Divergent Strategies Within a Shared Functional Framework Drive Niche Differentiation of Dominant Halophytes Along a Salinity–Alkalinity Gradient

How functional trait responses relate to niche differentiation among dominant halophytes along multidimensional environmental gradients remains unclear. This study investigated three co-dominant halophyte species in two representative saline–alkali habitats in northwestern China. We quantified niche differentiation and trait responses along gradients of soil salt content (SSC) and sodium adsorption ratio (SAR) and compared stress–trait–performance relationships among species. The three species differed in niche position within the SSC-SAR space, with overlapping niche distributions. Nitraria tangutorum Bobrov occupied habitats characterized by high SAR and low SSC, Salicornia europaea L. was associated with high SSC and low SAR conditions, and Kalidium foliatum (Pall.) Moq. occupied intermediate environments. Trait responses to SSC and SAR varied among species. N. tangutorum showed a negative association between acid detergent fiber (ADF) and biomass, consistent with a structure–growth trade-off, S. europaea showed strong coordination between osmotic regulation and nutrient traits, and K. foliatum exhibited relatively balanced trait coordination. Multi-group structural equation modeling identified three pathways with significant differences among species after correction for multiple testing. Overall, these findings suggest that niche differentiation among dominant halophytes is associated with variation in trait responses and selected trait relationships along SSC and SAR gradients.