Included here is the Tibetan Plateau bordered by the eastern Pamirs and eastern Hindo Kush Mountains to the west, the Karakorum Range and Himalayas to the south, the Bayan Khara Shan Mountains to the east and the Transalai (Zaalai) Range to the north It is the highest, largest and youngest plateau on Earth with a mean elevation exceeding 4500 m.

Tibetan High-Cold Stipa Steppe

Above an altitude of about 4400 m, the most extensive vegetation of the plateau is purple feathergrass (Stipa purpurea) steppe and has its centre of distribution on the Chiangtang Plateau. Often co-dominant with Stipa is the endemic sedge Carex moorcroftii (Cyperaceae).

Tibetan Xeric Shrublands

Often found in conjunction with steppe communities, these shrublands are characterized by low growing shrubs such as Potentilla fruticosa and several endemic species such as Caragana versicolor (Fabaceae) in the west and Lonicera tibetica (Caprifoliaceae) in the east. Other endemic, xerophilic shrubs found at these altitudes are Buddleia tibetica (Buddleiaceae) and Sophora moorcroftiana (Fabaceae). The latter is typically found on mobile dunes.

Tibetan Alpine Kobresia Meadow

Roughly between altitudes of 4600-5400 m in the east and 5000-5600 m in the west, high-cold meadows dominated by species of Kobresia are found. This is considered to be the world’s largest alpine ecosystem. The Tibetan Plateau also appears to be the diversity center for this genus with species permeating into the surrounding mountains, and most of them are endemic to Tibet or Central Asia. Kobresia pygmaea is often the main species, while others include K. curvata, K. humilis, K. macrantha, K. robusta, K. royleana and K. tibetica. Other characteristic species of these meadows are Anapholis xylorrhiza, Carex atrata var. glacialis, Leontopodium pusilum, Meconopsis horridula, Polygonum sphaerostachyum, P. viviparum, Potentilla stenophylla and the two endemic cushion plants Androsace tapete (Primulaceae) and Arenaria musciformis (Caryophyllaceae). On some of the more level areas with impeded drainage and in wet valleys, high-cold swampy meadows dominated by the mound-like growth forms of Kobresia littledalei occur, but by and large, these alpine Cyperaceae formations are dominated by endemic mats of Kobresia pygmaea. It is the smallest of the high Asian Cyperaceae usually growing to no more than about 2 cm tall, but in some places it can achieve up to 90% cover. It does this by forming dense clones that can spread vegetatively. The lowest altitude examples of Kobresia pygmaea mats have been recorded at about 3000 m in the montane zones of northeastern Tibet, while the highest examples have been recorded at 5960 m on the north face of Mount Everest. Kobresia pygmaea can occur in almost pure, completely closed stands but it is often interspersed with cushion forming species like Androsace tapeta, Arenaria bryophylla, A. kansuensis, Leontopodium nanum or barren turfs colonized by lichens. Between November and March, when temperatures can drop to – 30oC, the turf can remain frozen solid and even throughout the growing season it experiences nightly frosts. Needle-ice solifluctions are common and this together with gelifluctions can break up the turf. In places polygonal cracks can also appear probably caused by a combination of frost and desiccation. When the turf cover is completely destroyed the remaining sandy of silty substrate is usually colonized by pioneer communities of cushion plants of high alpine or steppe origin together with tall forbs such as Aconitum flavum, A. gymnandrum and Urtica hyperborea, and rosette species such as Eritrichium microcarpum, Lagotis brachystachya, Lasiocaryum densiflorum, Microcaryum pygmaeum, Oreosolen wattii, Persicaria glacialis, Potentilla bifurca and the endemic or near endemic Lancea tibetica (family?), Microgynoecium tibeticum (Chenopodiaceae), Microula tibetica (Boraginaceae), Pomatasace filicula (family?) and Przewalskia tangutica (Solanaceae). These pioneer communities have a high proportion of endemic monotypic genera. The extremely firm turf the Kobresia pygmaea produces is thought to protect large areas against erosion including the headwaters of the Brahmaputra, Huang He, Mekong, Salween and the Yangtze on which about one quarter of the world’s population depend. There are now concerns that excessive erosion caused by overgrazing could have a catastrophic impact on these major river systems affecting millions of people in the surrounding lowlands.

Tibetan High Cold Desert

In the north, such as the northernmost parts of Chiangtang (northern Tibet), high cold desert conditions prevail. The landscape shaped by glaciations and frost weathering, is largely composed of rocky slopes and rubble-strewn valleys. Here the two endemic species Carex moorcroftii (Cyperaceae) and Ceratoides compacta (Chenopodiaceae) make up the bulk of the vegetation. The coldest and driest climate in Tibet occurs in the northwestern parts of the Chiangtang Plateau. Here there is an almost non-existent growing season. The few scattered plants include the endemic Ceratoides compacta (Chenopodiaceae), which rarely achieves more than about 8 % ground cover, and its few companion species include Pegeophyton scapiflorum and the endemic Hedinia tibetica (Brassicaceae). In the slightly warmer southern areas, plants such as Pennisetum flaccidum and several endemic species such as Artemisia wellbyi (Asteraceae) and Orinus thoroldii (Poaceae) may be found.  On the western Ali Mountain plateau, the cold desert community mainly consists of Ajania fruticulosa, the suffrutescent Cerotoides latens (a relict of the ancient Tethys flora) and the endemic perennial Christolea crassifolia (Brassicaceae). Above a altitude of about 4500 m feathergrasses such as Stipa brevifolia, S. glareosa and S. subsessiliflora enter the community, and on Mount Chiangchenma this desert vegetation reaches an altitude of 5200, where it forms the world’s highest desert.

References

Chang, D. H. S. 1981. The vegetation zonation of the Tibetan Plateau. Mountain Research and Development, 1: 29-48.

Miehe, G., Miehe, S., Kaiser, K., Jianquan, L. & Zhao, X. 2008. Status and dynamics of the Kobresia pygmaea ecosystem on the Tibetan Plateau. Ambio, 37: 272-279.

Walter, H. & Box, E. O. 1983. Deserts of Central Asia. In: Ecosystems of the World 5 - Temperate Deserts and Semi-Deserts. Ed. N. E. West. Elsevier Scientific Publishing Company.

Wang, W., Wang, Q., Li. S. & Wang, G. 2006. Distribution and species diversity of plant communities along a transect on the northeastern Tibetan Plateau. Biodiversity and Conservation, 15: 1811-1828.

Ward, F. K. 1936. A sketch of the vegetation and geography of Tibet. Proceedings of the Linnean Society of London, 148: 133-160.