## 2 Materials and Methods
### Data Collation and Analysis  
Vegetation indices can be used to reflect canopy structure, pigment and physiological changes, and can be retrieved by vegetation reflectance at specific wavelengths [57]. To confirm the research time range, we selected NDVI as a reference to judge the start and end time of the growing season of Chinese cork oak in 2020. Pigment affects the photosynthetic activity of vegetation, and the MERIS terrestrial chlorophyll index (MTCI) can reflect the canopy chlorophyll content. The calculation equation of NDVI [58] and MTCI [59; 60] is as follows:  
\[\mathrm{NDVI}=\frac{\mathrm{R}_{\mathrm{NIR}}-\mathrm{R}_{\mathrm{red}}}{ \mathrm{R}_{\mathrm{NIR}}+\mathrm{R}_{\mathrm{red}}} \tag{2}\]  
\[\mathrm{MTCI}=\frac{\mathrm{R}_{\mathrm{NIR}}-\mathrm{R}_{\mathrm{rededge}}}{ \mathrm{R}_{\mathrm{rededge}}-\mathrm{R}_{\mathrm{red}}} \tag{3}\]where \(\rm R_{NIR}\) is the near-infrared reflectance range from 770 to 780 nm; \(\rm R_{red}\) is the red reflectance range from 650 to 660 nm; and \(\rm R_{reddeg}\) is the red edge reflectance range from 700 to 710 nm. We determined the time of reaching the extremum of the first derivative of NDVI as the beginning and end of the growing season.  
SIF is emitted by leaves after plants absorb light energy; it goes through the process of reabsorption in the canopy and then is finally observed at the top of canopy. Therefore, SIF is determined by absorbed photosynthetic active radiation (APAR), the SIF emission yield from all leaves in the canopy (\(\rm\phi F\)) and fluorescence escape ratio (\(\rm f_{esc}\)), as follows:  
\[\rm SIF=APAR\times\rm\phi F\times f_{esc} \tag{4}\]  
APAR is determined by the fraction of photosynthetically active radiation (fPAR), which is regulated by canopy structure and physiology. Since the red edge normalized difference vegetation index (\(\rm NDVI_{reddeg}\)) can characterize canopy structure and pigment changes [61], it has also been applied to the retrieval of fPAR. The calculation equation of APAR is as follows [62; 63]:  
\[\rm APAR=PAR\times fPAR \tag{5}\]  
\[\rm fPAR=1.37\times NDVI_{reddeg}-0.17 \tag{6}\]  
\[\rm NDVI_{reddeg}=\frac{R_{NIR}-R_{reddeg}}{R_{NIR}+R_{reddeg}} \tag{7}\]  
According to the recent research, \(\rm f_{esc}\) can be represented by near-infrared reflectance of vegetation (\(\rm NIR_{V}\)) and fPAR [41; 64], as follows:  
\[\rm f_{esc}\approx\frac{NIR_{V}}{fPAR} \tag{8}\]  
\[\rm NIR_{V}=NDVI\times R_{NIR} \tag{9}\]  
According to the above equations, it follows that \(\rm\phi F\) can be expressed as follows:  
\[\rm\phi F\approx\frac{SIF}{PAR\times NIR_{V}} \tag{10}\]  
Weather had an impact on the analysis. We chose the clearness index (CI) [65; 66] to represent the weather conditions (sunny or cloudy), and the calculation method was as follows:  
\[\rm CI=\frac{R_{g}}{R_{0}} \tag{11}\]  
\[\rm R_{0}=S_{0}\times\left[1+0.033\times\cos\left(\frac{2\pi\times DOY}{365} \right)\right]\times\cos\theta \tag{12}\]  
where \(\rm R_{g}\) is the total solar radiation; \(\rm R_{0}\) is the extraterrestrial radiation at the top of the atmosphere; \(\rm S_{0}\) is the solar constant, denoted as 1367 W\(\cdot\)m\({}^{-2}\); and \(\rm\theta\) is the solar zenith angle. In previous research, 0.5 was usually chosen as the critical value to divide sunny (daily CI \(>\) 0.5) and cloudy (daily CI \(\leq\) 0.5) days [67]. The daily values of the parameters in this section were calculated as the mean of the observations between 6:00 and 19:30.