Distribution pattern of trees along an elevation gradient of Eastern Himalaya, India

We examined the patterns of tree species richness, density and basal area in the Eastern Himalayan elevation gradient, and evaluated the roles of geometric constraints and environmental factors for the observed pattern. We followed the quadrat sampling method for this study at 23 different locations along a 300–4700 m elevation gradient in Sikkim. We used regression analysis to establish elevational pattern of trees (species richness, density and basal area) and to relate environmental factors with tree species and density. We also tested geometric constraints on species distribution by comparing empirical species richness with null model predictions generated by a Monte Carlo Simulation Programme (McCain, 2004). We observed a total of 3874 individual trees belonging to 171 species and 58 families. Tree species richness depicted unimodal pattern with a peak at 1500 m, whereas basal area peaked at 1900 m. Although density did not follow a uniform pattern, maximum value was observed at 3050 m. Climatic and energy related factors–rainfall, temperature, moisture index (MI) and actual evapotranspiration (AET)-correlated positively with tree species richness. The Mid-domain effect null model showed significant deviation of empirical richness from simulated species richness. Stepwise regression revealed that combined effects of energy (surrogated by temperature), productivity (surrogated by AET) and precipitation (reflected by MI) influenced observed tree species patterns in the Eastern Himalayan elevation gradient although historical and spatial factors might interplay.