## 4 Discussion  
### Variations in LUE at Different Time Scales  
The monthly mean hourly LUE increased sharply and reached a peak value in the morning at our study site (Figure 3c). We suspect that the following two factors are responsible for this phenomenon: (1) dark reaction lags behind the light reaction in the process of photosynthesis during the transition from dark to light, resulting in an increasing LUE after sunrise; (2) stomatal conductance increases sharply and reaches a peak value at around 8:30; thereafter, it decreases gradually in the morning [38; 39], and the LUE increases with stomatal conductance [31]. [PERSON] et al. (2018) found that the LUE decreased in a tropical rainforest ecosystem in the morning in southern China, and no major change in the LUE was observed in a rainfed spring maize field in the morning on the Loess Plateau, China [36]. The monthly mean diurnal course of LUE in the morning varies in different ecosystems, which could be due to different vegetation characteristics and environmental factors in different ecosystems. The monthly mean diurnal LUE increased gradually in the afternoon in this study (Figure 3c), which is consistent with the results in different terrestrial \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & **PAR** & **T\({}_{a}\)** & **VPD** & **P** & \multirow{2}{*}{**CF**} & & **GPP** & **LUE** & **LUE\({}_{\text{max}}\)** \\  & **(mol m\({}^{-2}\))** & **(\({}^{\circ}\)C)** & **(hPa)** & **(mm)** & & & **EF** & **CI** & **NDVI** & **(g C mol** & **(g C mol** & **photon\({}^{-1}\))** \\ \hline
2006 & 8048.67 & 15.03 & 8.49 & 559.70 & 0.45 & 0.37 & 0.37 & 1217.79 & 0.15 & 0.36 \\
2007 & 7755.90 & 15.11 & 9.17 & 490.90 & 0.43 & 0.36 & 0.37 & 1036.66 & 0.13 & 0.31 \\
2008 & 7670.72 & 14.64 & 9.19 & 600.10 & 0.42 & 0.36 & 0.37 & 1335.05 & 0.17 & 0.41 \\
2009 & 9451.69 & 14.52 & 9.01 & 456.40 & 0.43 & 0.45 & 0.38 & 1090.35 & 0.12 & 0.36 \\
2010 & 9212.84 & 14.26 & 8.13 & 512.10 & 0.43 & 0.44 & 0.37 & 1426.12 & 0.15 & 0.33 \\
2011 & 9200.02 & 13.81 & 8.39 & 860.00 & 0.39 & 0.43 & 0.37 & 1059.85 & 0.12 & 0.27 \\
2012 & 9792.10 & 14.38 & 9.02 & 613.30 & 0.41 & 0.45 & 0.38 & 1073.21 & 0.11 & 0.28 \\
2013 & 8902.24 & 15.04 & 8.79 & 474.60 & 0.40 & 0.42 & 0.34 & 981.28 & 0.11 & 0.25 \\
2014 & 9005.00 & 15.51 & 8.78 & 621.10 & 0.45 & 0.42 & 0.38 & 869.41 & 0.10 & 0.23 \\
2015 & 9807.15 & 15.02 & 8.35 & 664.80 & 0.38 & 0.43 & 0.39 & 842.37 & 0.09 & 0.21 \\
2016 & 9102.98 & 15.04 & 7.98 & 684.70 & 0.40 & 0.42 & 0.40 & 950.38 & 0.10 & 0.33 \\
2017 & 9596.15 & 15.32 & 8.87 & 620.10 & 0.38 & 0.45 & 0.39 & 1156.96 & 0.12 & 0.38 \\
2018 & 9527.76 & 15.12 & 8.87 & 686.20 & 0.41 & 0.45 & 0.38 & 1090.89 & 0.11 & 0.23 \\
2019 & 9249.70 & 15.45 & 10.04 & 546.60 & 0.38 & 0.44 & 0.36 & 1038.20 & 0.11 & 0.29 \\ Mean & 9023.07 & 14.88 & 8.79 & 599.33 & 0.41 & 0.42 & 0.38 & 1083.47 & 0.12 & 0.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Annual photosynthetically active radiation (PAR), mean air temperature (T\({}_{a}\)), mean vapor pressure deficit (VPD), precipitation (P), evaporative fraction (EF), clearness index (CI), normalized difference vegetation index (NDVI), gross primary productivity (GPP), light use efficiency (LUE), and maximal LUE (LUE\({}_{\text{max}}\)) in the Chinese cork oak plantation during 2006–2019. Figure 11: Correlation analysis (**a**) and path analysis (**b**) among annual biophysical factors and LUE in 2006–2019 in the Chinese cork oak plantation. * Significant at \(p<0.05\); ** significant at \(p<0.01\); *** significant at \(p<0.001\). ecosystems (e.g., meadow [13], forest [5], and cropland [36]).