Impact of deer on temperate forest vegetation and woody debris as protection of forest regeneration against browsing

Forest Ecology and Management 260 (2010) 429–437

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Impact of deer on temperate forest vegetation and woody debris as protection of forest regeneration against browsing Maryline Pellerin a,∗ , Sonia Saïd a,1 , Emmanuelle Richard a,b,1,2 , Jean-Luc Hamann a,1 , Cécile Dubois-Coli c,3 , Philippe Hum d,4 a

Office National de la Chasse et de la Faune Sauvage, Centre National d’Etudes et de Recherches Appliquées sur les Cervidés-Sanglier, “Montfort”, 01330 Birieux, France Université Lyon 1 CNRS UMR 5558, Laboratoire de Biométrie et Biologie Evolutive, Bâtiment G. Mendel, Université Claude Bernard Lyon1, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France c Office National des Forêts, Agence Travaux de Franche-Comté, 14 rue Planc¸on, BP 51581, 25010 Besanc¸on, France d Office National des Forêts, Direction Forêt, Cité administrative, 14 rue du Maréchal Juin, 67084 Strasbourg Cedex, France b

a r t i c l e

i n f o

Article history: Received 27 January 2010 Received in revised form 22 April 2010 Accepted 26 April 2010 Keywords: Temperate forest Plant species richness and diversity Regeneration Deer Browsing impact Woody debris

a b s t r a c t A 3-year field experiment with paired exclosure (fenced areas, excluding deer) and control plots (unfenced areas, free access to deer), with two treatments with and without woody debris, was carried out at two sites in a temperate forest in eastern France. The aim of the experiment was to assess the effect of browsing by roe deer (Capreolus capreolus) and red deer (Cervus elaphus) on the diversity and richness of plant species and to test the effectiveness of using woody debris to protect seedlings and saplings from deer browsing. In presence of deer, both plant species richness and diversity were reduced the first year of the study, but this negative impact of deer then disappeared after 3 years. Deer browsing mostly affected species composition of plant communities. We observed a decrease in the abundance of preferred species such as Carpinus betulus, Rubus fructicosus, Rubus idaeus, Anemone nemorosa and Epilobium angustifolium, and palatable species such as Acer spp., Carex spp., Festuca spp. and Mycelis muralis, whereas unpalatable species such as Lamium spp., or species particularly resilient to browsing such as grasses (Brachypodium spp. and Luzula spp.) increased in abundance. The use of woody debris as protection against browsing by deer did not limit damage to seedlings and saplings of the main commercially valuable species, Abies alba and Quercus spp. Instead of limiting deer impact, use of woody debris seemed to increase the negative effect of deer browsing on regeneration in control plots relatively to those without protection. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Following the extirpation of large predators (Breitenmoser, 1998) and changes in silviculture, agriculture, and game management (Kuiters et al., 1996; Waller and Alverson, 1997), native deer populations have reached historic peaks in abundance and have expanded their geographic range in recent decades in the northern hemisphere. Ungulate herbivory affects vegetation, from the plant organ to the landscape scale (Weisberg and Bugmann, 2003) with cascade mechanisms impacting bird and arthropod commu-

∗ Corresponding author. Tel.: +33 329 799 686; fax: +33 329 799 786. E-mail addresses: pellerin [email protected] (M. Pellerin), [email protected] (S. Saïd), [email protected] (E. Richard), [email protected] (J.-L. Hamann), [email protected] (C. Dubois-Coli), [email protected] (P. Hum). 1 Tel.: +33 329 799 686; fax: +33 329 799 786. 2 Tel.: +33 472 448 111; fax: +33 478 892 719. 3 Tel.: +33 381 657 898; fax: +33 381 650 887. 4 Tel.: +33 388 767 647; fax: +33 388 768 149. 0378-1127/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2010.04.031

nities (Allombert et al., 2005; Barrett and Stiling, 2007; Gill and Fuller, 2007), ground flora (Kirby, 2001) and trees and shrubs (Gill and Beardall, 2001). This situation causes damage to economically important human activities (e.g., agriculture, forestry). The damage caused by deer on plants through feeding is considered as a driver of plant community dynamics of the forest understory (Rooney and Waller, 2003) because it affects vegetation composition and structure (Gill and Beardall, 2001; Casabon and Pothier, 2008; Rooney, 2009; Boulanger et al., 2009). Deer browsing can directly alter plant shape (Drexhage and Colin, 2003), cover, growth and survival (Ammer, 1996; Joys et al., 2004; Stroh et al., 2008; Bergquist et al., 2009). It can also change plant distribution and species composition, by favouring species avoided by deer (unpalatable species) or resilient to deer browsing (Augustine and McNaughton, 1998; Horsley et al., 2003; Barrett and Stiling, 2006; Casabon and Pothier, 2008; Beguin et al., 2009; Mudrak et al., 2009; Rooney, 2009). Seed dispersal (via the coat, hoof or feces) or trampling of the soil may also directly alter both vegetation structure and composition (Malo and Suarez, 1995, 1998; Gill and Beardall, 2001). The effects of deer may also be indirect by modifying vegetation planting structure

