## 3 Results
### LUE at the Daily Time Scale and Influencing Factors  
Seasonal variations in the daily PAR, GPP, and LUE in 2018 and 2019 are shown in Figure 5. The daily PAR displayed a parabolic trend, peaked in late June, and sharply decreased on rainy/cloudy days (Figure 5a). Two peaks in the general trend in the daily GPP appeared in early May and mid-August. The daily GPP dramatically decreased on rainy/cloudy days and subsequently increased sharply on sunny days (Figure 5b). Similar to the daily GPP, the daily LUE generally first increased and then decreased within a year (Figure 5c).  
We selected the measured daily data from May to September in 2018 and 2019 to assess the effects of biophysical factors on the LUE at a daily time scale (Figure 6). The results of the correlation analysis between the biophysical factors and the LUE are shown in Figure 6a. All biophysical factors influenced the LUE at a significance level of \(p<0.001\); the \(r\) values between the EF and GPP and the LUE were \(0.53\) and \(0.56\), respectively, and the \(r\) values between the LUE and the PAR, CI, Ta, and VPD were \(-0.66\), \(-0.65\), \(-0.37\), and \(-0.36\), respectively. The effects of the PAR and GPP on the LUE were mainly direct; those of the EF, CI, Ta, and VPD on the LUE were mainly indirect, via PAR and/or GPP; the direct effect of the CI on LUE was \(-0.17\) (Figure 6b).  
Figure 4: Correlation analysis (**a**) and path analysis (**b**) among 30 min biophysical factors and LUE around noon (10:00–14:00) during May–September in 2018 and 2019 in the Chinese cork oak plantation. * Significant at \(p<0.05\); *** significant at \(p<0.001\).  
Figure 3: Monthly mean diurnal course (PAR \(>50\) μmm\({}^{-2}\) s\({}^{-1}\)) of (**a**) photosynthetically active radiation (PAR), (**b**) gross primary productivity (GPP), and (**c**) light use efficiency (LUE) in the Chinese cork oak plantation during the growing seasons of 2018 and 2019.  
The mechanisms by which the CI influences LUE are shown in Figure 7. In this study, the GPP rapidly increased when CI \(\leq\) 0.23, slightly increased when 0.23 < CI < 0.57, and rapidly decreased when CI \(\geq\) 0.57 (Figure 7a). The TD\({}_{t}\) was relatively unresponsive to the CI when CI \(\leq\) 0.23, and rapidly decreased when CI > 0.23 (Figure 7b). The PAR\({}_{r}\) was relatively unresponsive to the CI when CI \(\leq\) 0.23, and rapidly increased when CI > 0.23 (Figure 7c). The relationship between the PAR\({}_{t}\) and CI was a downward parabola, peaking at CI = 0.42 (Figure 7d). The GPP was insensitive to the PAR\({}_{r}\) (Figure 7e) but increased with increasing PAR\({}_{t}\) (Figure 7f), indicating that the PAR\({}_{t}\) affected the GPP more than the PAR\({}_{r}\) in this study. The relationships between the PAR\({}_{t}\) and GPP and between the CI and PAR\({}_{t}\) resulted in the relationship between the GPP and CI, and the negative relationship between the CI and LUE resulted from the different effects of the CI on the GPP and PAR.  
Figure 5: Seasonal variations in the daily PAR (**a**), GPP (**b**), and LUE (**c**) in the Chinese cork oak plantation in 2018 and 2019.  
Figure 6: Correlation analysis (**a**) and path analysis (**b**) among daily biophysical factors and LUE during May–September in 2018 and 2019 in the Chinese cork oak plantation. * Significant at \(p\) < 0.05; ** significant at \(p\) < 0.01; *** significant at \(p\) < 0.001.