## 4 Discussion
### Variations in LUE at Different Time Scales
The main reasons for this phenomenon are (1) the vegetation photosynthesis consists of two relatively independent processes, i.e., light reaction and dark reaction; the former and latter absorb solar energy and CO\({}_{2}\), respectively; (2) the vegetation can use relatively more chemical energy accumulated by earlier light reactions to absorb CO\({}_{2}\) during the transition from light to dark, resulting in a slower decrease in the GPP than the PAR in the afternoon [36]. The monthly mean diurnal course of LUE in the afternoon is similar in different ecosystems, indicating a common trend in the LUE increasing in the afternoon among different terrestrial ecosystems. The general trends in the daily and monthly LUE first increased and then decreased within a year in the Chinese cork oak plantation (Figures 5c and 8g), which agree with the results in temperate, subtropical, and tropical forests [5; 28], because the vegetation dynamics and activity determined the general trends in the daily and monthly LUE [36]. Consistent with previous studies [5; 36], day-to-day fluctuations were observed in the seasonal pattern of the daily LUE in this study; this is because the day-to-day fluctuations of the seasonal variations in the daily LUE were mainly controlled by meteorological factors [36]. Like the daily GPP, the daily LUE generally decreased between early May and mid-August in 2018 and 2019, and the mean monthly GPP and LUE were lower in June than in May and July, indicating that the decrease in the LUE in early summer is the norm in the Chinese cork oak plantation. The main reason for this phenomenon is that monthly VPD and EF in June were relatively higher and lower in the canopy closure period, respectively (Figure 8c,e), indicating that drought occurred in the same period, which inhibited vegetation activity. The start time for the phenomenon was determined by the water balance of the previous year and P during January-April of the current year, and the phenomenon ended with the beginning of the rainy season in the current year. During 2006-2019, the annual LUE was highest in 2008 (0.17 g C mol photon\({}^{-1}\)) and lowest in 2015 (0.09 g C mol photon\({}^{-1}\)), indicating that significant interannual LUE variation occurred in the Chinese cork oak plantation; this finding is consistent with previous studies conducted in different ecosystems [5; 12; 27]. The mean annual LUE in the Chinese cork oak plantation was different from that in different ecosystems [10; 20; 26]; for example, higher than that in a grassland (0.02 g C mol photon\({}^{-1}\)) and lower than that in an evergreen broadleaf forest (0.24 g C mol photon\({}^{-1}\)) in Australia [10]. This indicates that the annual LUE varies in different ecosystems due to different vegetation characteristics and natural conditions. In addition, the mean annual LUE was 0.11 g C mol photon\({}^{-1}\) in a plantation oak woodland in south-eastern England [40], similar to our results. Similar vegetation types are likely responsible for this phenomenon. Like the annual LUE, the annual LUE\({}_{\text{max}}\) showed great interannual variation (0.21-0.41 g C mol photon\({}^{-1}\)) in the Chinese cork oak plantation, due to the different hydrothermal conditions in each year, and agreeing well with previous findings obtained in various ecosystems [2; 5]. Therefore, the annual LUE\({}_{\text{max}}\) should be determined based on multi-year observation data when a carbon exchange model based on LUE is used to estimate the GPP at the regional scale. [PERSON] and [PERSON] (2018) [27], using measured net primary productivity, meteorological data, and remote sensing technology, estimated that the annual LUE\({}_{\text{max}}\) ranged from 0.06 to 0.55 g C mol photon\({}^{-1}\) in deciduous broad-leaved forests of China, and our results fall within this range.