FOREST ZONATION ALONG THE COMPLEX ALTITUDINAL GRADIENTS IN A DRY VALLEY OF PUNATSANG CHU, BHUTAN

Abstract

Forest zonations along the complex altitudinal gradients at Lungchozeykha series (1520 - 3370 m) in upper Lobesa, Bhutan were studied to evaluate the major environmental factors of temperature, moisture, human use, etc that characterize specific forest ecosystem traits from warm, dry, heavily disturbed valley bottom to cool, humid, less disturbed ridge tops. An inverse relationship between soil moisture and air temperature was observed along the altitudes. The floristic compositions of fifteen plot data selected at different altitudes were composed of thirty four (34) families with eighty three (83) tree species. The whole altitudinal series were grouped into five forest zones based on floristic similarity; 1, Warm-dry Pine; 2, Moist Evergreen Broad-leaved; 3, Moist/Wet Evergreen Broad-leaved; 4, Cool Coniferous and 5, Cold Coniferous forests. Maximum height and maximum diameter of trees increase with altitude from 14.6 m at 1520 m to 47 m at an altitude of 2950 m and from 35.5 cm at 1520 m to 132.5 cm at 3370 m, respectively. High number of species and high stem density were observed at mid-altitudes, 1750 m, 1860 m and 2820 m. The dominant species change along the altitude in relation to both temperature and moisture gradients, from warm-dry pine through warm/moist mixed broadleaved to humid conifers of Abies and Tsuga forests. Accordingly, from population structure of major forest dominants, three major regeneration types were identified as sporadic in the warm-dry pine zone, sporadic and inverse-J in the evergreen broad-leaved forest and uni-modal type in the humid conifer zone. The regeneration under each zones were affected by the human interventions particularly grazing, partly due to open access of cattle grazing. The forest biomass is estimated to be high in humid conifer and very low in warm-dry pine forest. The total basal area of 15.2 m2/ha at 1520 m are increased to 101.7 m2/ha at 3370 m and a litter accumulation of 1.1 t/ha in pine also increased to 14.8 t/ha in the cool humid coniferous forest. The organic carbon (C) and nitrogen (N) are in the increasing trend with the increase of altitude while pH decreases. High C:N ratio were inversely related with Ca:Mg ratio thus indicating high organic matter content and high leaching at higher altitude brought about by low temperature, low pH and high precipitation. The anthropogenic factors such as grazing, firewood and timber collection were also considered and the extents of impacts were measured to study the degree of human impacts on the natural forest. The result shows an intense and continuous human utilization of forest resources at the lower, but sporadic forest felling at the middle and weak but common grazing by cattle with infrequent forest fires at the higher altitudes.