From selection to complementarity: the shift along the abiotic stress gradient in a controlled biodiversity experiment.

The effects of principal mechanisms (selection and complementarity) of biodiversity on ecosystem functionality have been well studied. However, it remains unknown how environmental conditions affect the relative strength of these two mechanisms. To answer this question, a controlled pot experiment was conducted in which species diversity was manipulated in low (natural soil) and high stress (mine tailing) plots, respectively. Our results demonstrate that the principal mechanism underlying the increasing biomass shifts from the selection to complementarity with increasing abiotic stress. The shift occurs because species interactions varied with increasing abiotic stress. Competition prevails in low stress plots, while facilitation dominates in high stress plots. In low stress plots, the monoculture biomass of a specific species is a good indicator of the competitive ability of that species in the mixture, and the dominant species significantly affects the plot biomass. In high stress plots, the tolerance indexes of all individual species increase with the manipulated species richness, providing clear evidence for the increasing role of facilitation.