Plant Species Richness, Structure and Life Form Respond to an Altitudinal Gradient in Central Bhutan, Eastern Himalayas.

Abstract

Understanding how species richness changes with altitude is important for conservation management. In Central Bhutan, an altitudinal vegetation transect from 2985 m to 3790 m along an eastern spur on Kiki Phu, in Bumthang, was surveyed for plant species richness in autumn (November) 2014. Species richness, life form diversity, canopy cover and ground cover were measured every 100 m of altitude using two 25 m x 25 m quadrats (north and south). For the dominant tree species, height and diameter at breast height (DBH) were measured and compared against altitude. In total 114 species of vascular plant species were found, with a significant decrease in species richness occurring as altitude increased, from 42 species to 124 species, with altitude 3157 m showing a slight increase against this trend. Life form diversity varied significantly with altitude with forbs in particular decreasing in species richness. Maximum canopy cover, tree height and DBH of dominant species occurred at the mid elevations of 3255 m – 3461 m, and these were significantly different on the southern side of the ridge, where canopies were open and trees shorter. Mid altitude patterns may be related to moisture regimes and grazing with dampening effects on canopy cover, tree height and life forms particularly on the southern sides of the ridge. A Principal Coordinates Analysis showed species composition clustered with altitude. These results establish baseline floristic data for autumn which may be important for monitoring change over time and space to assess impacts of anthropogenic pressure and climate change.