Tree diversity and biomass carbon stock along an altitudinal gradient in old-growth secondary semi-evergreen forests in North East India

Forest structure and composition change are driven by complex interactions among local site factors, past site history, and region-specific socio-demographic factors. Previous studies suggest the structural, functional, taxonomic, and carbon storage potential of vegetation increases during succession. However, information on these attributes along an altitudinal gradient from the North East India is lacking for the old growth secondary forests. Therefore, the present study investigates (i) tree diversity and biomass carbon storage, and (ii) dominating and carbon important tree species, along an altitudinal gradient (30–700 m) in the tropical semi-evergreen forests of southern Assam. Random quadrats of 0.1 ha were laid over selected elevation gradient of secondary forest to evaluate tree diversity, density, basal area, and biomass carbon stock. Shannon–wiener index increases from 2.88 (at 30 m) to 3.33 (at 700 m) indicating an increase in species diversity with elevation. The stand density ranges from 920–1630 stems ha−1 and reciprocally related to the altitude. Palaquium polyanthum, Dysoxylum binectariferum, and Maniltoa polyandra were the most dominant species at 30 m, 300 m and 700 m, respectively. Biomass carbon stock decreases with the increase in altitude and ranges from 71.20 to 77.47 Mg ha−1. Palaquium ployanthum, Sterculia sp. and Syzygium cumini were the most carbon important species at 30 m, 300 m and 700 m, respectively. Present study suggests conservation and management of old-growth secondary forests have carbon storage and regional climate benefits. Management of such secondary forests can also enhance the resiliency of forest ecosystems to nature conservancy.