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© 2014 Elsevier B.V. The Tibetan highlands host the largest alpine grassland ecosystems worldwide, bearing soils that store substantial stocks of carbon (C) that are very sensitive to land use changes. This study focuses on the cycling of photoassimilated C within a Kobresia pygmaea pasture, the dominating ecosystems on the Tibetan highlands. We investigated short-term effects of grazing cessation and the role of the characteristic Kobresia root turf on C fluxes and belowground C turnover. By combining eddy-covariance measurements with 13CO2 pulse labeling we applied a powerful new approach to measure absolute fluxes of assimilates within and between various pools of the plant-soil-atmosphere system. The roots and soil each store roughly 50% of the overall C in the system (76MgCha-1), with only a minor contribution from shoots, which is also expressed in the root:shoot ratio of 90. During June and July the pasture acted as a weak C sink with a strong uptake of approximately 2gCm-2 d-1 in the first half of July. The root turf was the main compartment for the turnover of photoassimilates, with a subset of highly dynamic roots (mean residence time 20days), and plays a key role for the C cycling and C storage in this ecosystem. The short-term grazing cessation only affected aboveground biomass but not ecosystem scale C exchange or assimilate allocation into roots and soil.

The Tibetan Plateau has a significant role with regard to atmospheric circulation and the monsoon in particular. Changes between a closed plant cover and open bare soil are one of the striking effects of land use degradation observed with unsustainable range management or climate change, but experiments investigating changes of surface properties and processes together with atmospheric feedbacks are rare and have not been undertaken in the world's two largest alpine ecosystems, the alpine steppe and the Kobresia pygmaea pastures of the Tibetan Plateau. We connected measurements of micro-lysimeter, chamber, 13C labelling, and eddy covariance and combined the observations with land surface and atmospheric models, adapted to the highland conditions. This allowed us to analyse how three degradation stages affect the water and carbon cycle of pastures on the landscape scale within the core region of the Kobresia pygmaea ecosystem. The study revealed that increasing degradation of the Kobresia turf affects carbon allocation and strongly reduces the carbon uptake, compromising the function of Kobresia pastures as a carbon sink. Pasture degradation leads to a shift from transpiration to evaporation while a change in the sum of evapotranspiration over a longer period cannot be confirmed. The results show an earlier onset of convection and cloud generation, likely triggered by a shift in evapotranspiration timing when dominated by evaporation. Consequently, precipitation starts earlier and clouds decrease the incoming solar radiation. In summary, the changes in surface properties by pasture degradation found on the highland have a significant influence on larger scales.

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.