Spatial And Temporal Evolution Characteristics Of Carbon And Water Use Efficiencies In The Qinghai-Tibet Plateau And Their Responses To Climate Change And Human Activities

Carbon use efficiency(CUE)and water use efficiency(WUE)are key indicators of the carbon cycle,the water cycle and the carbon-water coupling process in terrestrial ecosystems,which respectively determine how vegetation allocates carbon and the degree of carbon and water coupling.Under the influence of climate change and human activities,the complexity and spatio-temporal variability of the terrestrial ecosystem carbon-water cycle have been significantly improved on the Tibetan Plateau.Therefore,investigating and revealing the impacts of climate change and human activities on terrestrial ecosystem CUE and WUE and the mechanisms can help to better understand the eco-hydrological processes and driving mechanisms on the Tibetan Plateau.In this study,our research provides the spatio-temporal variations of key eco-hydrological elements(such as gross primary productivity(GPP),Evaporation(ET),WUE and CUE)on the Tibetan Plateau based on ensemble simulations from 12 terrestrial ecosystem models,and quantitatively evaluates the respective effects of climatic factors,land use and land cover change(LULCC)and CO₂ fertilization on CUE and WUE on the Tibetan Plateau.The main conclusions are as follows:(1)The ensemble simulations showed that climate change can explain most of the changes in the GPP,while the direct effect of LULCC and rising CO₂ contributed 10%and–14%to the mean GPP values,37%and–20%to the interannual variations,and52%and–24%to the GPP's trend,respectively.(2)Average CUE value of the multi-model simulations(0.583±0.064)on the Tibetan Plateau is slightly lower than that derived from the satellite-based product,the Moderate Resolution Imaging Spectroradiometer(0.646).LULCC and CO₂fertilization contribute 4.24%and 0.79%of the annual mean CUE,respectively.Among the climatic factors,temperature and precipitation have positive correlations with CUE over most areas of the Tibetan Plateau while solar radiation shows a negative impact.(3)On the Tibetan Plateau,The simulated ET trend values are comparable from the upscaled product based on eddy covariance data(MTE,0.37 mm/yr²(p<0.01))and from the multi-model simulations(0.36mm/yr²(p<0.05)),while MODIS shows a significant larger downward trend(-5.87 mm/yr²(p<0.01)).From 1981 to 2010,the annual average ET on the Tibetan Plateau was less affected by CO₂ fertilization effect(-0.56%)and LULCC(-2.44%).In the Indus,the annual average value of ET is greatly affected by LULCC(11.50%),and the following in the Inner(7.76%).(4)The WUE value from multi-model ensemble is 1.470±1.052 g C/kg H₂O,WUE estimated by MODIS is 0.772 g C/kg H₂O,and the results of MTE and FLUXCOM are 1.880 g C/kg H₂O and 0.979g C/kg H₂O,respectively,so there is greater uncertainty.For the entire Tibetan Plateau,LULCC has led to a reduction for the annual average WUE(-7.39%).In most areas of the Tibetan Plateau,there is a positive effect of CO₂ fertilization on WUE,and the contribution rate to the annual average WUE is 11.13%.It is worth noting that the negative impact of LULCC on WUE and the positive impact of atmospheric CO₂ concentration on WUE can cancel each other out.