## 4 Discussion
### Impacts of Climate Factors on GPP, ET, and WUE  
Through partial correlation, geographic detector, and ridge regression analysis methods, this study qualitatively and quantitatively evaluated the impacts of climate and ecological environmental factors. The impacts of climate factors on GPP, ET, and WUE have obvious spatial distribution characteristics. In terms of PRE, which affects GPP, ET, and WUE in arid areas with water scarcity [58], especially in the mid-temperate semiarid region (II), it clearly contributes to the increase in ET. Recently, research has shown that PRE in northern China has increased, which provides water for the increase in ET in this region [59; 60]. Moreover, the vegetation in this area is also sensitive to precipitation [61]; thus, as PRE increases in this area, GPP also has a positive response [62; 63]. However, because the vegetation type in this area is mainly grassland, the increase in GPP is relatively limited, which also leads to a relatively weak response of WUE. Notably, increased PRE also has a significant positive effect on the plateau temperate semiarid region (IV). The authors in [64] reported that increasing ET was driven primarily by PRE on the Tibetan Plateau. The impacts of RAD on GPP, ET, and WUE are more pronounced in the northern subtropical humid region (VI), where water resources are abundant. However, the weather in this area is mainly cloudy [65; 66]. Therefore, an increase in RAD can provide energy for water vapor evaporation. Moreover, the vegetation types in the area are mainly forestland and shrubs, with high vegetation coverage. An increase in RAD not only accelerates plant transpiration but also effectively regulates the photosynthetic rate of vegetation, promoting an increase in GPP [67]. However, because the increase in ET is much greater than that in GPP, the impact of RAD on WUE is negatively correlated in the northern subtropical humid region (VI). The impacts of TEM on GPP, ET, and WUE are relatively small compared with those of precipitation and solar radiation, with a more significant contribution occurring in the plateau temperate semiarid region (IV). With global climate warming, the TEM has experienced amplified warming in plateau temperate semiarid regions (IV) [68; 69], which has resulted in increasing GPP and ET and has led to a decline in WUE with a sporadic distribution in the central region of plateau temperate semiarid regions (IV) [70; 71].  
The relationship between VPD and WUE is negatively correlated in central China (i.e., the northern subtropical humid region and the marginal temperate humid region). Previous studies have shown that an increase in VPD affects both vegetation and the environment. For plants, an increase in VPD can directly induce stomatal closure to prevent excessive transpiration. However, stomatal closure also inhibits the absorption of CO\({}_{2}\) by plants, reducing the cumulative GPP. In terms of the environment, an increase in VPD leads to increased atmospheric evaporative demand, thereby stimulating soil evaporation [72; 73; 74]. On the Qinghai-Tibet Plateau, previous studies have indicated that in ecosystems located in cold and water-scar regions, WUE is more sensitive to VPD than to soil moisture, and VPD is often the dominant factor driving seasonal changes in ET [75], which is consistent with our findings. The impact of soil moisture is mainly observed in northern China, specifically in the semiarid regions of the mid-temperate zone.