fig. 3. Predicted mean overall browsing and leader browsing incidence to Scots pine saplings in relation to sapling height during (a) summer and (b) winter at Glen Tanar.

characteristics. The probability of browsing varied significantly with sapling height, although this accounted for very little of the variation, and the variation that was explained was between visits (the plot  year scale) rather than between individual

saplings, as sapling heights varied between plots. Browsing incidence was highest for saplings at around 1.3 m tall and leader browsing incidence at 1 m tall (Fig. 3a). When interactions across random strata were permitted in the model, interactions of roe deer dung

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count with sapling height became significant, and the proportion of plot by year variation increased. In contrast, over winter at Glen Tanar, there was less variation in browsing and leader browsing incidence between plots than in summer (Table 4). However, as in summer, if interactions across strata were not included, dung counts were not significant, and very little of the variation was explained by the models. For saplings less than about 1.5 m tall, browsing incidence was on average below 10%, but it increased on taller saplings (Fig. 3b), whereas leader browsing incidence declined with sapling height during winter. Inclusion of interactions across strata failed to account for much more of the variation, although there was a significant positive relationship of browsing incidence with the interaction of red deer dung and the index of pine shoot availability. 3.6.2. Browsing severity For saplings which had been browsed during winter at Glen Tanar, the estimated mean percentage of shoots browsed was 10.5%, and it did not differ between years. Although plot means across years varied between 2 and 28%, annual plot means were not related to winter dung counts, ground vegetation characteristics or the availability of pine shoots on the plot. Summer plot means across years ranged between 3 and 20% (overall mean 7.9%), but, as in winter, they were not related to dung counts or plot characteristics.

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The corresponding values at Glen Affric were considerably higher, averaging 36.6% for summer 1999 (plot range from 18 to 58%) and 28.3% for winter 2000 (plot range from 7 to 85%), but sample sizes were small, and browsing severity could not reliably be compared with use of the plots by deer. 3.7. Utilisation of ground vegetation Estimated mean annual utilisation of heather was below 10% at both sites (Table 5) in all years. The 1997 estimates were higher than for the following 2 years, which could be in part because the spring visits were later, and at Glen Affric because sheep had full access to the site in 1997 but not thereafter. In any case, 1997 data were not used in plot-based comparisons of utilisation and dung counts, as FA counts were not made in that year. At both sites, grazing levels of graminoids were similar in 1998 and 1999. Browsing of blaeberry and bog myrtle at Glen Affric varied considerably between years, and the proportion of blaeberry shoots grazed at Glen Tanar declined from 1997 to 1999. At Glen Affric, the annual utilisation of heather and the biomass removal index at the plot scale increased with red deer dung count and with both heather and blaeberry cover (Table 6). The proportion of blaeberry shoots grazed was not related to dung counts or vegetation cover, but the proportion of graminoid

Table 5 Annual utilisation of ground vegetation as estimated in spring at (a) Glen Affric and (b) Glen Tanar Year

n plots

(a) Glen Affric 1997 1998 1999

20 22 22

Mean

64

(b) Glen Tanar 1997 1998 1999

18 21 21

Mean

60

Mean date

07 May 23 April 18 April

19 May 15 April 14 April

Heather

Blaeberry

Bog myrtle

Graminoids

U (%)

B

G (%)

B

G (%)

B

G (%)

I (%)

9.5 4.9 5.6

2.7 1.5 1.8

69.0 61.3 33.2

7.6 7.7 4.3

52.6 33.0 54.6

3.2 2.3 3.7

3.9 6.9

28 42 58

6.4

1.9

54.6

6.6

45.0

3.0

7.8 6.7 6.5

2.0 1.5 1.2

44.7 30.5 20.5

5.4 4.1 3.2

7.0

1.6

32.3

4.3

43

5.2 5.5

58 67 66 65

U: utilisation (sensu Grant, 1971); G: proportion of shoots grazed; B: biomass removal index, calculated as the proportion grazed (U or G) by the estimated cover in the plot; I: cover-weighted proportion of quadrats in which grazing had occurred; bog myrtle cover was zero for all Glen Tanar plots.

