## 2 Data and Methods  
### Study Sites  
The two flux tower sites include Dinghushan subtropical evergreen broadleaved forest (CN-Din) and Qianyanzhou subtropical evergreen needleleaf forest (CN-Qia). Both sites are located in the subtropical region of southeast China, which is an important part of the Chinese terrestrial ecosystem flux research network. The data for the two sites were available from the FLUXNET2015 dataset [24]. Dinghushan and Qianyanzhou sites are representatives for subtropical ecosystems in China and have enough data coverage to span longer than 3 years. Table 1 lists the geographic, vegetation, and climate conditions at the two sites, including the annual climate conditions and summer climate conditions in the daytime. Overall, the meteorological conditions are similar to the moderate temperature and adequate precipitation, reflecting the characteristics of a subtropical monsoon climate. The histogram of meteorological variables in the summer over 2003-2005 is shown in Figure 1. Owing to the close proximity, solar radiation at the two sites was similar (Figure 1a), although the mean solar radiation at Dinghushan was \(\sim\)50 W m\({}^{-2}\) lower than at Qianyanzhou. Notably, Dinghushan is at a higher elevation (300 m) than Qianyanzhou (100 m); thus, air temperature and VPD there are lower than those at Qianyanzhou. In contrast, precipitation at Dinghushan is higher than at Qianyanzhou. Accordingly, summertime soil is much moister at Dinghushan than at Qianyanzhou. These differences in meteorological conditions can lead to different responses in WUE and C-W coupling to meteorology, which are discussed in Section 3.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Dinghushan** & **Qianyanzhou** \\
**Sites** & **(CN-Din)** & **(CN-Qia)** \\ \hline Vegetation type & Evergreen broadleaved forest & Evergreen confiferous forest \\ Latitude (\({}^{\circ}\)N) & 23.17 & 26.74 \\ Longitude (\({}^{\circ}\)W) & 112.53 & 115.06 \\ Elevation (m) & 300 & 102 \\ Mean annual temperature (\({}^{\circ}\)C) & 19.6 & 19.0 \\ Mean annual precipitation (mm) & 1618.1 & 1466.8 \\ Leaf area index (LAI, m\({}^{2}\)/m\({}^{2}\)) & 4.0 & 3.5 \\ Mean daytime solar radiation (W m\({}^{-2}\)) in summer & 555.6 & 592.0 \\ Mean daytime air temperature (\({}^{\circ}\)C) in summer & 29.1 & 31.6 \\ Mean daytime vapor pressure deficit (H\({}^{\circ}\)Par) in summer & 14.7 & 22.4 \\ Mean daytime soil water content (\%) in summer & 21.26 & 12.43 \\ Dominant species & Pinus massoniana [PERSON], Pinus & Schima superb, Castanea henry, Pinus massoniana Lamb et al. \\  & & lancoolate et al. \\ Frequency & Half-hour & Half-hour \\ Time series & 2003–2005 & 2003–2005 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of site characteristics.  
Figure 1: The frequency of summer daytime shortwave radiation (SR, **(a)**), air temperature (Ta, (b)), vapor pressure deficit (VPD, **(c)**), and soil water content (SWC, **(d)**) in the Dinghushan evergreen broadleaved forest (red solid line) and Qianyanzhou evergreen confiferous forest (cyan solid line). The dashed lines show the mean value of the meteorological variables at Dinghushan (red dashed line) and Qianyanzhou (cyan dashed line) during summer daytime; the specific values are presented in Table 1.