## 3 Results
### How Do Environmental Factors Affect SIF? To clarify the influence mechanism of the environment on SIF, we established a reliable structural equation model based on the half-hour and daily scale of the growing season. The goodness-of-fit index (GFI), the comparative fit index (CFI) and the incremental fit index (IFI) of the obtained models were all closer to or above 0.9, and the root mean square error of approximation (RMSEA) was less than 0.2. This means that the models have high reliability. Figure 7 shows the influence of the environmental factors on the APAR, \(\phi\)F and \(\mathrm{f_{esc}}\). At the daily scale, the PAR, SM and Ws were not the main reasons for the fPAR variation; the VPD had no significant effect on \(\phi\)F; and the Ws and SM had no significant effect on the \(\mathrm{f_{esc}}\). At the half-hour scale, all five of the environmental factors had significant effects. In other words, the half-hour scale SIF was more susceptible to environmental factors. According to the influence coefficients in Tables 1 and 2, the direct influence of the environment on APAR mainly came from PAR, especially at the half-hour scale (0.938). At the same time, the environmental factors also affected APAR by changing fPAR. Ta and  
Figure 6: The partial correlation coefficient between different seasonal fluorescence parameters and environmental factors, including Ta (purple bar) and VPD (green bar) in different seasons at the daily (**a**) and half-hour (**b**) scales, with PAR as the control variable. Note: *—significant correlation at \(p<0.05\) and **—**significant correlation at \(p<0.01\). the VPD drove the fPAR variation. The PAR and Ta affected \(\phi\)F. The SM affected the daily scale \(\phi\)F, while the VPD affected the half-hour scale of \(\phi\)F. It is worth noting that PAR had opposite effects on the two-time scales of \(\phi\)F. The total influence coefficients of Ta on the f\({}_{\rm esc}\) were \(-0.673\) and \(-0.187\) at the daily and half-hour scales, respectively, and Ta was the dominant factor of the f\({}_{\rm esc}\). In addition, PAR and the VPD affected the f\({}_{\rm esc}\) at the daily scale, and the VPD and SM affected the f\({}_{\rm esc}\) at the half-hour scale. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Effect** & **Variable** & **PAR** & **Ta** & **VPD** & **SM** & **Ws** & **fPAR** \\ \hline \multirow{4}{*}{Standardized Direct Effects} & VPD & \multicolumn{4}{c}{0.591} & \multirow{4}{*}{\(-0.303\)} & \multirow{4}{*}{0.107} & \multirow{4}{*}{0.06} \\  & fPAR & \(0.138\) & \(0.818\) & \(-0.303\) & \(0.107\) & \(0.06\) \\ \cline{1-1}  & APAR & \(0.843\) & & & & \\ \cline{1-1}  & \(\phi\)F & \(0.344\) & \(-0.333\) & \(0.068\) & \(0.253\) & \(0.25\) \\ \cline{1-1}  & \(\rm f_{esc}\) & \(0.208\) & \(-0.549\) & \(-0.209\) & \(0.021\) & \(-0.099\) \\ \hline \multirow{4}{*}{Standardized Indirect Effects} & VPD & \multicolumn{4}{c}{\(-0.179\)} & \multirow{4}{*}{\(-0.092\)} & \multirow{4}{*}{0.033} & \multirow{4}{*}{0.018} \\  & fPAR & \(0.042\) & \(0.194\) & & & \\ \cline{1-1}  & \(\rm\phi\)F & \multicolumn{4}{c}{\(0.04\)} & & & \\ \cline{1-1}  & \(\rm f_{esc}\) & \multicolumn{4}{c}{\(-0.124\)} & & & \\ \hline \multirow{4}{*}{Total Effects} & VPD & \multicolumn{4}{c}{\(0.591\)} & \multirow{4}{*}{0.107} & \multirow{4}{*}{0.06} \\  & fPAR & \(0.138\) & \(0.639\) & \(-0.303\) & \(0.107\) & \(0.06\) \\ \cline{1-1}  & APAR & \(0.884\) & \(0.194\) & \(-0.092\) & \(0.033\) & \(0.018\) \\ \cline{1-1}  & \(\rm\phi\)F & \(0.344\) & \(-0.293\) & \(0.068\) & \(0.253\) & \(0.25\) \\ \cline{1-1}  & \(\rm f_{esc}\) & \(0.208\) & \(-0.673\) & \(-0.209\) & \(0.021\) & \(-0.099\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The relationships between the effects of fluorescence variables and environmental variables at daily scale in the SEM. Figure 7: Structural equation model (SEM) path diagram of APAR, \(\phi\)F, f\({}_{\rm esc}\) and environmental factors in daily **(a–c)** and half-hour **(d–f)** scales. The thickness of the line represents the value of the path coefficient.