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Table 6 Relationships of heather utilisation and biomass removal index, and the proportion of blaeberry and graminoid shoots grazed with FA dung counts and vegetation cover during winter at (a) Glen Affric and (b) Glen Tanar (F statistics shown are calculated ignoring terms fitted later in the model, and the direction of the relationship is shown for significant covariates) Effect

Heather utilisation

Heather biomass removal index

Blaeberry proportion grazed

Graminoid proportion grazed

(a) Glen Affric Red deer dung count Roe deer dung count Sheep dung count Mean heather cover Mean blaeberry cover Mean graminoid cover Mean bog myrtle cover Year

F 50.1** þ 1.4 0.1 29.8** þ 11.2** þ 1.0 0.0 0.0

F 91.8** þ 2.0 0.3 54.6** þ 19.2** þ 0.1 0.6 0.0

F 0.5 0.6 0.0 2.6 2.6 0.5 n/ia 18.6**

F 35.3** þ 0.6 5.6* þ 0.0 1.4 0.3 2.3 0.8

(b) Glen Tanar Red deer dung count Roe deer dung count Mean heather cover Mean blaeberry cover Mean graminoid cover Year

F 5.1* þ 11.6** þ 43.7**  6.7*  0.2 0.1

F 23.0** þ 13.1** þ 0.0 2.2 0.3 3.8

F 0.1 1.7 0.0 23.0** þ 1.9 11.1**

F 0.0 0.2 0.0 0.2 0.5 0.1

a

Not included. P < 0:05. ** P < 0:01. *

shoots grazed increased significantly with both red deer dung and sheep dung. At Glen Tanar, heather utilisation and the biomass removal index increased with both red and roe deer dung counts, but utilisation decreased with both heather and blaeberry cover. However, the cover-weighted biomass removal index was independent of the cover of either species. The proportion of blaeberry shoots grazed increased significantly with its cover but was not related to dung counts, and neither was the proportion of graminoids shoots grazed related to dung counts.

4. Discussion 4.1. Dung accumulation period The direct pairwise comparison of SC and FA dung counts offers an alternative method for estimating the mean accumulation period, having the advantage that all dung is deposited naturally over time and space by the population of deer under study. Our results indicate that, for a SC dung count conducted at the end of winter (typically in April) on sites in the wetter and milder deer forests of western Scotland, such as Glen

Affric, an accumulation period of no longer than six months should be used for converting dung counts to deer density. It is possible that a shorter period may be more appropriate, but that cannot be determined from our data. In the drier eastern half of Scotland, as typified by Glen Tanar, a longer accumulation period of around eight months is recommended. It is stressed that these recommendations apply only to similar habitat types to those in which the study was conducted, i.e. to Caledonian pinewood sites. The method allows controlling for plot scale (or larger) variation in habitat when estimating the accumulation period. In this study, however, inclusion of habitat variables did not alter the estimated accumulation period. The method would be improved by making more frequent visits to clear FA plots, which would facilitate the estimation of shorter accumulation periods at sites with rapid decay, and by increasing the number of plot pairs to increase the power to detect habitat-dependent variation in decay rate. 4.2. Seasonal occupancy and browsing of Scots pine Estimation of site occupancy by deer and sheep from dung counts also depends on knowledge of

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Table 7 Estimated seasonal occupancy of sites at Glen Affric and Glen Tanar, 1998–2000, from FA dung counts and published defecation ratesa

Glen Affric Winter: mean accumulation period ¼ 186 days Mean FA dung count (groups/100 m2) Defecation rate (groups/day) Occupancy (animals/km2) Summer: mean accumulation period ¼ 172 days Mean FA dung count (groups/100m2) Defecation rate (groups/day) Occupancy (animals/km2) Glen Tanar Winter: mean accumulation period ¼ 175 days Mean FA dung count (groups/100 m2) Defecation rate (groups/day) Occupancy (animals/km2) Summer: mean accumulation period ¼ 181 days Mean FA dung count (groups/100 m2) Defecation rate (groups/day) Occupancy (animals/km2) a

Red deer

Roe deer

Sheep

11.3 23–25 24–26

0.64 17–20 1.7–2.0

0.70 16 2.4

0.82 21–25 1.9–2.3

0.48 19–20 1.4–1.5

2.6 16 9.5

2.5 23–25 5.7–6.2

6.2 17–20 18–21

1.0 21–25 2.2–2.6

1.7 19–20 4.7–5.0

Welch (1982), Mitchell and McCowan (1984), Mitchell et al. (1985), Ratcliffe and Mayle (1992) and Mayle and Staines (1998).

