Resilience or vulnerability? Vegetation patterns of a Central Tibetan pastoral ecotone

The question of the resilience or vulnerability of Tibetan highland pastures is notonly of regional importance but also of global relevance for the Earth's energy budgetand atmospheric circulation. A climate and grazing driven environmental change of theTibetan highland albedo could induce feedback effects within an ecotone ofapproximately 200 km between Cyperaceae-dominated grazing-lawns and alpine steppesstretching over 2000 km between the Qilian Shan (38°N) and the Himalaya (28°N).Understanding the vegetation structure of this region would enable accurate modelingapproaches for global change scenarios. We analyzed the Central Tibetan pasture vegetation in the wider Nam Co area usingphytosociological vegetation surveys and remote sensing techniques (ASTER, SRTM).The survey described in this chapter recorded a total of 105 vegetation relevés, with atotal of 233 species. The species-based approach using DCA and phytosociologicalclassification distinguishes six plant communities including juniper woodlands andjuniper open dwarf shrublands, alpine steppe communities, Kobresia pygmaeacommunities and transitional mosaics with fragments of Kobresia turf and alpine steppe.The most common community was the transitional mosaic of Kobresia pygmaea matsand alpine steppe. Except for the phanerophytic communities and the grazing lawns ofKobresia pygmaea, plant communities are characterized by low overall plant cover withtwo thirds of species showing values lower than 1%. An analysis of plant functional typesshows that most species are adapted to intense grazing regimes. The prevalence of dwarfgrowth forms, rhizomes and repellent characteristics shows that the pastures are grazingresilient. By contrast, the fragmented turf cover is vulnerable and endangered by intensegrazing, digging by small mammals, turf removal and periglacial weathering.