## 4 Discussion
### SIF and GPP Relationship and Environmental Drivers  
Previous studies have consistently demonstrated a significant linear relationship between GPP and SIF across a variety of ecosystem types. Strong correlations have been observed in deciduous broadleaf forests, such as beech (_Fagus grandifolia Ehrl_) and red oak (_Quercus rubra_), at the Harvard Forest site in the United States [24], as well as in key agricultural systems, including soybean (_Glycine max_ L. _Merr_) fields in Iowa [19], rice (_Oryza sativa_ L.) paddies in Jurong, China [26], and maize (_Zea mays_ L.) fields in both Maryland, USA [25], and Zhangye, China [41]. In agreement with these findings, our study revealed a significant linear correlation between SIF and GPP in the temperate evergreen needleleaf forest (ENF), with a coefficient of determination (\(R^{2}\)) of 0.678. In the ENF, the relationship between SIF and GPP is jointly influenced by absorbed photosynthetically active radiation (APAR), as well as by the distinct stability of the canopy structure and the seasonal dynamics of leaf-level physiology [27]. Compared to crops and deciduous forests, ENFs exhibit minimal seasonal variation in canopy structural parameters, such as leaf area index (LAI), leaf angle distribution, and leaf clumping, leading to a relatively stable canopy escape fraction (\(f_{\mathrm{esc}}\)) [42]. However, this relationship was not temporally uniform. Seasonal changes in environmental conditions, particularly radiation and precipitation, were found to influence the strength of the SIF-GPP coupling. For example, due to more frequent cloudy and rainy days, the correlation in August was markedly weaker than that observed in September and October, which were characterized by higher radiation levels. In ENFs, seasonal variation in SIF is primarily driven by changes in the chlorophyll fluorescence yield (\(\Phi_{\mathrm{F}}\)), which is regulated by physiological adjustments in response to environmental stresses. For instance, during winter, ENF trees activate photoprotective mechanisms that increase the carotenoid/chlorophyll ratio, enhancing sustained non-photochemical quenching (\(\Phi_{\mathrm{sNPQ}}\)) to prevent photodamage [43]. As environmental conditions improve in spring, this mechanism is reversed, allowing for higher \(\Phi_{\mathrm{PS(II)}}\) and reversible NPQ (\(\Phi_{\mathrm{PNPQ}}\)) to dominate energy partitioning [44]. In contrast, GPP is mainly regulated by canopy light use efficiency (\(\mathrm{LUE_{P}}\)) and leaf gas exchange. Thus, the SIF-GPP relationship in ENFs reflects a unique coupling between stable canopy structure and seasonally dynamic leaf physiology [21]. These results suggest that the SIF-GPP relationship is modulated by environmental drivers, with SIF and GPP exhibiting different sensitivities to microclimatic factors. While GPP reflects net carbon assimilation and is influenced by both radiative and physiological constraints, SIF is more directly linked to absorbed light and its subsequent dissipation. Our continuous ground-based observations during autumn and early winter provided detailed insight into canopy SIF responses under variable weather conditions. The diurnal patterns of SIF differed seasonally, with stronger signals in autumn and near-zero levels in winter. This is consistent with observations in coniferous forests in Colorado [21]. Notably, SIF exhibited a continuous increase with PAR throughout the growing season, with no evidence of midday depression or saturation. In contrast, GPP was primarily regulated by SWC. This partial decoupling of SIF and GPP under certain conditions is consistent with findings from rice paddies, where SIF showed a stronger correlation with absorbed light than GPP did [45]. The divergence arises from energy allocation mechanisms, as SIF serves as an important non-photochemical dissipation pathway that protects the photosynthetic apparatus under excess light, whereas GPP depends on additional biochemical and water-related constraints. Furthermore, [PERSON] et al.