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and consequently light and interspecific competition (Hobbs, 1996; Augustine and McNaughton, 1998); changing plant species diversity through changes in primary productivity (Stewart et al., 2009); altering soil by trampling (Persson et al., 2000); modifying litter, and then soil composition, due to changes in shrub stratum and feces deposition (Hobbs, 1996; Augustine and McNaughton, 1998; Persson et al., 2000; Harrison and Bardgett, 2003); and by affecting soil microfauna and invertebrates (Suominen, 1999; Suominen et al., 1999a,b; Feber et al., 2001). These direct or indirect effects are often positive at low or intermediate deer densities, through increasing favourable areas for colonization and germination of vegetation species, but are negative at high deer densities, by hampering vegetation establishment, growth and succession (Rooney and Waller, 2003; Côté et al., 2004). The type and magnitude of damage depend on the ungulate community, the density of large herbivores, the season, the vegetation available to herbivores, the history of the site, and forestry management (Côté et al., 2004). One of the various silvicultural techniques used for limiting deer impact consists in leaving woody debris, after logging or storms, to protect seedlings and saplings from browsing. Though Fredericksen et al. (1998) and Kupferschmid and Bugmann (2005) did not find a significant effect of woody debris on the browsing intensity on seedlings or saplings, more recent studies have shown that this protection was efficient for reducing browsing by ungulates on tree seedlings and for improving the etablishment and the growth of palatable species (de Chantal and Granström, 2007; Casabon and Pothier, 2007; Hunn, 2007; Relva et al., 2009). Here, we assess the impact of deer populations (roe deer (Capreolus capreolus) and red deer (Cervus elaphus)), exhibiting increasing abundance, on temperate forest vegetation. We used fencing (paired exclosure-control plots), a common experimental approach for studying deer impacts by manipulating deer densities or vegetation (Risenhoover and Maass, 1987; Trumbull et al., 1989; Healy, 1997; Castleberry et al., 2000; Nomiya et al., 2002; Palmer et al., 2004; Tremblay et al., 2006; Rooney, 2009). We compared species richness and diversity of woody and herbaceous vegetation (ground layer) and the regeneration of two commercially valuable tree species between four treatments: exclosures (without deer) – cleared of woody debris; exclosures – with woody debris; control plots (with deer) – cleared of woody debris; or control plots – with woody debris. We tested the following hypotheses: H1. High deer browsing pressure reduces plant species richness and diversity (Tilghman, 1989; Horsley et al., 2003), when no protection is used to protect seedling and saplings. We predicted that without woody debris, both species richness and diversity would be reduced, whereas with protection (woody debris), we would not find significative difference between exclosure and control plots because natural protection by woody debris would limit browsing. In absence of deer (i.e., in exclosures), woody debris will reduce both species richness and diversity (Krueger and Peterson, 2009) by modifying environmental variables for plant species, e.g. by reducing light and increasing soil humidity. The following gradient in plant species richness and diversity was thus expected: exclosures – cleared of woody debris > exclosures – with woody debris = control plots – with woody debris > control plots – cleared of woody debris. H2. We predicted that deer browsing would change plant community composition, favouring non-palatable species or species particularly resilient to browsing to the detriment of highly palatable species. H3. We expected the same gradient for the number of seedlings and saplings of the main commercially valuable species, oak (Quer-

