# Responses of Water Use Efficiency to Drought in Southwest China
## 4 Discussion  
Many studies have examined the effects of drought on WUE. Some researchers believed that drought could lead to a significant increase in the WUE of natural ecosystems [24; 50] while others suggested that drought could reduce the WUE of most ecosystems [51; 52]. The responses of water and the carbon cycle to drought in the terrestrial ecosystem are affected by many biological and abiotic factors. Previous studies have pointed out that the WUE had a significant intrinsic variability and plasticity, which may be caused by inheritance [53]. For example, C4 plants generally have a stronger water use ability than C3 plants [54]. The long-term living environment will affect the adaptability of plants to drought. Some plants living in arid regions have a high drought tolerance gene [55], which can improve their resistance to drought stress; some plants (e.g., cactus) increase their resistance to drought stress through metamorphic leaves [56]. In wet areas, high temperature and high evaporation can lead to the decline of WUE while for plants in alpine areas, the WUE is often relatively low because of the high wind speed and solar radiation [50]. Our study shows that overall, drought slightly increased the WUE of most forest, grassland, and shrubland ecosystems while there were turning points when drought became more severe. The increase in the WUE suggests that under certain water stress, plants can achieve higher productivity with  
Figure 11: Variations of annual WUE, GPP, and ET with changing wet or dry conditions (scPDSI) over the period 2000–2017 for forests (**a–c**), shrublands (**d–f**), grasslands (**g–i**), and croplands (**j–l**). the same amount of water lost to ET or the same productivity with lower ET; when the degree of water stress exceeds the ability of plant self-regulation, however, drought will reduce the normal physiological metabolism level of plants [57]. Moreover, drought has different inhibition points on WUE for different vegetation types. On the whole, croplands had the lowest resistance to drought, which is also related to the vulnerability and higher productivity of these ecosystems. Therefore, croplands could require higher stability of water availability, and mild drought can cause fluctuations of WUE. The WUE of grasslands was also sensitive to drought, and moderate drought (scPDSI = \(-\)3 to \(-\)2) could reduce ecosystem WUE. This is consistent with previous research results [58], which found that grasslands can maintain the normal water metabolic demand through root extension under mild drought, and improve WUE by controlling transpiration. The grass root system is relatively shallow, and its drought mediation ability is limited, and more serious water deficit could lead to the decline of grasslands productivity and WUE. The forests and shrublands in Southwest China are dominated by perennial deciduous and evergreen forests that have deeper roots and stronger ability to utilize groundwater; therefore, short-term drought or mild drought could enhance the WUE of forests and shrublands while more severe or long-term drought could decrease WUE. The responses of WUE to drought varied with region, season, and vegetation type. The drought areas in 2010 were characterized by mixed areas of croplands, grasslands, and forests, but the proportion of croplands was relatively large. With the increase of drought intensity, WUE exhibited a significant quadratic curve pattern (Figure 9b), which is different from the overall analysis results that drought led to the decline of WUE in croplands (Figures 6 and 10d). However, WUE and scPDSI showed a negative correlation (WUE decreased due to drought) from January to June (early and middle growth period) and positive correlation (drought enhanced WUE) during July to August (mature period).