Functional redundancy changes along a drought stress gradient for the shift of selection effect to complementarity effect in experimental plant communities

Species interactions alter with environmental change, which influences the relationship between biodiversity and ecosystem functioning (BEF). How BEF relationships change with environments requires knowledge of the changes of functional attributes in individual species in response to environmental change. Here, we assembled experimental plant communities with different species richness (1, 2, 4, 8, and 16) and exposed them to a drought stress gradient (control, moderate and intensive drought stress). Functional redundancy decreased with increasing drought stress for a shift from selection effect to complementarity effect. In response to drought stress, species interactions shifted from competition to facilitation and the role of light-asymmetric competition in the competitive ability of species gradually decreased. Many species that are redundant under control conditions make essential contributions to biomass production under moderate and intensive drought stress conditions. Consequently, biodiversity effects should be evaluated from a predicted climate change model and redundant species should be carefully defined.