# Comprehensive Effects of Atmosphere and Soil Drying on
## 4 Discussion  
The results showed the effects of the decoupling of VPD and SWC on the stomatal behavior of the vegetation. It is evident that Gs in the five vegetation types has a significant relationship with VPD and SWC, as Gs decreases significantly with increasing VPD and decreasing SWC. The limitation of the VPD is mostly manifested as a decrease in Gs with increasing VPD across the five vegetation types, indicating that the elevated VPD restricts Gs to varying degrees in different vegetation types, a result that has been confirmed by numerous studies [13; 15; 40]. VPD dominates the decrease in Gs, GPP, and NPP in most of the vegetation, which has been confirmed in previous studies. In addition, we found that SWC also plays a non-negligible role, which is reflected by the fact that the decrease in SWC limits the overall levels of Gs, GPP, and NPP. While previous studies focused on a single vegetation type (e.g., crop, forest) or separated the effects of VPD and SWC from all types of flux sites as a whole, this paper separates the sites of five representative vegetation types to study their responses to VPD increase and SWC decrease, respectively. Different vegetation types were limited by SWC in a different way, among which CRO and GRA were most limited by SWC, and it is also noted that there is an exceptional response of SAV to SWC, which showed that Gs at low SWC was greater than Gs at high SWC. With respect to VPD, VPD stress is more pronounced in agricultural and forest areas than in shrub and grassland areas. CRO and DBF exhibit negative variations in Gs, GPP, and NPP when SWC is below the 30 th percentile, while GRA and SAV exhibit stronger adaptability to decreased SWC and only exhibit negative variations when SWC is below the 60 th percentile. VPD has a more significant impact on the stomatal behavior of vegetation in CRO, DBF, and ENF, while its impact is smaller in GRA and SAV and mainly concentrated in the extremely high VPD range (90-100%). Similar conclusions have been drawn in previous studies, suggesting that VPD stress is stronger in agricultural and forest areas than in shrub and grassland areas, which is due to VPD limitations being more apparent in humid areas than in dry areas [40; 41]. Our study also confirms that an increase in VPD has a dominant effect on the increase in WUE, which is similar to the conclusion drawn by [PERSON] et al. [18]. However, it is worth noting that for the DBF and GRA vegetation types, the decrease in SWC has a greater effect on WUE than the increase in VPD. In addition, we introduced CUE to quantify the carbon sequestration capacity of plants affected by VPD and SWC, and the results showed that an increase in VPD dominated the decrease in CUE in the CRO, DBF, ENF, and GRA vegetation types. VPD and SWC could affect vegetation WUE and CUE by influencing plant physiological functions in many ways. Under the condition of high VPD and low SWC, plants may be limited in their ability to perform photosynthesis, the process through which plants convert carbon dioxide and water into organic matter using sunlight, producing nutrients and energy for plant growth and metabolism. When water is scarce, plants may close their stomata to reduce water transpiration, thereby reducing the rate of carbon dioxide entry into the leaves, leading to limited photosynthesis and lower water use efficiency. Moreover, when soil moisture is inadequate, plant growth rate may decrease, resulting in reduced efficiency of water use for growth. Plants may adjust their physiological metabolism to adapt to water-limited environments under drought conditions. For example, plants may synthesize and accumulate antioxidant substances to counteract oxidative stress caused by drought. However, the synthesis of these substances requires energy and nutrients, which may lower the total primary productivity and net primary productivity of plants, thereby affecting WUE and CUE.