## 3 Results and Discussion
### Meteorological Influences on Daily Water Use Efficiency  
In summer, high solar radiation, air temperature, and dry air all tend to reduce daily WUE. Such WUE responses to meteorological conditions in summer are rather different from those in other seasons. This is the case for both broadleaved and needleleaf forests. The responses of WUE\({}_{\text{dt}}\) to four meteorological factors in summer are shown for the broadleaved forest (Figure 4a-d) and coniferous forest (Figure 4e-h). In the two forests, WUE\({}_{\text{dt}}\) decreased with increases in radiation, air temperature, and VPD. However, the degree of the WUE\({}_{\text{dt}}\) responses to meteorological factors were different between the two forests. In the broadleaved forest, high VPD can reduce WUE\({}_{\text{dt}}\) most significantly. In the coniferous forest, the negative effects of VPD and air temperature on WUE\({}_{\text{dt}}\) are nearly equal, and they are the main factors affecting WUE\({}_{\text{dt}}\). As for soil water content, WUE\({}_{\text{dt}}\) in both forests showed slight increases with increasing soil moisture, and the sensitivity is larger in the coniferous forest than in the broadleaved forest. Meteorological factors affect the variation in WUE by controlling GPP and ET [5]. Overall, VPD is the dominant factor for WUE variation, as discussed. The negative effect of VPD on WUE was reported [2; 12; 29]. High VPD tends to reduce stomatal conductance in plants, limiting GPP and ET to some extent [30]. Meanwhile, if the water vapor gradient between leaves and air is high, ET still increases [31], leading to a decrease in WUE. Figure 3: Histogram of WUE\({}_{\text{dt}}\) in the broadleaved forest at Dinghushan and in the coniferous forest at Qianyanzhou on summer no-rain days over 2003–2005. Solar radiation is an important factor driving photosynthesis and transpiration [15]. Under high radiation, photosynthesis saturates even if water is sufficient [32], while ET increases with radiation almost linearly [33], and thus, WUE decreases with an increase in radiation. For the coniferous forest at Qianyanzhou, the negative effect of temperature on WUE is significant. Generally, transpiration increases with air temperature. However, photosynthesis varies with temperature roughly in a Bell curve that peaks within an optimum range of air temperature. When air temperature is higher than this range, photosynthesis decreases with temperature, but transpiration still increases with temperature, and thus WUE decreases with temperature [34]. Such a negative correlation between temperature and WUE has also been documented [2,4,35]. However, [PERSON] et al. [36] reported that the forest annual WUE generally decreases with temperature in the latitude of 10\({}^{\circ}\)-25\({}^{\circ}\) N and 40\({}^{\circ}\)-55\({}^{\circ}\) N but increases with temperature in the latitudes of 25\({}^{\circ}\)-40\({}^{\circ}\) N. Examining the results from [PERSON] et al. [36] closely, we noticed that the three latitudinal zones were drawn approximately. Qianyanzhou is located at 26.76\({}^{\circ}\) N, where it is likely within the zone with a negative temperature-WUE relationship extended from 10\({}^{\circ}\) to 25\({}^{\circ}\) N, at least in summer. The sensitivity of WUEdt to various meteorological factors was assessed by the regression coefficients of the ridge regression (Figure 5). For the Dinghushan broadleaved forest, WUEdt is mostly modulated by VPD, followed by radiation, while air temperature and soil water status showed no significant effect. For the Qianyanzhou coniferous forest, radiation, air temperature, and VPD all have similarly negative effects on WUEdt. Soil moisture showed a positive but insignificant impact on WUEdt. Figure 4: Variation in WUEdt with meteorological conditions (SR, T\({}_{a}\), VPD, and SWC) in the broadleaved forest at Dinghushan (**a–d**) and in the coniferous forest at Qianyanzhou (**e–h**), where (**a,c**) are WUEdt variations with SR, (**b,f**) are for T\({}_{a}\), (**c,g**) are for VPD, (**d,h**) are for SWC.