defecation rates, which can vary according to species, time of year and diet. Ideally these should be estimated under similar conditions to those prevailing in the study sites (Putman, 1984), but that was beyond the scope of this study. Instead, we used seasonal ranges determined from published studies under similar conditions, from which we derived estimated seasonal ranges in site occupancy (Table 7). The estimated winter occupancy from FA dung counts by red deer at Glen Affric was around 25 animals/km2. Clearly red deer occupancy of the site was much lower in summer, although possibly higher than the estimate of about 2 animals/km2, perhaps owing to faster dung decay during summer. Roe deer occupancy was low throughout the year, below 2 animals/km2. Roe deer generally perform badly in Glen Affric, having low carcass weights and high spring mortality of first year animals (R. Cooper, personal communication). Sheep were mostly present during spring and summer, at up to about 10 animals/km2. Browsing of Scots pine saplings at Glen Affric was almost entirely limited to the winter months, and can therefore be assumed to have been largely due to red deer, as red deer winter dung density was roughly ten

times greater than roe deer and sheep winter dung densities. Browsing incidence over four winters averaged only 14%, little more than at Glen Tanar, yet pine was not regenerating at Glen Affric. To a large extent this may have been due to the winters throughout the period of the study having been consistently milder than the long-term average, with very little longlasting snow cover, and hence lower than average browsing rates. Indeed, during the winter of 1995/ 1996, which was colder than average, very high browsing incidence was recorded in another part of Glen Affric about 5 km from the study site (Palmer et al., 1998). Moreover, pine saplings on the site were small and stunted, at least three-quarters showed signs of previous browsing damage, and when they were browsed during the study, the severity of damage was relatively high (on average 28% of shoots bitten in spring 2000). The winter occupancy estimates for Glen Tanar were surprisingly high, at about 6 red deer/km2 and 20 roe deer/km2, especially in comparison with recent estimates for the whole area of pine woodland on the estate of less than 2 animals/km2 of each species (M. Bruce, unpublished data). Certainly the study site

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appeared to be more heavily used than other parts of the estate during winter, yet winter browsing incidence was relatively low, at an average of no more than 10% of saplings under 1.5 m, and many pine saplings on the site were growing well. There was an indication that browsing of pines by red deer in winter might be directed towards areas of relatively high sapling density, which is consistent with the patterns shown by moose (Alces alces (L.)) in Sweden (Andre´ n and Angelstam, 1993). Occupancy estimates for the Glen Tanar site during summer were considerably lower than for winter. Seasonal variation in site occupancy cannot be ruled out, particularly as the site was in part bounded by the main track leading into the glen, possibly resulting in higher disturbance during summer, but in part this pattern may have been due to more rapid disappearance of dung during summer (Welch et al., 1990). Nevertheless, a greater proportion of pine saplings was browsed during summer (23%) than during winter (9%), although the level of damage to browsed saplings was similar (8% versus 10% of shoots bitten). Most of the damage during summer was to the leading shoots of saplings around 1 m tall, rather taller than the optimum height for browsing leading shoots of Sitka spruce Picea sitchensis in a forestry plantation with both deer species present (Welch et al., 1991). Many of the damaged Scots pine saplings were very bushy, with multiple leading shoots, indicating that they had been browsed many times in the past, as has been observed for Sitka spruce (Welch et al., 1992). Browsed shoots were concentrated in groups, suggesting that the browsing had occurred at the time of shoot elongation in June, as pine shoots started to lengthen in a tight group before spreading out later in the summer. At this time, the shoots would have been tender and, presumably, had high concentrations of nutrients. 4.3. Habitat use by red and roe deer There was no evidence from either site that the availability of pine browse had any influence on habitat use at the plot scale (i.e. the 200 m grid) over the course of a full winter by either deer species. Indeed, at Glen Tanar, winter dung counts were not related to any of the habitat variables measured, either in terms of shelter or cover of the main components of the ground vegetation. This may be because the site

