Climate Change and Ecological Projects Jointly Promote Vegetation Restoration in Three-River Source Region of China

As the source of the Yellow River, Yangtze River, and Lancang River, the Three-River Source Region (TRSR) in China is very important to China's ecological security. In recent decades, TRSR's ecosystem has degraded because of climate change and human disturbances. Therefore, a range of ecological projects were initiated by Chinese government around 2000 to curb further degradation. Current research shows that the vegetation of the TRSR has been initially restored over the past two decades, but the respective contribution of ecological projects and climate change in vegetation restoration has not been clarified. Here, we used the Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) to assess the spatial-temporal variations in vegetation and explore the impact of climate and human actions on vegetation in TRSR during 2001-2018. The results showed that about 26.02% of the TRSR had a significant increase in EVI over the 18 yr, with an increasing rate of 0.010/10 yr (P < 0.05), and EVI significantly decreased in only 3.23% of the TRSR. Residual trend analysis indicated vegetation restoration was jointly promoted by climate and human actions, and the promotion of human actions was greater compared with that of climate, with relative contributions of 59.07% and 40.93%, respectively. However, the degradation of vegetation was mainly caused by human actions, with a relative contribution of 71.19%. Partial correlation analysis showed that vegetation was greatly affected by temperature (r = 0.62, P < 0.05) due to the relatively sufficient moisture but lower temperature in TRSR. Furthermore, the establishment of nature reserves and the implementation of the Ecological Protection and Restoration Program (EPRP) improved vegetation, and the first stage EPRP had a better effect on vegetation restoration than the second stage. Our findings identify the driving factors of vegetation change and lay the foundation for subsequent effective management.