cus spp.) and silver fir (Abies alba), as for plant species richness and diversity. 2. Material and methods 2.1. Study area The study was carried out in la Petite-Pierre National Hunting and Wildlife Reserve (NHWR), a 2700-hectare unfenced forest area located in the Vosges mountain range, north-eastern France (48.5◦ N, 7◦ E), at a mean elevation of 300 m. The climate is continental with oceanic influence, leading to mild winters and cool summers (mean January and July temperatures are respectively 0.6 ◦ C and 18.4 ◦ C) (Bonenfant et al., 2005). Snow accumulation is rare. The sandstone substrate in la Petite-Pierre NHWR produces acidic and poor soils, resulting in a poorly diversified vegetation of low nutritive quality for herbivores. The forest is structured in even-aged clusters of trees, and comprises roughly equal proportions of broadleaved, mainly beech (Fagus sylvatica), and coniferous trees, mainly silver fir (Abies alba), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) (Hamann et al., 1997). 2.2. Deer populations The NHWR is free of big game predators and ungulate populations of red and roe deer, present within the reserve, are managed through hunting with quota. Red and roe deer population densities have been maintained at relatively constant levels since 1984 (Garel et al., 2010; Richard et al., 2010) and in the past 10 years, an average of 40 red deer and 50 roe deer were harvested annually in la Petite-Pierre NHWR. We applied spotlight counts in order to estimate deer populations in the study area (Garel et al., 2010). We counted deer at night when driving a car along a road twice a month between December and April. We sampled deer along three independent roads of variable length. We counted both deer species using a powerful spotlight (100 W) from 1978 to 2008. As the number of observed deer increased with the road length we expressed the count in terms of number of deer seen per km. This method has been shown to provide a reliable assessment of population cinetics in the studied population (Garel et al., 2010). In the study area, the mean number of deer seen per km was 0.54, 0.46, 0.55 and 0.55, respectively, in 2005, 2006, 2007 and 2008 for roe deer (mean = 0.53 ± 0.04; Richard et al., 2010) and 0.69, 0.79, 0.87 and 0.86, respectively, in 2005, 2006, 2007 and 2008 for red deer (mean = 0.80 ± 0.08; Garel et al., 2010). 2.3. Data collection The paired-plot methodology is often used for estimating the amount of vegetation browsed by deer (Stromberg, 1995): animals are excluded from one of each pair of plots (fenced plots = exclosure) while the other may receive normal use (non-fenced plots = control plot). The comparison between exclosure and control plot is only feasible when initial conditions of both sets are identical, i.e., they are members of the same population. We used a design-based approach to respect this assumption since our plots were random samples in which every member of the whole population had an equal and independent chance of being in the sample. Both exclosure and control sets were stratified by habitat type and were characterised by the same sample size. Random placement of plots justifies the extrapolation of sample statistics to the population of interest. We set up the experimental design at two different sites: (i) a beech wood/fir plantation dominated by fir (51%), site

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1, and (ii) a beech wood/oak grove, dominated by oak (59%), site 2. Site 1: forest with beech wood and fir plantation We placed 30 paired plots (1 m2 with and without woody debris of beech) in the control plot (1 ha). We sampled the same number of paired plots (1 m2 ) in the exclosure (1 ha enclosed with a 2.3 m fence high, set up in March 2005) where no deer had access.

Table 1 Results of Two-Way ANOVAs (df = degree of freedom, F = F value, P = p-value) used to estimate the effect of the four treatments (exclosures – cleared of woody debris, exclosures – with woody debris, control plots – cleared of woody debris, and control plots – with woody debris) and the site (1 and 2) on Shannon’s diversity index (H ) and species richness (N, the total number of plant species recorded in the homogeneous plot), for each year of the study and for different vegetation types: for all the species, for woody species and for herbaceous species. Year

Treatment df

Site 2: forest with beech wood and oak grove It was the same as site 1 but with 21 paired plots (with and without woody debris of oak) placed in the control plot (1.5 ha) and 21 paired plots in the exclosure (1.5 ha, set up in March 2005). All plots measured 1 m2 and were circular. On each plot, we carried out a phytoecological investigation to estimate the structural complexity, in May 2005, 2006 and 2008. This investigation included an evaluation of the cover of main vegetation layers (herbaceous layers <0.5 m, low shrubs between 0.5 and 2 m, high shrubs between 2 and 8 m and trees >8 m). For each plot and layer, we recorded the abundance-dominance of all vascular plant species with a scale of 7 levels (Braun-Blanquet, 1932): absence, rare and cover <5%, abundant and cover <5%, 5 < cover < 25%, 25 < cover < 50%, 50 < cover < 75%, 75 < cover < 100%. We also estimated the number of seedlings or saplings in each plot. On both sites, we tested a silvicultural technique for limiting access to seedlings and saplings. This method consisted in leaving woody debris (tree tops) after logging to protect the natural regeneration (seedlings and saplings) from deer browsing. The area covered by woody debris was approximatively 1 m2 and the height was 1 m.

431

2005 2006 2008

2005 2006 2008

2005 2006 2008

F

Site P

Woody and herbaceous species H 3 25.2 0.012 N 3 28.8 0.010 H 3 8.5 0.056 N 3 5.8 0.091  3 2.0 0.290 H N 3 13.4 0.030 Woody species 3 4.8 0.114 H N 3 9.3 0.050 3 0.3 0.836 H N 3 38.4 0.007  3 0.7 0.612 H N 3 1.9 0.298 Herbaceous species 3 11.9 0.036 H N 3 34.4 0.008 3 2.9 0.200 H N 3 1.9 0.311  3 1.1 0.472 H N 3 8.8 0.054

df

F

P

1 1 1 1 1 1

1.0 0.0 0.6 3.4 1.0 0.4

0.382 0.908 0.507 0.164 0.390 0.587

1 1 1 1 1 1

3.6 25.4 7.6 72.9 1.0 4.6

0.153 0.015 0.071 0.003 0.382 0.120

1 1 1 1 1 1

2.7 5.9 0.6 0.0 2.1 10.7

0.198 0.094 0.492 0.979 0.241 0.047

P-values < 0.05 are in bold type.

