SOME PROBLEMS ON THE [ALTITUDINAL BELTS IN SOUTHEASTERN QINGHAI-XIZANG (TIBETAN) PLATEAU
Zheng Du Yang Qin-ye (Institute of Geography, Chinese Academy of Sciences)
The study area is mainly covered by montane coniferous forests, alpine scrubs and meadows. It is the most humid region in the Qinghai-Xizang Plateau. In general, four types of the altitudinal belt, all pertaining to the oceanic system, may be recognized. The differentiation of the altitudinal belts is correlated with the basal belt. Prom southeastern margin to the interior of the study area, with increasing altitude, in addi-tion to differences of the basal belt there is a decrease in number of altitudinal belts and a simplification of spectra. The regional variation of zonal structures and their al-titudinal limits is conspicuous. Compared with other parts of the world, the upper for-est limit in the eastern Tibet is the highest. A number of dry valleys located in the gorge 'bottoms is one of the striking geo-ecologieal phenomena. The extent, distribution and upper limit of the dry valleys as well as their relationship to the horizontal zonation are discussed. Altitudinal belts of the study area are economically important in providing different areas for agriculture, animal husbandry, forestry, fuel supply and others. The long-term' results of human impacts on fragile mountain environments may be summarized as the expansion of scrubs belt of the dry valley, the lowering of upper forest limits, de-forestations, and dramatical acceleration of soil erosions. Although the population pressure is one of the underlying causes of environmental and natural resource problems in the mountainous areas, the deforestation and misma-nagement of forests are directly responsible for ensuing environmental problems.