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The Tibetan Plateau (TP) is one of the most important areas for the study of the carbon budgets of terrestrial ecosystems. However, the estimation of the net ecosystem productivity (NEP) remains uncertain in this region due to its complex topographic properties and climatic conditions. Using CO2-eddy-covariance-flux data from 1982 to 2018 at 18 sites distributed around the TP grassland, we analyzed the spatial–temporal patterns of the grassland NEP and its driving factors from 1982 to 2018 using a random forest (RF) model. Our results showed that the RF model captured the size of the carbon sink (R2 = 0.65, p < 0.05) between the observed and simulated values for the validation samples. During the observation period, the grassland acted as a carbon sink of 26.2 Tg C yr−1 and increased significantly, by 0.4 g C m−2 yr−1. On a regional scale, the annual NEP gradually increased from the northwest to the southeast, and a similar pattern was also observed in the long-term trends. Furthermore, the moisture conditions, such as the specific humidity and precipitation, were proven to be the main driving factors of the carbon flux in the southeastern areas, while the temperature predominantly controlled the carbon flux in the northwest. Our results emphasize the net carbon sink of the TP and provide a reliable way to upscale NEP from sites. [ABSTRACT FROM AUTHOR]

Greening of the Earth is among the world's most important ecological changes in recent decades, but its effects and triggers are multifaceted and varied across the globe. Climate change is the primary driver for greening of the Tibetan Plateau (TP), where vegetation is of great significance for resident survival, carbon sequestration and climate feedbacks. However, the spatial variability of greening and its drivers are not well-understood on the TP. Here, we compiled multi-source vegetation indices, climate and atmospheric data to investigate the greenness spatiotemporal variability and its linkage with large-scale climate oscillations. We found that greening dominated the TP during 1980s–2010s and positive trends in satellite-based vegetation indices account for >70% of the TP. Specifically, the arid and semi-arid TP experienced a significant and higher greening rate due to the remarkable increase in precipitation. We further clarified the domains of the South Asian monsoon (SAMI1 and SAMI2), the westerly wind (WI) and the plateau monsoon (TPMI) on the TP, and realized that the wetness increase in the arid and semi-arid was correlated with a weakening WI. Detecting the linkage between WI with large-scale climate oscillations, we suggested that the decadal weakening WI was induced by a warmer North Atlantic SST (positive AMO phase), which triggered wave train anomalies and regulated zonal and meridional vapor transport across Eurasia. Generally, during the weak/negative WI phase, anticyclone anomalies centered over Mongolia may strongly reduce vapor output across the east boundaries, while cyclone anomalies centered over the northwest outside the TP likely enhance meridional vapor transport into the western TP from the south ocean. Eventually, precipitation over the arid and semi-arid TP increased significantly in the past four decades, that stimulated plant growth and greening pattern on the TP. Our results established the linkage among climate oscillations, regional climate alteration and vegetation greening, providing a more thorough understanding in terrestrial greening. • Approximately 70% areas of Tibetan Plateau (TP) were greening at the past four decades. • The greening of TP was dominated by the wetting in arid and semi-arid areas. • The decadal weakening of westerly wind dominated wetting over the TP, thus actuated the greening of TP. [ABSTRACT FROM AUTHOR]

• Water conditions dominated plant phenology and photosynthetic capacity. • Plant phenology and photosynthetic capacity co-determined to annual GPP. • Plant photosynthetic capacity dominated annual GPP. Under global climate change, climate warming and changing precipitation patterns would greatly affect plant phenology and photosynthetic capacity and hence affect the gross primary productivity (GPP) of the sensitive alpine meadow. To deeply understand the variations in GPP, it is important to clarify the joint effects of plant phenology and photosynthetic capacity. In this study, 10-year continuous flux and microclimatic data and in situ phenology period observation data were used to explore the roles of growing season length (GSL) and the seasonal maximal capacity of carbon uptake (GPP max) on annual cumulative gross primary productivity (GPP ann). The results indicated that the temperature and water conditions, especially water conditions, dominated GSL and GPP max and hence determined GPP ann. Both GSL and GPP max were dominated by annual precipitation, and the soil water content in August also showed a strong influence over them. GSL and GPP max together could explain 87% of the variation in GPP ann. Compared with GSL, GPP max had stronger effects on the interannual variability (IAV) in GPP ann. This study provides a new perspective on clarifying the proximate causes of IAV in GPP ann. The IAV in GPP ann of alpine ecosystems could be determined by these two key underlying processes, the length of the carbon uptake period and amplitude. These results are of critical importance for improving our ability to predict future changes in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]