Location and General Description
The alpine zone in Tibet includes all areas where the average temperature during July, the warmest month of the year, is not more than 10oC. In fact, almost all of the plateau except the southern river valleys–the Indus, Sutlej, and Tsangpo (including the Lhasa Valley)–falls within this "high cold" alpine zone. Within the alpine zone, moisture determines whether a region supports meadow, steppe or alpine desert vegetation. As precipitation decreases northwestward, vegetation changes from dense scrub to meadow to steppe to desert. In Tibet, forests are confined to valleys. Forests never occur on the plateau due to cold, continental climate and the fact that the plateau lies above the 10o isotherm.
The broad Yarlung Tsangpo (Zambo) River Valley, aligned along the juncture of the Asian and Indian continental plates, lies to the north of the Central Himalayan Crest. It is contained by the Himalaya on one side and Tibetan marginal ranges on the other. The latter includes the Nyenching Tangula and Gangdise Mountain Ranges that run parallel to the Himalaya about 150 to 200 km north.
Due to the rain shadow effect, annual precipitation in the valley ranges from 200 to 500 mm, decreasing westward. Mean annual temperatures are 4 to 8oC and reach a value of 16oC in the Lhasa Valley during July, the warmest month of the year. The presence of a strong foehn (catabatic wind) contributes to warming and drying the Tsangpo Valley. On the south slope of the Himalaya, adiabatic cooling of rising air is offset by the latent heat released as water vapor condenses into fog. Dry air descending the north slope, however, warms at nearly its full catabatic potential as there is little heat expended to evaporate water in this dry environment. The consequence is warmer temperatures in the Tsangpo Valley than occur at a similar elevation on the south slope of the Himalaya.
The Yarlung Tsangpo River flows at a gentle gradient for more than 1,000 km, and as it gradually descends, the vegetation in the valley-bottom undergoes a transition from cold desert to steppe to deciduous scrub vegetation, finally changing to conifer-rhododendron forest at about 93.5oE longitude. Tree line is as low as 3,200 m, probably attributable to lack of moisture rather than low temperature.
Along the river’s middle reaches, vegetation in the valley-bottom consists of grasses and scattered forbs and shrubs that may be scattered or clumped depending on water availability. Typical vegetation on both the north and south slopes of the valley consists of steppe and shrublands. Dominant plants of the steppe include grasses and forbs such as Stipa bungeana, Pennisetum flaccidum, Aristida triesta, Orinus thoroldii, Artemisia webbiana, and Trikeraia hookeri. Of these, the first two occur throughout Central Asia while the latter four are endemic to the Tibetan Plateau. Shrubs include Sophora moorcroftiana, Leptodermis sauranja, Ceratostigma griffithii, and others, most of which are endemic to the Tibetan Plateau.
At elevations above 4,400 m on the sides of the valley, cold steppe vegetation replaces the temperate steppe shrublands of the lower slopes. Species richness declines and dominant plants include the grass Stipa purpurea and shrubs Potentilla fruticosa and Lonicera tibetica in the east, with Caragana versicolor in the west. Where hills extend above 5,000 m, Kobresia pygmaea and cushion plants like Arenaria, Androsace, and Oxytropis dominate stable slopes.
The Tsangpo River Valley, its tributaries and nearby basins to the south are the main human population center for Tibet. Local people grow crops (barley, buckwheat, and potatoes), and animal husbandry is an important economic and subsistence activity. As a result of this human activity, the Tsangpo Valley supports less wildlife than colder, less populated areas to the north. Argali (Ovis ammon) are reported from historical records, and blue sheep (Pseudois nayaur) and Tibetan gazelles (Procapra picticaudata) occur here now. White-lipped deer (Cervus albirostris) are also recorded in the part of this ecoregion that lies to the east of Lhasa, but the other deer of eastern Tibet are probably confined to the forest and alpine scrub areas. Tibetan eared-pheasant (Crossoptilon harmani) and the closely related white eared-pheasant (Crossoptilon crossoptilon) are also reported from steppe habitat at the lower end of the Tsangpo River Valley.
Climate change during the Holocene has had a significant effect on the vegetation of the Yarlung Tsangpo Valley and on Tibet as a whole. Areas of steppe today supported forests of birch (Betula spp.), hemlock (Tsuga spp.), and oak (Quercus spp.) until about 3,000 years ago, but since that time the climate has become too cool and dry to support this kind of forest vegetation.
Evidence also exists for more recent anthropogenic change as exemplified by conditions in the Lhasa Valley, along a tributary of the Yarlung Tsangpo River. Here temperature (15.5oC average for July) and precipitation (450 mm per year) predict forest similar to that of a Himalayan inner valley. The fact that trees are absent here is probably due to the long history of human activity in the Lhasa Valley.
According to Georg Miehe (1988) it is possible to see evidence today of lower and upper belts of alpine vegetation that are floristically similar to that expected in a Himalayan valley. Here the upward transition to the alpine zone is marked at 4,100 m by darker soils and a higher plant cover, compared to the slopes below where tree cover has been removed. The transition to the upper alpine zone is indicated at 4,300 m by replacement of tall forbs by flat or hemispherical cushion plants and patches of Kobresia sedge, depending on wind exposure.
Types and Severity of Threats
Wild mammal populations have been reduced in this area by centuries of human occupation. Recent human population growth and social change in this most densely settled part of the Tibetan Plateau has had a further detrimental effect on wildlife and remaining habitat.
Justification of Ecoregion Delineation
Ecoregion lines were derived from a digital map of Tibetan rangelands (Commission on Integrated Survey of Natural Resources 1992). The region combines ‘warm and cool semi-arid montane rangeland,’ ‘warm and semi-arid montane scrub grassland,’ and ‘warm, cool, and moist subalpine sparse forest and scrub meadow and grassland.’
Chang, D.H.S. 1981. The Vegetation Zonation of the Tibetan Plateau. Mountain Research and Development 1(1): 29-48.
Commission on Integrated Survey of Natural Resources. 1992. Rangeland Types
of Xizang (Tibet) (1:2,500,000). Beijing, China.
Miehe, G. 1988. Geoecological reconnaissance in the alpine belt of Southern Tibet. GeoJournal 17: 635-648.
Schaller, G.B. 1998. Wildlife of the Tibetan Steppe. The University of Chicago Press, Chicago.
Prepared by: Chris Carpenter
Reviewed by: In process