Response Of Vegetation Net Primary Productivity To Climate Change On The Tibetan Plateau

Based on an extensive synthesis of both empirical and modeling research, the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) unequivocally concludes that global climate is changing. Global climate changes underscore the necessity of understanding how climatic change may affect global biomes. Various data sources (e.g., meteorological records, remote sensing data) and methods (statistics, e.g., singular value decomposition; Geographical Information System, e.g., ArcGIS;ecosystem model, e.g., Biome-BGC) were applied in this study. Based on observed climate change, we designed climate change (mainly precipitation and temperature changes) scenarios to simulate Net Primary Productivity (NPP) with an ecosystem process model (Biome-BGC) in order to assess response of various ecosystems to climate changes. The main progress is as following:NPP at 11 terrestrial ecosystems of four biomes across the Tibetan Plateau in a 40-year period (1969-2008) were simulated by the Biome-BGC model. The relationship between NPP and annual precipitation was analyzed. The sensitivities (change in NPP divided by change in precipitation) of ecosystems decreases across biomes as precipitation increase. However, during the driest years at each site, there is convergence to a common maximum rain use efficiency (RUEmax).Water-limited dry and intermediate sites have RUE that close to RUEmax, whereas non-water-limited mesic sites have RUE that deviates significantly from RUEmax. Decrease in growing season precipitation and/or increase in air temperature resulted in site-level RUE approaching RUEmax. The existence of RUEmax suggests a potential response of biosphere to climate change (precipitation and air temperature).In recent decades, interannual precipitation variability increased in West China, with more frequency of wet and dry events. In addition to an increase in interannual temperature variability, a warming trend is also evident in recent decades. Interannual precipitation and air temperature variabilities decrease across biomes with increased mean annual precipitation. Interannual NPP variabilities increase across biomes with increased interannual precipitation and temperature variabilities. Scenarios were designed to assess response of vegetation productivity to increased interannual precipitation and temperature variabilities. The responses of NPP at 11 terrestrial ecosystems under these scenarios were evaluated. The results suggested that interannual NPP variabilities increased with increasing interannual precipitation and temperature variabilities. Increased interannual precipitation variations resulted in decrease of 40-year mean NPP at sites with low sensitivity to precipitation change, while had marginal impact at high sensitivity sites. Increased between-year air temperature variabilities have marginal impacts on 40-year mean NPP at all sites.Greenhouse-gas-induced climate warming is expected to increase intra-annual precipitation variability, such as a higher frequency of extreme rainfall events, a lower frequency of rainfall events, and longer intervening dry periods. Our analysis suggested that intra-annual precipitation variability decreased across biomes with increasing mean annual precipitation. Under the same warming background, simulated NPP slightly declined at low sensitivity sites as intra-annual precipitation variability increasing, which might be attributed to decreased water availability resulted from substantial increase in outflow at these sites; in contrast, NPP increased at intermediate and high sensitivity sites with increased intra-annual precipitation variability, probably due to increased soil water recharge at these sites. However, more field and simulation works need to be done.