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Résumé
Les populations abondantes de grands herbivores affectent grandement la composition, la structure et la fonction des écosystèmes. Par contre, le rétablissement de la végétation après l’arrêt du broutement en relation avec la productivité des sites reste peu compris. Nous avons d’abord comparé les trajectoires de composition végétale dans les parcelles soumises au broutement du cerf de Virginie (Odocoileus virginianus Zimmermann) à celles d’exclos sur une période de huit ans dans quatre types d’habitats tourbeux (bog, fen ouvert, fen arbustif et lagg) sur l’île d’Anticosti (Québec, Canada) en utilisant les courbes de réponse principales (PRC). Ensuite, nous avons examiné les changements de diversité bêta selon un gradient de minérotrophie, qui a été utilisé comme un estimé de la productivité pour comprendre la réponse de la végétation à l’arrêt du broutement selon plusieurs régimes de productivité. Les analyses ont révélé que la composition végétale des exclos a changé significativement dans le temps par rapport aux parcelles avec cerfs dans les fens arbustifs et les laggs. Aucun changement n’a été observé dans les bogs et les fens ouverts qui sont des habitats composés de plantes résistantes et tolérantes au broutement. Les exclos ont favorisé la croissance d’espèces broutées par le cerf, soit Sanguisorba canadensis et Betula pumila alors que des espèces tolérantes au broutement comme Dasiphora fruticosa et Trichophorum cespitosum ont bénéficié du broutement. La hauteur des arbustes était 27% et 34% supérieure dans les exclos que dans les parcelles avec cerf dans les fens arbustifs et les laggs alors que le couvert en herbacées était 43% plus grand dans les exclos que les parcelles avec cerfs dans les laggs. La diversité bêta était plus grande dans les exclos que les parcelles avec cerfs pour les tourbières à fort pH, ce qui suggère que les cerfs favorisent l’homogénéisation biotique dans les sites les plus productifs. En somme, nous résultats indiquent que les communautés végétales des tourbières se rétablissent lentement et montrent une réponse d’accélération le long d’un gradient de minérotrophie.
Abstract
Overabundance of large herbivores strongly affects the composition, structure and function of ecosystems. However, the recovery of vegetation after browsing control in relation to site productivity remains less understood. We first compared the trajectories of vegetation composition in plots subjected to browsing by white-tailed deer (Odocoileus virginianus Zimmermann) to plots protected by exclosures over a period of eight years in four types of peatland habitats (bog, open fen, shrub fen, lagg) on Anticosti Island (Québec, Canada), using principal response curves (PRC). We then examined changes in beta diversity in relation to site minerotrophy, which was used as a surrogate for plant productivity, to assess the response of vegetation to browsing under different productivity regimes. Vegetation composition changed significantly over time in unbrowsed plots compared to browsed plots in shrub fens and laggs, while no changes were observed in bogs
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and open fens, habitats composed of deer-resistant and tolerant plant species. Exclosures promoted the growth of palatable species such as Sanguisorba canadensis and Betula pumila, while tolerant species like
Dasiphora fruticosa and Trichophorum cespitosum benefited from browsing. Shrub height was 27% and 34%
higher in unbrowsed plots than browsed plots in shrub fens and laggs, respectively, while herbaceous species cover was 43% greater in unbrowsed plots than browsed plots in laggs. Beta diversity in exclosures was higher than in browsed plots for peatlands with high pH, suggesting that deer use leads to biotic homogenization of more productive sites. Overall, our results indicate that peatland plant communities can recover from browsing, and show a stronger response along a minerotrophic gradient.
Introduction
Large herbivores have modified vegetation composition in a wide range of habitats worldwide (de Vos et al. 1956, Vázquez 2002, Côté et al. 2004, Nunez et al. 2010). These modifications are more pronounced in environments that have evolved without herbivory (Milchunas et al. 1988) or under a new type or level of herbivory when an introduced herbivore reach higher densities than an native one (Hobbs & Huenneke 1992). Prominent vegetation-browsing patterns can be illustrated by the well-known cases of the Haida Gwaii archipelago (Canada), where introduced black-tailed deer (Odocoileus hemionus) have contributed to simplify vegetation into few browsing tolerant species (Martin et al. 2010); of the Palma Island in the Canary Islands, where seedling recruitment declined under high browsing pressure from rabbits and goats (Oryctolagus
cuniculus and Capra hircus) (Irl et al. 2012); and of many New Zealand communities, where multiple pressures
are experienced by plants and particularly indigenous species following several introductions of mammal herbivores (Wardle et al. 2001).
