Revisiting the role of migratory caribou in the control of shrub expansion in northern Nunavik (Québec, Canada)

Abstract

Widespread shrub expansion has been observed at the circumpolar scale in response to climate change. This phenomenon could be inhibited by large herbivores through selective browsing that reduces shrub growth. Our objective was to assess the dynamics of shrub species in the Deception Bay area, in Nunavik (Quebec, Canada), a region in the summer range of the Riviere-aux-Feuilles migratory caribou herd. We surveyed abundance and stem mortality of all shrub species along systematically located transects in a 54 km2 area. Our results showed that shrubs covered 11.5% of the area, and were dominated by prostrate Betula glandulosa (8.7%). Stem mortality of established individuals was greater for Salix planifolia (45.6%) and Salix glauca (46.5%) than for B. glandulosa (9.3%). Using dendrochronological analysis, we determined the age structure of the two dominant erect shrub species in ten 10 m × 10 m plots. We recorded a high number of B. glandulosa and, to a lesser extent, of S. planifolia recruits (71.0 and 4.6 per 100 m2, respectively) established after 1999, when the Riviere-aux-Feuilles herd was abundant. Thus, our results do not support the hypothesis that large herbivores counteracted the positive effects of climate change on shrub recruitment. However, since the structure of the recruits are prostrate rather than erect, as is usually observed with shrub expansion in the warming Arctic, it is likely that herbivory, and/or, the harsh climate, is constraining habitat change.