## 4 Discussion
### Uncertainties and Limitations
Some studies have explored the performance of early water stress detection using leaf-level measurements of chlorophyll fluorescence and found that chlorophyll fluorescence decreased for plants with water stress relative to well-watered plants, while the filled watering experiment stated that chlorophyll fluorescence levels of maize under water stress were similar to those of well-watered maize [84]. [PERSON] et al. [45] also reported that the field-scale SIF yield (\(\phi_{F}\)) data showed water deficit stress from the comparison between irrigated and rain-fed corn field at three different spatial scales, and \(\phi_{F}\) was positively correlated with canopy-scale stomatal conductance, a reliable indicator of plant physiological condition. [PERSON] et al. [42] found that decoupling of GPP and SIF under water stress conditions may also alter the GPP/SIF ratio, and the rapid response of the stomatal conductance to water stress may explain the decline in the GPP/SIF ratio under water stress conditions. Under optimal temperature conditions, the GPP/SIF ratio may be dominated by other environmental variables or changes in canopy structure, whereas under a wide temperature range including low-temperature conditions, the GPP/SIF ratio may be largely influenced by low temperatures [34]. In this study, we found that the GPP/SIF ratio was positively correlated with temperature for group A, indicating that the GPP-SIF relationship is affected by a wide temperature range, including low temperatures. In addition, the potential of using SIF to detect forest stresses, especially the defoliation of the plant by primary insects or foliar infection by pathogens, is of great interest [85]. However, current studies generally ignore the crown heterogeneity caused by pests. The impact of structural properties of damaged foliage on canopy SIF, as well as the response of the GPP/SIF ratio to pest-damage, has not been well-explored. Despite the potential effects of environmental pressures, the GPP/SIF ratio is also affected by the leaf age, leaf phenology, and leaf chlorophyll content during the growing period [30]. Some studies have reported that the seasonal peak timing of APAR played a dominated role in the seasonal peak timing of far-red SIF, whereas the seasonal peak timing of chlorophyll content contributed more to the seasonal peak timing of GPP [30,46]. The seasonal difference between far-red SIF and GPP is, thus, possibly more strongly determined by the seasonal changes in the APAR and canopy chlorophyll content [30]. Therefore, the seasonal GPP/SIF variation may also be affected by the seasonal variations in vegetation function traits. Canopy structure can also play a role in forming the seasonal pattern of the GPP/SIF ratio [18,25,27]. The seasonal variations in canopy structure can lead to the changes in the escape probability (\(f_{esc}\)), which further affects the observed SIF. The \(f_{esc}\) is generally lower when the canopy is denser during the growing season. Some studies have reported that a denser canopy structure can also lead to a higher GPP/SIF ratio [86]. Remote-observed SIF is only part of the total emitted SIF by leaves, which can experience a large number of scattering and reabsorption effects [87]. To reduce the effects of the canopy structure, we explored the seasonal variations in the ratio of GPP to the total SIF (defined as GPP/SIF, Figures S1-S3). A similar \"hump-shaped\" seasonal pattern of GPP/tSIF was found in many PFTs, while the \"bowl-shaped\" seasonal pattern of GPP/SIF for some PFTs was largely corrected. Although this indicates that the canopy structure affects the seasonal variation in GPP/SIF, a decline in GPP/tSIF was also found under low-temperature conditions (Figures S1-S3). By reducing the potential contribution of the canopy structure to the change in GPP/SIF, we can better explore the seasonal pattern in GPP/SIF under changeable climates.