## 3 Results
### Impacts of Drought on Forest Ecosystem Measures of NPP, ET and WUE
However, in the extreme drought region, WUE under drought conditions were close to the values under normal conditions in the first half of the drought period and then dramatically exceeded the baseline WUE in the second part of the drought period. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Drought Class** & \multicolumn{2}{c}{**D1**} & **D2** & **D3** \\ \hline \multirow{4}{*}{**Baseline condition**} & NPP & 762.8 \(\pm\) 321.63c & 854.7+391.94a & 794.7 \(\pm\) 370.0b \\ \cline{2-5}  & ET & 635.4 \(\pm\) 171.9b & 637.4 \(\pm\) 160.1a & 576.8 \(\pm\) 146.6c \\ \cline{2-5}  & **WUE** & 1.19 \(\pm\) 0.48c & 1.31 \(\pm\) 0.51b & 1.35 \(\pm\) 0.55a \\ \hline \multirow{4}{*}{**Drought condition (2009)**} & NPP & 684.6 \(\pm\) 310.8c & 780.1 \(\pm\) 383.6a & 721.2 \(\pm\) 358.5b \\ \cline{2-5}  & ET & 607.1 \(\pm\) 173.6b & 623.8 \(\pm\) 160.7a & 555.1 \(\pm\) 137.6c \\ \cline{2-5}  & **WUE** & 1.24 \(\pm\) 0.56c & 1.37 \(\pm\) 0.62b & 1.38 \(\pm\) 0.67a \\ \hline \multirow{4}{*}{**Drought condition (2010)**} & NPP & 746.9 \(\pm\) 320.3c & 824.0 \(\pm\) 380.6a & 750.8 \(\pm\) 352.4b \\ \cline{2-5}  & ET & 633.8 \(\pm\) 176.9b & 639.1 \(\pm\) 162.3a & 565.5 \(\pm\) 142.1c \\ \cline{2-5}  & **WUE** & 1.30 \(\pm\) 0.55c & 1.41 \(\pm\) 0.59b & 1.42 \(\pm\) 0.65a \\ \hline \hline \end{tabular} Note. Values sharing the same letters in row are not different at \(p\) < 0.05 (by LSD test).  
\end{table} 
Table 1: Mean annual net primary productivity (NPP) (gC/m\({}^{2}\)/yr), ET (mm/yr) and WUE (gC/mm) under baseline conditions and drought conditions by three drought class. Drought class: D1: Moderate drought; D2: Severe drought; D3: Extreme drought. As illustrated by Figure 7, \(\triangle\)NPP and \(\triangle\)ET (the difference between drought and baseline NPP and ET) during the drought period in the three drought class regions were all smaller than  
Figure 6: The time series of NPP (**a–c**), ET (**d–f**), and WUE (**g–i**) under the normal condition and drought condition extracted by drought class. Drought class: D1: Moderate drought; D2: Severe drought; D3: Extreme drought. zero, which indicated that the drought reduced the NPP and ET of south-western China's forests. Furthermore, \(\triangle\)NPP and \(\triangle\)ET during the drought period in severe and extreme drought class regions are lower than that under the moderate drought class region. The decline of \(\triangle\)NPP and \(\triangle\)ET in the three drought class regions persisted until July, 2010. In the second half of 2010, \(\triangle\)NPP and \(\triangle\)ET increased to positive values, which indicated that NPP and ET under the drought condition recovered to the normal condition. Meanwhile, \(\Delta\)WUE in December 2009 and January 2010 in the D3 region was larger than zero; meanwhile in the D1 and D2 regions, \(\triangle\)WUE in January 2010 was larger than zero. We found that there existed a 4- to 6-month lag between most severe drought stage that occurred in January 2010 and maximum NPP and ET decline in 2010 (Figure 7). Moreover, the time-lag effect also existed between the drought event and ET declines (Figure 7a,b). By contrast, the extent of the maximum decline in NPP behaved more severely than that of ET in the drought area. Monthly \(\triangle\)NPP, \(\triangle\)ET and \(\triangle\)WUE in the extreme drought region were clarified into three vegetation types (coniferous forests, broadleaf forests, and mixed forests) (Figure 8). The changes in the \(\triangle\)NPP and \(\triangle\)ET for each vegetation type were similar, especially in the drought period (Sep. 2009-Mar. 2010). A similar finding about the time lag between drought and vegetation declines of NPP and ET has also been observed for all vegetation types. Furthermore, the extent of the maximum decline in NPP and ET after drought periods were similar for three vegetation types. \(\triangle\)WUE for  
Figure 7: The time series of the difference between ecosystem measures under the drought condition (2009 and 2010) and normal condition.