Climate warming and pastoral land use change: Implications for carbon cycling, biodiversity and rangeland quality on the Northeastern Tibetan Plateau

I investigated the independent and combined effects of climate warming and grazing on community, ecosystem and biogeochemical properties on the Northeastern region of the Tibetan Plateau. I worked at two habitats---distinguished by the dominant vegetation and the season of use by local pastoralists---crossed with two grazing history levels. Within each of four sites, I established a fully factorial experimental design with four-fold replication. I simulated warming using fiberglass open top chambers (OTCs) and simulated the defoliation effects of grazing through selective clipping. OTCs elevated growing-season averaged air temperature by 0.6--2.0°C (+10cm), soil temperature by -0.5 to +0.9°C (-12cm), increased nitrogen availability, and had mixed effects on soil moisture.; Warming decreased total species richness at all sites, decreased total ANPP and BG biomass at the winter grazed meadow sites; the summer grazed shrubland sites were more resilient to these changes in productivity. Warming decreased graminoid ANPP and increased shrub ANPP; forbs responded individualistically to warming. Warming increased soil carbon storage at the meadow sites and decreased soil carbon storage at the shrubland sites. Over the long-term, I predict both habitats will be carbon sinks; the shrubland sites will be a more stable, long-term carbon sink than the meadow sites.; For many variables, grazing yielded the opposite direction of response than warming. Moreover, non-additive interactions between warming and clipping were also sometimes present, illustrating that global change effects can be unpredictable from studying these effects separately. These responses were also often context dependent: the outcomes varied by year, habitat and site grazing history.; My research illustrates climate warming can have relatively rapid and dramatic effects on the Tibetan Plateau ecosystem, with potentially large feedbacks to climate and rangeland quality. Rangeland degradation that has previously been attributed to overgrazing may, in fact, be a response to ongoing climate warming. Since ecosystem properties that enhanced carbon sequestration actually decreased rangeland quality, it is important to carefully weigh the global goal of carbon sequestration versus the local goal of maintaining rangeland quality. A flexible pastoral management strategy will be an essential tool under a changing climate on the Northeastern Tibetan Plateau.