3. Results

2.4. Data analysis

3.1. Plant species diversity and richness

We estimated plant diversity and richness using the Shannon’s diversity index (H ; Shannon and Weaver, 1949) and species richness (N, the total number of plant species recorded in the homogeneous plot), respectively, for all plots and the following functional plant groups: woody species, herbaceous species, and all species combined. One-Way and Two-Way ANOVAs were used to determine whether diversity and richness of vegetation differed between the four treatments: exclosures – cleared of woody debris, exclosures – with woody debris, control plots – cleared of woody debris, and control plots – with woody debris; and according to the site. We used Tukey’s Honest Significant Difference (HSD) to determine which treatment(s) was(were) different. We performed separate analyses for each study year to assess variation through the study period. We then tested for differences in species composition between the four treatments, the two sites and the 3 years of the study, using permutational multivariate analysis of variance using a distance matrix (“permutational MANOVA”, formerly “nonparametric MANOVA”; Anderson, 2001). We used percent cover of each species per plot to compute an analysis of variance on a Horn–Morisita distance matrix (Horn, 1966; Chao et al., 2006). To classify species in palatable, unpalatable species or preferred species, we used results of studies on diet composition and selectivity of roe and red deer (Tixier and Duncan, 1996; Storms et al., 2008). Finally, we estimated the average abundance (i.e., average number per plot) of seedlings and saplings of Abies alba and Quercus spp., by site (1 and 2), for the four treatments and for each study year. One-Way ANOVAs and Tukey’s Honest Significant Difference were used to determine the differences between the treatments. As with diversity and richness, we performed separate analyses for each study year. The whole analysis was performed using the software R (R Development Core team, 2008).

We compared Shannon’s diversity index and species richness between the four treatments on the two sites 1 and 2 (Table 1 and Fig. 1). The treatment significantly affected richness of woody species, herbaceous species and all species combined, and diversity of herbaceous species and all species in 2005. Only richness of woody species in 2006 and richness of all species combined in 2008 were influenced by the treatment (Table 1 and Fig. 1). We found a significant effect of the site on richness of woody species in 2005 and 2006 (site 1 > site 2) and richness of herbaceous species in 2008 (site 1 < site 2) (Table 1 and Fig. 1). During the first year of the study (2005), richness of woody and herbaceous species and diversity of herbaceous species significantly differed between exclosures and control plots (Fig. 1), underlining a negative effect of the presence of deer: exclosures – cleared of woody debris > control plots – cleared of woody debris (ANOVA and Tukey’s HSD, p = 0.072 and 0.078 for N of woody and herbaceous species); and exclosures – with woody debris > control plots – with woody debris (ANOVA and Tukey’s HSD, p = 0.008 and 0.033 for N and H of herbaceous species). Differences in diversity and species richness between exclosures and control plots were higher in plots with woody debris than in those without protection, except for richness of woody species (Fig. 1; difference in plots with woody debris/without protection = 0.40/0.25 and 0.45/0.15 for diversity of woody and herbaceous species, 0.33/0.47 and 1.13/0.50 for richness of woody and herbaceous species). Differences between exclosure and control plots decreased through the study period owing to an increase, especially from 2005 to 2006, in species diversity and richness (especially among herbaceous species) in control plots (Fig. 1). In fact, between 2005 and 2006, several herbaceous species occurred in the control plots (cleared or with woody debris) and some woody species occurred in the cleared control plots, whereas we found no new

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woody debris), with the species changing according to the site (Fig. 2): Carpinus betulus, Anemone nemorosa (site 1, Fig. 2A and C), Rubus fructicosus, Rubus idaeus (sites 1 and 2, Fig. 2A and B) and Epilobium angustifolium (site 2, Fig. 2D). Some other species were also more abundant in exclosures than in control plots: Galeopsis tetrahit, Milium effusum, Moehringia muralis, Poacea spp. (site 1, Fig. 2C); Acer spp., Carex spp., Festuca spp. and Mycelis muralis (site 2, Fig. 2D). In contrast, some herbaceous species were more present in control plots: Brachypodium spp., Luzula spp. (site 1, Fig. 2C) and Lamium spp. (site 2, Fig. 2D). Most of the woody species present in plots without woody debris were absent or rare in both exclosures and control plots with woody debris (Fig. 2A): Cytisus scoparius, Picea abies, Pinus sylvestris, Pseudotsuga menziesii, Quercus spp. (site 1). Some herbaceous species were also rare in plots with woody debris: Brachypodium spp. (sites 1 and 2) and Epilobium angustifolium (site 2) (Fig. 2C and D). Three nitrophilic and hygrocline herbaceous species occurred in exclosures with protection: Circaea lutetiana (site 1), Scrophularia spp. (site 2) and Urtica dioica (sites 1 and 2) (Fig. 2C and D). 3.3. Abundance of key species’s seedlings/saplings

