Location and General Description
The Tsangpo (Zangbo) River originates near Mount Kailas in the southwestern part of the Tibetan Plateau and flows for more than 1,000 km at elevations above 3,000 m before entering the deeply eroded river gorge area of southeastern Tibet. Here the Tsangpo cuts through the Himalaya, dropping more than 2,000 m over a short distance to emerge on the southern slopes as the Brahmaputra River. While the upper reaches of the Tsangpo lie in the rain shadow of the Himalaya and support arid steppe vegetation, the middle section flows through valleys that draw considerable moisture from the South Asian monsoon. Here, extensive subalpine coniferous forests occur on the valley slopes of the Tsangpo and its tributaries. Similar forests are distributed throughout the eastern Himalayan region where elevation and precipitation patterns are appropriate, usually from 3,000 to 4,000 m elevation in valleys that are partially shielded from the monsoon.
Subalpine coniferous forests occupy Himalayan "inner valleys" where the force of the South Asian monsoon is diminished by intervening mountain ranges, but where precipitation is still adequate to support lush forest vegetation. These forests tend to occur within an elevation belt of 2,500 to 4,200 m. Vertical relief in this area is exceptionally high, and the boundaries of this ecoregion are as convoluted as the corresponding contour intervals on a topographic map of the area. Furthermore, because of the altitudinal zonation that occurs here, the distribution of ecoregions here is highly stratified.
Within the subalpine coniferous forest belt, dominant trees (in order of elevation) include hemlock (Tsuga dumosa), spruce (Picea smithiana), fir (Abies spp.), and other species. Where spruce and fir grow together in the same watershed, it is easy to distinguish the two by the color of their foliage The smaller spruce are a pale blue-green while the larger fir appear almost black. Near tree line, pure stands of arboreal juniper (Juniperus indica, J. recurva) or shrub juniper (J. recurva, J. squamata) may occur on dry sites. Other conifers that occur in this zone include the winter deciduous larch (Larix griffithiana, L. potaninii), pine (Pinus wallichiana), and yew (Taxus baccata).
Below the subalpine belt, coniferous forests give way to stands that are dominated by broad-leaved trees (either evergreen or deciduous), although some lower-elevation conifers like Tsuga dumosa, T. chinensis, Taxa baccata, or Pinus wallichiana may also occur. The upper limit of the subalpine forest is defined by an altitudinal tree line, generally the elevation at which the maximum temperature during the warmest month of the year is no more than 10oC. Above tree line, moist alpine scrub of juniper and rhododendron together with deciduous shrubs such as Berberis spp., Rosa spp., Salix spp., and Lonicera spp. occupies moist, sheltered sites. Turfs of Kobresia spp. sedge and a rich assemblage of alpine forbs grows on drier, more exposed sites.
From a distance, the subalpine forests appear to consist entirely of conifers because these trees are tall enough to obscure other vegetation. In fact, it is common, especially in the wetter areas, to encounter a flourishing lower canopy of broad-leaved deciduous trees. Birch (Betula utilis) is probably the most common of these broadleaved subalpine associates. Others include various species of Acer spp., Magnolia spp., Sorbus spp., Viburnum spp., and various high elevation species of Lauraceae and Aaralliaceae.
Rhododendron spp. are also an extremely important component of the northeastern Himalayan subalpine coniferous forest ecosystem. The Eastern Himalaya is the global center of diversity for this genus, and species richness seems to increase above 2,000 m to a maximum in the subalpine forest belt. According to F. Kingdon Ward who wrote about the area in 1926 (quoted by Robert Fleming) "If we include the gorge of the Tsangpo in Pemakö, the number of Rhododendrons alone amounts to some sixty species."
Some Rhododendron species form shrubs, but many such as R. grande and R. arboreum grow as large trees. Higher still, at alpine elevations, Rhododendron species like R. setosum and R. nivale form dwarf shrubs above 4,500 m with leaves less than 1 cm in length. In contrast, the leaves of R. hodgsonii, an associate of subalpine fir forests 1,000 m lower, have leaves that may exceed a half meter in length.
