## 4 Discussion
### Regulation of SIF by Environmental Factors  
In the daily scale, SIF was not only affected by the canopy structure and pigment content, but also fluctuated according to the environmental factors. This was especially true when the canopy structure was stable, as environmental factors dominated the dynamics of SIF. SIF contains information about APAR and the environmental stresses (e.g., temperature and water stress) related to photosynthetic light energy utilization [14; 80]. Compared with the vegetation index, SIF had more obvious daily dynamics and was more sensitive to the variations in PAR, Ta, VPD and SM [69; 81]. To clarify the influence path of the environmental factors on SIF, we analyzed the responses of the APAR, \(\phi\)F and the \(\mathsf{f_{esc}}\) to the PAR, Ta, VPD, SM and Ws. The results showed that PAR was the most closely related factor to the SIF in the Chinese cork oak canopy, which significantly affected the three components in most cases, indicating that the SIF was driven by incident solar radiation. The ability of PAR to drive SIF has been recognized in many studies [82; 83; 84; 85]. However, there is another explanation for what drives the diurnal dynamic of SIF: the genes that encode the chlorophyll binding proteins may be up-regulated at dawn and down-regulated before sunset [86]. This suggests that the circadian rhythm may be another factor in determining the dynamics and in changing the relationship between fluorescence and photosynthesis. The opposite effect of PAR on \(\phi\)F at two time-scales may be related to the diurnal dynamics of PAR. When PAR exceeds the electron transport chain capacity, plants will protect their leaf structure through NPQ [87], which will reduce their fluorescence yield [88; 89]. The analysis of the relationship between the environmental factors (e.g., PAR, Ta and precipitation) and the forest SIF, which was based on remote sensing data, found that the 16-day and day-scale SIF in the Greater Khingan Mountains were only related to the Ta [30]. The satellite data were instantaneous, taken from one moment in time, and thus, they did not include the diurnal dynamics of PAR. In this condition, there was no significant relationship between SIF and PAR, suggesting that the circadian rhythm could affect this relationship. On a regional scale, the Ta regulated SIF by influencing photosynthesis. We found that the Ta had a consistent and significant effect on SIF. It had a positive effect on APAR and a negative effect on the \(\phi\)F and \(\mathsf{f_{esc}}\), yet the contribution of APAR to SIF was higher than that of \(\phi\)F and the \(\mathsf{f_{esc}}\); therefore, the Ta had a positive effect on SIF. After eliminating the covariant of Ta and PAR, the effect of the Ta on the daily SIF was weakened or had disappeared. In the spring and summer, the Ta was higher than the optimum photosynthetic temperature, and it had a negative effect on SIF. High temperatures reduce the photosynthetic rate, and radiation promotes the temperature; therefore, the compensation of PAR to SIF covered the inhibition effect of the high temperatures on SIF. While the temperature in autumn was low, Ta became a photosynthetic limiting factor and influenced the rhythm of the vegetation [75]. The soil moisture of this study site was insufficient and the change of intra-day Ta impacted on leaf morphology (high temperature will make the leaf curl), which may be the reason why Ta significantly affected \(\mathsf{f_{esc}}\) at the half-hour scale [90]. The reasons for the significant influence of Ta on \(\mathsf{f_{esc}}\) at a daily scale can be explained from two aspects. On the one hand, Ta was related to the canopy structure through synergistic changes with the phenological pattern in deciduous broadleaf forest. On the other hand, continuous high Ta along with less precipitation in May-June (late spring and early summer) regulated canopy structure.