Fig. 1. Mean values for the two sites (1/2) of (A) Shannon’s diversity index H and (B) Species richness N, for the four treatments: exclosures – cleared of woody debris (in white), exclosures – with woody debris (in ligth grey), control plots – cleared of woody debris and control plots (in dark grey) – with woody debris (in black), for each year of the study and for different vegetation types: for all the species, for woody species and for herbaceous species. Standard deviations are represented by segments. * indicated significant differences between the treatments (treatment factor: p-value < 0.05 in Two-Way ANOVAs).

species occurrences during the study period in the exclosures (Fig. 2). In 2005 and 2006, there was no effect of woody debris in exclosures on diversity and richness of woody species and herbaceous species (exclosures – cleared of woody debris = exclosures – with woody debris) (ANOVA and Tukey’s HSD, 2005: p > 0.5 and 2006: p > 0.8 for H and N of woody and herbaceous species), whereas richness of woody species was higher in cleared control plots than in those with woody debris in 2006 (ANOVA and Tukey’s HSD, p = 0.007) (the same trend but less marked was found for richness of herbaceous species in 2005, ANOVA and Tukey’s HSD, p = 0.091) (Fig. 1). In 2008, this negative impact of woody debris persisted in control plots for richness of woody species, but was less important (ANOVA and Tukey’s HSD, p = 0.354) and occurred in control plots and exclosures on richness of herbaceous species (Fig. 1), although it was not significant (ANOVA and Tukey’s HSD, p = 0.127 in control plots and p = 0.383 in exclosures). 3.2. Plant community composition The analyse of values of similarity index between plots shown signficant effects of the treatment, the year and the site on species composition (“permutational MANOVA”, p < 0.01 for the three factors). The abundance of some preferred species, sensitive to browsing, was larger in exclosures than in control plots (cleared or with

In 2005, the abundance of Abies alba seedlings was lower in plots with woody debris compared to cleared plots (i.e., a negative effect of woody debris; ANOVA and Tukey’s HSD, p < 0.001), but did not differ between exclosures and control plots (i.e., no effect of deer presence) (Fig. 3A). In 2006, there was still a negative effect of woody debris (mostly in control plots) (ANOVA and Tukey’s HSD, p < 0.001), but also the occurrence of a positive effect of deer presence since the number of Abies alba seedlings was higher in control plots than in exclosures (ANOVA and Tukey’s HSD, p = 0.002), especially in plots without woody debris (Fig. 3A). In 2008, the high increase in the abundance of Abies alba seedlings in cleared control plots increased the difference between the two types of control plots, cleared and with woody debris. This negative effect of woody debris also occurred in exclosures but was less marked (difference cleared/with woody debris in control plots = 10.8 and in exclosures = 1.9). There was still a positive effect of the presence of deer on the number of Abies alba seedlings (ANOVA and Tukey’s HSD, p < 0.001), especially in cleared plots where the effect was more marked than in 2006 (difference cleared exclosure/control plot in 2006 = 4.4 and in 2008 = 9.6) (Fig. 3A). The abundance of Quercus spp. seedlings was approximatively the same between the four treatments in 2005 (ANOVA, p = 0.838) and in 2008 (ANOVA, p = 0.433) (Fig. 3A). A very small positive effect of deer occurred in 2006 (ANOVA, p = 0.066) (Fig. 3A). The abundance of Abies alba saplings was similar in 2005, in cleared exclosures and in those with woody debris, and in control plots and exclosures cleared of woody debris (Fig. 3B). In contrast, it was higher in cleared control plots than in those with woody debris, showing a negative effect of woody debris (Fig. 3B), although difference was not significant (ANOVA and Tukey’s HSD, p = 0.470). In 2006, we found two trends (not significant differences): a small negative effect of woody debris, for both control plots and exclosures, and a small negative effect of the presence of deer, for both cleared plots and those with woody debris, due to an increase in sapling abundance in cleared exclosures and in control plots with woody debris (Fig. 3B). In 2008, an increase in the number of Abies alba saplings in cleared exclosures and in control plots both cleared and with woody debris led to higher values in cleared plots and in those with woody debris, for both exclosures and control plots (ANOVA, p = 0.015), and led to similar values in exclosures and control plots, for both cleared plots and those with woody debris (ANOVA, p = 0.783). The abundance of Quercus spp. saplings in 2005 was lower in control plots compared to exclosures (ANOVA, p = 0.034), but did

