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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]

• Elevated CO 2 significantly advanced spring phenology in kobresia pygmaea. • Alpine plants on the tibetan plateau are more sensitive to elevated CO 2. • Elevated CO 2 reduced overlapping flowering between species. • Advancement in K. pygmaea's phenology was linked to higher water-use efficiency. Phenological changes in global vegetation are often attributed to climate warming. However, climate warming and elevated atmospheric CO 2 concentration (e CO 2) are two co-occurring global change factors, and how e CO 2 would affect vegetation phenology has received less attention. The partial pressure of atmospheric CO 2 on the Tibetan Plateau (TP) is lower than that in regions of lower altitudes. Consequently, the growth and phenology of alpine plants in this region could be more sensitive to e CO 2 , but this hypothesis is not yet supported by empirical evidence. Here we explored the effect of e CO 2 on plant phenology (including phenophases of green-up, budding, and flowering) through a 5-year field manipulation experiment in a high-altitude (4600 m above sea level) alpine grassland on the TP. Our results showed that e CO 2 significantly advanced the spring phenology of an early-flowering species (Kobresia pygmaea), while it had no impact on the phenology of two mid-flowering species (Potentilla saundersiana and Potentilla cuneata). Compared to other low-altitude regions, plant phenology on the TP underwent greater alterations under e CO 2 , which supports our hypothesis that the growth of high-altitude plants is more sensitive to e CO 2. Furthermore, we found that e CO 2 significantly reduced the overlapping of flowering between contrasting plant species, mainly due to the phenological advancement of the K. pygmaea induced by e CO 2. The observed advancement of the spring phenology in K. pygmaea under e CO 2 was associated with increasing ecosystem water-use efficiency (WUE), thereby advancing its subsequent phenological development, such as budding and flowering. Our findings provide experimental evidence that atmospheric CO 2 enrichment can accelerate plant growth processes in high-altitude regions, and suggest that large-scale model simulations should consider the effects of elevated atmospheric CO 2 concentration on plant growth and phenology. [Display omitted] [ABSTRACT FROM AUTHOR]

• This is the CO 2 enrichment field experiment conducted on the highest elevation. • Elevated CO 2 (eCO 2) stimulates both GEP and ER, and causes a neutral effect on NEP. • Simultaneous N addition and eCO 2 stimulate carbon sink. • Excessive precipitation suppresses the eCO 2 effects on carbon fluxes. It's generally believed that elevated CO 2 (eCO 2) could stimulate plant growth and the ecosystem carbon (C) sink. However, great uncertainties exist in terms of the CO 2 fertilization effect (CFE) magnitude, and how it is regulated by other global change factors. The lack of experimental evidence from the Alpine Region also limits our cognition on the CFE. By conducting a five-year manipulative field experiment in a semi-arid grassland of the Tibetan Plateau, we are aimed to explore the behavior of ecosystem C exchange in response to eCO 2 and N availability under contrasting natural precipitation regimes. The experiment showed that eCO 2 stimulated both gross ecosystem productivity (GEP) and ecosystem respiration (ER), and resulted in a neutral effect on net ecosystem productivity (NEP). The reduction of leaf N concentration under eCO 2 constrained the eCO 2 effects on C fluxes, especially on GEP and NEP. As N addition replenishes N availability in soil and leaf, GEP benefited more from the N addition than the ER. The eCO 2 strengthened the C sink when exogenous N was added simultaneously. Furthermore, precipitation variability played an importance role in mediating the eCO 2 effect among growing seasons. The eCO 2 effects on C fluxes tended to decline with increased water availability. The CFE was suppressed with excessive precipitation when the water-use efficiency (WUE) response was weak and eCO 2 -induced water-saving disappeared. The negative impact of precipitation on the CFE may also be attributed to the short precipitation intervals and insufficient radiation caused by high-frequency precipitation. Our study demonstrates that eCO 2 only stimulates net C uptake under conditions of N addition or during drier periods. Given the widespread N limitation, the efficacy of terrestrial ecosystems in mitigating climate change under rising CO 2 may be weaker than projected and is closely related to the precipitation variability. [ABSTRACT FROM AUTHOR]

• The soil water content and temperature determined the maximum photosynthesis. • Sufficient soil water elevated the optimal temperature and maximum photosynthesis. • The optimal environmental configuration for the alpine meadow was detected. • If soil water is high, higher temperature and VPD are beneficial for carbon uptake. The changes in the carbon sink function of sensitive and fragile alpine ecosystems under climate change are widely concerned. Radiation, temperature, and water resources play important roles in regulating carbon assimilation. Therefore, it is important to detect the optimal configuration of environmental factors for the photosynthesis of alpine meadow ecosystems. In this study, based on 10-year flux and corresponding environmental observations in a typical alpine meadow ecosystem, the optimal environmental configuration that could realistically occur in the natural world for photosynthesis was detected. The results indicated that the temperature and water conditions were the primary limiting factors, as radiation resources are abundant on the Tibetan Plateau. The optimal temperature for photosynthesis (TA opt) was determined to be 11.92 ℃ based on the entire observation dataset, with a maximum photosynthetic capacity (GPP max) of 0.20 mg CO 2 m−2 s−1. However, the TA opt and the corresponding GPP max could be elevated when the soil water content (SWC) was abundant. When SWC exceeded 0.26 m3 m−3, TA opt increased to 13.41 ℃, and the corresponding GPP max reached as high as 0.30 mg CO 2 m−2 s−1, which is the maximum the ecosystem could reach. Thus, the optimal environmental configuration for photosynthesis that could realistically occur in the natural world in this alpine meadow was air temperature (TA)=13.41 ℃, SWC=0.28 m3 m−3, vapor pressure deficit (VPD)=0.61 kPa, net radiation (RN)=411.53 J m−2 s−1. According to this study, it could be inferred that within a reasonable range, higher temperatures and VPD would benefit photosynthesis in this alpine meadow as long as the soil water supply is sufficient. The climate on the Tibetan Plateau is arid with high VPD and is experiencing warming due to climate change. Consequently, alpine ecosystems situated in relatively humid regions with sufficient soil water resources may exhibit greater potential for carbon sequestration than previously estimated. [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]