## 4 Discussion
### Effect of Biophysical Factors on LUE at Different Time Scales  
At the 30 min time scale, the measured 30 min data around noon in the canopy closure period of 2018 and 2019 were used to assess the effects of biophysical factors on the LUE in our study. The CI was calculated using Equation (3), and the ratio of PAR to S\({}_{r}\) was quite constant [41; 42], resulting in a high \(r\) value between the PAR and CI (0.94, Figure 4a). As observed in previous studies [43; 44], the CI was negatively correlated with the LUE in the Chinese cork oak plantation. The effects of the EF, T\({}_{a}\), and VPD on the LUE were mainly indirect, via PAR and GPP (Figure 4b); the T\({}_{a}\) and VPD had negative effects on the GPP, due to higher T\({}_{a}\) and VPD resulting in stomatal closure around noon (Figure 4a),which agrees well with previous studies [42; 44]. Many studies have shown that water stress can inhibit vegetation photosynthesis in various ecosystems [3; 42]; hence, a positive relationship between the EF and LUE existed in our study (Figure 4a), as the EF is an indicator of vegetation moisture conditions in the canopy closure period. In addition, the effects of the PAR, CI, T\({}_{a}\), and VPD on the EF and GPP were negative (Figure 4a), illustrating that the light saturation phenomenon caused by water stress often occurred around noon during the canopy closure period in the Chinese cork oak plantation. The effects of biophysical factors on the LUE at the daily time scale in our study were also found in rainforest and savanna ecosystems [3], because measurement data in the canopy closure period were used to eliminate the influence of vegetation growth on the LUE. The biophysical factors had similar effects on the LUE at the 30 min and daily time scales in the Chinese cork oak plantation (Figures 4a and 6a), but the effects of the PAR, CI, and VPD on the GPP were different at the two time scales, as the light saturation around noon could not determine the vegetation photosynthesis throughout the day. The difference in penetration of PAR\({}_{r}\) and PAR\({}_{f}\) in the canopy and the different effect of the PAR\({}_{r}\) and PAR\({}_{f}\) on the GPP were the root causes of the LUE increasing with CI decreasing, due to the following factors: the PAR\({}_{r}\) could only illuminate a fraction of leaves in the upper canopy, which were often light-saturated [35]; the PAR\({}_{f}\) could provide energy for most leaves shaded by upper leaves in the canopy, which were generally light-limited [20; 44]. With increasing clouds and/or aerosols in the sky, the PAR received by the canopy decreased, but the TD\({}_{f}\) increased, producing a more uniform irradiance in the canopy. This led to a faster decrease in the PAR than the GPP, which resulted in an increase in the LUE [43]. Because the monthly NDVI was around 0.5, the Chinese cork oak plantation had a relatively dense canopy in the canopy closure period, which was essential for the phenomenon where the LUE increased with a decreasing CI. When the ecosystem had a thin canopy, the PAR\({}_{r}\) could illuminate most leaves in the canopy, and the effects of the PAR\({}_{f}\) on the canopy's photosynthesis would consequently disappear [43]. The effects of biophysical factors on the LUE at the monthly time scale were different from those at the 30 min and daily time scales, as NDVI, an indicator of vegetation characteristics, was considered when analyzing the relationship between the biophysical factors and the LUE; similar to our study, vegetation growth had a positive effect on the LUE at the monthly time scale in various ecosystems [5; 9; 36]. The monthly PAR, T\({}_{a}\), VPD, P, and EF had positive effects on the monthly LUE in our study (Figure 9), which agrees well with the results in deciduous temperate forests and evergreen temperate forests [3; 5; 31].