| Chapitre 1 Long-term effects of white-tailed deer on the structure and composition of heavily browsed boreal plant communities on Anticosti island | ||
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| Chapitre 1 Long-term effects of white-tailed deer on the structure and composition of heavily browsed boreal plant communities on Anticosti island | ||
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Table des matières
Browsing by large herbivores may directly and indirectly alter forest structure (Frelich and Lorimer 1985, Koh et al. 1996, Virtanen et al. 2002), and/or plant community composition (Augustine and Frelich 1998, Crête et al. 2001, Kirby 2001). At a fine scale, herbivores affect individual plants on which they feed. Herbivory may reduce plant growth, survival, competitive ability and ultimately alter plant fitness (Putman et al. 1989, Crawley 1997, Rooney 1997, Augustine and Frelich 1998). Although some plant species may compensate for tissue loss (McNaughton 1983, Belsky 1986, Paige and Whitham 1987, du Toit et al. 1990), this compensation becomes impossible when herbivore pressure reaches a certain level (Bowen and Vuren 1997, Stromayer and Warren 1997, Anderson et al. 2001). At a larger scale, ecological processes such as nutrient cycling, soil mineralization as well as litter quality and quantity also may be affected (Pastor et al. 1993, Hobbs 1996, de Mazancourt and Loreau 2000, Wardle et al. 2001). Through these direct and indirect effects, herbivores may ultimately alter forest succession (Davidson 1993, Bowers 1997) as illustrated by browsing by moose ( Alces alces ) on Isle Royale (Risenhoover and Maass 1987). In this case, the herbivore altered the pattern of succession through changes in nutrient cycling so that the conifers took over from deciduous species in forest stands (McInnes et al. 1992, Pastor et al. 1993).
Cervids have greatly increased in abundance over the last decades in many temperate ecosystems of Europe and America, as exemplifed by white-tailed deer in northeastern United States and more recently in southern Québec, Canada. White–tailed deer are selective browsers that feed on a wide variety of plants. They influence the abundance and diversity of preferred browsed species (Anderson and Katz 1993, Augustine and Jordan 1998, Anderson et al. 2001, Rooney 2001, Russell et al. 2001). This process usually results in the supression or elimination of more palatable or more sensitive species and competitive release of unpalatable or browse-tolerant species (Gill 1992, Augustine and McNaughton 1998, Wardle et al. 2001, Bergquist et al. 2003). The abundance of understory perennials such as Trillium grandiflorum , Clintonia borealis , Arisaema triphyllum, Maïanthemum canadense was found to be negatively correlated with intensity of deer browsing (Anderson 1994, Balgooyen and Waller 1995, Rooney 1997, Fletcher et al. 2001). The understory plant community may shift from diversified woody and herbaceous species to a reduced number of species dominated by ferns, grasses and woody plants that are browse-tolerant or not preferred by deer (Horsley and Marquis 1983, deCalesta 1997, Horsley et al. 2003). High densities of deer also have been shown to have cascading effects throughout ecosystems, adversely affecting other groups of species such as songbirds, small mammals or invertebrates (Putman et al. 1989, Baines et al. 1994, deCalesta 1994, Flowerdew and Ellwood 2001).
The paradigm concerning the management of forest resources, including wildlife, has changed markedly during the last decades. Today, managers have to take into account the multiple resources of a territory in their planning. In this context, the management of herbivore populations may represent a great challenge. On the one hand, deer can represent a threath for the conservation of biodiversity and forest regeneration but on the other hand, deer abundance can be an attractive asset for hunters and tourists year after year, being a major local economic activity in many instances. Integrated management decisions, whose main objective is to bring deer density to a level compatible with the conservation of the natural flora and maintenance of an economically profitable forest, have to deal with these socio-economic considerations.
The presence of white-tailed deer at a high density over 50 years in the boreal environment of Anticosti Island provided a unique opportunity to assess the impacts of high herbivore density on the vegetation. The presence of Mingan Archipelago 35 km to the north offers an accurate control site because Mingan and Anticosti were similar in terms of geology, climate and biology prior to deer introduction, and deer are absent from Mingan. The objectives of our study were: 1) to assess the long-term effects of white-tailed deer browsing on the structure and composition of plant communities in four habitat types of Anticosti Island: mature balsam fir stands, windfalls in balsam fir stands, fens and bogs, by comparing their vegetation with that of Mingan islands, and 2) to assess the potential of recovery of the herbaceous and shrub layers from prolonged high browsing pressure, based on an exclosures experiment.
Anticosti island is a 7943 km2 boreal forest area located in the Gulf of St. Lawrence (49°03’- 49°55’N and 61°45’-64°35’W). Anticosti lies beyond the northern limit of white-tailed deer distribution (Gaspé peninsula).
Some 200 deer were introduced in 1896 on Anticosti, a predator free island. The population increased and spread rapidly until the early 1930’s when the first major winter die-offs were reported. In summer 2001, the population was estimated at 127 000 deer (16/km2) (Rochette et al. 2003). The first negative effects of browsing by deer on the island were reported about 30 years after its introduction (Rousseau 1950, Pimlott 1963, Marie-Victorin and Rolland-Germain 1969). Several woody plants were reduced or completely extirpated and the shrub layer below 250 cm, which corresponds to most intensive browsing interval for white-tailed deer, was virtually eliminated (Pimlott 1954a, 1954b, Huot 1982, Potvin 1985). Nowadays, the most dramatic modification concerns the decline of balsam fir ( Abies balsamea ) stands. Before the introduction of deer, fir stands were estimated to cover 40 % of the island but they have decreased by 50 % over the last century (Potvin et al. 2000). Forest harvesting, insect outbreaks, windfalls and fires are responsible for opening the primitive forest, but the main cause of the decline is browsing by deer that prevent balsam fir regeneration.
