## 3 Results and Discussion
### Meteorological Influences on C-W Coupling  
Meteorological conditions can impact GPP, ET, and the GPP-ET relationship individually and collectively in complicated ways (Figure 2). Due to differences in meteorological conditions at the two sites, the relative importance of meteorological conditions to C-W coupling can be different in the two forests. Taking all the data, we show in Figure 7 how the \(r\) value varies with each of the daily meteorological factors for the broadleaved forest (Figure 7a-d) and coniferous forest (Figure 7e-h), although this figure cannot fully separate the independent impact from each of the meteorological factors. In the broadleaved forest at Dinghushan (Figure 7a-d), VPD is shown to be the most influential factor for C-W coupling. High VPD conditions tend to increase the vapor gradient between the leaf and air but lower stomatal conductance, which can reduce GPP but may increase ET. Therefore, C-W coupling is compromised. The \(r\) value also decreases with increases in air temperature but is less responsive to variations in solar radiation. Interestingly, soil moisture has a negative effect on \(r\). In the coniferous forest at Qianyanzhou, \(r\) decreases with radiation, air temperature, and VPD. However, \(r\) increases slightly with soil moisture, as the soil at this site is much drier than at Dinghushan (Figure 1d), which results from the different amounts of precipitation in the summer at the two sites. The relative importance of each meteorological factor can be assessed with regression coefficients through ridge regression analysis (Figure 8). For the broadleaved forest at Dinghushan, VPD showed the most significant effect on C-W coupling, followed by SWC, while T\({}_{\text{a}}\) and solar radiation showed insignificant negative and positive effects, respectively. Increases in VPD led to decreases in C-W coupling. More soil moisture also tended to reduce C-W coupling, due to plenty of soil water at the site (Figure 1d). In the coniferous forest at Qianyanzhou, increasing VPD can lead to a decrease in C-W coupling even more significantly than in the broadleaved forest. In fact, VPD is the only meteorological variable that can significantly impact the C-W coupling strength at Qianyanzhou. Figure 6: Histogram of diurnal C–W coupling in the broadleaved forest at Dinghushan and in the coniferous forest at Qianyanzhou on summer no-rain days over 2003–2005. Figure 8: Regression coefficients of meteorological conditions (SR, T\({}_{a}\), VPD, and SWC) with diurnal C–W coupling in the broadleaved forest at Dinghushan and in the coniferous forest at Qanyanzhou. “** indicates that \(r\) is at the 90% significance level (\(p<0.10\)), and “***” at the 99% level (\(p<0.01\)). Figure 7: Diurnal C–W coupling varying with meteorological conditions (SR, T\({}_{a}\), VPD, and SWC) in the broadleaved forest at Dinghushan (**a–d**) and in the coniferous forest at Qanyanzhou (**e–h**), where (**a,c**) are WUE\({}_{\text{dt}}\) variations with SR, (**b,f**) are for T\({}_{a}\), (**c,g**) are for VPD, (**d,h**) are for SWC. The means and standard deviations of C–W coupling varying with meteorological conditions are marked with dots and error bars, respectively. studied the diurnal variation relationship between GPP and ET in tea plantations in hilly areas of southeast China, and they showed that high VPD reduced stomatal conductance and regulated photosynthetic rate, resulting in asynchronous phenomenon between GPP and ET and the weakened coupling of GPP and ET. With the increasing VPD and temperature throughout the day, both forests reduce the stomatal opening to prevent excessive water loss due to transpiration [37], which also leads to the decline of photosynthesis. However, if soil moisture in the forests is sufficient, the stomata are not completely closed [38]. Therefore, partial stomatal closure cannot effectively limit transpiration when VPD is high. If a decrease in stomatal conductance is proportional to an increase in VPD, transpiration still increases; meanwhile, GPP decreases [39].