## 4 Discussion
### The Impact of Meteorological Variables on Vegetation WUE  
Meteorological factors influence both GPP and ET, which in turn drive variations in WUE. Our study identifies VPD as the primary factor affecting WUE changes across China, contributing more significantly--both relatively and absolutely--than precipitation, temperature, and shortwave radiation. This finding aligns with [PERSON] et al. [23] and highlights that VPD generally exerts a positive effect on WUE. The mechanism likely involves rising atmospheric temperatures, which increase the saturation vapor pressure and elevate VPD. Higher VPD reduces stomatal conductance, decreasing both transpiration and photosynthesis rates. However, the reduction in ET exceeds that of GPP, leading to an overall increase in WUE [73; 74]. Moreover, VPD's impact on WUE is more pronounced in SC compared to other regions, the mechanisms behind this may differ. The reason is that global warming, on one hand, increases vegetation ET and reduces VPD in the South China region, while on the other hand, it enhances vegetation photosynthesis, leading to an increase in GPP. In humid regions, GPP is more sensitive to VPD than ET, and the rate of increase in GPP is faster than that of ET. As a result, VPD changes contribute positively to WUE, promoting its increase [75; 76; 77]. Contrarily, some studies suggest that increasing VPD may promote GPP, which accelerates transpiration and lowers WUE [78]. Precipitation also plays a critical role in WUE variations, particularly in arid and semi-arid regions where it serves as a vital moisture source for vegetation [79]. [PERSON] et al. [17] found a positive correlation between WUE and precipitation in China's arid and semi-arid regions, based on data from 77 flux sites. Similarly, our findings reveal that precipitation contributes more significantly to WUE changes in the Tibetan Plateau than in other regions. Other studies corroborate the influence of both precipitation and VPD on WUE, consistent with our results. However, they propose that factors such as leaf area index or interactions with CO\({}_{2}\) concentration are the dominant drivers of WUE changes in the Tibetan Plateau, diverging from our conclusions [80; 81; 82]. This discrepancy may arise from differences in the climatic factors considered, as our study does not account for biotic influences. The choice of environmental variables significantly impacts the analysis of WUE drivers. Temperature exerts a dual effect on WUE. It can enhance stomatal conductance and increase transpiration while simultaneously raising the saturation vapor pressure, which elevates VPD. Higher VPD reduces stomatal conductance and water loss, introducing uncertainty regarding temperature's net impact on WUE [83; 84]. A more detailed analysis of temperature's effects on WUE is warranted. Shortwave radiation also affects WUE by promoting vegetation and soil evaporation. Excessive radiation, however, inhibits photosynthesis, reducing GPP and lowering WUE [85; 86]. Consequently, regions with high elevations in SW and subtropical areas in SC show a larger proportion of negative contributions from shortwave radiation to WUE changes compared to other regions (Figure 8 and Table 3). In addition, factors such as soil moisture and CO\({}_{2}\) concentration also influence WUE. However, first, considering that soil moisture data are stratified and often derived from remote sensing products, their consistency is relatively low. Second, soil moisture is not a meteorological variable; its variation is driven by precipitation and evaporation, and including it in regression analysis alongside meteorological variables could compromise the robustness of the model [87]. As for CO\({}_{2}\) concentration, studies on its impact on vegetation carbon-water cycles typically require long-term time series [88]. Moreover, the data are often derived from station measurements, which introduces high uncertainty when studying it at a 0.5\({}^{\circ}\) grid scale.