Mingan is a 97 km2 archipelago about 85 km long. It is located 35 km north of Anticosti Island and is part of Park Canada network. It is composed of five major islands, Grande Île (25,4 km2), île à la Chasse (16,5 km2), île du Havre (8,6 km2), île Quarry (8,3 km2) and île Niapiskau (6,3 km2) and about thirty smaller islands.
We used the Archipelago of Mingan as a control site because it is similar to Anticosti in terms of environmental conditions. The underlying bedrock is mainly composed of Ordovician and Silurian limestomes that derived from the same original shelf running along the north shore of the Gulf of St. Lawrence to Newfoundland. The channel between Mingan and Anticosti is due to erosion (Marie-Victorin 1938). Both sites benefit from the same type of sub-boreal climate with mild, wet and windy weather for much of the year due to the marine influence. The mean temperature ranges from –14°C in February to 15°C in July. The climate is characterised by cool summers and mild winters with abundant snow falls compared to the mainland. Finally, Anticosti Island and Mingan islands form one biological unit that differs markedly from the Gaspé Peninsula, the nearest mainland to the south, and from the North shore of the St. Lawrence (Roberge 1996).
The natural vegetation is typically boreal on both sites, with extensive stands of balsam fir and black spruce ( Picea mariana ) dominating the landscape. On Anticosti, white spruce ( P. glauca ), which is far less palatable to deer, is currently replacing balsam fir. Peatlands, dominated by ericaceous species, and black spruce and tamarack ( Larix laricina ) shrubs, are also abundant throughout both areas. On Anticosti, deciduous species such as paper birch ( Betula papyrifera ), trembling aspen ( Populus tremuloides ) and balsam poplar ( P. balsamifera ) are mostly found in the tree stratum only. The forests of Mingan islands are more diversified with maples ( Acer spp. ), American mountain ash ( Sorbus americana ), Canada yew ( Taxus canadensis ) and red-osier dogwood ( Cornus stolonifera ) in the shrub layer (Dryade 1986a). Many botanists described the native and unique flora of Anticosti and Mingan (Marie-Victorin and Rolland-Germain 1969, MTF 1975, Grondin et al. 1983, Grondin et al. 1986) and many of them considered the two sites as a same ecological unit named Anticosti-Minganie (Marie-Victorin and Rolland-Germain 1969).
Two types of peatlands are present: fens, dominated by sedges, herbaceous plants and ericaceous dwarf shrubs ( Sanguinaria canadensis, Sarracenia purpurea, Potentilla fruticosa, Myrica gale, etc. ), and bogs, dominated by cloudberry ( Rubus chamaemorus ), black crowberry ( Empetrum nigrum ) and Vaccinium spp . The vegetation of the former is generally more diversified because of the higher mineral content of the water percolating through it (Payette and Rochefort 2001).
Field surveys were conducted from late June to late August 2001 in the western part of Anticosti and on five islands of the Mingan Archipelago (île du Havre, île du Fantôme, île Niapiskau, île Quarry, Grande Île) selected for their large size, their similarities with Anticosti in terms of forest composition and their accessibility (Fig. 1). We separated our sampling effort among four natural habitats of different ecological value for deer on Anticosti: mature fir stands because of their high utilization by deer in winter, recent (< 10 years) windfalls because they represent fir stands at an early stage of succession, and fens and bogs for their extended coverage on Anticosti and because they are also used by deer (22%). We sampled three or four replicates for each of these habitats in each site (Anticosti and Mingan). Three replicates were sampled for bogs because of their homogeneity among and within study sites. We only sampled three replicates for windfalls also because of their scarcity on Mingan. Overall, a total of 14 areas were therefore surveyed in each site (4 mature fir stands, 4 fens, 3 bogs and 3 windfalls). Coniferous trees occupied more than 75% of the total basal area in mature fir stands (MFS), and balsam fir composed at least 75% of the basal area of coniferous trees. MFS and windfalls (W) were selected based on their age (MFS³ 60 years and W£ 10 years), height (MFS³ 12 m and W£ 4 m) and density (MFS and W, balsam fir cover³ 60%) as described on forest maps. These variables are the most accurate available on forest maps (Dussault et al. 2001). We used forest maps from the ministère des Ressources naturelles du Québec obtained from the interpretation of aerial photographs taken in 1998 and 1999 (Québec 1999). We also selected the fens and bogs from forest maps. No criterion was used for bogs because of the great similarity among sites, but we selected fens whose black spruce cover did not exceed 10% because this habitat type is avoided by deer. We randomly selected the habitats to survey among a series of easily accessible potential sites which had the required characteristics. In each of the selected site, we randomly positioned 20 sample points using the software ArcView GIS 3.2 (ESRI inc., Saratoga, Calif.). We used 20 m as the minimum distance between sample plots and between sample plots and habitat edges. These specifications also imposed a minimal area required for each habitat.
