Plant–plant interactions could limit recruitment and range expansion of tall shrubs into alpine and Arctic tundra

Species in cold-limited biomes are expected to expand their distribution ranges in response to climate warming. For plants, range shifts can only occur via successful recruitment beyond their current distribution limit. However, many environmental and ecological filters can act upon recruitment and establishment, thereby potentially limiting the expected climate-driven shifts. In this study, we investigate potential mechanical and chemical constraints that vegetation above the tall shrubline in alpine and Arctic tundra could impose upon the successful establishment of willow species in the Canadian Western Arctic. We collected willow seeds from an alpine and an Arctic shrubline and conducted germination trials to test (1) for seedbed preferences among three natural and one experimentally scarified seedbeds, and (2) for vulnerability to allelopathic chemicals produced by ericaceous dwarf shrub species. We found that germination was almost four times higher on manually exposed bare ground than on intact, herbaceous vegetation. Seeds of two willow species, Salix arctica and Salix pulchra, were not affected by leaf extracts from dwarf shrubs, Cassiope tetragona and Vaccinum uliginosum, but the germination of Salix richardsonii was reduced by as much as 24% in the presence of chemicals from C. tetragona. Our results suggest that biotic interactions could limit the predicted expansion of tall shrubs in the tundra by interfering with germination. Seemingly species-specific responses highlight the need for replicated studies across a wider range of species combinations. Potential range shifts may not occur as a uniform translocation of the shrubline, but could change the composition of the plant community by filtering out certain species.