## 4 Discussion
### Climatic Controls on GPP and WUE  
Ta and Pr were the main climatic factors contributing to the increase in vegetation GPP in each aridity gradient. Ta maintained an increasing trend under all aridity gradients across China (Figure 8c), and GPP also kept a high positive correlation with Ta across arid-humid zones and vegetation types (Figure 13c). This study concluded a stronger positive correlation between Ta and GPP than Pr, which differs from [PERSON] et al. [55]. [PERSON] et al. [56] recorded that the sensitivity of GPP to precipitation decreased in recent years with the gradual increase in rainfall. As can be seen in Figures 9d and 11, this phenomenon was more pronounced in the humid zone where Pr was higher, and the positive correlation between GPP and Pr gradually increased as the humid zone transitioned to the arid zone. Furthermore, Figure 13d also revealed that wetland GPP showed a negative correlation with Pr in humid and semi-humid zones. Research has shown that excessive Pr can limit nutrient uptake by reducing the oxygen supply to the soil. In wetland ecosystems, prolonged inundation can cause oxygen stress on plant roots, leading to a decrease in GPP [57]. RH is the ratio of actual humidity to saturation humidity. In general, as the temperature rises, the air can hold more moisture and the RH decreases [58]. A negative correlation between RH and GPP can be found in Figure 13a for each arid-humid zone and vegetation type. This can reflect the contribution of rising Ta to GPP from the perspective of RH. Rn also exhibited a decreasing trend in the country (Figure 8b). Lower Rn could affect the photosynthesis of vegetation and consequently its GPP [59]. But the promotion of GPP by other factors (e.g., Ta and Pr) caused a negative correlation between Rn and GPP in the arid-humid zones where Rn mostly showed a decrease (Figure 13b). In addition, high WS can increase transpiration and respiration rates of plants, resulting in more water loss through leaves and offsetting some of the carbon gained through photosynthesis. WS has mostly shown a decreasing trend in all arid-humid zones since 2001, resulting in a negative correlation between GPP and WS for multiple vegetation types in each aridity gradient. The correlation pattern between WUE and the five climatic factors was obvious in the aridity gradient, with the highest correlation coefficients with the two thermal factors Ta and Rn, followed by the moisture factors Pr and RH, and the lowest with WS. (Figure 11). Among all vegetation types, except wetland and shrubland, all other vegetation types also showed a high correlation with thermal factors (Figure 12). WUE was positively correlated with the thermal factor and negatively correlated with the moisture factor. This phenomenon gradually diminished from the humid zone to the arid zone (Figure 14). There is a critical value for the effect of Ta on WUE, and either too high or too low Ta is detrimental to plant WUE [60]. As shown in Figure 8c, the intensity of Ta increase in China is higher in arid and semi-arid zones than in humid and semi-humid zones. The study inferred that excessive warming led to inconsistent positive and negative correlations of WUE to Ta for each vegetation type in arid and semi-arid zones, while a uniform positive correlation was observed in humid and semi-humid zones (Figure 14c). In maintained a consistent positive correlation with WUE across all aridity gradients. Since 2001, Rn has shown a decreasing trend in the humid, semi-humid and semi-arid zones of east-central China (Figure 8), which may be the main climate reason for the decrease in WUE in this region (Figure 3). Excessive precipitation is also detrimental to the WUE of vegetation. In recent years, increased Pr in China has led to a negative correlation between Pr and WUE in several arid-humid zones (Figure 11).