Explaining the mechanisms behind niche dimensionality and light-driving species diversity based on functional traits.

Two prevalent ecological mechanisms, niche dimensionality and light asymmetry, may well explain species loss with fertilization gradients in grassland communities. Although there is still controversy surrounding the two competitive mechanisms that maintain species coexistence, few studies have examined the patterns of change in dissimilarity in species composition (β-diversity) and the relative explanatory contributions of plant functional traits to α- and β-diversity when multiple resources are added. To clarify this knowledge gap, we conducted a 6-year experiment of resource addition in an alpine meadow on the Qinghai-Tibet Plateau to assess how species richness and spatial β-diversity are affected by increasing numbers of added resources (NAR) and light limitation. Our results found that both NAR and light limitation led to decreased species richness, suggesting that niche dimensionality and light asymmetry may contribute equally to species loss, rather than either alone. Moreover, NAR is the primary factor responsible for the increase in β-diversity, which exhibits a negative relationship with species richness. Furthermore, the increase in height is the most likely explanation for β-diversity, while the increase in SLA is the most likely explanation for species richness, thereby indicating the changes in species richness and composition can be effectively explained by the response of certain morphological functional traits with the addition of multiple resources. Future research should focus on the complex interactions of different ecological mechanisms that contribute to the maintenance of biodiversity in grassland ecosystems all over the world.