## 4 Conclusions  
In this study, we investigated carbon and water relationships at two subtropical forests in southeast China: an evergreen broadleaved forest at Dinghushan (23.17\({}^{\circ}\) N, 112.53\({}^{\circ}\) E, 300 m) and an evergreen coniferous forest at Qianyanzhou (26.74\({}^{\circ}\) N, 115.06\({}^{\circ}\) W, 102 m). Besides the differences in vegetation types and elevations, the meteorological conditions are also different at the two sites; Dinghushan has lower radiation, temperature, and VPD  
Figure 11: The hourly GPP and ET relationship in the coniferous forest at Qianyanzhou with different C–W couplings (\(r\)): (**a**) 0.0–0.2, (**b**) 0.2–0.4, (**c**) 0.4–0.6, (**d**) 0.6–0.8, and (**e**) 0.8–1.0. Coefficient of determination R\({}^{2}\) is listed in each panel. All values are on summer no-rain days over 2003–2005. Figure 10: The hourly GPP and ET relationship in the broadleaved forest at Dinghushan with different C–W couplings (\(r\)): (**a**) 0.0–0.2, (**b**) 0.2–0.4, (**c**) 0.4–0.6, (**d**) 0.6–0.8, and (**e**) 0.8–1.0. Coefficient of determination R\({}^{2}\) is listed in each panel. All values are on summer no-rain days over 2003–2005. but higher soil moisture than Qianyanzhou. We focused on short-term carbon and water relationships in summer by assessing the daytime water use efficiency (WUE\({}_{\text{dt}}\)) and the correlation of hourly GPP and ET on a day (\(r\)), using the FLUXNET2015 dataset. The conclusions for this study are drawn as follows. 1. The WUE\({}_{\text{dt}}\) in the subtropical forests in southeast China is moderate in the summer. The daily WUE\({}_{\text{dt}}\) in the two forests ranges from 4 to 14 mg CO\({}_{2}\) per g H\({}_{2}\)O. The daily WUE \({}_{\text{dt}}\) is lower and more concentrated in the broadleaved forest at Dinghushan than in the coniferous forest at Qianyanzhou. 2. Daily WUE\({}_{\text{dt}}\) is closely modulated by daily meteorological conditions. In general, high radiation, air temperature, and humidity reduce summertime WUE\({}_{\text{dt}}\) in the two forests. For the broadleaved forest, the most influential factor on WUE\({}_{\text{dt}}\) is VPD, followed by radiation, while in the coniferous forest, VPD, air temperature, and radiation are almost equally important. 3. The \(r\) values for C-W coupling on an hourly time scale are similar for the two forests, in terms of the mean and its histogram. The skewed normal distribution of \(r\) peaks around 0.6-0.8. This distribution pattern differs from that for WUE\({}_{\text{dt}}\) at the two sites. The mean \(r\) is 0.5-0.6, suggesting asynchronous diurnal variations in GPP and ET. The C-W coupling in the coniferous forest is slightly stronger than in the broadleaved forest. 4. Air dryness (VPD) plays a key role in regulating hourly C-W coupling in the two forests. The drier the air is, the weaker the coupling is. 5. The terms of WUE\({}_{\text{dt}}\) and \(r\) reflect the C-W relationship from different perspectives. WUE\({}_{\text{dt}}\) reflects the C-W relationship on a daily scale, while \(r\) is an indicator of the C-W relationship at sub-daily scale. Both showed the strongest response to VPD among all the daily meteorological factors. However, WUE\({}_{\text{dt}}\) and \(r\) responded to VPD with different levels of sensitivity. The same was observed for the other meteorological factors. Our study highlights the role of meteorological factors in controlling the short-term C-W relationships in subtropical forests in southeast China. As known, subtropical forests in China play an important role in the global carbon and water cycle. Yet, there are few studies on the C-W relationship in this region on such a time scale. Therefore, the C-W relationship and its response to meteorological conditions in this region are still unclear on a short-term scale. Under climate change, the day-to-day fluctuation is intensified. As a result, the interaction between climate and forest on a shorter time scale gained increasing attention.