Plant richness pattern in an elevation gradient in the Eastern Himalaya

In spite of many theoretical explanations, the plant richness pattern along the mountain elevation gradient is still debatable. Here we considered East district of Sikkim state in the Eastern Himalaya to study plant richness pattern of vascular plants along an elevation gradient of 500–3300 m at 100, 200 and 300 m elevation steps. The study evaluates species-area relationships along the elevation gradient, spread over four major forest types i.e., low (tropical to sub-tropical, up to 900 m), middle (subtropical, 750–1500 m), upper (wet temperate, 1500–2700 m) and Rhododendron-Conifer Zone (cold temperate/sub-alpine, 2700–3600 m) with 664 species from 367 genera and 131 families. Pteridium aquilinum was found to be the most diverse species occurring in all 28-elevation steps along 500–3300 m, followed by Polystichum lentum, Polygonum runcinatum and Nephrolepis cordifolia those occured in 23 elevation steps. In general, a hump shaped pattern was observed for plant richness of all life forms along the elevation gradient. The peaks became prominent, with the increase in scale of extent from 100 m (R² = 50%) to 300 m (R² = 78%) through 200 m (R² = 55%) elevation steps, using 2nd order polynomial fitting. The reduction in tree height and richness was noticed beyond 2300 m allowing dominance of herbs owing to climatic constraints. Using generalized additive model, temperature could explain the maximum deviance of > 47%. Soil explained 36.4% deviance, followed by precipitation (21.6% deviance) in the plant richness. However, the ecotone effect of different forest types explained the mid-elevation peaks in plant richness more prominently than the geographic area availability. Further, inclusion of disturbance and biotic interactions may improve ecological understandings on the plant richness pattern along the elevation gradient.