Aboveground facilitation and not complementary resource use cause overyielding among grasses in Tibetan alpine ecosystems

Although plant-plant interactions and their effects on ecological functioning have been examined in communities composed of different functional groups, they have rarely been studied in communities that contained a single functional group. We established a five-year experiment using three grass species with different nutrient addition and mowing treatments to explore overyielding and species interactions. We found strong overyielding caused by trait-independent complementarity effects. Nutrient addition increased overyielding via increasing trait-independent complementarity effects whereas mowing eliminated overyielding. One possible mechanism underlying the responses observed is the changes of positive aboveground species interactions. This result is consistent with the considerable literature identifying facilitation in alpine ecosystems by which coexisting plant species improve the microclimate and thereby promote the performance of all species. Complimentary resource use and selection effect have been demonstrated frequently as the mechanisms driving overyielding in grasslands. However, as we identify here, and positive interactions may be especially important in driving overyielding and positive interactions may be especially important in stressful environments such as alpine grasslands.