Over the last decades, deer populations have greatly increased in most temperate habitats of North America and Europe (Côté et al. 2004). Deer browsing has been shown to modify forest structure or plant community composition by reducing the abundance of more palatable species (Augustine & Frelich 1998, Royo et al. 2010) and favoring browsing-tolerant or resistant plants (Tremblay et al. 2007). In certain circumstances, browsing can alter succession of ecosystems and alternative successional stages can be reached (Connell & Slatyer 1977, Suding et al. 2004). This is due to the establishment of a recalcitrant understory layer (Royo & Carson 2006) and the impoverishment of seed banks (Maron & Crone 2006) that hinder vegetation recovery after herbivore density control (Hidding et al. 2013). Chronic browsing can in fact lead to biotic homogenization, that is the process by which natural or anthropogenic factors reduce the variation of species composition in space (beta diversity) over time (McKinney & Lockwood 1999, Olden & Rooney 2006). Rooney (2009) indeed suggested that deer can constitute a causal factor of biotic homogenization of the ground-layer vegetation of these forests, with the cover of grasses and sedges increasing from 10% to 83% within 16 years of high herbivore abundance (> 16 deer/km2). In northern Wisconsin forests, deer have modified vegetation
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communities and have caused a 9% increase in similarity of species composition among sites (Rooney et al. 2004). As well, long history of browsing in late succession and old-growth stands in Pennsylvania has contributed to the biotic homogenization of the sapling layer, resulting in low understory density, low diversity and abundance of browse-tolerant species (Schumacher & Carson 2013).
The process by which vegetation communities are homogenized by browsing may depend on the rate of production of new vegetation biomass by the ecosystem (Allaby 2010). Indeed, browsing impact may increase with productivity, due to more intense feeding activity at high productivity or because plants are more resistant in low productive sites (Daskin & Pringle 2016). Compensation has been shown for plants growing both at high and low productivity and will also influence the outcome of browsing on plant communities (Daskin & Pringle 2016). When an herbivore is extirpated from an ecosystem, the subsequent recovery of vegetation could also depend on site productivity. While highly productive ecosystems can be more intensively grazed, they may be more resilient, with vegetation recovering more rapidly from browsing pressure (Maschinski & Whitham 1989). Clearly, the response of ecosystems to browsing in function of their productivity is only partially understood. Because peatlands offer the great advantage of exhibiting a wide range of productivity from ombrotrophic to minerotrophic habitats (Zoltai & Vitt 1995), they represent an ideal context for assessing the response of vegetation to deer browsing along a productivity gradient. We based our assessment on pH, which is highly correlated with the minerotrophic gradient of peatlands, and thus their productivity (Vitt & Chee 1990). While browsing effects on forests and grasslands have been largely studied (Côté et al. 2004), the impact of wild large ungulates on peatlands is less understood. In these habitats, most studies have focussed on livestock rather than wild animals (Rawes & Hobbs 1979, e.g. Grant et al. 1985). Grazing and trampling by sheep and cattle have been shown to reduce the cover of ericaceous shrubs while favoring graminoids (Rawes & Hobbs 1979, Rawes 1983, Birnie & Hulme 1990, Ward et al. 2007) and decrease the total vegetation biomass (Ausden et al. 2005). Trampling also reduced the cover and richness of bryophytes while increasing the cover of bare peat (Arnesen 1999, Stammel & Kiehl 2004). Furthermore, only one study has evaluated the effects of browsing exclusion in the context of different types of peatlands and took place in ombrotrophic peatlands (bogs) in northern England, where the cessation of grazing by livestock for a period of 14 years led to the replacement of original species by species associated with drier conditions (Smith et al. 2003). Vegetation changes were also greater at the margins of the bog, suggesting this difference was associated with productivity (Smith et al. 2003). Few studies have examined the impact of grazing on minerotrophic peatlands (fens) (Middleton 2002, Stammel et al. 2003) while grazing impacts may differ between ombrotrophic and minerotrophic peatlands (Middleton et al. 2006).
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In a context of high herbivore densities and lack of predators, open habitats like peatlands can represent potential feeding grounds (Massé and Côté, 2009). For example, Pellerin et al. (2006) conducted an observational study of the impact of deer browsing at high density on peatland vegetation by comparing peatland plant communities of Anticosti Island (Québec, Canada) with those on the deer-free Mingan archipelago located nearby; they found a greater impact of browsing in fens than in bogs, with a loss of diversity in fens, principally for shrubs, sedges and liverworts. Although this study focused on open peatlands, it showed that browsing could differ between habitats. Laggs, which are ecotones between open peatland expanses and adjacent mesic forests, are susceptible to more intensive use by deer (Massé & Côté 2009). This is likely due to their high productivity and thus could be more affected than other peatland types. On Anticosti Island, peatlands are highly used by introduced white-tailed deer, and, in the absence of predation, fine-scale habitat selection by deer is determined more by forage quantity available than by protective canopy cover, which fosters the use of open or semi-forested peatlands adjacent to forests (Massé & Côté 2009). The presence of white-tailed deer at a high density in peatlands of Anticosti Island for several decades provides a unique opportunity to evaluate the response of peatlands to browsing in relation to the habitat type and productivity. We set up a deer exclosure experiment in 53 peatlands and assessed the response of vegetation over eight years in order to evaluate the impact of deer browsing on four types of habitats: bogs, open fens, shrub fens and laggs. In the context of high herbivore densities and absence of predators, we asked the following questions: 1) does response to deer exclusion vary between habitat types? and 2) can deer browsing induce biotic homogenization of peatland plant communities and is this process related to site productivity? Specifically, we predicted that under no deer browsing: (i) cover of plants non-resistant to herbivory and preferred species will increase through time, especially in habitats that are more intensively browsed by deer (shrub fens and laggs), and (ii) herbaceous as well as cover and height of shrubs will increase over time. We also predicted that beta diversity would be greater under deer exclusion than in browsed sites and that this difference would increase with the productivity of peatlands since browsing is likely to be intensified in productive sites.