Aim: To present a first description of plant communities of the Tibetan alpine steppes based on floristically complete vegetation records as a baseline reference for future ecological and palaeoecological studies. These constitute the world's largest alpine biome, but their vegetation is virtually unknown. Due to their vast extent, they are relevant for functioning of large-scale climatic systems. In turn, arid and alpine biomes are suspected to be highly sensitive to ongoing climate change, underwent climate-driven changes during the Last Glacial Maximum and have been subject to overgrazing and desertification.Location: Northwestern Tibetan highlands (China: Xizang, Qinghai), 4200 to 5400 m a.s.l., total area ca. 800 000 km 2 .Methods: Two hundred and fifty-three vegetation records with absolute percentage cover were classified based on expert knowledge and analysed by DCA; composition of plant functional types related to grazing resilience was also assessed.Results: Ten communities of alpine steppe were distinguished. A set of 11 alpine steppe species is distributed throughout the highlands, with a precipitation gradient between 350 mm yr À1 (southeast) and 20 to 50 mm yr À1 (northwest). The elevational range of more than 80% of species is larger than 1000 m. The data set comprises 30% endemic species, with ten endemic genera.
Conclusions:The wide thermal and hygric range of many species and high rate of endemism do not support the idea of high sensitivity to climate change or occurrence of past climate-driven extinctions. The prevailing plant functional types are grazing resilient, and evidence for overgrazing is very limited. Cushion plants and dwarf shrubs, however, become increasingly rare around settlements, because they are uprooted for fuel. Hence, the world's largest arid alpine biome is apparently resilient to climatic changes and grazing. This contradicts common perceptions about arid and alpine ecosystems and therefore deserves intense multi-disciplinary research efforts.
Arid and Alpine ecosystems are known for extreme environmental changes during the Late Quaternary. We hypothesize that the world's largest Alpine arid ecosystem however, the Alpine Steppes of the Tibetan highlands, remained ecologically stable during the LGM and the mid-Holocene. This hypothesis is tested by distributional range of plant species, plant life forms and rate of endemism. The set of character species has a precipitation gradient between 50 and 350 mm/a, testifying for resilience to precipitation changes. 83% of the species have a wider vertical range than 1000 m used as a proxy for resilience to temperature changes. 30% of the species are endemic with 10 endemic genera, including plate-shaped cushions as a unique plant life form. These findings are in line with palaeo-ecological proxies (δ18O, pollen) allowing the assumption that Alpine Steppes persisted during the LGM with 3 to 4 K lower summer temperatures.During the mid-Holocene, forests could have replaced Alpine Steppes in the upper catchments of the Huang He, Yangtze, Mekong, Salween and Yarlung Zhangbo, but not in the interior basins of the north-western highlands, because the basins were then flooded, suppressing forests and supporting the environmental stability of this arid Alpine grassland biome.
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