At each sampling point, we determined the species composition and relative abundance of woody and herbaceous vegetation including mosses and lichens. Specimens were identified to the species level in most cases, except for 17 genera ( Amelanchier, Aster, Carex, Eriophorum, Galium, Goodyera, Hieracium, Juncus, Ribes, Salix, Solidago, Spiranthes, Stellaria, Thalictrum, Trifolium, Urtica, Viola ) and six broader groups (grasses, ferns, mosses, lichens, horsetails and lycopods). The size of our plots differ between the stands according to the type of habitat and the layer sampled. The ground cover of shrubs and ericaceous species was estimated in 1 x 10 m plots in mature fir stands and in 1 x 5 m plots in windfalls and peatlands whereas the cover of herbs, ferns and mosses was estimated in 1 x 1 m plots. We counted tree seedlings by height class ( 1 : 0-5cm, 2 : 5-10 cm, 3 : 10-30 cm) in a 4 m2 circular plot. We visually estimated percent ground cover (vertical projection of the plant species to the ground) for each species according to the following cover classes: trace,<1%, 1-5%, 5-10%, 10-15%, 15-25%, 25-35%, 35-45%, 45-55%, 55-65%, 65-75%, 75-85%, 85-90, 90-95% and 95-100%. We also estimated the mean height of each vascular species using four classes: 1 : 0-10 cm, 2 : 10-30 cm, 3 : 30-60 cm, 4 : 60-100 cm in each plot.
We determined the composition of the tree layer by estimating the basal area (m2 . ha-1) of all tree species with a factor 2 metric wedge prism in each plot. The diameter at breast height (DBH) of trees was also recorded which allowed us to estimate stem density (ha-1) by species (Grosenbaugh 1952). Trees <10 cm at DBH were not recorded because the factor 2 wedge prism is not accurate for such small trees. We measured the height and age of two representative trees in 50% of the plots. Finally, we estimated the canopy closure (%) by counting the number of points every 1 m along a 20 m transect that overlapped with the canopy (Vales and Bunnell 1988, Potvin et al. 1999).
We estimated the intensity of browsing on each available woody stem between ground level and 200 cm according to these classes: 0% (none of the twigs browsed), 1-25%, 25-80%, 80-99% and 100%. Browsing by deer was easily distinguished from the sharp, angled cut browsing by the snowshoe hare ( Lepus americanus ), the only other browser inhabiting the study sites. We counted the number of pellet groups (old and recent ones) for deer and individual pellets for hare in each larger plots to obtain a relative index of habitat use by theses herbivores.
At least, we used four pairs of 4 m2 exclosures and control plots established in 1996 in mature fir stands of Anticosti to assess vegetation changes following the removal of deer browsing pressure. These exclosures were located in the same fir stands that were sampled, allowing the comparaison of the vegetation inside the exclosure with the plants outside and on Mingan islands.
We calculated species richness (number of species) and Shannon diversity index (H) (Magurran 1988) by habitat type and study site for the shrub and herbaceous strata:
where
is the proportion of cover for each species in the sample. We also used Morisita index (Horn 1966) as a measure of overlap between the two sites, habitat types and strata:
where S is the total number of species in both samples, species ‘i’ is represented xi times in population X and yi times in Y, and
is the Simpson’s index of diversity (Simpson 1949).
varies from 0 when the samples are completely distinct (no species in common) to 1 when the samples are identical (Morisita 1959). For the calculation of
and H, we only used species which occurred in more than 20% of the sampling plots and those covering on average more than 1% of the ground. All the statistical analyses were performed by habitat type because of the large differences in terms of species composition between these. We used the median of each cover and height classes as dependant variables. We used permutation tests (Legendre and Legendre 1998; pp 17-26) to compare diversity indexes (species richness and Shannon index), and height between study sites, by habitat types and strata. The permutation test is similar to an exact test but does not postulate any distribution of data. We calculated a browsing index by combining the occurrence and intensity of browsing to assess deer plant preference on Anticosti. We used a Kruskall-Wallis ANOVA to determine wether the number of herbivore pellets differed among habitat types and another non parametrical comparison analyses to test if there were differences between habitats (Sherrer 1984; pp 540-549). Statistical analyses were conducted with Systat (version 9.0) and SAS (version 8.2). A rejection level of 0.05 was used in all tests.
A total of 280 plots were sampled on Anticosti and on Mingan in an equal number of replicates for each site. We found a total of 86 and 84 kinds of vascular plant species (each vascular plant identified to genera and grasses accounted for one species) on Anticosti and Mingan respectively, regardless of habitat type and stratum (Morisita index = 0.84). However, the total number of species recorded on the two sites was 114, a first indication that species composition differed between sites.
For all species, there were differences in presence/absence among habitats, so that species seem to be characteristic of the habitat type. The results are therefore presented by habitat type and stratum because composition differed markedly among habitat types and differences between study sites varied among strata.
Except for total stem density and basal area, tree species composition of mature fir stands did not differ much between Anticosti and Mingan. Balsam fir accounted for > 80% of the total tree proportion in all fir stands. Other tree species were Betula papyrifera, Picea glauca, Populus tremuloides and Sorbus americanus . Picea glauca was recorded only on Anticosti whereas Sorbus americanus was found only in Mingan. Stand structure however differed between the two sites (Table 1) because of the higher proportion of small fir trees (10 ≤ DBH ≤ 20 cm) in Mingan (Mingan average density = 2,200 stems/ha and Anticosti average density = 460 stems/ha; t =-2.38; P = 0,03). On average, trees with DBH ≥ 10 and < 20 cm accounted for 87% and 64% of the total tree density on Mingan and Anticosti, respectively (Fig. 2). The same trend for smaller trees to be less abundant on Anticosti than on Mingan also existed within the 10-20 cm DBH tree class.