Although tree species richness is probably less than in the temperate forest at lower elevation, standing biomass in these forests is very high due to the large size of the trees. In similar forests of eastern Nepal, the species richness of vascular epiphytes is less than the temperate forests of lower elevation, but the subalpine forests appear to have a very rich assemblage of lichens and epiphytic mosses. Certain taxa such as Ericaceae (Rhododendron spp., Vaccinium spp. and others) have many species at these elevations.
Threatened mammals include red pandas (Ailurus fulgens), takins (Budorcas taxicolor), musk deer (Moschus moschiferus), and other Himalayan forest species including red goral (Naemorhadus baileyi), Asiatic black bears (Ursus thibetanus), and leopards (Panthera pardus). Musk deer are especially abundant in the valley of the Yi’ong River and its tributaries (Tangmai County, Tibet) where they are hunted by the local people. Red goral occurs primarily in southeast Tibet where authorities have established a reserve specifically for this goat-antelope. It is also found in Assam, Yunnan, and northern Burma.
The lower reaches of the Tsangpo River and its tributaries form the eastern part of the Southern Tibet endemic bird area. Two restricted-range species endemic to southern Tibet occur in the subalpine zone and in adjacent alpine scrub at elevations of 3,000 to 5,000 m. Both of these, the vulnerable Tibetan eared-pheasant (Crossoptilon harmani) and the closely related white-eared pheasant (Crossoptilon crossoptilon) are reported to be locally common. The giant babax (Babax waddelli) is another restricted range species that inhabits the coniferous forests and alpine scrub of this ecoregion.
Types and Severity of Threats
In many areas, the extent of timber harvesting is limited by the steepness and inaccessibility of the terrain. Local people may take some trees for their own use (mainly construction timber and fuelwood), but commercial felling is impractical. In other areas, especially the river valleys of southeastern Tibet, road construction enabled much commercial timber harvesting to occur in the 1980s and early 1990s. More recently, the government of the Tibetan Autonomous Region has begun to express a greater interest is forest conservation.
Justification of Ecoregion Delineation
These subalpine conifer forests of pine, hemlock, spruce and fir grow on valley slopes where rivers of the Tibetan Plateau cut through the Himalaya. Ecoregion lines were derived from a digital map of Tibetan rangelands (Commission on Integrated Survey of Natural Resources 1992) using a combination of ‘warm and moist forest grassland’ and ‘warm, cool and moist subalpine sparse forest and scrub meadow and grassland.’ According to the CVMCC (1979) Vegetation Map of China, this area is represented as cold temperate, coniferous forest mix (4a, 4b).
Chang, D.H.S. 1981. The Vegetation Zonation of the Tibetan Plateau. Mountain Research and Development 1(1): 29-48.
Chinese Vegetation Map Compilation Committee. 1979. Vegetation map of
China. Map (1:10,000,000). Science Press, Beijing, China.
Commission on Integrated Survey of Natural Resources. 1992. Rangeland Types
of Xizang (Tibet) (1:2,500,000). Beijing, China.
Mackinnon, J., M. Sha, C. Cheung, G. Carey, Z. Xiang, and D. Melville. 1996. A biodiversity review of China. World Wide Fund for Nature, Hong Kong.
Schaller, G. B. 1998. Wildlife of the Tibetan Steppe. The University of Chicago Press, Chicago.
Stattersfield, A. J., M.J. Crosby, A.J. Long and Devid C. Wege 1998 Endemic Bird Areas of the World: Priorities for Biodiversity Conservation. BirdLife International, Cambridge, UK
Ward, F. K. 1926. The Riddle of the Tsangpo Gorges. Edward Arnold, London.
Prepared by: Chris Carpenter
Reviewed by: In process