Heterogeneous Stress Driven by Environmental Filtering Maintains Plant Diversity Along an Arid Riparian Gradient.

Environmental stress is generally expected to suppress biodiversity, yet how heterogeneous environmental filtering regulates plant community assembly in arid riparian ecosystems remains poorly understood. Here, we investigated herbaceous plant communities and associated soil factors along riparian environmental gradients of the Ulungur River in arid northwestern China, aiming to assess whether and how severe but spatially heterogeneous stress conditions can maintain high local diversity. Non-metric multidimensional scaling (NMDS), generalized linear models (GLM), and random forest regression were used to disentangle the relative contributions of spatial location and soil stress in shaping species turnover and diversity patterns. Our results showed that local plant alpha diversity unexpectedly peaked in downstream river segments characterized by severe soil salinization-alkalization and phosphorus limitation (pH up to 8.88, total phosphorus as low as 0.70 g/kg), challenging the widespread negative stress-diversity relationship. NMDS ordination revealed pronounced species turnover along the riparian gradient, with the greatest spatial dispersion observed in upstream plots (mean Bray-Curtis dissimilarity = 0.569) due to high habitat heterogeneity. GLM further revealed that elevation was the strongest common negative factor for all diversity metrics (standardized coefficients