## 4 Discussion
### Temporal and Spatial Dynamics of Seasonal eWUE in Response to Hydrothermal Conditions  
CO\({}_{2}\) concentration, leaf area index, air temperature, precipitation, soil moisture, etc., will all have an impact on regional eWUE [42; 43; 44]. The areas with negative correlations between eWUE and precipitation in spring and autumn in Inner Mongolia were mainly concentrated in the middle and high mountains (Figure 8). This was because the vegetation in the middle and high mountains grows poorly due to the low temperature caused by its high altitude, the vegetation water demand is limited and the precipitation increases. This has little significance in terms of vegetation growth but will increase the evaporation of the site [45]. Therefore, water use efficiency was negatively correlated with precipitation in this region. However, in these areas, eWUE in Inner Mongolia was positively correlated withprecipitation in summer, mainly because most of these areas had higher altitudes and lower evapotranspiration rates, while the higher the altitude in eastern Inner Mongolia, the better the vegetation growth [46]. That is to say, the greater the potential of vegetation to sequester carbon, the more the eWUE of vegetation will increase when the temperature satisfies the requirements for the development of vegetation, which coincides with an increase in the amount of precipitation. [47; 48; 49; 50]. The performance of eWUE varies in regions with different precipitation levels. In China, areas with annual precipitation levels less than 627 mm showed significant positive correlations (R\({}^{2}\) = 0.9, \(p<\) 0.01), and areas with annual precipitation levels greater than 627 mm showed significant negative correlations [51]. When the annual precipitation level was less than 2352 mm, eWUE would increase with the increase in precipitation. However, when the annual precipitation was more than 4450 mm, the effective water use efficiency would decrease with the increase in precipitation [52]. In Europe, the WUE of Quercus suber with a precipitation gradient of 491-1299 mm decreased with increasing precipitation [53]. In areas which experience less precipitation, the increase in precipitation would promote plant photosynthesis, resulting in a substantial increase in GPP, while the increase in ET would be small, resulting in an increase in WUE in areas which experience less precipitation. Meanwhile, for areas which experience more precipitation, the same increase would be observed. Precipitation contributed less to GPP but greatly increased the amount of water available for evaporation, resulting in lower eWUE in the region [54]. Excessive precipitation would gradually decrease the eWUE [55]. In the study area, the response of water use efficiency of ecosystems to air temperature in different seasons was different. In spring, the areas with positive correlations between eWUE and temperature were mainly distributed in the northeastern part of the study area. More dry matter mass was present, thereby improving vegetation eWUE [56]. In summer, the areas with positive correlations between eWUE and temperature were mainly distributed near Hulun Lake and Erenhot, and the areas with negative correlations were mainly distributed in the agro-pastoral cross belt near Tongliao and Chifeng and the Hetao Plain. As the response of eWUE to meteorological elements depends on the dominant process driving plant photosynthesis and ecosystem water loss, that is, the relative change of GPP to ET [57], the correlation between eWUE and temperature is mainly determined by vegetation ecosystems and depends on the degree of development. Although a warmer climate usually results in better vegetation growth, the southwestern part of the study area was mostly located in low latitudes and had relatively high temperatures. Excessive temperatures will cause plant respiration to be stronger than photosynthesis, stunting plant growth and even causing plants to die.