Plant species diversity and functional diversity relations at different levels of salinization in desert-oasis transition zone of the Hexi Corridor

Soil salinization, a typical form of land degradation in arid regions, poses a serious threat to ecosystem stability. However, systematic understanding of the coupling relationship between species diversity and functional diversity and its underlying mechanisms along salinization gradients remains limited. In this study, we investigated plant community characteristics and soil physicochemical properties along a salinization gradient in the desert-oasis transition zone of the eastern and central Hexi Corridor in northwestern China. By integrating Random Forest (RF), Redundancy Analysis (RDA), and Partial Least Squares Path Modeling (PLS-PM), we examined variations in species and functional diversity, their interrelationships, and their responses to environmental factors during the salinization process. Results showed that the soil environment exhibited clear ion differentiation along the salinization gradient, with the accumulation of Na + , Ca 2+ , Mg 2+ , SO₄ 2− , and Cl − serving as key drivers distinguishing salinization levels. As salinization intensified, species richness and community density decreased significantly. Environmental filtering was strongest at the moderate salinization stage, where species diversity was lowest and functional traits were most convergent. Functional diversity, measured by functional dispersion (FDis) and Rao's quadratic entropy (RaoQ), first decreased and then increased with increasing salinization, indicating a shift in plant trait responses from convergence under moderate stress to divergence under severe stress. Accordingly, the coupling relationship between species diversity and functional diversity exhibited a typical “weak–strong–weak” pattern, indicating that the correspondence between taxonomic and functional dimensions of biodiversity was strongest at intermediate salinization levels and weaker at both low and high salinization levels. PLS-PM results further showed that salinization was directly associated with reduced species diversity and indirectly related to both species and functional diversity through soil pH and total soil salt content (TSC). Overall, these findings suggest that increasing salinization may strengthen environmental filtering and be associated with shifts in community assembly processes, from environmental filtering dominance toward differentiation of salt-tolerance strategies, providing scientific support for vegetation restoration and management of saline-alkali lands in arid regions.