Interactive impacts of grazing pressure and climatic context on plant functional diversity and ecosystem structure

Livestock grazing is a pervasive driver of dryland ecosystem dynamics, yet its ecological consequences often depend on environmental context. Community assembly theory predicts that broad-scale climatic filtering constrains the range of viable species and trait strategies, while grazing modifies local competitive interactions within those environmental limits. Using a trait-based framework, we evaluated how grazing intensity interacts with climatic context to influence taxonomic composition, functional diversity, trait composition, and vegetation structure across five topo-climatic zones in a protected mountainous rangeland of southeastern Iran. We quantified Functional Richness (FRic), Functional Evenness (FEve), Functional Divergence (FDiv), community-weighted mean traits, and total canopy cover across 75 plots spanning a standardized grazing disturbance gradient. Species composition differed significantly among climatic zones and grazing intensities, with a strong grazing × climate interaction (PERMANOVA, p < 0.001), indicating context-dependent compositional turnover. Constrained ordination of community-weighted mean traits revealed significant shifts in trait composition across climatic and grazing gradients, demonstrating that grazing selectively reorganized dominant ecological strategies within climate-specific assemblages. Functional richness exhibited climate-dependent responses, increasing with grazing in cooler and more mesic zones but remaining constrained in hotter, more arid environments. In contrast, functional evenness and divergence were comparatively stable across the grazing gradient. Vegetation cover responded strongly to grazing, although the magnitude and direction of these responses varied among climatic zones. Together, these findings support a hierarchical assembly framework in which climate establishes the primary template for community organization, while grazing acts as a secondary, context-dependent filter modifying species composition and trait dominance patterns. The partial decoupling between taxonomic turnover and functional diversity further suggests that functional redundancy can buffer ecosystem structure against species replacement. Accounting for climatic context is therefore essential for predicting grazing impacts and designing adaptive management strategies in heterogeneous dryland landscapes.