## 4 Discussion
On the other hand, decrease in precipitation (drier) would trigger plants' water-saving adaptive strategy (partly close their stomata) and further reduce photosynthesis. The ecosystem resilience would thus decrease under drought stresses due to decreased WUE induced by reduced GPP and almost unchanged ET in such regions [11; 64]. The croplands in humid and semi-humid regions (e.g., Dongbei Plains, Huabei Plains, Yangtze River basin and some regions of South China) also showed relatively lower resilience to drought (the WUE for this region showed positive response to drought) due to their vulnerability to water stresses. The crops would wilt or even die under water deficiency conditions, thus the WUE will decrease with increased ET and declined ecosystem GPP [65; 66]. In some humid and semi-humid regions (Northeast China, the west of Inner Mongolia and some many areas of southern China) with abundant vegetation (i.e., DBF and DNF), energy was the main factor affecting vegetation growth. The incoming shortwave solar radiation was reduced when SPEI increased, hindering the absorption of carbon [23]. And soil evaporation and vegetation transpiration enhanced owing to additional water input, so the WUE decreased. When drought event happened, the ecosystems of those regions could absorb the disturbances from drought and sustain its GPP by increasing WUE because of their strong ecosystem resilience [26]. The above findings are generally consistent with [PERSON] et al. (2015) except for the central region of Inner Mongolia, which may be induced by the differences of data sources and time periods used [23]. Moreover, the anthropogenic factors (e.g., afforestation and deforestation) could also affect the ecosystem resilience in the west parts of in North China and Northwest China, the central of Inner Mongolia as well as some regions of Tibetan Plateau. In our study, different vegetation types showed different WUE and their WUE showed different responses to drought. It may because GPP and ET in different vegetation types showed different sensitivities to climate change leading to the difference in carbon uptake and water consumption [65; 66]. For example, grassland showed lowest WUE (Figure 2c), but its positive correlation between SPEI and WUE was the highest (Figure 4b). The growth and activity of grassland was highly dependent on water supply [67] and ecosystem productivity in grassland was closely related to precipitation [19; 67]. Therefore, grassland had lowest GPP and lower ET [65; 66] resulting in lowest WUE due to water limitations in arid and semi-arid regions. The growth rate and activity of grassland would decline when SPEI decreased (drought) [68] and the decreased GPP was higher than ET since GPP was more sensitive to drought than ET [26], leading to a decreased in WUE in the dry years. Moreover, DNF showed high WUE (Figure 2c), but its negative correlation between SPEI and WUE was the highest (Figure 4b). DNF was mainly distributed in humid and semi-humid regions, where energy supply was the main driver affecting vegetation functions [23; 69]. Drought usually resulted in more incoming solar radiation (less precipitation and less cloud cover) and therefore in these radiation-limited, humid regions, drier, but also sunnier, conditions would imply a larger increase in GPP compared to ET, resulting in higher WUE [26; 69]. Climate factors such as precipitation and temperature usually have lagging effects on vegetation growth [70; 71]. In this study, we found that droughts also had lagging effects on ecosystem WUE in some regions (e.g., the east of Inner Mongolia and Northeast China, the west and east regions in North China and central parts in Tibetan Plateau). [PERSON] et al. (2016) suggested that the relationship between WUE and SPEI in different years (previous-year and current-year) were in the opposite direction in all vegetation types over the world [26]. However, [PERSON] et al. (2017) found that the impacts of drought in current-year and previous-year on WUE were in the same direction [20].