## 4 Discussion
### The Temporal Variation Characteristic of SIF  
After vegetation absorbs PAR, fluorescence is emitted from the leaves, which is finally observed at the top of the canopy through scattering and reabsorption within the canopy [68]. These processes are mainly affected by the canopy structure, the chlorophyll content and the environmental conditions [69]. The influence of the canopy structure on SIF is mainly reflected in two processes: radiation absorption (fPAR) and scattering and reabsorption (f\({}_{\text{esc}}\)) [70]. In this study, the canopy SIF in a Chinese cork oak plantation showed an obvious seasonal dynamic and showed a general trend of first increasing and then decreasing in the growing season (Figure 1). Similar trends have been observed in studies on farmlands and temperate mixed forests [18; 19]. As a deciduous forest, the Chinese cork oak plantation showed dramatic changes in its canopy structure, especially during the periods of leaf expansion and defoliation. The fPAR and the f\({}_{\text{esc}}\) of the Chinese cork oak had obvious seasonal characteristics. The f\({}_{\text{esc}}\) was mainly affected by the canopy structure and was weakly affected by the chlorophyll content. The seasonal trend and the variation amplitude of the fPAR were between the MTCI and NDVI, indicating that the fPAR was affected by a combination of the canopy structure and the chlorophyll content. The SIF in mixed forests and farmlands have been seen to decrease with the plants' senescence [12]. Vegetation usually enters the senescence stage before the deciduous stage begins. At this stage, the photosynthetic activity of the leaves decreases, accompanied by chlorophyll \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Effect** & **Variable** & **PAR** & **Ta** & **VPD** & **SM** & **Ws** & **fPAR** \\ \hline \multirow{4}{*}{Standardized Direct Effects} & VPD & \multirow{4}{*}{\(-0.128\)} & \multirow{4}{*}{\(-0.225\)} & \multirow{4}{*}{\(0.134\)} & \multirow{4}{*}{\(0.071\)} & \multirow{4}{*}{\(0.205\)} \\  & fPAR & \(-0.128\) & & & & & \\  & APAR & \(0.938\) & & & & & \\  & \(\phi\)F & \(-0.232\) & \(-0.198\) & & & & \\  & f\({}_{\text{esc}}\) & \(0.068\) & \(-0.301\) & & & & \\ \hline \multirow{4}{*}{Standardized Indirect Effects} & VPD & \multirow{4}{*}{\(-0.154\)} & \multirow{4}{*}{\(-0.046\)} & \multirow{4}{*}{\(0.027\)} & \multirow{4}{*}{\(0.014\)} & \multirow{4}{*}{\(0.205\)} \\  & fPAR & & & & & & \\  & APAR & \(-0.026\) & & & & & \\  & \(\phi\)F & & & & & & \\  & f\({}_{\text{esc}}\) & & & & & & \\ \hline \multirow{4}{*}{Total Effects} & VPD & \multirow{4}{*}{\(-0.128\)} & \multirow{4}{*}{\(0.659\)} & \multirow{4}{*}{\(-0.225\)} & \multirow{4}{*}{\(0.134\)} & \multirow{4}{*}{\(0.071\)} \\  & fPAR & \(-0.128\) & & & & & \\  & APAR & \(0.912\) & & & & & \\  & \(\phi\)F & \(-0.232\) & \(-0.142\) & & & & \\  & \({}_{\text{esc}}\) & & \(0.068\) & \(-0.187\) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The relationships between the effects of fluorescence variables and environmental variables at half-hour scale in the SEM. decomposition to protect the photosynthetic structure from the destruction of excess light energy [71]. Chlorophyll also affects the canopy \(\mathrm{f_{esc}}\) and the fPAR. Overall, the chlorophyll content affects every stage, from radiation absorption to fluorescence signal acquisition. The increase in the chlorophyll content promotes photosynthesis and fluorescence emission, leading to an increase in the SIF and maximum carboxylation rate [72]. The fluorescence yield of wheat leaves has shown a stable trend seasonally, but has also shown a decrease consistent with the chlorophyll content in the late senescence [73]. The secretion of abscisic acid, which can induce stomatal closure, is a self-protection strategy in senescence-period plants, which is often accompanied by chlorophyll decomposition. During this period, the SIF and photochemical reactions decrease and NPQ increases [74].