# Comprehensive Effects of Atmosphere and Soil Drying on
## 1 Introduction
There is a significant negative correlation between WUE and SWC in the ecosystem, and the negative correlation is more obvious at higher VPD [16; 17]. One study points out that the WUE of the ecosystem will increase by 10-35% at the end of this century with the rise in VPD under Representative Concentration Pathway 8.5 (RCP8.5) [18], which is supported by both observation and model simulation [19; 20; 21]. In contrast, the research on how plant carbon use efficiency (CUE) is affected by VPD and SWC is not sufficient, especially the systematic and comprehensive discussion on CUE and its influencing factors [22]. However, WUE and CUE efficiency can be influenced by various factors, such as environmental conditions, plant species, and physiological processes [23]. Under drought conditions, plants may reduce transpiration to minimize water loss, which can lead to lower GPP and NPP, as plants are limited by water availability [24; 25; 26]. This may also result in lower vegetation water use efficiency, as plants need to use more water to produce the same amount of biomass. There is ongoing debate about whether VPD or SWC has more influence on plant water stress, given that they interact with each other and other climatic variables in land-atmosphere interactions [27; 28; 29]. For instance, precipitation directly affects atmospheric temperature and SWC, and in return, SWC feeds back into the atmosphere relative humidity, which could determine the change in VPD with the concurrent effect of the atmospheric temperature [8; 9; 30; 31]. Furthermore, with future global warming, the compound adverse effects of increasing VPD and decreasing SWC on plants and ecosystems may make the problem more complex and critical [15; 32]. Therefore,research is needed to better understand the complex interactions between GPP, NPP, photosynthesis, vegetation water use efficiency, and carbon use efficiency. This knowledge can provide insights into plant productivity and resource utilization in ecosystems, and contribute to better management and conservation strategies for natural resources. In summary, to clarify the relationship between VPD and SWC and plant stomatal behavior and carbon and water use efficiency, and systematically evaluate the response of different ecosystems to water stress, we used the hourly observation data of 36 flux towers covering five plant functional types (PFTs) from around the world, namely crops, deciduous broad-leaved forest, evergreen coniferous forest, grassland, and savanna. This paper delves into the impact of VPD and SWC on the carbon and water cycles of ecosystems, specifically through their influence on plant stomatal behavior. Additionally, this paper aims to elucidate how different PFTs are differentially affected by VPD and SWC, and to identify the conditions under which certain PFTs are most susceptible. Through a detailed investigation of VPD and SWC's impact on plant stomatal behavior, CUE, and WUE in varying PFTs, this paper seeks to enhance our understanding of the relationship between hydrological and climatic factors and ecosystem functioning. Ultimately, the findings of this study can serve as an important reference for future research on the role of plants in ecosystem carbon and water cycling.