No shrubs were recorded in the plots located in the fir stands on Anticosti (Table 2) whereas shrub species commonly found in the boreal forest were present in Mingan. The number of fir seedlings of 0-5 cm (P = 0.1) and 5-10 cm (P = 0.7) did not statistically differ between the two sites. However, fir seedlings of 10-30 cm were over 100 times more abundant in Mingan (P < 0.01) (Table 3).
The diversity of herb species (t = 3.08, P = 0.02 with HAnticosti = 1 and HMingan = 0.8; Fig. 3) was higher on Anticosti than on Mingan. Ground cover of Trientalis borealis (P = 0.5) and Maïanthemum canadense (P = 0.2) did not differ between Anticosti and Mingan. Even if the occurrence of Cornus canadensis in the sample plots did not differ between the two sites (Table 2), ground coverage of the species was higher in Mingan (P = 0.04). Cover of Linnaea borealis , a common but small herb species also did not differ between sites (P = 0.5; Table 2). Clintonia borealis was detected only in Mingan (Table 2). Finally we found no difference in height for any species between sites.
Ground cover of ferns (P = 0.4) and mosses (P = 0.3) did not differ between Mingan and Anticosti fir stands.
Because we selected severe windfalls, trees were scarce or completely absent in these areas. In the shrub layer, we observed the same pattern as in mature fir stands, with very few woody species on Anticosti; white spruce was dominant, but balsam fir and white birch were also found on the island (Table 4). White spruce was not recorded in the windfalls of Mingan (Table 4) where balsam fir was the most abundant shrub species, followed by Rubus idaeus and Ribes sp.. Balsam fir seedlings recorded on Anticosti did not exceed 30 cm high (Table 3). Large woody debris (fallen trees) apparently protected balsam fir from browsing. The number of fir seedlings recruited to the 10-30 cm class was more abundant in windfalls than in mature fir stands on Anticosti (t = -2.21, P = 0.02) because they were protected by fallen trees. In the herbaceous stratum, we observed the same trends in windfalls as those observed in mature fir stands, with a higher diversity on Anticosti (t = 4.94, P < 0.01 with HAnticosti = 0.978 and HMingan = 0.445; Fig. 3). As in mature fir stands, some herb species were found only on Anticosti ( Cirsium arvense, Urtica sp., Rumex acetosella, Cerastium vulgare ) whereas other herb species were found only on Mingan ( Clintonia borealis, Solidago sp., Aralia nudicaulis ) (Table 4). We did not detect differences in height and cover of all herb species present in both sites except for Linnaea borealis and Cornus canadensis for which ground coverage was higher on Mingan (t = -3.95, P < 0.01 and t = -2.94, P = 0.02).
The most common shrub and ericaceae species in fens and bogs were Potentilla fruticosa , Betula pumila , Myrica gale , Kalmia angustifolium , K. polyfolia, Ledum groenlandica , Cassandra caliculata and Andromeda glauca . These species were found in both sites, with similar occurrence and ground cover (Tables 5 and 6). However, species commonly found in fens and bogs ( Potentilla fruticosa , Betula pumila, Myrica gale and Kalmia polyfolia ) were also eaten by deer on Anticosti (Table 7).
We found no difference in cover and height for herb species between sites in fens and bogs. Finally, ground cover of lichens was greater in bogs of Mingan compared to Anticosti (50% and 17%, t =-3.21, P = 0.04).
Species such as Abies balsamea, Rubus idaeus, Sorbus americanus recovered within five years when protected from deer browsing (Table 8). Indeed, while these species were not found in the control plots accessible to deer, all were found in the fir stands of Mingan. Cirsium sp . and grasses were recorded only in browsed plots of Anticosti where as Taxus canadensis and Goodyera sp. (Orchidaceae) were found only in Mingan.
Windfalls are the most intensively used habitat by deer, followed by mature fir stands, bogs and fens (Z = 76.9, P < 0.01; Table 9). Pellet of hares were very scarce compared to deer pellet groups on Anticosti. We did not find differences between number of pellets of hares between Anticosti and Mingan. Estimation of browsing shows that balsam fir was the most intensively browsed species (Table 7). Betula sp. were also systematically browsed by deer but most surprisingly, even shrub species such as Potentilla fruticosa, Myrica gale , or Kalmia spp ., commonly found in peatlands but not considered deer forage, were browsed with variable intensities in summer.
Mature balsam fir stands and recent windfalls were the habitats that differed most between Anticosti and Mingan in terms of vegetation. Deer browsing affected species abundance and composition in all height strata. At the species level, Abies balsamea, Rubus spp., Ribes spp., Clintonia borealis, Potentilla fruticosa and Betula spp. were the most affected species on Anticosti.
