LAUSR

At high population size, migratory caribou (Rangifer tarandus) are regulated by forage abundance in their summer range. Climate warming likely affects forage availability by increasing productivity and advancing phenology of vegetation. Our objective was to investigate the combined effects of browsing and climate warming on the availability of dwarf birch (Betula glandulosa). We simulated direct (warming, with open-top chambers) and indirect (increased nutrient cycling) effects of climate warming in interaction with simulated browsing (leaf stripping) from 2009 to 2013 in Nunavik, Canada. We measured the effect of treatments on dwarf birch biomass and phenology. Moderate and heavy browsing reduced the estimated biomass of birch leaves by 14% and 34%, respectively. Fertilization did not increase the biomass of birch leaves, but increased the biomass of another forage, Poaceae. The warming treatment advanced the opening of birch leaves by 4 days (95% CI: [3, 6]) in 2011 and 7 [5, 8] days in 2013, the two years colder than average. The absence of significant phenological shifts in warmed plots during warmer springs suggests that established dwarf birches may have reached a threshold in a limiting resource, likely soil moisture, under which they cannot respond to further warming. Our results demonstrate that browsing can reduce forage biomass, but the variability in caribou populations could provide windows of opportunity for shrub growth.