Forest vegetation of the Himalaya

Abstract

This review deals with the forest vegetation of the Himalaya with emphasis on: paleoecological, phytogeographical, and phytosociological aspects of vegetation; structural and functional features of forest ecosystem; and relationship between man and forests. The Himalayan mountains are the youngest, and among the most unstable. The rainfall pattern is determined by the summer monsoon which deposits a considerable amount of rain (often above 2500 mm annually) on the outer ranges. The amount of annual rainfall decreases from east to west, but the contribution of the winter season to the total precipitation increases. Mountains of these dimensions separate the monsoon climate of south Asia from the cold and dry climate of central Asia. In general, a rise of 270 m in elevation corresponds to a fall of 1°C in the mean annual temperature up to 1500 m, above which the fall is relatively rapid. Large scale surface removals and cyclic climatic changes influenced the course of vegetational changes through geological time. The Himalayan ranges, which started developing in the beginning of the Cenozoic, earlier supported tropical wet evergreen forests throughout the entire area (presently confined to the eastern part). The Miocene orogeny caused drastic changes in the vegetation, so much so that the existing flora was almost entirely replaced by the modern flora. Almost all the dominant forest species of the Pleistocene continue to maintain their dominant status to the present. Presently the Himalayan ranges encompass Austro-Polynesian, Malayo-Burman, Sino-Tibetan, Euro-Mediterranean, and African elements. While the Euro-Mediterranean affinities are well represented in the western Himalayan region (west of 77°E long.), the Chinese and Malesian affinities are evident in the eastern region (east of 84°E long.). However, the proportion of endemic taxa is substantial in the entire region. A representation of formation types in relation to climatic factors, viz., rainfall and temperature, indicates that boundaries between the types are not sharp. Formation types often integrate continuously, showing broad overlaps. Climate does not entirely determine the formation type, and the influence of soil, fire, etc., is also substantial. The ombrophilous broad leaf forests located in the submontane belt (<1000 m) of the eastern region are comparable to the typical tropical rain forests. On the other extreme, communities above 3000 m elevation are similar to sub-alpine and alpine types. From favorable to less favorable environments, as observed with decreasing moisture from east to west, or with decreasing temperature from low to high elevations, the forests become increasingly open, short-statured and simpler, with little vertical stratification. Ordination of forest stands distributed within 300-2500 m elevations of the central Himalaya, by and large indicates a continuity of communities, with scattered centers of species importance values in the ordination field. Within the above elevational transect, sal (Shorea robusta) and oak (Quercus spp.) forests may be designated as the climax communities, respectively, of warmer and cooler climates. The flora of a part of the central Himalayan region is categorized as therohemigeophytic and that of a part of the western Himalayan region as geochamaephytic. An analysis of population structure over large areas in the central Himalaya, based on density-diameter distribution of trees, suggests that old-growth forests are being replaced by even-aged successional forests, dominated by a few species, such as Pinus roxburghii. Paucity of seedlings of climax species, namely Shorea robusta and Quercus spp. over large areas is evident. The Himalayan catchments are subsurface-flow systems and, therefore, are particularly susceptible to landslips and landslides. Loss of water and soil in terms of overflow is insignificant. Studies on recovery processes of forest ecosystems damaged due to shifting cultivation or landslides indicate that the ecosystems can recover quite rapidly, at least in elevations below 2500 m. For example, on a damaged forest site, seedlings of climax species (Quercus leucotrichophora) appeared only 21 years after the landslide. In the central Himalaya, the biomass of a majority of forests (163-787 t ha^-1) falls within the range (200-600 t ha^-1) given for many mature forests of the world, and the net primary productivity (found in the range of 11.0-27.4 t ha^-1 yr^-1 is comparable with the range of 20-30 t ha^-1 yr^-1 given for highly productive communities of favorable environments. In most of the forests of this region, the litter fall values 2.1-3.8 t C ha^-1 yr^-1 are higher than the mean reported for warm temperate forests (2.7 t C ha^-1 yr^-1). Of the total litter, the tree leaves account for 54-82% in the Himalayan forests. The rate of decomposition of leaves in some broadleaf species of submontane belt (0.253-0.274% day^-1) are comparable with those reported for some tropical rain forest species. Because of the paucity of microorganisms and microarthropods in the forest litter and soil, high initial C:N ratio and high initial lignin content in leaves, the rate of leaf litter decomposition in Pinus roxburghii is markedly slower than in other species of the central Himalaya. The fungal species composition of the leaf litter of Pinus roxburghii is also distinct from those of other species. A greater proportion of nutrients is accumulated in the biomass component of the Himalayan forests than in the temperate forests. Although litter fall is the major route through which nutrients return from biomass to the soil pool, a substantial proportion of the total return is in the form of throughfall and stemflow. Among the dominant species of the central Himalaya, retranslocation of nutrients from the senescing leaves was markedly greater in Pinus roxburghii than in Quercus spp. and Shorea robusta. Consequently, the C:N ratio of leaf litter is markedly higher in Pinus roxburghii than in the other species. Immobilization of nutrients by the decomposers of the litter with high C:N ratio is one of the principal strategies through which Pinus roxburghii invades other forests and holds the site against possible reinvasion by oaks. Observations on the seasonality of various ecosystem functions suggest that Himalayan ecosystems are geared to take maximum advantages of the monsoon period (rainy season). Most of the human population depends on shifting-agriculture in the eastern region and on settled agriculture in the central and western regions. Either of these is essentially a forest-dependent cultivation. Each unit of agronomic energy produced in the settled agriculture entails about seven units of energy from forests. Consequently, forests with reasonable crown cover account for insignificant percentage of the land. Tea plantations and felling of trees for timber, paper pulp, etc., are some of the major commercial activities which adversely affected the Himalayan forests.