On the relevance of facilitation in alpine meadow communities: An experimental assessment with multiple species differing in their ecological optimum

It has been recently shown that co-occurring species with different stress tolerance and ecological optima may show differential responses to the same neighbors in a given community. Despite this, most facilitation studies have evaluated a given plant-plant or plant-group of plants interactions. We conducted a removal experiment in an alpine meadow of the Qing-Hai Tibet Plateau to test two hypotheses: (i) facilitation will be the dominant plant-plant interaction in this stressful environment; and (ii) the magnitude of positive interactions among species will differ, with those species closer to their ecological optimum (i.e. those species more abundant in the community) showing competitive or neutral interactions. The experiment was conducted using seven co-occurring species differing in their relative abundances within the community. Neighbors positively affected the growth in biomass and height of all the studied species. Thus, and as predicted by our first hypothesis, facilitation was the dominant interaction within the studied community. According to our second hypothesis, biomass growth responses to neighbor removal were species-specific, with the dominating species showing a weak facilitative response. Our results contribute to advance our knowledge on the interplay of facilitation and competition in multi-species communities, and can be used to refine current conceptual models regarding the outcome of plant-plant interactions and abiotic stress.