## 4 Discussion  
In our study, NPP and ET showed synchronized declines in drought periods (Figure 6). Evidence justified the idea that climate extremes such as droughts could lead to a decrease in ecosystem carbon stocks at regional and global scales [3], and had the potential to modify carbon budgets [17; 60]. This can be explained by the fact that water stress decreases stomatal conductance and then affects photosynthetic biochemical processes [61]. Accompanied with the depression of productivity, canopy transpiration is usually decreased due to stomatal closure. So, reduction in precipitation alone could reduce forest ET because of the reduction in canopy interception, soil evaporation, and tree transpiration [62]. Transpiration and photosynthesis are tightly coupled via leaf stomatal conductance and environmental stress, such as water availability, temperature, and so on [63]. Furthermore, drought-induced NPP decline was more extensive than ET reduction (Figures 6 and 7). This can be explained by NPP decreasing faster than ET with an increase in aridity in the regions of lower precipitation. Some studies indicated that different sensitivity of the ecosystem process to changes in hydro-climatic conditions for sub-humid ecosystems was controlled by biological process (i.e., GPP) [32]. Moderate droughts generally do not cause large decrease in NPP due to the buffering capacity of forest soils and shallow groundwater [64]. However, extreme droughts greatly reduced the NPP of forests in this study. In our study, the extreme drought event has the same time-lag impact on productivity and ET of the subtropical forest ecosystem in south-western China after spring 2010 (Figure 7). Furthermore, the more severe the extent of drought, the larger the time-lag impacts on NPP (Figure 7a). However,  
Figure 8: The time series of the difference between ecosystem measures under the drought condition (2009 and 2010) and normal condition in extreme drought (SPI \(<-2.0\)) for three vegetation types (coniferous forest, broadleaf forest, and mixed forest). (**a**) \(\triangle\)NPP = NPP2009 or 2010-baseline NPP in D3 (extreme drought); (**b**) \(\triangle\)ET = ET2009 or 2010-baseline ET in D3 (extreme drought); (**c**) \(\triangle\)WUE = WUE2009 or 2010-baseline WUE in D3 (extreme drought). the time-lag impacts on ET were similar between drought classes (Figure 7b). Evidence is mounting that a time-lag effect generally exists between the onset of a water shortage and the identification of its consequences [30]. For instance, hydraulic redistribution that occurred in ecosystems containing woody plants with dimorphic root systems may facilitate the nutrition acquisition and delay the onset of soil drying during the drought period [65; 66]. The time-lag effect of drought on the carbon sequestration of forests can be explained by the evidence that the impact of drought on vegetation does occur instantaneously but is cumulative [67]. Meanwhile, drought persisting to next year's spring may constrain annual carbon uptake by regulating the availability of soil moisture during the summer season [68; 69]. Furthermore, the time lag between drought events and greenness index as the vegetation indicators has also been reported in many studies. For example, the 3-month SPI has the best correlation with normalized difference vegetation index NDVI, indicating lag and cumulative effects of drought on vegetation [66]. [PERSON] et al., (1997) and [PERSON] et al., (2003) found that NDVI response to precipitation was 5-7 weeks lag for all ecosystem types in the U.S.A and 4-8 weeks in the central Great Plains, USA, respectively [70; 71]. Similar findings about time-lag between drought and tree declines have also been observed in tropical [22] and in temperate regions [72]. Many studies showed that drought leads to a significant uptrend in the WUE of forests [21; 33; 73; 74]. However, it was more complex than we have assumed in humid regions [75].