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Fig. 2. Relative abundance of woody species (A and B) and herbaceous species (C and D) by site (1 and 2) for the four treatments: exclosures – cleared of woody debris (in white), exclosures – with woody debris (in ligth grey), control plots – cleared of woody debris and control plots (in dark grey) – with woody debris (in black). * indicated that species occurred in 2006 in control plots (i.e., it was absent in 2005).

not differ between cleared plots and those with woody debris (ANOVA, p = 0.602) (Fig. 3B). In 2006, due to an increase in sapling abundance in control plots with woody debris, we found two trends (not significant differences): a small negative effect of woody debris in control plots and a small negative effect of deer presence for cleared plots (Fig. 3B). In 2008, there was a negative effect of deer presence for both cleared plots and those with woody debris (ANOVA, p = 0.077), and a small negative effect (not significant) of woody debris for both exclosures and control plots (ANOVA, p = 0.208), due to an increase in the number of Quercus spp. saplings

in exclosures both cleared and with woody debris and in cleared control plots (Fig. 3B). 4. Discussion 4.1. Diversity, richness and abundance of vegetation This study demonstrated that the deer effect on species richness and diversity of forest vegetation was not constant throughout the study period. In the first year of the experiment, we found

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Fig. 3. Average abundance (i.e., average number per plot) of (A) Seedlings and (B) Saplings of oak (Quercus spp.) and silver fir (Abies alba), by site (1 and 2), for the four treatments: exclosures – cleared of woody debris (in white), exclosures – with woody debris (in ligth grey), control plots – cleared of woody debris and control plots (in dark grey) – with woody debris (in black), for each year of the study. P* indicated significant differences between the treatments “exclosure” and “control plot” (pvalue < 0.05 in One-Way ANOVAs), W* indicated significant differences between the treatments “cleared of woody debris” and “with woody debris” (p-value < 0.05 in One-Way ANOVAs) and PW* indicated significant differences between the four treatments (p-value < 0.05 in One-Way ANOVAs).

a strong and immediate negative effect of the presence of deer on both species richness and diversity in plots without protection (cleared of woody debris), as predicted in our first hypothesis H1, and also in those with woody debris, contrary to our expectation that natural protection by woody debris would limit browsing H1. These results on roe and red deer corroborate those of Tighman (1989) and Horsley et al. (2003) on white-tailed deer. They respectively found that browsing at high deer densities (≥18 deer/259 ha; i.e., ≥7 deer/km2 ; Tighman, 1989) reduced the diversity of tree seedlings and that richness and diversity of tree species were negatively related to deer density (in Horsley et al., 2003: Shannon’s diversity index of trees varied from 1.5 at 4 deers/km2 to 0.75 at 25 deers/km2 ). In our case, the difference in species diversity and richness between exclosures and control plots was maximal during the first year of the experiment, indicating a strong immediate effect of fencing excluding deer on the vegetation. The reduction of the difference the following years revealed an attenuation of the deer browsing impact on species diversity and richness. After 3 years, plant diversity and richness were similar in control plots and in exclosures. This attenuation was due to an increase, especially in the second year, of both woody and herbaceous species diversity and richness in control plots. In fact, between 2005 and 2006, several woody and herbaceous species occurred in control plots whereas we found no new species occurrences during the study