Saplings of all tree species were almost absent on Anticosti as compared to Mingan which explains why almost no balsam fir stand has become established since 1930 (Potvin et al. 2003). The shrub layer has also completely disappeared from the fir stands on Anticosti. Species such as Taxus canadensis, Acer spicatum, A. rubrum and Sorbus americanus, which are common on Mingan, have been extirpated or were severely reduced on Anticosti. Boreal woody plants vary greatly in terms of palatability to mammalian herbivores (Bryant and Kuropat 1980, Bryant et al. 1983, Bryant et al. 1989). Deciduous species are particularily affected by browsers because they are usually preferred over conifers. Because of deer selective herbivory, deciduous stems on Anticosti were present in conifer stands only in the tree stratum and we did not encounter any stems of deciduous species in the shrub layer in our 80 sample plots within mature balsam fir stands. The regeneration of balsam fir also is seriously hampered and even prevented in some locations (Potvin and Laprise 2002). On Anticosti, fir stands offer food and shelter to deer during winter (Huot 1982), but balsam fir is actually browsed all year-long (Potvin and Laprise 2002). Nevertheless, the density of small fir seedlings (0-5 and 5-10 cm) on Anticosti was comparable to Mingan which suggests that differences are not related to poor seed production or germination rate. The problem is rather aused by deer browsing which inhibits fir growth as indicated by the scarcity of balsam fir in the 10-30 cm height class in all habitat types. According to Horsley and Marquis (1983) and Horsley et al. (2003), deer browsing should indirectly benefits grasses, sedges and ferns and a dense ground cover by these species should further inhibit the establishment of tree seedling. We detected a greater abundance of grasses on Anticosti compared to Mingan. Ferns, however, did not show this trend in the western part of the island.
Herbaceous vegetation differed between sites with a higher diversity of herbs on Anticosti in the mature fir stands. The differences observed in understory forest composition and structure on Anticosti could be explained by the cumulative effects of absence of fir regeneration, the reduction in shrub density and more light reaching the ground in the mature balsam fir stands. The increased penetration of light through the canopy on Anticosti may at least partly explain why species richness was greater there. However, even if herbaceous plant diversity is greater on Anticosti, herb morphology may be altered, especially for preferred species. For example, stem and scape height of Trillium grandiflorum and Clintonia borealis have been reported to be negatively correlated with the intensity of deer browsing (Anderson 1994, Balgooyen and Waller 1995). Deer may affect the vigor and the reproductive success of browsed species by removing large flowering individuals (Anderson et al. 2001). With repeated browsing, plants become smaller and flowering may cease due to the reallocation of resources used for growth and reproduction (Anderson 1994, Shelton and Inouye 1995, Augustine and McNaughton 1998). In our study, we did not detect any difference in height of the different species between Anticosti and Mingan. Narrower height classes and an index of reproductive success (number of flowers and fruits) might have been more powerful to assess the effect of browsing in this stratum. We recommand also a measure of foliar area for such survey instead of a measure of height. Clintonia borealis , a typical boreal species, was not found in any of the sample plots on Anticosti. We suggest that this is apparently the result of high selective browsing by deer on this species (Balgooyen and Waller 1995). Other understory species such as Maïanthemum canadense , Cornus canadensis, Linnaea borealis or Trientalis borealis , did not seem to be affected as much as Clintonia borealis. However, species belonging to the same family, such as Maïanthemum and Clintonia may show strong differences in their reproductive and growth strategies and response to herbivory (Lamoureux 2002). Finally, some common small size boreal species: Coptis groenlandica, Oxalis montana and Mitella nuda were only recorded in our sample plots on Anticosti although these species are present on Mingan (Dryade 1986b). Such differences may be explained by local site characteristics or by a combination of their ability to tolerate browsing and a reduction of other competitive species.
The only balsam fir stems present in the shrub layer on Anticosti were in windfalls; although they were also severely browsed (100% of the twigs) and had a typical “bonsaï-like” morphology (Chouinard and Filion 2001). Normally, we should have found a dense balsam fir regeneration, after a disturbance such as a windfall in a mature balsam fir stand with a sufficient bank of seedlings. However, our results show a very low density of regeneration in this habitat type on Anticosti as compared to Mingan. A large proportion of seedlings are apparently protected from deer browsing by woody debris, but when they reach about 30 cm they become exposed to browsing during the snowfree period. Chouinard and Filion (2001) reported browsing rates of 37 % on seedlings <30 cm and 100% on saplings (>30 cm) in a semi-closed second growth site in Anticosti. Potvin et al. (2003) recently estimated that 26 % of the small fir seedlings are browsed in the forests during the snowfree period, so that very few can ever reach 30 cm.
The replacement of balsam fir by white spruce could be explained by a lower browsing rate on spruce. White spruce was absent from our sampling plots of windfalls on Mingan but was present at the shrub stage in mature fir stands. This suggests that balsam fir may have a natural competitive advantage over white spruce, which would prevents its germination or growth in closed fir stands (Lieffers et al. 1996). After a windfall, early succession plants find optimum sunlight conditions for germination and establishment. Herbs also differed between sites with a higher diversity in Anticosti. Contrary to fir stands where no difference was observed, we found differences in cover of Cornus canadensis and Linnaea borealis in windfalls. Because windfalls are more intensively used by deer than fir stands in summer, the impact on vegetation may be heavier. After 5-6 years, heliophilous herbs and woody species had colonized the windfalls on both sites. On Mingan, the absence of deer explains the presence of Rubus idaeus, Ribes spp., Solidago spp. and Aralia nudicaulis . On Anticosti, as a result of heavy browsing, especially on woody species, the site is left open to invasion by other opportunistic and low palatability species such as Cirsium arvense, C. vulgare , Urtica sp., Rumex acetosella, Cerastium vulgare and grasses. Those invasive species appear to be favored in presence of deer compared to native species as they were not present in our sample plots on Mingan.
