# Water Use Efficiency Spatiotemporal Change and Its Driving Analysis on the Mongolian Plateau
## 4 Discussion
This shows that the temperature has no significant impact on WUE under general conditions; however, once the temperature changes at the lower end, its impact on the WUE and water content becomes significant. In warm low-latitude areas, high temperatures lead to water evaporation, which not only causes water stress and affects vegetation development but also increases ET, thus placing double pressure on WUE. However, forest areas at high latitudes are cold and humid, and the ET is usually high, whereas low temperatures inhibit the photosynthesis of vegetation, thus reducing the NPP. Water anomalies had the same effect (either increasing or decreasing) on the NPP and ET, which may mean that water has a stable influence on WUE; thus, WUE has strong resistance to water disturbances. The DSI and NDVI had a spatial distribution that was similar to that of WUE, especially regarding the difference between forests and grasslands; however, the opposite trend was observed. The NDVI in forest areas was positively correlated with WUE, which indicates that the poorer the forest vegetation growth, the more WUE will be promoted, while drought anomalies in forest areas will increase the WUE (Figure 9b,f). This trend is reversed in grasslands, which shows that the abnormal synergism of drought and vegetation has significantly different effects on WUE in areas with different vegetation types. From the perspective of spatial differentiation, the degree of interpretation of vegetation change regarding WUE was higher than that for meteorological factors (Figure 8a), which is consistent with previous findings [78]. In addition, delaying the rejuvenation period will eventually affect WUE to some extent. The earlier the SOS in spring, the earlier the plants begin to photosynthesize, leading to a rapid and sustained increase in GPP. However, the changes in ET during spring are unclear. Earlier leaf expansion or a larger leaf area in spring can increase plant transpiration; however, evaporation decreases because of the reduced surface temperature and solar radiation. This resulted in a smaller increase in ET than in GPP, ultimately leading to an improvement in WUE in spring. Therefore, in summary, the effect of ET, which was more significantly affected by the vegetation phenology, on WUE was compromised by NPP. Moreover, the cooperation of phenological parameters and meteorological factors had a nonlinear enhancing effect on WUE. The WUE of terrestrial ecosystems reflects the interactions between vegetation productivity and water availability. Because climate change leads to increased precipitation variability, precipitation infiltration directly affects the replenishment of soil moisture. These factors significantly influence ecosystem evapotranspiration processes, thereby impacting vegetation growth and the water-carbon cycle and ultimately resulting in changes in WUE across different vegetation types. Future climate warming will further exacerbate the imbalance between the water supply and demand in terrestrial ecosystems. Therefore, understanding the changes in WUE under different future scenarios is crucial to ensure the sustainable use of water resources. Future studies should consider precipitation, soil moisture, and evapotranspiration under 1.5 \({}^{\circ}\)C and 2.0 \({}^{\circ}\)C warming scenarios as key indicators for the assessment of ecosystem WUE, thereby improving our understanding of the impacts of climate change on the ecosystems of arid and semiarid regions on the Mongolian Plateau.