FOREST ECOSYSTEM STRUCTURE AND COMPOSITION ALONG AN ALTITUDINAL GRADIENT IN THE WESTERN GHATS, SOUTH INDIA

Abstract

Vegetation structure and composition of deciduous and evergreen forest ecosystems were studied along an altitudinal gradient (250-1150 m) at Kodayar in the Western Ghats of South India. Plants >10cm in DBH were enumerated to measure the diversity, dominance, similarity, evenness and species richness indices of plant communities and also to assess the regeneration status of tree species. A total of 58, 77,125 and 105 plant species belonging to 30, 28, 52 and 45 families were recorded in moist deciduous forests (MDF, sites I & II), an evergreen forest (EF, site III) and a forest at higher elevation (HEF, site IV) respectively. Species diversity indices (Shannon index) of tree community were 2.20, 2.37, 2.65 and 2.48, ranking low compared to other studies available in the Western Ghats. Terminalia paniculata, Pterocarpus marsupium and Aporosa lindleyana were the dominant species in the moist deciduous forests, whereas Hopea parviflora, Vateria indica and Xanthophyllum flavescens dominated in the evergreen forest. The forest at higher elevation was dominated by Agrostistachys meeboldii, Cuttenia excelsa and Drypetes oblongifolia. Stem
density and basal area of the evergreen forest (748 trees ha^-1, 81.38 m² ha^-1) were twice those of the moist deciduous forests (352-450 trees ha^-1,28.05-33.77 m² ha^-1), while the forest at higher elevation showed higher density and lower basal area (1173 trees ha^-1,72.72 m² ha^-1). The "L" shaped curve of different DBH classes of trees and saplings indicated good regeneration in these forests. Eupatorium and Ageratum are two dominant exotic weeds that form part of the ecosystem in open and disturbed sites here. The changes in species composition are largely due to transition in vegetation types influenced by anthropogenic perturbations and other abiotic factors.