# Responses to the Impact of Drought on Carbon and Water Use Efficiency in Inner Mongolia
## 4 Discussion
### Effects of Drought on CUE and WUE  
Drought affects CUE and WUE mainly through stomatal and nonstomatal factors. The stomatal factor is the water stress imposed by drought on plants, which, in turn, changes stomatal conductance to affect CUE and WUE. Stomatal conductance is the degree of stomatal opening and closing on the surface of plant leaves, which controls transpiration, respiration, and photosynthesis [49]. Under drought conditions, stomatal conductance decreases in plants to reduce transpiration water losses, but at the cost of a lower CO2 con  
Figure 8: Correlation coefficients of CUE and WUE with DSI for different plant types in Inner Mongolia from 2001 to 2020. (**a**) Correlation coefficients of CUE and DSI; (**b**) correlation coefficients of WUE and DSI. centration in the leaves as well. Since the diffusion resistance of water vapor is greater than that of carbon dioxide [50], when stomatal conductance decreases, the decrease in water vapor escaping from the leaves is less than the decrease in carbon dioxide uptake, leading to an increase in WUE; accordingly, CUE and WUE decrease when stomatal conductance increases [51]. Nonstomatal factors refer to the inhibition of the physiological functions of plants' photosynthetic organs by drought, the decrease in photosynthetic enzyme activity, the decrease in the photosynthetic rate, and the decrease in carbon sequestration capacity, which ultimately affect CUE and WUE [52; 53]. The drought index (DSI) was negatively correlated with the CUE of vegetation in 97.96% of the regions in Inner Mongolia, while it was negatively correlated with water use efficiency in only 54.21% of the regions, indicating that the occurrence of drought events was the main limiting factor for the reduction in the CUE of vegetation in Inner Mongolia, and the occurrence of drought events could increase the average ratio of the dark respiration rate to the photosynthesis rate of vegetation while reducing vegetation's photosynthesis. The occurrence of drought events reduces photosynthesis in vegetation and increases the average ratio of the dark respiration rate to the photosynthesis rate, which leads to a reduction in the CUE of vegetation. The impact of drought events on ecosystems varies [54; 55; 56], and even under the same severe drought conditions, the effects on ecosystems can vary greatly depending on the ecosystem and the climatic context. Using the Boreal Ecosystem Productivity Simulator (BEPS), [PERSON] et al. [57] determined the WUE of China's terrestrial ecosystems, finding that, from 2000 to 2011, the WUE in northeastern China and central Inner Mongolia decreased. [PERSON] et al. [58] observed that WUE rose during drought in the forested regions of northeastern China, northeastern Inner Mongolia, and southern China, but fell in the forested regions of northwestern and central China. During 2009 and 2010, a drought in southwestern China reduced plant growth in the provinces of Yunnan, northern Guangxi, Guizhou, and eastern Sichuan, according to research by [PERSON] et al. [59]. According to the findings of a study carried out by [PERSON] et al. [60], which investigated the associations between global WUE and humidity indices, the value of WUE rose in arid ecosystems during droughts but declined in semi-arid/sub-humid ecosystems over the same time period. This difference is likely attributable to the varying degrees of sensitivity of these ecosystem types to shifts in the hydro-climatic conditions. Drought has a significant impact on northern China's dryland vegetation for a number of reasons, including the fact that forest and grassland ecosystems react differently to drought, and because the forests in the region are more vulnerable to water scarcity and carbon depletion than grasslands [61].