Quantitative assessment of forest attributes and the processes structuring forest are crucial not only in understanding a forest but also in developing adaptive conservation and management plans, notably for the climatically vulnerable forest of the Himalayas. However, studies in this field are limited, particularly from Sikkim, a north-eastern state of India. The current study explores 83, 0.1ha plots, ranging from 900m to 3200m above sea level, using taxonomic and phylogenetic indices. The result from the study provides the elemental understanding of the: (i) forest attributes, (ii) factors associated with the composition and structure of forest trees, (iii) pattern of forest structure in terms of phylogenetic relatedness, and (iv) significance of environmental and spatial processes in driving compositional shift (beta diversity) along an altitudinal gradient.

The second chapter explores the diversity, composition, and structure of forest trees along the altitudinal gradient. The result showed a decrease in species diversity with an increase in altitude. The forest composition varied, with a unique association of species along the altitudinal gradient. Overall, Fagaceae trees reported the highest importance value index, with an extensive contribution to the total basal area. The forest structure overall showed an ideal reverse J-shaped distribution; however, with disproportionate distribution along the altitudinal gradient.

The third chapter assesses the role of anthropogenic disturbance, climate, topography, and soil nutrient variables in driving forest structure and composition along the altitudinal gradient. The result revealed a strong association of topography, climate, and anthropogenic disturbance with the forest composition and structure, however, the strength of associations along the altitudinal gradient varied.