We found no difference in height and cover of shrub and herb species in bogs and fens. In these habitats, the species composition was similar in both sites, which could be explained by the dominance of ericaceous species, usually not consumed by deer. However, even such an usually unsuitable habitat is disturbed by deer. Shrubs such as Potentilla fruticosa , Betula pimula , Myrica gale and even ericaceaous shrubs such as Kalmia spp. or Andromedra glauca , are browsed by deer at varying intensity. Moreover, we found that lichens are less abundant in bogs on Anticosti which might be generated by trampling. Little is known about the suitability of peatlands to deer, but our results suggest that white-tailed deer on Anticosti use all available habitats. More research is needed to determine the value of peatlands to white-tailed deer on Anticosti. Recent aerial surveys indicated that deer density in the eastern part of the island, where fens and bogs dominate the landscape, is comparable and even higher than that measured in the large areas dominated by balsam fir stands (Rochette et al. 2003)
Deer modified the plant community composition and structure through selective herbivory on Anticosti. This change favors the spreading of unpalatable and tolerant plant species. Cirsium spp. are favored by their spines which make them unpalatable and are known for their ability to invade sites, whereas grasses can tolerate high grazing intensities (McNaughton 1979). Contrary to other herbaceous species, whose meristem (living parts) is located at the upper stem of the plant, grasses cumulate their meristem at the bottom of the stem, which constitutes an advantage to tolerate browsing.
In the mature fir stands, some woody and herbaceous species reappeared during the first five years after release of browsing in exclosures. These species such as Abies balsamea, Sorbus americanus, Rubus idaeus were still present in the seed bank or on the ground as very small seedlings. Several studies have shown that browse-intolerant species have the potential to reestablish themselves following removal of browsing pressure, but recovery may be slow (Ross et al. 1970, Anderson and Katz 1993). Species found in Mingan but that did not recover inside the exclosures may not be definitively extiparted on Anticosti. Due to its slow growth rate, Taxus canadensis may require longer periods of time to reestablish (Allison 1990). The absence of rare species such as Goodyera sp. in the exclosures may be explained by its natural scarcity.
Pellet results showed that windfall is the most intensively used habitat. Aerial surveys indicate that mature fir stands are most intensively used in winter by deer (Potvin and Gingras 2002) and that peatlands are less used compared to the other habitats at any season even if they are apparently occasionally frequented on Anticosti compared to the mainland. Windfalls, as other open habitats, are attractive to deer for their food availability and nearly escape cover in summer.
Deer browsing affected species abundance and composition on Anticosti island. It is unlikely that the differences between the vegetation of Anticosti and Mingan are due to factors other than deer browsing. Indeed, herbaceous and woody species rapidly recovered in exclosures, which clearly demonstrates the effects of deer browsing on vegetation communities. Moreover, since 1930, many studies have already reported deer impacts on vegetation on Anticosti (Pimlott 1963, Huot 1982, Chouinard and Filion 2001, Hébert and Jobin 2001, Potvin et al. 2003). No accessible sites were released by deer on Anticosti so Mingan operates as the best available control site for the comparison. We based our interpretation on species that were most frequently observed in our sample plots and that are characteristic of the boreal forest. Marie-Victorin and Rolland-Germain noted 845 vascular taxa on Anticosti and 496 taxa on Mingan during their reconnaissance excursions between 1917 and 1926 which corresponds to the fisrt negative reports of deer impacts on vegetation (Marie-Victorin and Rolland-Germain 1969). The surveys were only qualitative at that time and one could argue that the greater diversity on Anticosti is explained by its greater size (MacArthur and Wilson 1967). Majority of species used in our comparisons (103/114 species), were identified by these botanists at the beginning of the XXth century on both sites, except for Urtica sp. and Veronica officinalis (Marie-Victorin and Rolland-Germain 1969). However, nine of the 114 species identified in our field surveys in 2001 and only present on Anticosti, have been already only recorded on Anticosti during the first botanizing excursions. We therefore suggest that deer browsing is the major disturbance factor explaining differences between sites. Deer, however, may not be directly responsible for the reduction of all species that differed between sites. Deer may have affected some species indirectly by modifying environmental conditions (light, soil characteristics, competition between species) so that habitat became less suitable to some species.
More research on plant-herbivore interactions is required to properly manage abundant deer populations with respect to economic activities and ecosystem sustainability. Indeed, hunting, logging and recreation tourism represent the major economic activities on Anticosti but their success relies on deer abundance. It is a priority to explore management avenues to bring deer densities to levels compatible with the conservation of the natural flora and balsam fir regeneration maintaining hunting as the major lucrative activity. Developping management solutions to this problem is a major ecological challenge of the next decades.
