Background/Question/Methods   Climate warming on the Tibetan Plateau is predicted to be several times greater than the global mean. In addition, extreme events, such as large snowstorms, are also predicted to increase. Concurrent with these climate changes, governmental policies are altering the nature of pastoral land use across the Tibetan Plateau. We are examining the independent and combined effects of these global changes on the alpine meadow, shrubland, and steppe communities of the Tibetan Plateau. Our questions include: How do separate and interacting climate changes affect the composition, production and structure of these systems? Can grazing make these systems more resilient to the effects of climate change? To address these questions, we have set up two experiments on the Tibetan Plateau: a long-term experimental warming x simulated grazing experiment on a relatively mesic, lower elevation meadow-shrubland system of the northeastern plateau; and a new experimental warming x snow addition x yak grazing x pika experiment on the relatively arid, higher elevation steppe-shrubland system of the central plateau. We have also sampled grazed and non-grazed (40 years) plots across a climate transect. Finally, we are using the CENTURY ecosystem model to simulate the interactions of climate change and grazing in the meadow and steppe ecosystems.

Results/Conclusions   Experimental warming decreased total aboveground production (ANPP) at the alpine meadow site, with no effect at the shrubland site since at the shrubland site, an increase in shrub ANPP compensated for a decrease in forb and graminoid ANPP. At both sites, warming decreased species diversity and the delivery of key ecosystem services, such as the number of medicinal and palatable plant species. Grazing generally mediated the negative effects of warming. Preliminary results indicate that the steppe vegetation may be similarly vulnerable to warming, but interactions with the snow addition and grazing treatments have yet to emerge. Across the climate transect, mean annual precipitation, rather than mean annual temperature, was most strongly correlated with species richness and ANPP. Grazing had positive effects at the mesic sites, but negative effects at the drier sites. The model supports the fieldwork and demonstrates that grazing interacts with warming, but the effects on ANPP depend on precipitation regime.