Ecological drivers of tree diversity and distribution along an elevation zone in Mamley watershed of Sikkim, India

Tree species diversity plays a crucial role in forest structure. This study assessed the species diversity, floristic composition, and vegetation structure along three elevation zones (lower elevation (LE) (<1200 m), middle elevation (ME) (1200–2100 m), and upper elevation (UE) (>2100 m)) and identified the ecological drivers causing changes in vegetation attributes in the Mamley watershed, Sikkim. The mean tree density ha−1 was highest at UE (1322.22 tree ha−1) while girth class distribution showed a reversed J-shape at all elevations. The study showed the highest Shannon diversity index at ME (2.69) and lowest at LE (2.18). In contrast, the highest importance value index was recorded by Schima wallichi (90.09) at LE, Quercus lamellosa (42.22) at ME, and Viburnum erubescens (48.39) at UE. Moreover, Principal Component Analysis (PCA) and Redundancy Analysis (RDA) indicated that the variation in species composition among the elevation zones was primarily driven by canopy height and Normalized Difference Vegetation Index (NDVI). The community-based indicator value analysis identified clear diagnostic taxa for each elevation zone, including Shorea robusta and Celtis tetrandra at LE, Viscum album at ME, and Cryptomeria japonica at UE. The findings of this study underscore the biological distinctiveness of each elevation zone and its correspondence with environmental gradients.