Table 1. General characteristics of the four habitat types (mature fir stands, windfalls, fens and bogs) sampled on Anticosti (deer) and on Mingan (no deer)
|
Anticosti |
Mingan |
P |
|
|
Mature fir stands Dominant tree species |
Abies balsamea, Picea glauca |
Abies balsamea, Betula papyrifera |
|
|
Total density (stems ha-1)a |
850 ± 290 |
2,300 ± 590 |
0.02 |
|
Basal area (m2 ha-1)a |
40 ± 4 |
46 ± 4 |
0.02 |
|
Tree age (years)a |
76 ± 4 |
84 ± 10 |
0.2 |
|
Height (m)a |
15 ± 1 |
13 ± 1 |
0.06 |
|
Canopy closure (%)a |
68 ± 7 |
75± 3 |
0.1 |
|
Windfalls |
|||
|
Regeneration age (years) |
5b |
5-6c |
|
|
Fens |
|||
|
Dominant species |
Potentilla fruticosa, Myrica gale |
Potentilla fruticosa, Myrica gale |
|
|
Bogs |
|||
|
Dominant species |
Rubus chamaemorus, Empetrum nigrum, Sphagnum fuscum and Cladina sp. |
Rubus chamaemorus, Empetrum nigrum, Sphagnum fuscum and Cladina sp. |
a Values are means ± SE
b Windfalls of 1996 identified from forest maps
c Age based on regeneration of deciduous shrubs
Table 2. Occurrence (%, n = 80) and cover (%) of woody and herbaceous species in mature fir stands of Anticosti (deer) and Mingan (no deer)
|
Species |
Occurrence (%) |
Covera (%) |
||
|
Anticosti |
Mingan |
Anticosti |
Mingan |
|
|
Woody species (< 200 cm) |
||||
|
Abies balsamea |
-b |
89 |
- |
30.5 |
|
Acer spicatum |
- |
9 |
- |
0.5 |
|
Betula papyrifera |
- |
21 |
- |
2.5 |
|
Picea glauca |
- |
5 |
- |
0.5 |
|
Sorbus americanus |
- |
19 |
- |
0.01 |
|
Taxus canadensis |
- |
15 |
- |
20.5 |
|
Herbaceous species |
||||
|
Clintonia borealis |
- |
74 |
- |
8 |
|
Coptis groenlandica |
47 |
- |
2.5 |
- |
|
Cornus canadensis |
69 |
81 |
0.5 |
2.5 |
|
Grasses |
35 |
- |
2.5 |
- |
|
Linnaea borealis |
30 |
66 |
2.5 |
2.5 |
|
Maïanthemum canadensis |
90 |
72 |
2.5 |
2.5 |
|
Mitella nuda |
36 |
- |
0.5 |
- |
|
Oxalis montana |
26 |
- |
2.5 |
- |
|
Trientalis borealis |
53 |
69 |
0.5 |
2.5 |
a Cover data correspond to the mode for each species
b "– " means that this species was never found in our plots
Table 3. Mean number of fir seedlings ± SE (ha-1) according to their height classes, in the mature fir stands and in the windfalls of Anticosti (deer) and Mingan (no deer)
|
Fir seedlings height classes (cm) |
0-5 |
P |
5-10 |
P |
10-30 |
P |
|
Mature fir stands |
||||||
|
Anticosti |
79,500 ± 28,000 |
0.1 |
24,000 ± 7,000 |
0.8 |
200 ± 200 |
<0.01 |
|
Mingan |
134,000 ± 68,000 |
26,000 ± 8,000 |
24,000 ± 12,000 |
|||
|
Windfalls |
||||||
|
Anticosti |
5,000 ± 3,000 |
<0.01 |
7,000 ± 3,000 |
0.6 |
2,000 ± 400 |
<0.001 |
|
Mingan |
14,000 ± 2,000 |
9,000 ± 4,000 |
5,000± 200 |
* p < 0.05
** p < 0.01
a fens and bogs were grouped as peatlands because there was no difference between these habitats
Table 4. Occurrence (%, n = 60) and cover (%) of woody and herbaceous species in windfalls of Anticosti (deer) and Mingan (no deer)
|
Species |
Occurrence (%) |
Covera (%) |
||
|
Anticosti |
Mingan |
Anticosti |
Mingan |
|
|
Woody species (< 200 cm) |
||||
|
Abies balsamea |
20 |
71 |
2.5 |
40.5 |
|
Acer rubrum |
-b |
22 |
- |
0.5 |
|
Acer spicatum |
- |
21 |
- |
30.5 |
|
Betula papyrifera |
3 |
29 |
0.5 |
8 |
|
Picea glauca |
56 |
- |
30.5 |
- |
|
Ribes sp. |
- |
24 |
- |
20.5 |
|
Rubus idaeus |
3 |
48 |
0.5 |
13 |
|
Sorbus americanus |
- |
19 |
- |
2.5 |
|
Herbaceous species |
||||
|
Aralia nudicaulis |
- |
10 |
- |
2.5 |
|
Cerastium vulgare |
11 |
- |
0.5 |
- |
|
Cirsium arvense |
20 |
- |
8 |
- |
| Clintonia borealis |
- |
62 |
- |
8 |
|
Cornus canadensis |
40 |
60 |
2.5 |
20.5 |
|
Grasses |
52 |
- |
2.5 |
- |
|
Linnaea borealis |
20 |
62 |
0.5 |
20.5 |
|
Maïanthemum canadensis |
65 |
43 |
2.5 |
0.5 |
|
Oxalis montana |
35 |
- |
2.5 |
- |
|
Rubus pubescens |
20 |
8 |
0.5 |
2.5 |
|
Rumex acetosella |
18 |
- |
2.5 |
- |
|
Solidago sp. |
- |
18 |
- |
13 |
|
Trientalis borealis |
40 |
34 |
0.5 |
0.5 |
|
Urtica sp. |
8 |
- |
8 |
- |
