## 3 Results
### Temperature Responses of GPP and SIF during Overwintering Period  
Firstly, we found that GPP and \(cSIF\) both responded to \(T_{a}\) (Figure 6a-c). However, GPP and red SIF (\(cSI_{688}\)) showed a relatively more obvious change in response to \(T_{a}\) than \(cSI_{760}\) did. We can also see that \(fPAR\) showed a horizontal trend with \(T_{a}\) with the correlation coefficient (r) of \(-0.27\) (\(p\)-value \(>\) 0.05, Figure 6g), and \(Red_{o}\) exhibited a significant decreasing trend with increasing \(T_{a}\) (Figure 6e), while \(NIR_{v}\) was not sensitive to \(T_{a}\) (Figure 6f). In this study, \(f_{esc\_688}\) was estimated by \(Red_{v}/fPAR\), and the changes in \(Red_{o}\) and \(fPAR\) could indirectly reflect dynamics in \(f_{esc\_688}\) (Figure 6h). Similarly, the \(f_{esc\_760}\) was estimated by \(NIR_{v}/fPAR\), and the changes in \(NIR_{v}\) and \(fPAR\) could indirectly reflect dynamics in \(f_{esc\_760}\) (Figure 6i). In addition, we found that \(cSI_{760}\) was enhanced at about \(-10\)\({}^{\circ}\)C (Figure 6c). We further analyzed how the \(LUE_{p}\) and fluorescence quantum yield (\(cSI_{yield}\) and \(\Phi_{F}\)) changed in response to \(T_{a}\) (Figure 7). From Figure 7a, we can see that \(LUE_{p}\) was strongly related to \(T_{a}\) with a high r value of 0.83. This result indicates that photosynthetic activity substantially decreased at low temperatures (Figure 4b). \(cSI_{yield\_688}\) and \(\Phi_{F\_688}\) were also strongly related to \(T_{a}\) (r = 0.72, 0.71; \(p\)-value \(<\) 0.05, Figure 7b,d). However, it should be noted that \(cSI_{yield\_760}\) and \(\Phi_{F\_760}\) exhibited different responses to \(T_{a}\), and the fitted lines were almost invariant (r = \(-\)0.23, 0.22; \(p\)-value \(>\) 0.05). \(cSI_{yield\_760}\) and \(\Phi_{F\_760}\) at a low temperature (about \(-10\)\({}^{\circ}\)C) were greater than that at about \(0\)\({}^{\circ}\)C, which might result in  
Figure 5: Seasonal dynamics of daily mean \(f_{esc\_688}\) (**a**) and \(f_{esc\_760}\) (**b**). The blue shaded rectangle represents the overwintering periods. Local regression was used to smooth the data. The black shaded area indicates the 95% confidence interval. the far-red SIF quantum yields not being sensitive to low temperature. In addition, the \(fPAR\) did not correlate with \(T_{a}\) (r = \(-\)0.27, \(p\)-value > 0.05), which had less impact on the estimation of \(cSI_{yield}\) and \(\Phi_{F}\) during the overwintering period. We also explored the different responses of photosynthesis and fluorescence to \(T_{a}\). From Figure 8, we can see that \(\frac{LUE_{p}}{cSI_{yield}}\) (equals to \(\frac{GPP}{cSIF}\)) and \(\frac{LUE_{p}}{\Phi_{F}}\) (equals to \(\frac{GPP}{cSIF}\)) were well correlated with \(T_{a}\). This indicates that photosynthesis and fluorescence responded differently to \(T_{a}\). However, it should be noted that the increase in \(\frac{LUE_{p}}{cSI_{yield}.68}\) and \(\frac{LUE_{p}}{\Phi_{F,68}}\) with \(T_{a}\) mainly resulted from the different physiological responses of \(LUE_{p}\) and \(\Phi_{F,688}\) to \(T_{a}\), while the increase in \(\frac{LUE_{p}}{cSI_{yield}.760}\) and \(\frac{LUE_{p}}{\Phi_{F,760}}\) in response to \(T_{a}\) largely contributed to the temperature response of \(LUE_{p}\) (Figure 7). These results suggest that SIF-based GPP estimation should consider the effects of temperature. Figure 6: The relationships of daily mean \(GPP\) (**a**), \(cSI_{688}\) (**b**), \(cSI_{760}\) (**c**), WDRVI (**d**), \(Red_{0}\) (**e**), \(NIR_{b}\) (**f**), \(fPAR\) (**g**), \(fesc_{688}\) (**h**), and \(fesc_{760}\) (**i**) with air temperature (\(T_{a}\) ). The data were averaged over intervals of 2 \({}^{\circ}\)C, and the error bars indicate the standard deviation. The 95% confidence levels for prediction are represented by the grey-shaded zones. * represents a significance level of 0.05 and ** represents a significance level of 0.01.