## 3 Results
### Regulations of the Biophysical Variables on WUE
#### 3.3.2 Biophysical Regulations on WUE of Each Vegetation Type  
Figure 12 accessed the relative importance of key driving factors on each vegetation cover type. As for CRO, VPD is the most crucial factor with an IncNodePurity value of approximately 3.0, indicating its key role in regulating the WUE of cropland. Rn and PPT followed VPD, collectively affecting cropland water use efficiency (Figure 12a). The high sensitivity of cropland to VPD might be related to its high transpiration rate and sensitivity to water conditions. For CSH, Temp is the primary influencing factor. Temperature directly affects plant transpiration rates and photosynthetic efficiency, thereby impacting water use efficiency (Figure 12b). As for DBF, WUE is mainly influenced by Temp and VSWC. These two factors play a crucial role in regulating WUE in these forests, as temperature and soil water conditions directly affect tree transpiration and photosynthesis, thus influencing water use efficiency (Figure 12c). WUE is primarily affected by VSWC in DNF, indicating that soil water conditions are a key factor for water use in this kind of forest. Moreover, LAI and EVI are also important factors for DNF, reflecting the impact of vegetation structure and growth conditions on WUE (Figure 12d). For ENF, EVI is the most important factor. EVI reflects vegetation health and productivity, indicating that healthy vegetation significantly influences water use efficiency. WUE is mainly affected by VPD for GRA, and Temp and Rn followed afterwards, reflecting the combined effect of environmental conditions on WUE of grassland (Figure 12f). Temp is a key factor in regulating water use efficiency in mixed forests, with other important factors including LAI and Rn (Figure 12g). Rn and EVI together exhibited weak controls on WUE for OSH, as the IncNodePurity was relatively small (Figure 12h). WUE is mainly influenced by Temp and VPD in SAV, both with IncNodePurity values of around 3.0. Temp and VPD directly affect plant transpiration rates and water use efficiency (Figure 12i). In WSA, WUE is primarily affected by Temp, with an IncNodePurity value of about 12, indicating that temperature is the determining factor for water use efficiency in semi-arid grasslands. VPD and LAI follow behind Temp, reflecting the importance of these factors in semi-arid environments (Figure 12j).  
Figure 12: Importance ranking of driving factors in predicting water use efficiency (WUE of each vegetation type in the study period based on a random forest model. (**a**) CRO: cropland, (**b**) CSH: closed shrubland, (**c**) DBF: deciduous broadleaf forest, (**d**) DNF: deciduous needle forest, (**e**) ENF: evergreen needle forest, (**f**) GRA: grassland, (**g**) MF: mixed forest, (**h**) OSH: open shrubland, (**i**) SAVs: savannas and (**j**) WSAs: woody savannas.  
Overall, the main driving factors affecting WUE differ across vegetation cover types, with Temp and VPD being key factors in most vegetation types. Moreover, VSWC and LAI also showed an important influence on WUE. Additionally, other factors like Rn, PPT, and EVI also play significant roles in certain ecosystems.