## 4 Discussion
### Explanations and Limitations of the CCC Approach and the Backward Approach  
A non-linear logarithmic relationship (\(\mathrm{R}^{2}>0.9\), \(p<0.05\)) was shown between simulated \(\mathrm{SIF}\) by SCOPE model and CCC calculated by measured LAI and leaf Cab in a deciduous mixed forest plantation [25]. Based on this, the CCC approach was presented to estimate spatial \(\mathrm{SIF}\) using the empirical relationship. However, the CCC did not capture the variations of \(\mathrm{SIF}\) with the logarithmic function in our study (Figure 7). We infer that the different result is caused by the much fewer changes in the canopy chlorophyll that occurs in the evergreen forest than that in deciduous forest. In contrast to crops and deciduous plants, the canopy structure of forests is generally considered to be constant throughout the day. The LAI of the subtropical evergreen forests, which this study focused on, rarely even had large seasonal changes (Table 1). Therefore, the diurnal changes of \(\mathrm{SIF}\) for the same canopy would hardly be disturbed by the canopy structure. The diurnal \(\mathrm{SIF}\) dynamics estimated from the canopy chlorophyll concentration (CCC) mainly depended on the diurnal changes of leaf chlorophyll content. However, changes in the leaf chlorophyll content have rarely been found during the season (Table 1) nor the days (Figure 6b). There is a link between the amount of chlorophyll in leaves and the fluorescence emitted by chlorophyll, but the CCC typically represents a kind of potential fluorescence emission when all available chlorophyll is used at maximum efficiency. In fact, many regulation mechanisms make the efficiency could not fully used. The fewer canopy chlorophyll changes in the evergreen forest might be regulated by long-term environmental adaptation in the subtropical area. Radiance,rather than canopy chlorophyll, matters for the fluorescence emission for evergreen forests. CCC could be a valuable approach to estimate spatial SIF at landscape scale using the empirical logarithmic relationship between CCC and SIF [25]. Still, the application of the CCC approach needs more evidence for the temporal SIF estimation, especially in the evergreen forests. The backward approach estimates canopy SIF through an inverse way from [26], in which the estimation of GPP from the forward model using tower-based SIF and leaf-level F\({}_{\text{s}}\) measurements agreed well with flux tower observations of GPP (R\({}^{2}\) = 0.68, \(p\) < 0.0001). We proved that the backward approach improved with the correction factor k could capture the diurnal dynamics of canopy SIF under normal weather conditions. Although the coefficient of determination of SIF\({}_{\text{BACK}^{\star}}\) and SIF\({}_{\text{760}}\) was higher than that of SIF\({}_{\text{BACK}^{\star}}\) and SIF\({}_{\text{687}}\), but the values of SIF\({}_{\text{BACK}^{\star}}\) and SIF\({}_{\text{687}}\) were much closer. This might be caused by the different scattering and reabsorption effects on different wavelengths. One assumption of the backward approach is that both photosynthesis and fluorescence can be calculated as a product of the absorbed radiance and light conversion efficiency across different spatial levels. However, the top-of-canopy SIF observed from the tower is not merely the accumulative signal of SIF emitted by all leaves of the forest canopy but contains an additional term quantifying the effect of canopy scattering and reabsorption. Considering the escape probability of all SIF photons emitted from all leaves to the outer canopy (f\({}_{\text{esc}}\)), we have SIF = APAR \(\times\)\(\epsilon_{\text{FC}}\times\) f\({}_{\text{esc}}\)[4; 50; 51]. In addition, the f\({}_{\text{esc}}\) differed with the wavelength [52], which caused different degrees of underestimation of SIF\({}_{\text{687}}\) and SIF\({}_{\text{760}}\) using the backward approach (Figure 9).