FOREST STRUCTURE UNDER HUMAN INFLUENCE NEAR AN UPPER-ELEVATION VILLAGE IN NEPAL

by Bolton, Gary Howard.

Indigenous human use of forest products may be altering upper-elevation forest structure and resource availability in Nepal. Subsistence villagers extract hand-cut treeleaf fodder for livestock, fuelwood, and poles for construction of herders’ shelters. In a broadleaf evergreen montane forest in west-central Nepal, I described tree species composition and species size-class structures, and investigated relationships among patterns of forest structure and metrics of human use. Mean (SE) density of trees > 10 cm dbh was 817 (30) stems ha-1 and mean (SE) total basal area was 44 (3.2) m2 ha-1. Cluster analysis of density data for 32 tree species suggested the presence of three forest communities: a Symplocos-Quercus community, a Symplocos-mixed evergreen community, and a Rhododendron-Symplocos community. Tree size-class frequency distributions for the most abundant species, S. ramosissima, differed significantly among communities. Size-class distributions of important fodder resource oak species suggest Quercus lamellosa is in decline, but Q. oxyodon and Q. semecarpifolia may be reproducing successfully. Mean tree diameters of both S. ramosissima and Lindera pulcherrima were inversely related to a cutting index of forest product harvest intensity. Cutting index was inversely correlated with distance to village and with elevation. Ordination by principal components analysis of pole, tree, and total species densities and 35 tree basal area indicated a relationship between community structure and harvest intensity, suggesting that extraction of forest products may be an important factor shaping forest composition and structure. Introduction Vegetation research begins with descriptive studies of species composition and vegetation structure. While vegetation in some regions of the world has been described in detail, much of the vegetation in the Himalaya has only been characterized in broad qualitative terms. Further research quantifying Himalayan vegetation types is prerequisite for understanding vegetation dynamics relative to past and current uses, and for creating a baseline for monitoring subsequent vegetation change resulting from climate change and human impacts. Early research in vegetation ecology focused on “pristine” vegetation communities to study composition, structure, and vegetation dynamics free from human influence. Throughout much of the 20th century, plant succession was viewed by many as a predictable pathway leading to a stable climax community (Clements 1916, 1937). In recent decades, however, researchers have increasingly recognized the fundamental role of disturbance in maintaining dynamic nonequilibrium conditions that shape plant communities (e.g., Pickett and White 1985; Denslow 1987; Glenn-Lewin et al. 1992). Increasingly, researchers have also recognized that much vegetation formerly regarded as pristine has been under the influence of indigenous human disturbance for centuries to millennia (Turner et al. 1990). 36 Several authors suggest that indigenous extraction of forest products in the Nepal Himalaya may have had widespread influence on forest composition and structure for more than a century (Mahat et al. 1986a, 1986b; Soussan et al. 1995). In this research, I quantitatively describe tree species composition and community structure of a mixed evergreen-deciduous broadleaf montane forest in west-central Nepal. I then investigate relationships between patterns of forest composition and structure and metrics of indigenous human use of forest products. Forests of Nepal Nepal is a mountainous, landlocked country, stretching 800 kilometers east to west along the central Himalaya, with an average width of 150 kilometers from south to north. Divided into three long strips, Nepal includes a southern lowland plain (Terai), the high Himalaya, and in between, the “Middle Hills,” a rugged region of ridges and valleys between 500 m and 3000 m in elevation. More than half of Nepal’s human population resides in the Middle Hills. Nepal’s climate is controlled largely by the South Asian Monsoon, which brings heavy precipitation to much of the country between June and September. The monsoon approaches Nepal from the Bay of Bengal to the southeast, creating a gradient of decreasing monsoon intensity from east to west. Vegetation in Nepal is controlled by a dramatic gradient of increasing elevation (and consequently decreasing temperature) from south to north, as well as the strong gradient of decreasing precipitation from east to west (Ives and Messerli 1989). 37 Qualitative descriptions of forest vegetation of the Nepal Himalaya were made by