## 4 Discussion
In our study, we found higher WUE occurred in extreme and severe drought regions but the changes in WUE from baseline condition to drought condition were not obvious. It seems that the forest ecosystems in subtropical China were relatively stable. In detail, WUE in the D2 and D3 regions were higher than that in D1 region in the drought period (Figure 6) or in whole year (Table 1). The results indicated that NPP was more sensitive to drought than that of ET. An increase of WUE in January 2010 was found in extreme, severe and moderate drought regions related to the baseline condition, with a value of 19%, 11% and 13%, respectively. These values were a little higher than another study in which an increase (7.09%) of WUE was observed in evergreen forests under drought stress based on global MODIS GPP and ET products [18]. An increasing trend in WUE for different drought severity (D1-D3) provided a great capacity to maintain productivity when plants were limited by water stress [33], which indicated how the forest ecosystems behaved a drought-resilient ecosystem [32; 71]. Severe droughts enhance ecosystem resistance and resilience in responding to water deficiency [73], which likely contributed to the high WUE in the drought period in south-western China. Meanwhile, the synthetic declines of NPP and ET in the growing season of 2010 resulted in no obvious change of WUE in the growing season after the spring drought in 2010. We also tested the different responses of WUE to drought for each forest type (Figure 8). Similar patterns can be observed for the changes in WUE for each forest types in the extreme drought region. Furthermore, a substantial increase in WUE in the forest can also be influenced by other processes, i.e., CO\({}_{2}\) fertilization. But increasing atmospheric CO\({}_{2}\) should lead to an increase in both photosynthetic uptake and WUE [76], which was different to drought that would reduce the forests' productivity and increase the WUE. Although forest ecosystems were sensitive to drought and temperature extremes in extreme climate events [77], our findings indicated these forests in subtropical regions were less vulnerable than commonly assumed, which can be ascribed to the rich vegetation and species diversity and high temperature and precipitation after extreme climate events in south-western China [37]. The resilience of the forest ecosystem is associated with its biological resources, species composition and ecological functions under changing climatic conditions [78]. High species diversity can partly offset the effect of extreme climate events on the carbon cycle of forests [79]. Meanwhile, it could be partly attributed to the existence of cross-scale interaction between ecological processes that will balance or offset the losses caused by extreme events. To some extent, the tree species diversity of subtropical forests in southern China alleviates the impacts imposed by extreme events, and thus the subtropical forests have higher potential to recover from disturbances. Thus, with increasing frequencies and intensities of climate extremes in the future, studies on the response of the ecosystem carbon cycle to various levels of climate extremes are of high priority. The gap between experiments, remote-sensing information and ecosystem modeling still exists [80]. It is necessary to monitor strategies and variables which are most useful for quantifying resilience and thresholds in wide range of ecosystems [81]. Monitoring systems with high accuracies and improvements on spatio-temporal resolution are strongly recommended to obtain better insights into the response of forest ecosystem to extreme events and feedbacks between them. [PERSON] and [PERSON] designed the framework of this study; [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] collected and processed the data. [PERSON], [PERSON], [PERSON] and [PERSON] analyzed the data and wrote the paper. [PERSON], [PERSON] and [PERSON] provided suggestions on the structure of the manuscript.