period in the exclosures. This high turnover of species in control plots may have been caused by disturbances due to the presence of deer, which were prevented by fencing in exclosures. Due to these new occurrences of species in control plots, the presence and abundance of some species differed between exclosures and control plots. Exclosures (cleared or with woody debris) contained a higher abundance of preferred species such as Carpinus betulus, Rubus fructicosus, Rubus idaeus, Anemone nemorosa (only present in exclosures) and Epilobium angustifolium and palatable species such as Acer spp., Carex spp., Festuca spp. and Mycelis muralis, whereas some grasses, Brachypodium spp. and Luzula spp., and unpalatable herbs, Lamium spp., were more present in control plots. Hence, deer browsing affected species composition of plant communities by decreasing the presence of highly preferred or palatable species and favouring unpalatable species or species resilient to browsing, in accordance with our second hypothesis H2. Our results showed a greater abundance of Rubus spp., a highly preferred species, in sites without deer which is consistent with the findings of Tilghman (1989), Horsley et al. (2003) and Casabon and Pothier (2008). Concerning herbaceous species, the diversity was lower in control plots (with deer) than in exclosures but the abundance of grasses, browse-tolerant species, was higher in plots with deer, which is consistent with previous studies on deer browsing impact (Horsley et al., 2003; Rooney and Waller, 2003; Tremblay et al., 2006; Casabon and Pothier, 2008; Mudrak et al., 2009; Rooney, 2009). This result suggests an apparent competitive gain by grasses under browsing presure (Augustine and McNaughton, 1998; Horsley et al., 2003; Rooney and Waller, 2003; Tremblay et al., 2006). Growth of grasses may also be stimulated by grazing by deer. As growth takes place from basal meristems, which are protected from herbivory, regrowth can start immediately after damage without activation of new meristems (Georgiadis et al., 1989; Moser and Shütz, 2006). This higher abundance of grasses in presence of deer may also be due to the effectiveness of the seed dispersal by these animals of some plant species such as grasses or small herbs (Gill and Beardall, 2001). Contrary to findings of most of these studies, sedges (Carex spp.) were not more abundant in presence of deer. Though not much consumed by deer, they were more abundant in exclosures (without deer). This may due to a competition with abundant grasses in the exclosures. In 2008, our results revealed a negative impact of the use of woody debris on richness of herbaceous species, in exclosures, as predicted in our first hypothesis H1, and also in control plots on richness of woody and herbaceous species, contrary to our prediction H1. This negative effect of woody debris was consistent with the negative correlation between slash abundance and woody diversity and richness found by Krueger and Peterson (2009). The negative impact of woody debris in control plots occurred from 2005 to 2008. Instead of limiting deer impact, the use of woody debris increased the effect of deer browsing in control plots relatively to those without protection. In fact, the abundance of preferred species such as Carpinus betulus, Rubus fructicosus and Rubus idaeus was lower in control plots with protection than in those without, in the beech wood/fir plantation site (site 2). In contrast, exclosures with woody debris had a higher abundance of Anemone nemorosa in this site, and a higher abundance of Rubus fructicosus and Rubus idaeus in the beech wood/oak grove site (site 1), than exclosures without protection. In all cases, use of woody debris increased the difference in plant diversity and abundance between exclosures and control plots instead of decreasing it (as predicted in our hypothesis H1). In absence of deer (exclosures), woody debris may be considered as microsites that provide a microclimate (light, temperature, wetness) to saplings and permit a better regeneration, which explains the higher abundance of some species such as Anemone nemorosa and Rubus spp. in exclosures with protection than in those without. In control plots with deer,

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in a poor environment such as la Petite-Pierre Reserve where preferred/palatable species were not very abundant, we may assume that saplings just emerging from woody debris are more visible to deer, leading to a stronger browsing pressure on these young trees instead of protecting them from deer. Finally, the appearance of nitrophilic/hygrocline herbs in plots with protection (Circaea lutetiana, Scrophularia spp. and Urtica Dioica) was due to woody debris that made soil more wet and hence appropriate for the establishment of these species. The negative effect of deer on the abundance of vegetation in la Petite-Pierre Reserve is conditioned by habitat type. In fact, species that were more abundant in exclosures or in control plots differed according to the site. Deer browsing reduced Carpinus betulus and Anemone nemorosa abundance in the beech wood/fir plantation site (site 2) and the abundance of Acer spp., Carex spp. and some herbs (Epilobium angustifolium, Festuca spp. and Mycelis muralis) in the beech wood/oak grove site (site 1). Rubus spp. was affected by deer browsing in both site types, suggesting that this species is a good indicator of deer impact. Deer browsing rate on Rubus has been used to monitor deer impact in a French forest (Morellet et al., 2001). The differences in plant communities occurring in the sites and in site characteristics (light, ground composition) conditioned the effects of deer on the vegetation (Gill and Beardall, 2001; Palmer et al., 2004). 4.2. Effectiveness of woody debris as protection for regeneration of Abies alba and Quercus spp. Impact of deer browsing was very different on seedling and sapling abundance, whereas the use of woody debris always decreased it. The presence of deer seemed to positively affect the number of seedlings whereas the use of woody debris had a negative impact. These effects were particularly marked for seedlings of Abies alba. The negative effect of woody debris occurred thoughout the study period, especially in control plots, whereas the deer effect was less immediate since it appeared in the second year. These two effects on seedlings became more marked with time. The higher number of seedlings in control plots, cleared or with woody debris, is in contradiction with our third expectation H3. This could be explained by the effectiveness of seed dispersal of some plant species by these animals, via the coat, clog or feces (Malo and Suarez, 1995, 1998), and also by the modifications of the soil due to trampling which may improve seed germination and thus increase the abundance of seedlings. In exclosures or in control plots, woody debris seemed to modify environmental variables for plant species, e.g. by reducing light and increasing soil humidity, and reduced the regeneration of some species such as Abies alba, corroborating our third prediction for exclosures and rejecting it for control plots H3. Moreover, in control plots with deer, we may assume that woody debris limit the access for deer and thereby prevent the beneficial effects of seed dispersal and trampling by deer. Contrary to seedling abundance, the number of saplings was negatively affected by deer browsing, for both Abies alba and Quercus spp., as predicted in our third hypothesis H3. For Abies alba, this negative impact of the presence of deer occurred in the second year of the experiment and then disappeared, whereas for Quercus spp. it occurred throughout the study period. This difference in deer impact between seedlings and saplings may be due to the greater height of saplings compared to seedlings, making them more visible and thus more sensitive to browsing by deer. As with seedlings, the use of woody debris decreased the abundance of saplings, in both exclosures and control plots, and this effect became stronger with time. As with seedlings, this negative effect seemed to be due to modifications of environmental variables that reduced the regeneration of Abies alba and Quercus spp., in accordance with our third