a Cover data correspond to the mode for each species
b "– " means that this species was never found in our plots
Table 5. Occurrence (%, n = 80) and cover (%) of ericaceae and herbaceous species in fens of Anticosti (deer) and Mingan (no deer)
|
Species |
Occurrence (%) |
Covera (%) |
||
|
Anticosti |
Mingan |
Anticosti |
Mingan |
|
|
Ericaceae species |
||||
|
Andromeda glauca |
76 b |
78 |
0.5 |
0.5 |
|
Betula pumila |
- |
9 |
- |
0.5 |
|
Cassandra caliculata |
31 |
8 |
8 |
2.5 |
|
Kalmia angustifolium |
20 |
25 |
2.5 |
0.5 |
|
K. polyfolia |
16 |
20 |
0.5 |
0.5 |
|
Larix laricina |
59 |
49 |
20.5 |
13 |
|
Ledum groenlandica |
57 |
58 |
0.5 |
0.5 |
|
Myrica gale |
80 |
98 |
20.5 |
8 |
|
Potentilla fruticosa |
61 |
70 |
13 |
13 |
|
Herbaceous species |
||||
|
Carex sp. |
76 |
78 |
13 |
2.5 |
|
Eriophorum sp. |
13 |
28 |
2.5 |
13 |
|
Sanguisorba canadensis |
42 |
47 |
0.5 |
2.5 |
|
Sarracenia purpra |
28 |
33 |
0.5 |
2.5 |
|
Smilacina trifolia |
20 |
46 |
0.5 |
0.5 |
a Cover data correspond to the mode for each species
b "– " means that this species was never found in our plots
Table 6. Occurrence (%, n = 60) and cover (%) of ericaceae and herbaceous species in bogs of Anticosti (deer) and Mingan (no deer)
|
Species |
Occurrence (%) |
Covera (%) |
||
|
Anticosti |
Mingan |
Anticosti |
Mingan |
|
|
Ericaceae species |
||||
|
Andromeda glauca |
40 b |
5 |
0.5 |
0.5 |
|
Cassandra caliculata |
85 |
87 |
20.5 |
20.5 |
|
Empetrum nigrum |
92 |
97 |
2.5 |
13 |
|
Kalmia angustifolium |
93 |
98 |
2.5 |
2.5 |
|
K. polyfolia |
78 |
78 |
0.5 |
0.5 |
|
Ledum groenlandica |
98 |
98 |
2.5 |
8 |
|
Vaccinium oxycoccos |
90 |
68 |
0.5 |
0.5 |
|
Vaccinium angustifolium |
40 |
40 |
2.5 |
2.5 |
|
Herbaceous species |
||||
|
Rubus chamaemorus |
97 |
98 |
2.5 |
2.5 |
|
Drosera rotundifolia |
90 |
65 |
0.5 |
0.5 |
|
Sarracenia purpra |
24 |
14 |
0.5 |
0.5 |
a Cover data correspond to the mode for each species
b "– " means that this species was never found in our plots
Table 7. Median browsing index for woody species according to habitat, Anticosti island, summer 2001
|
Habitat type |
Species |
Browsing Index b |
|
Mature fir stands |
- a |
- |
|
Windfalls |
Abies balsamea |
100 c |
|
Betula papyrifera |
52.5 |
|
|
Fens |
Potentilla fruticosa |
52.5 |
|
Betula pumila |
52.5 |
|
|
Myrica gale |
13 |
|
|
Kalmia polyfolia |
13 |
|
|
Bogs |
K. polyfolia |
13 |
|
K. angustifolium |
13 |
|
|
Andromedra glauca |
13 |
a No shrub or ericaceous species were available to deer in fir stands.
b Browsing index was recorded according to the following classes: 0% (none of the twigs were browsed), 1-25 %, 25-80%, 80-99% and 100% in each plot.
c Cover data correspond to the mode for each species
Table 8. Occurrence (Ï) of tree seedlings, shrubs and herbaceous species in mature fir stands of Anticosti and Mingan. Occurrence (presence/absence) on Anticosti was estimated in areas protected (exclosures) and not protected from deer browsing (open)
|
Species |
Anticosti (open) |
Anticosti (exclosures) |
Mingan (no deer) |
|
Abies balsamea (shrub) |
Ï |
Ï |
|
|
Epilobium angustifolium |
Ï |
Ï |
|
|
Rubus idaeus (shrub) |
Ï |
Ï |
|
|
Sorbus americanus |
Ï |
Ï |
|
|
Cirsium arvense |
Ï |
||
|
C. vulgare |
Ï |
||
|
Grasses |
Ï |
||
|
Goodyera sp. |
Ï |
||
|
Taxus canadensis |
Ï |
Table 9. Mode and range of number of pellet groups (100 m-2) of deer and snowshoe hares in the 4 habitats of Anticosti (deer) and Mingan (no deer)
|
Deer |
Hare |
|||
|
Anticosti |
Mingan |
Anticosti |
Mingan |
|
| Mature fir stands |
30 aa [0-230] |
- b |
0 a [0-50] |
0 a [0-30] |
| Windfalls |
40 a [0-420] |
- |
0 a [0-20] |
0 a [0-100] |
|
Fens |
0 b [0-100] |
- |
- |
- |
|
Bogs |
0 b [0-325] |
- |
- |
0 a [0-25] |
a Values in the same column not sharing a common letter are significantly different (P< 0.05)
b "– " indicates that pellets were never found in our plots
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