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Les changements environnementaux influencent la végétation nordique, mais également les herbivores qui s’en nourrissent. Comme ces changements s’accentueront, un état de la situation au Nunavik (Québec, Canada) devient nécessaire. Nous discutons ici de l’influence qu’exercent les herbivores vertébrés sur les communautés végétales, et décrivons l’état des populations des deux principales espèces d’herbivores de cette guilde au Nunavik. Réparti en deux troupeaux, le caribou migrateur (Rangifer tarandus) est un élément structurant des écosystèmes du Nunavik. À forte densité, il peut réduire la productivité végétale dans son aire d’estivage et limiter la croissance des espèces consommées. Espèce introduite dans les années 1960, le boeuf musqué (Ovibos moschatus) colonise maintenant l’essentiel des côtes de la région. Quoiqu’il soit connu pour limiter l’expansion arbustive ailleurs dans l’Arctique, l’influence du bovidé au Nunavik demeure marginale jusqu’à maintenant. Sauf pour la bernache du Canada (Branta canadensis) qui niche en grandes colonies, l’influence des autres herbivores vertébrés sur les communautés végétales sera vraisemblablement localisée. L’augmentation du couvert arbustif procure des ressources alimentaires et un abri à diverses espèces qui pourraient voir leur aire de répartition se déplacer vers le nord. Les préférences alimentaires de ces herbivores pourraient moduler la réponse des espèces végétales aux changements climatiques., Environmental changes influence northern plant communities and the populations of herbivores that feed on them. Given the predicted increased rates of change in the north, it is essential to understand the situation in Nunavik (Québec, Canada). This review outlines the influence of vertebrate herbivores on plant communities and describes the population status of the two main members of this guild in the region. The migratory caribou (Rangifer tarandus) is a keystone species, and two herds are present. At high densities, it can reduce plant productivity in its summer range and limit the growth of the species consumed. The muskox (Ovibos moschatus), which was introduced in the 1960s, has now colonized most of the region’s coastal area. Although it may limit shrub expansion elsewhere in the Arctic, to date, its influence in Nunavik remains marginal. Except for the Canada goose (Branta canadensis), which nests in large colonies, the impact of other vertebrate herbivores on plant communities is likely localized. Increases in shrub cover will provide food and shelter for various herbivores that may experience a northward shift in range. Finally, the food preferences of herbivores present in Nunavik is expected to influence the response of plant species to climate change.

Summary NDVI analysis and repeated aerial photographs have revealed significant shrub expansion in many subarctic regions. While the recent increase in temperature is usually considered to be the main driver of this phenomenon at regional scales, very little is known about the local heterogeneity of shrub responses across the landscape. In this study, we aim to identify the climatic factors controlling the growth of the largely distributed shrub species Betula glandulosa in three types of environments (terrace, hilltop and snowbed). We also aim to evaluate the relationship between B. glandulosa growth and the NDVI data for the Boniface River region, in north-western Quebec, where the study took place. In the field, we harvested 180 B. glandulosa individuals (20 per site, 3 sites per type of environment). We constructed specific growth-ring width chronologies and mean axial growth rate chronologies for each site and used them for dendroclimatic analysis (response functions). We also used linear regressions to evaluate the relative influence of dwarf birch growth on the NDVI trend. We found a sharp increase in B. glandulosa radial growth in the 1990s followed by a sharp decreasing trend from 2002 on terraces and hilltops, while growth remained high in snowbeds. Betula glandulosa growth was positively correlated with summer temperatures on terraces and hilltops, whereas winter precipitation promoted growth on snowbeds. The NDVI trend was largely correlated to B. glandulosa growth on terraces and hilltops for the period between 1986 and 2002 (71–80% explained variance). Synthesis. Our results suggest that topography plays a major role in B. glandulosa growth and therefore in shrub community dynamics. Because terraces and hilltops represent 70% of the land surface, the sharp B. glandulosa growth increase at these sites promoted an important overall expansion of the shrub community in the region. However, the decline in B. glandulosa growth observed after 2002 suggests that the expansion could be slowed down in the near future, therefore limiting shrub growth contribution to the regional NDVI signal.

Shrub densification has been widely reported across the circumpolar arctic and subarctic biomes in recent years. Long-term analyses based on dendrochronological techniques applied to shrubs have linked this phenomenon to climate change. However, the multi-stemmed structure of shrubs makes them difficult to sample and therefore leads to non-uniform sampling protocols among shrub ecologists, who will favor either root collars or stems to conduct dendrochronological analyses. Through a comparative study of the use of root collars and stems of Betula glandulosa, a common North American shrub species, we evaluated the relative sensitivity of each plant part to climate variables, and assessed if this sensitivity is consistent across three different types of environments in northwestern Quebec, Canada (terrace, hilltop and snowbed). We found that root collars had greater sensitivity to climate than stems, and that these differences were maintained across the three types of environment. Growth at the root collar was best explained by spring precipitation and summer temperature, whereas stem growth showed weak and inconsistent responses to climate variables. Moreover, sensitivity to climate was not consistent among plant parts, as individuals having climate sensitive root collars did not tend to have climate sensitive stems. These differences in sensitivity of shrub parts to climate highlight the complexity of resource allocation in multi-stemmed plants. Whereas stem initiation and growth are driven by micro-environmental variables such as light availability and competition, root collars integrate the growth of all plant parts instead, rendering them less affected by mechanisms such as competition and more responsive to signals of global change. Although further investigations are required to determine the degree to which these findings are generalizable across the tundra biome, our results indicate that consistency and caution in the choice of plant parts are a key consideration for the success of future dendroclimatological studies on shrubs. This article is protected by copyright. All rights reserved.

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we (1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and constraints on shrub increase, (3) explore causes, feedbacks and implications of the increased shrub cover in tundra ecosystems, and (4) address potential lines of investigation for future research. Satellite observations from around the circumpolar Arctic, showing increased productivity, measured as changes in ‘greenness’, have coincided with a general rise in high-latitude air temperatures and have been partly attributed to increases in shrub cover. Studies indicate that warming temperatures, changes in snow cover, altered disturbance regimes as a result of permafrost thaw, tundra fires, and anthropogenic activities or changes in herbivory intensity are all contributing to observed changes in shrub abundance. A large-scale increase in shrub cover will change the structure of tundra ecosystems and alter energy fluxes, regional climate, soil–atmosphere exchange of water, carbon and nutrients, and ecological interactions between species. In order to project future rates of shrub expansion and understand the feedbacks to ecosystem and climate processes, future research should investigate the species or trait-specific responses of shrubs to climate change including: (1) the temperature sensitivity of shrub growth, (2) factors controlling the recruitment of new individuals, and (3) the relative influence of the positive and negative feedbacks involved in shrub expansion.