Trait Convergence Dominates Community Assembly Across Contrasting Himalayan Habitats: Integrating Above- and Belowground Strategies With Phylogenetic Structure

Understanding the mechanisms that structure 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 typically signalling strong environmental filtering and divergence reflecting niche partitioning or competitive exclusion. In extreme environments such as cold, arid mountains, it remains unclear whether persistence is driven 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, enabling separation of adaptive responses from evolutionary constraints in shaping community composition. We analysed community assembly across 482 sites in six contrasting Himalayan habitats: salt marshes, wet grasslands, shrublands, steppes, screes, and alpine tundra, spanning 3,700-5,850 m. Using community-weighted means (CWM) and standardized effect sizes (SES) for 13 above- and belowground traits, including leaf morphology, stem anatomy, clonality, root carbohydrates, and nutrient storage, alongside a multi-locus phylogeny of all species, we linked functional and phylogenetic structure with species richness. Most communities showed strong functional convergence (negative SES) in stress-tolerant traits (e.g., high Leaf Dry Matter Content, Root Phosphorus Content, and Root Non-structural Carbohydrates), particularly in species-poor habitats such as salt marshes and alpine tundra. Divergence (positive SES) was more common in species-rich wet grasslands and steppes, where milder conditions likely facilitate niche differentiation. Phylogenetic overdispersion frequently accompanied functional convergence, indicating repeated evolution of similar trait syndromes in distantly related lineages under shared environmental constraints. Assembly processes had contrasting effects across traits: in alpine and scree communities, conservative traits converged while clonality and {delta}15N diverged, suggesting parallel action of filtering and differentiation; in nutrient-enriched systems, convergence in Specific Leaf Area coincided with divergence in root traits, highlighting trait-specific responses to stress and competition. We conclude that environmental filtering dominates Himalayan community assembly but is frequently modulated by niche differentiation. The consistent decoupling between functional and phylogenetic diversity demonstrates that only by integrating multidimensional trait data with phylogeny can we disentangle adaptation from evolutionary constraint and robustly predict biodiversity responses to environmental change.