Forest structure and tree diversity along an altitudinal gradient in Sikkim Himalaya

Abstract

The vegetation types of Sikkim Himalayas more particularly Southern Sikkim has not been studied before. This study is the first comprehensive one on the tree species diversity and composition across four forest types (Tropical semi evergreen forest (TSEF), Sub-tropical broad mixed leaved hill forest (STBLHF), Himalayan wet temperate forest (HWTF) and Sub alpine Forest (SAF) in the region. We employed quadrat sampling and all the trees > = 10 cm DBH at 1.37m height were considered for the purpose. A total of 130 tree species belonging to 91 genera and 52 families were recorded in the study sites of which Moraceae and Lauraceae represented the highest number of species (8 each). The tropical semi evergreen forest (TSEF) harboured 61 species, followed by sub-tropical broad mixed leaved hill forest (STBLHF) (59), Himalayan wet temperate forest (HWTF) (43) and sub-alpine forest (SAF) (25). The Shannon-Weiner diversity index (H’) in TSEF, STBLHF, HWTF and SAF was 3.31, 2.92, 2.82, and 2.34 respectively. The H’ showed a linear relation with elevation (R2 = 0.9265, p < 0.05) and the Margalef’s species richness (S) indicated a significant quadratic relation (R2 = 0.9996, p < 0.05) with elevation. A hump-shaped pattern was observed for tree species richness while the DBH size class showed a reverse J shaped pattern. It concluded that tree species diversity in South Sikkim Himalayas is primarily influenced by elevation, showing a linear decline in H′ and a hump-shaped pattern for species richness, peaking at mid-elevations. The reverse J-shaped DBH distribution indicates active regeneration, further supporting the observed diversity pattern.