## 4 Discussion
### Response of WUE to the Driving Factors  
Previous studies have demonstrated that ecosystem WUE is affected by both biotic factors, including vegetation growth, and abiotic factors, including the climatic environment [19; 20; 51]. The impact of climatic variables on WUE changes has been extensively demonstrated. Warming affects vegetation productivity by extending the growing season in favor of greenery. The ET rate remains constant or increases gradually owing to greenery [8; 52]. Therefore, the warming of the STC has a positive effect on the WUE. An increase in solar radiation will lead to an increase in GPP, and radiation is positively correlated with canopy conductance and transpiration [52; 53]. In the STC, changes in solar radiation stimulated GPP more significantly than ET, which ultimately led to changes in WUE. Precipitation is the dominant factor in surface ecohydrological processes. Increased precipitation promotes vegetation growth and surface evapotranspiration (both vegetation transpiration and surface evaporation). The response of vegetation to changes in precipitation may exhibit a threshold effect; when precipitation exceeds a certain threshold, the GPP's response to increased rainfall is weakened [54], Thus, in the humid STC, fluctuatingincreases in rainfall have a negative impact on WUE. In humid areas, moisture is usually not a limiting factor for vegetation growth. Other studies have shown that SWC is not a major driver of humid ecosystem GPP and ET [35; 55]. The effect of VPD on WUE is well-known, with elevated VPD leading to decreased WUE, which is also consistent with previous studies [27]. Elevated VPD can lead to a decrease in the stomatal conductance of vegetation, resulting in a decrease in the photosynthetic rate and GPP. Simultaneously, reduced stomatal conductance reduces ET. Studies have shown that under high humidity conditions, GPP is more sensitive to VPD changes than ET, resulting in a negative impact of VPD increases on WUE [56] (Figure 6). LAI, as an indicator reflecting the biological characteristics of vegetation affected by climate change and human activities, varies in different landscapes. LAI changes affect the rate of absorption of photosynthetically active radiation and the allocation of radiation among latent and sensible heat fluxes. Many previous analyses have demonstrated that LAI shows a strong positive influence on GPP, whereas LAI shows a greater positive impact on seasonal ET but a weaker effect on long-term ET [56; 57]. Thus, on an interannual scale, the impact on GPP by LAI exceeds that on ET [58]. Several studies have also reported a positive relationship between LAI and ecosystem WUE trends [59; 60]. In the STC, strong  
Figure 6: Time series of annual (**a**) WUE and GPP, (**b**) ET and LAI, (**c**) Ca and SWC, (**d**) VPD and MAT, and (**e**) MAP and Rg in the STC. spatial heterogeneity was observed in the contribution of LAI variations to the WUE trend due to the LAI trend and the spatial heterogeneity of WUE sensitivity to LAI. Elevated atmospheric CO\({}_{2}\) concentration directly affects photosynthesis (i.e., the fertilization effect of increased CO\({}_{2}\)) by raising the CO\({}_{2}\) concentration gradient between cells and leaf surfaces [27; 60]; moreover, an increase in the concentration of CO\({}_{2}\) will impact the stomatal conductance of the surface by altering the number, shape, and density of stomata [19]. In addition, when leaf stomatal CO\({}_{2}\) uptake rate reaches a level of saturation, there may be a reduction in transpiration due to decreases in the diameter of leaf stomata. These twin processes can result in a reduction in transpiration while vegetation productivity is maintained or improved, thereby increasing WUE. However, a higher CO\({}_{2}\) concentration over a long time is also expected to increase the effective evaporation area of the leaf and surface, which can increase the total evaporation at the ecosystem level [61].