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hypothesis for exclosures and in contradiction to it for control plots H3. In control plots with deer, as for plant species abundance (see Section 4.1), saplings just emerging from woody debris were very attractive to deer, leading to a stronger browsing pressure on these young trees instead of protecting them from deer. Problems caused by deer damage are well known to foresters who exploit a variety of techniques for the local control of deer impact: use of individual plastic tube or wire fencing (efficient but costly), electric fencing (less expensive but less efficient), repellents (e.g., predator urine) (Côté et al., 2004). The use of woody debris as protection in a poor environment such as la Petite-Pierre Reserve did not prove to be an effective technique for limiting deer browsing on young trees and permitting regeneration, as in the study conducted by Fredericksen et al. (1998) on the use of slash against browsing by white-tailed deer on seedlings and saplings of several tree species, and in Kupferschmid and Bugmann (2005)’s study on the use of logs to protect saplings of Picea abies from browsing by ungulates. Our results were not consistent with those of more recent studies that underlined the effectiveness of woody debris as protection of seedlings and saplings from browsing by ungulates. de Chantal and Granström (2007) showed that aggregations of dead wood, formed by windthrow or fire-killed trees, limited browsing by moose (Alces alces L.) and roe deer (C. capreolus) on seedlings of Populus tremula and Salix caprea. Casabon and Pothier (2007) and Hunn (2007) found that logging debris protected tree seedlings from browsing by white-tailed deer, and Relva et al. (2009) showed that red deer and roe deer use was limited by fallen logs, allowing palatable species to establish and grow. A similar technique was tested by Rammig et al. (2007) in a simulation model. They evaluated the effects of two management strategies on forest succession and structure, i.e., clearing the fallen logs or leaving the sites untouched (“uncleared”) after the heavy storm “Vivian” (in 1990). They found that uncleared sites had a higher potential to recover from high browsing pressure due to the high amount of favourable microsites that are provided by decaying logs, whereas at cleared sites, ungulate browsing delayed tree regeneration. Hence, it would be interesting to test the woody debris technique in richer environments than la Petite-Pierre Reserve, where highly palatable species are relatively abundant, to determine whether woody debris may provide favourable microsites for regeneration under deer pressure. We could also modify the height and the cover of woody debris to test their effect on seedling and sapling abundance.

5. Conclusion Our results suggest that even in a site with medium deer number (0.53 roe deer and 0.80 red deer seen/km in la Petite-Pierre Reserve), deer browsing has an effect on temperate forest vegetation by changing species composition. Moreover, the regeneration of oak and silver fir, two main timber species, seemed to be affected by deer, in a positive way for seedlings and a negative one for saplings. The use of woody debris as protection was not effective and had a higher negative effect than deer presence on diversity, species composition and regeneration. To evaluate the consequences of an increase in browsing pressure on the two main timber species, future research should use control plots in which deer densities are controlled at various levels. Moreover, in addition to seedlings and saplings, it would be interesting to take into account stump sprouts to estimate the regeneration of oak under deer pressure as regeneration from stump sprouts are preferable for most hardwood species. Finally, in this study we treated the short-term effect of deer on vegetation since our experiment covered only three years after fencing. Further years would be required to assess the deer impact over the medium and long term and underline potential changes in plant succession.

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Acknowledgements The study was conducted by the Office National de la Chasse et de la Faune Sauvage in collaboration with the Office National des Forêts. We thank Julie Lorand and Aurélie Martzoff for the field work. We are most grateful to Jean-Michel Gaillard, Anders Mårell, Vincent Boulanger and Franc¸ois Klein for constructive comments on the manuscript. We thank Catherine Carter for revising the English manuscript.

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