Soil moisture dominates the interannual variability in alpine ecosystem productivity by regulating maximum photosynthetic capacity across the Qinghai-Tibetan Plateau

The alpine ecosystems on the Qinghai-Tibetan Plateau are quite sensitive to climate change. The increasing temperature and changing precipitation patterns greatly affect the gross primary productivity (GPP) and disturb the carbon balances of these alpine ecosystems. To clarify the impacts of future climate change across the Qinghai-Tibetan Plateau, it is important to address the scientific issue "Which factor would dominate interannual variability (IAV) in GPP and through which path does it work?" To clarify this issue, two key processes, growing season length (GSL) and maximum photosynthetic capacity (GPPmax), were introduced to reveal the underlying mechanisms, and which of the environmental factors dominated their variations were studied specifically based on the flux and corresponding environmental observation data obtained in different types of alpine ecosystems across the Qinghai-Tibetan Plateau in this study. The results indicated that across the temperature- and waterlimited alpine ecosystems, the temperature controlled the GSL, but the water conditions dominated the variations in GPPmax. The soil water content (SWC) dominated GPPmax, which could explain 89% of the variation in GPPmax. The GSL alone was incapable of explaining IAV in GPP. Conversely, GPPmax is robust in explaining IAV in GPP, which could explain 94% of the annual GPP. Therefore, climate change would probably drive IAV in GPP through the path of "SWC -* GPPmax -* annual GPP" on the Qinghai-Tibetan Plateau. In addition, GPPmax together with GSL (GSL x GPPmax) could explain 99% of IAV in GPP, as they indicated the length of the carbon uptake time and the capacity of carbon sequestration, respectively. This study provides a new perspective on the predominant causes of IAV in GPP in alpine ecosystems, indicating that the changing precipitation patterns under future climate change will play a dominant role in affecting the carbon sink function of the Qinghai-Tibetan Plateau.