Environmental stress shifts community assembly from niche differentiation to trait convergence across Himalayan plant communities

Abstract Understanding mechanisms structuring plant communities is fundamental for predicting biodiversity responses to global change. Functional trait‐based approaches can reveal assembly processes by linking species' adaptations to community‐level patterns, with convergence signalling strong environmental filtering and divergence reflecting niche partitioning or competitive exclusion. In extreme environments, including cold, arid mountains, it remains unresolved whether persistence is governed primarily by abiotic filtering (low temperatures) or by competition for scarce resources (soil moisture, nutrients) that promotes niche differentiation. Disentangling these mechanisms requires an integrated framework combining multidimensional trait data with phylogenetic structure. We analysed community assembly across 482 sites in six Himalayan habitats: salt marshes, wet grasslands, shrublands, steppes, screes and alpine tundra, spanning 3700–5850 m. Using null model‐based standardized effect sizes, we quantified convergence or divergence across 13 above‐ and below‐ground traits that capture resource use, persistence and stress tolerance, including nutrients, stable isotopes, carbohydrate storage, clonality and stem anatomy. Functional patterns were evaluated alongside phylogenetic structure to distinguish convergent adaptation from shared ancestry. Functional convergence predominated in the most stressful habitats (salt marshes, screes and alpine tundra), consistent with environmental filtering imposed by salinity, substrate instability and cold. In contrast, trait divergence was most pronounced in steppes and shrublands, where reduced abiotic constraints coincided with greater functional differentiation among co‐occurring species. Across habitats, functional convergence often co‐occurred with phylogenetic overdispersion, indicating repeated evolution of similar trait syndromes in distantly related lineages. Assembly signals were trait‐specific: conservative traits linked to persistence and storage (LDMC, NSC, lignification) tended to converge, whereas traits associated with competition or resource‐use differentiation (height, clonality, foliar δ 15 N) more often diverged, even within the same communities, demonstrating that filtering and niche differentiation can act simultaneously on different functional dimensions. Synthesis . Himalayan plant community assembly reflects a shifting, trait‐specific balance between environmental filtering and niche differentiation, with trait convergence predominating under strong abiotic filtering and divergence increasing in more productive, less stressful environments. By integrating multidimensional traits with phylogenetic structure across contrasting habitat types, our results provide a mechanistic framework for anticipating functional reorganization of high‐mountain ecosystems as climate change alters the strength of abiotic filters and biotic interactions.