Contributions of climate change, elevated atmospheric CO2 and human activities to ET and GPP trends in the Three-North Region of China

Quantifying and distinguishing the contributions of climate change, elevated atmospheric CO2 concentration (eCO(2)) and human activities to evapotranspiration (ET) and gross primary production (GPP) changes is of great significance for assessment of water shortage and land degradation. Based on the Vegetation Interface Processes (VIP) model, the spatial-temporal changes of ET and GPP in the Three-North Region (TNR) of China from 1982 to 2017 were estimated. The results showed that annual ET and GPP increased slightly with a rate of 0.71 mm yr(-1) (P < 0.001) and 0.002 gC m(-2) yr(-1) (P < 0.001), respectively. The ridge regression presented that the contributions of climate change, eCO(2) and human activities to ET and GPP trends were varying noticeably in the four sub-regions, namely: in the Mongolia-Xinjiang Sub-region (MXS), ET and GPP trends were basically attributed to increasing precipitation; in the Loess Plateau Sub-region (LPS), vegetation greening resulted from the Grain to Green Program was the dominant driving factor for GPP enhancement, while the relative contribution of vegetation greening to ET intensifying was lower than the raising net radiation; in the North China Sub-region (NCS), the negative effect of net radiation decline on ET was offset by the positive effect of vegetation greening, -and eCO(2) and vegetation greening dominated the upward trend of GPP; in the Northeast China Sub-region (NES), the declines in net radiation and precipitation showed negative impacts on ET and GPP trends. These findings are crucial for better understanding the complicated influences of driving factors on water and carbon cycles over the TNR, which is favorable for evaluation of ecological projects effects and management of water resources.