# Responses of Water Use Efficiency to Drought in Southwest China
## 4 Discussion
In the early stage of crop growth (e.g., the jointing stage of corn, the heading stage of rice, the flourishing and long term of flue-cured tobacco), the cropland growth is faster and is sensitive to water demand, and therefore the water deficit leads to a serious decline in productivity and WUE. During July to August (the maturity stage of crops), the plants mainly accumulate dry materials through photosynthesis; as the roots of crops are well developed, the absorption capacity of roots to soil effective water is the strongest, and the high ground canopy coverage can also effectively inhibit soil evaporation; thus, the ecosystem WUE of croplands is relatively high. GPP and ET are two components that directly influence the change of ecosystem WUE [59; 60; 61]. The different sensitivity of ecosystem GPP and ET to drought results in different responses of WUE to drought. It has been reported that the ecosystem WUE increased under drought stress because the decrease of GPP was less than that of ET [62]. Contrary to some previous studies [63], our results showed that the relationships between WUE and drought varied with vegetation type in Southwest China, indicating the different sensitivity of different vegetation types to drought. The responses of WUE to water deficit during drought has been reported in previous studies, but few studies have been conducted on the changes of WUE after the end of persistent drought events [64; 65]. Studying the responses of WUE following drought can help us understand the ability of different ecosystems to recover after drought and to cope with another drought [66]. Vegetation has lagged responses to environmental factors, such as rainfall and temperature, and ecosystem WUE also has lagged responses to drought [8]. During the drought stress period, the drought in the previous year and current year both affected the WUE of terrestrial ecosystems (forests, shrublands, grasslands, and croplands). The regionally averaged WUE had a stronger correlation with drought in the previous year than with drought in the current year (Table 4). This indicates that the drought in the previous year could have larger effects on ecosystem WUE than drought stress in the current year. After drought stress, WUE in the current year was more closely related to the current-year drought than to the previous-year drought. Therefore, the lag effect of drought was not obvious under normal water conditions [67; 68]. The differences of ecosystem WUE under different drought conditions found in this study were consistent with previous results [69]. The spatial differences of WUE were mainly determined by the environmental factors and the heterogeneity of physiological characteristics among plant species [6]. In addition, the drought resistance of ecosystems depends on various factors, such as drought severity, drought duration, drought extent, vegetation type, plant growth rate, and inter-species competition, and scale differences in drought observation. The stability of ecosystems is related to not only the resistance of plants to water stress but also the recovery of plants (i.e., resilience) [70]. Our results showed that croplands had a weak resistance to drought. Usually, croplands under a water deficit would reduce photosynthesis and aboveground biomass and increase underground biomass, and this is also a survival strategy of ecosystems in arid and semi-arid areas. After severe drought, an ecosystem needs a recovery period [71]. If another drought occurs, the recovered ecosystem can respond quickly and to some extent ensure the stability of the ecosystem WUE level [9]. The resilience of ecosystem WUE found in this study is consistent with previous studies [26; 69]. To some extent, ecosystems can adapt to changing hydro-climatic conditions by adjusting its WUE, and after drought, ecosystems can restore their pre-drought status through self-regulation with improved soil water availability [72].