Diversity components and assembly patterns of plant functional traits determine community spatial stability under resource gradients in a desert steppe

The diversity–stability relationship has been addressed and debated for decades, but how this relationship is affected by nutrient availability remains contentious. In the present study we assessed the effects of plant diversity, in terms of species richness, functional group composition and functional trait diversity, on the spatial stability of net primary productivity (NPP) following nitrogen and phosphorus application. In addition, we explored how functional traits at the species level contribute to the spatial stability of NPP. The results support the hypothesis that greater diversity leads to higher spatial stability. This relationship was highly dependent on soil nutrient availability, and increasing species richness or functional trait diversity significantly increased spatial variation of NPP under a high N fertilisation level. The effects of high mineral fertilisation rates may perhaps have masked the effects of plant diversity. Although species richness or functional trait diversity of the original and modified communities from which species with particular functional traits had been removed were significantly different, there were no differences in the coefficients of variation in the NPP of those communities. The lack of difference demonstrated that the relationship between spatial variability and biodiversity depended on the measure of diversity applied and that the functional group composition exerted a stronger effect than other diversity measures. Further analyses revealed that spatial stability of NPP was enhanced with increased diversity in vegetative plant height, rooting depth and the presence of legume, and diminished with diversity in the root system type and life cycle under some fertilisation treatments. The present study demonstrates that the relationship between biodiversity and ecosystem functioning is variable with different diversity, identity and environmental factors. Evaluating the contribution of particular traits to community stability will ultimately help us better understand the mechanisms underlying the diversity–stability relationship.