# Comprehensive Effects of Atmosphere and Soil Drying on
## 3 Results
### Influence of VPD and SWC on WUE and CUE of Different PFTs  
In the previous section, we found that VPD and SWC limit the Gs, GPP, and NPP of vegetation, which directly affect the WUE and CUE of vegetation. To further investigate the effects of VPD and SWC on WUE and CUE, we used a similar analytical approach as in the previous section and focused on the possible impact of VPD and SWC on WUE and CUE. From Figure 5, it is clear that the linear relationship between WUE/CUE and VPD/SWC is  
Figure 4: Changes in Gs (**a–e**), GPP (**f–j**), and NPP (**k–o**) affected by VPD and SWC for five PFTs (CRO, DBF, ENF, GRA, SAV). The color in the figure indicates the abnormality of Gs, GPP, CUE, and WUE for each VPD and SWC percentile. For different vegetation types, the difference between these variables and the average value of the multi-year growing season is calculated as the abnormal value on the hourly scale. not as strong as that between Gs/GPP and VPD/SWC. Overall, in the five vegetation types, WUE generally increases with increasing VPD, except for when the soil water content is extremely high (90-100%) in the DBF sites and when the soil water content is between 15 and 30% in the GRA sites under the condition that VPD is greater than 3 kPa, where WUE shows a significant decreasing trend. The effect of SWC is more ambiguous, as WUE in the DBF and ENF sites with lower soil water content (15-30%) is higher in than those with higher soil water content (90-100%). These conclusions are also supported by Figure 6, which shows that positive changes in WUE are mainly distributed in the upper part of the 10 \(\times\) 10 grid, while negative changes are distributed in the lower part. The area with the greatest positive change in WUE among the five vegetation types is mainly located in the 0-50% soil water content percentile and VPD greater than the 40 th percentile interval (positive variation of 20-30%), and it is noteworthy that GRA also shows an abnormal positive change in WUE in the 80-90% soil water content percentile interval. As GPP and NPP have a similar distribution with respect to SWC and VPD among the five vegetation types, the specific changes in CUE are difficult to determine from Figures 5 and 6. However, in the previous section, we found that in ENF, the increase in NPP in the 30-50% soil water content percentile range with increasing VPD and the significant decrease in the soil water content percentile range of 70-100% and 0-15% with increasing VPD indirectly leads to an increase or decrease in CUE in these soil water content percentile ranges with increasing VPD. To further analyze the impact of increased vapor pressure deficit (VPD) and decreased soil water content (SWC) on five different types of vegetation, we quantified the water use efficiency (WUE) and carbon use efficiency (CUE) of different vegetation types under the influence of VPD and SWC, respectively. Figure 7 quantifies the stress of WUE (CUE) under VPD and SWC by subtracting the maximum WUE (CUE) in the highest VPD chamber from the minimum WUE (CUE) in the lowest VPD chamber or by subtracting the minimum WUE (CUE) in the lowest SWC chamber from the maximum WUE (CUE) in the highest SWC chamber (represented as \(\Delta\)WUE(CUE)(VPD \(|\) SWC) and \(\Delta\)WUE(CUE)(SWC \(|\) VPD), respectively). Figure 5: The WUE and CUE of five PFTs (CRO (**a**,**b**), DBF (**c**,**d**), ENF (**e**,**f**), GRA (**g**,**h**), SAV (**i**,**j**)) when VPD and SWC are decoupled. The six colors represent different percentile ranges of soil water content (0–15%, 15–30%, 30–50%, 50–70%, 70–90%, and 90–100%). The interval of each point on VPD is 0.2 kpa. Figure 6: Changes in WUE (**a-e**) and CUE (**f-j**) affected by VPD and SWC for five FFTs (CRO, DBF, ENF, GRA, SAV). Figure 7: Changes in (**a**) Gs, (**b**) GPP, (**c**) NPP, (**d**) WUE and (**e**) CUE for FFTs (CRO, DBF, ENF, GRA, SAV) under the guidance of increasing VPD (VPD | SWC) and decreasing SWC (SWC | VPD), respectively.