Richness–biomass relationships change with increasing acid rain intensity by shifting from positive to negative selection

The species interactions can be affected by environmental stress; however, how such change affects the plant species richness–biomass relationship is still unclear. Here, experimental plant communities with species richness levels of 1, 2, 4, and 8 were assembled and subjected to a simulated acid rain (SAR) intensity gradient (pH 7.0, 4.5 and 2.5). The species richness–biomass relationship was investigated and the absolute growth rate and survival rate were measured under each SAR intensity. With the increasing SAR intensity, the species richness–biomass relationships changed from positive to negative as a result of a shift in the biodiversity effects from positive to negative selection. Under a SAR intensity of pH 7.0, the absolute growth rate determined the biomass ratio of species in the mixtures. Mixtures that included Sedum lineare (with rapid growth) had a lower soil total nitrogen (TN), soil total phosphorus (TP), and greater aboveground biomass than mixtures that did not include the species, which led to a positive selection effect. Under a SAR intensity of pH 2.5, the survival rate determined the biomass ratio of species in the mixtures. Mixtures that included Sedum emarginatum and Phedimus spurius (with high survival rates) had less aboveground biomass than mixtures that did not include them, which led to a negative selection effect. Our results indicated that the mechanism by which a plant gains competitive superiority (i.e., a shift from rapid growth to a high survival rate) in response to environmental stress will affect the biodiversity effect. Considering future climate change scenarios, we suggest that changes in the competition mechanism should be integrated into future studies of the relationship between plant diversity and ecosystem functioning.