## 4 Discussion  
### The Spatial Characteristics of the Dominant Factors Driving WUE Extreme Variation  
The dominant factors influencing extreme variations in WUE differ significantly across climatic zones. From arid to humid regions, the sensitivity of GPP to precipitation increases, while that of ET decreases, illustrating a clear spatial gradient (Figure 3). This pattern can be attributed to greater water availability in humid regions, which supports more efficient photosynthesis. Consequently, the increase in GPP is more pronounced in these regions compared to arid areas. Under extreme WUE conditions, climatic fluctuations typically cause GPP to respond more quickly than ET, as GPP is inherently more sensitive to environmental changes. In humid areas, GPP is better able to maintain high photosynthetic rates, further amplifying this trend [65; 66]. In arid regions, however, [PERSON] et al. [17] found that WUE is driven by gross ecosystem productivity rather than ET, diverging from our conclusions. This difference may stem from limited water availability and low soil moisture in arid regions, which hinder plant growth under extreme WUE conditions. Compared to vegetation in humid regions, both GPP and ET are lower in arid zones. While vegetation may still retain some capacity for carbon sequestration, transpiration is highly constrained, resulting in ET being smaller than GPP. Increased precipitation accelerates changes in ET more than in GPP, suggesting that ET is more sensitive to precipitation [28; 66; 67]. In semi-arid and semi-humid regions, the impact of precipitation on GPP diminishes. This may be due to unstable water availability, which increases uncertainty in the trade-off between GPP and ET, the two components of WUE. The complexity of vegetation types in these regions, including both isohydric and anisohydric plants, leads to varied sensitivities to atmospheric water demand under water stress [17]. This diversity further complicates the relationship between vegetation carbon sequestration and precipitation. [PERSON] et al. [68] assessed the responses of GPP, ET, and WUE to seasonal drought using correlation coefficients and found that in arid ecosystems, WUE is driven by evaporation, while in semi-arid or humid environments, variations in water use efficiency are primarily influenced by carbon assimilation or biological activity. This supports our conclusions. However, the study by [PERSON] et al. [69], which used the SPEI index and site-interpolated meteorological, GPP, and ET data in China, presents findings that contradict ours. This discrepancy may be related to vegetation characteristics and growth environments. Under extreme drought conditions, vegetation reduces photosynthesis and external gas exchange to minimize water loss [70]. In contrast, in humid environments, non-isohydric species can maintain higher productivity than isohydric species, making them more sensitive to external environmental factors [71]. The regions most sensitive to extreme WUE variations influenced by climate are in SC. WSN is mainly distributed in SC and NE. [PERSON] et al. [72] demonstrated at the site scale that seasonal droughts significantly affect carbon-water coupling in subtropical forests in these areas. Our findings show that, on an annual scale, extreme WUE in SC, EC, and