Quantitative assessment of relative roles of climate change and human activities on grassland net primary productivity in the Three-River Source Region,China
The Three-River Source Region(TRSR),a region with key importance for the ecological security of China,has been affected by climate change and by changes in human activities driven by a series of ecological restoration projects in recent decades.In this study,we analyzed land use and cover change(LUCC)in the TRSR and the consequent changes in grassland net primary productivity(NPP).For this analysis,we used the CASA (Carnegie-Ames-Stanford Approach)model,a climatic production model,and land use data for the study area from 2001and 2012,which were derived from the MODIS global land cover product.The contributions of LUCC,land management measures,and climate change to NPP variations were analyzed quantitatively. The results showed that,during the 12years,the net increase in grassland area was 6749km~2,and the av- erage NPP and total NPP of grassland decreased by 5.47%and 5.00%,respectively.New grassland contributed 1293.12Gg C(Gg=10~9 g)to total NPP,while the loss of grassland led to a decrease of 215.42Gg C.The climatic total NPP of grasslands decreased by 20.27%and the anthropogenic total NPP increased by 41.51%. These results indicate that climate conditions have negatively affected vegetation growth,whereas human activities have favorably affected vegetation recovery.The contributions of climate change,LUCC,and improved management measures to the variation in total NPP were-165.28Tg C,2.22Tg C,and 140.03Tg C,respectively.Therefore,improving grassland management measures can significantly reduce grassland deterioration due to climate change.