Ecological filtering explains species distribution pattern and regeneration potential in Western Himalayan forests

Understanding how environmental heterogeneity shapes species composition is essential for managing fragile Himalayan ecosystems. We assessed patterns of tree and shrub diversity, forest structure, regeneration dynamics, and soil physicochemical properties along an elevation gradient and across north- and south-facing slopes in 36 forest stands of Western Himalaya. We recorded 3,486 individuals from 47 tree and 34 shrub species. Floristic composition revealed four distinct forest community types along elevation: sub-tropical Sal-dominated (300-900 m), sub-tropical Pine (900–1500 m), lower temperate mixed (1500–2400 m), and upper temperate mixed (2400–3000 m) forests, indicating strong species turnover. Tree density and basal area increased with elevation, peaking in upper temperate forests (∼1128 ind ha⁻¹; ∼68 m² ha⁻¹), whereas species richness and Shannon diversity peaked in Sal forests. Shrub density showed a unimodal response, peaking at mid-elevations (∼1600 m). Tree seedling density was greater on north-facing slopes, whereas sapling density peaked at mid-elevations (∼1200 m). Soil moisture, organic carbon, and available potassium increased with elevation, with higher values on north-facing slopes. However, bulk density was greater on south-facing slopes. Multivariate ordination identified soil nutrients and elevation as the main drivers of community composition. Redundancy Analysis explained 53% of variance (adj. R² = 0.59, p < 0.001), with elevation, soil moisture, organic carbon, and available potassium emerging as the strongest predictors. Climatic variables primarily influenced Sal forests. This study highlights the dominant role of soil and topography in governing community composition in Western Himalaya, with climate exerting as secondary but reinforcing driver at lower elevations.