Positive plant-plant interactions expand the upper distributional limits of some vascular plant species

Biotic interactions can shape species' distributions through their impact on species' realized niches, potentially constraining or expanding the range of conditions under which species occur. We examine whether fine-scale plant-plant interactions scale up to shape broad-scale species' distributions, using Azorella selago, a widespread cushion plant that facilitates other species, and the rest of the vascular flora of sub-Antarctic Marion Island as a model system. We compared the upper elevational distributional limit of each species when growing on vs. away from A. selago to test how the interaction with this cushion plant species affects species' ranges. Three out of 19 vascular plant species occurred at higher altitudes in the presence of A. selago than in the absence of A. selago: Acaena magellanica (+26 m higher), Colobanthus kerguelensis (+37 m higher), and Lycopodium saururus (+19 m higher). Therefore, A. selago's fine-scale impacts scaled up to shape the distribution of a subset of the vascular flora of Marion Island. Plant-plant interactions thus have the potential to expand species upper distributional limits by increasing the niche space that a species can occupy, although the influence of these interactions may be strongly species-specific.