offered relatively good quality shelter and forage availability throughout. However, the data suggested that roe deer may have shown a preference during summer at Glen Tanar for parts of the site with relatively high cover of blaeberry and availability of pine browse. In contrast, the Glen Affric site had a high degree of habitat variation, and the use made by red deer of the site during the winter months was strongly correlated with the first principal component of plot characteristics, i.e. deer made much greater use of the lower altitude plots with higher shelter afforded by mature trees than the open higher altitude plots. This shows both similarities with and differences to the patterns observed for red and roe deer in a commercial Sitka spruce plantation, in which open areas, such as rides and restocked blocks, were the most heavily used, although it was in those open areas that the availability of heather was highest (Welch et al., 1990). The degree of shelter and availability of winter browse in the form of heather and blaeberry were correlated on the Glen Affric site, and it was therefore difficult to assess the relative importance of shelter and forage availability in determining winter habitat use. The data suggested that forage availability was the better determinant during the course of this study, although that might not be the case during more inclement winters. Certainly shelter has been shown to be important to red deer on open hill ground (Staines, 1976), but in order to estimate its importance to red deer in more heterogeneous habitats, in which open and wooded habitats are available, a physiological modelling approach would be recommended (see Armstrong and Robertson, 2000). Likewise, such an approach would allow other factors, such as the energetic costs of foraging (e.g. Wallis de Vries, 1996) and disturbance to be taken into account. A further insight into the contrasting ways in which deer used the two sites is apparent from the patterns of grazing of the ground vegetation. Although annual heather utilisation increased with deer use at the plot scale on both sites, as would be expected, utilisation and an index of biomass removal increased with heather cover at Glen Affric, whereas at Glen Tanar, utilisation decreased with heather cover and biomass removal was independent of cover. These results indicate that red deer at Glen Affric concentrated their grazing of heather in areas where it was most plentiful,

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which were also the parts of the site offering best shelter. The spatial distribution of heather grazing by red and roe deer in Glen Tanar was much more even, with similar quantities grazed in different areas regardless of availability. The influence of blaeberry availability on habitat use must not be overlooked. It was more heavily browsed at Glen Affric than at Glen Tanar, but, in contrast to utilisation of heather, grazing of blaeberry was independent of its availability in Glen Affric, but increased with its availability in Glen Tanar (albeit with smaller sample sizes than for heather at both sites). This could have been due to differences in the herbivores present or to seasonal variation in the importance of blaeberry in the diet (Henry, 1978; Staines and Welch, 1984; Welch et al., 1994; Latham et al., 1999). Clearly different patterns of forage utilisation (both tree browse and ground vegetation) can occur at different sites, and depend on interactions between herbivore numbers, environmental characteristics and availability and quality of different types of forage. Such interactions operate at a range of spatial scales and vary on an annual and seasonal basis. Hence relationships between deer numbers and browsing rates are difficult to predict (Putman, 1996). Furthermore, it is not possible to control adequately for all possible factors in a field-based observational study. A process-based modelling approach, including a physiological sub-model to allow for the influences of shelter on foraging behaviour, would enable the integration of the relationships established in this and other studies with more general theories of herbivore foraging strategy.

165

pine saplings are small, and, possibly, subjected to occasional higher levels of damage in more severe winters, as small saplings may take several years to recover from heavy browsing damage. Although dung counts can be an effective method of estimating seasonal occupancy by large herbivores at the site scale, weak relationships were found between dung counts and levels of browsing on Scots pine saplings at the plot scale within sites, although grazing on heather was related to dung counts. This was probably because tree browsing occurred episodically over short time periods, whereas dung counts integrated habitat use over longer periods. Thus, successful regeneration of Scots pine cannot readily be predicted simply in terms of a given deer density. Many other factors must be taken into account, including the likely spatial and temporal foraging patterns of the deer, alternative forage available, the occurrence of severe weather (especially snow) and the size and potential growth rates of pine saplings present.

Acknowledgements We are grateful to Michael Bruce for permission to work on Glen Tanar Estate and to Forest Enterprise for access to Glen Affric. Tanja Bla¨ sing, Peter Dullaghan, Gudrun Heckermaier, Joe Hope, Tom Ings, Steve Kirkby and Rachel Thwaites assisted in data collection, and David Elston provided statistical advice. Helen Armstrong, Phil Hulme, Dave Scott, Rene´ van der Wal and David Welch commented on the manuscript. This work was supported by an award under the Scottish Executive Rural Affairs Department’s Flexible Fund Scheme.

5. Conclusions This work is, as far as we are aware, unique in examining simultaneously the possible attraction of young trees to free-ranging deer and their impact on those trees. It has confirmed that natural regeneration of Scots pine may occur at levels of local occupancy by red deer in winter at least as high as those previously suggested and, furthermore, with a high winter occupancy by roe deer. However, at higher red deer occupancy during winter, regeneration may be prevented by generally relatively low browsing rates if the

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