## 4 Discussion  
This study quantitatively assessed the impact of land cover change on the ecosystem WUE under the background of climate change. In general, these geographic characteristics of WUE were mainly due to different climatic conditions [66; 67], which was consistent with the results of existing research on WUE [15; 65]. The favorable water and heat conditions were beneficial for vegetation growth, and the ecosystem WUE was relatively high [18]. The WUE values of the different vegetation types varied widely in this study, probably due to differences in carbon uptake and water consumption [15; 66]. Large variations in the WUE were also evident for the same land cover types in different regions, probably because of the different climate conditions and the impact of human activities. The WUE of the forests in the eastern TNR was higher than that of the croplands [30]. In the arid areas of the western TNR, the forest WUE was lower than that of the croplands, probably because of drip irrigation [38; 39], plastic mulch [40], gravel-sand mulch [41], and other land management practices that made the increase in productivity greater than that of evaporation [39]. In terms of the spatial distribution, the regions with significant changes in WUE showed greater spatial variability. To better highlight the WUE variation in small regions, we chose five subregions with obvious changes in the WUE-K to further explore the reasons for the changes in WUE from the perspective of the trends in the GPP and ET. The greater spatial variability of the trends in the WUE was mainly due to the different directions and amounts of GPP and ET in the different regions (Figure S5). In the four different regions from west to the east (regions I, II, III and IV), the factors that most contributed to the WUEs were the mean temperature, mean precipitation, mean precipitation and LCC, respectively. The most important factor affecting the WUE in region I was the mean temperature. Region I was located in both arid and semiarid regions, where strong evaporation and low precipitation limit the vegetation growth [15]. The effect of the temperature on the WUE was largely reflected in the soil evaporation and the vegetation transpiration due to the sparse vegetation coverage [11; 68; 69]. The factor that contributed most to the WUE in region II was the mean precipitation. The results based on the site and the model both showed that the WUE of the Chinese terrestrial ecosystems increased quickly with precipitation and then reached saturation [15; 64], which was generally consistent with our results from examining the effect of the precipitation on the trends of the WUE (Figure S4). In region III, the vegetation was sensitive to precipitation [15; 39; 70; 71]. The WUE increased rapidly and then maintained a high rate with the increasing temperature (Figure S4) in region IV. This may have been due to the presence of a broad temperate grassland in northeast China that is mainly controlled by temperature [15]. In addition to climate factors, human activities also played an important role in the WUE. The variation trend of the WUE was different in the different regions, which was closely related to the changes in land cover (Figure 4b-e). From 1985 to 2012, the TNR experienced intense human activities, such as afforestation, which increased the forested area from approximately 10 million to 34 million hectares [34; 35]. The WUE-K from the croplands to the grasslands was higher than that to forests in regions II to IV, and the WUE-K of the stable grasslands was higher than that of the forest that was converted from grasslands in all four regions, which implied that returning croplands to grasslands or protecting the original grasslands may be superior to afforestation and reforestation for maintaining  
Figure 5: Importance of the variables to the WUE trends in northern China via the RF. The seven factors in the RF were LCC, the mean precipitation (Mp), temperature (Mt) and solar radiation (Mr), and their trends (Kp, Kt and Kr, respectively). higher WUE.