## 4 Discussion
### The Relationship Between SIF and Tr and Their Environmental Response  
In this study, SIF and Tr in three plantations both show significant correlation. Compared to traditional vegetation indices, SIF demonstrates better correlation and consistency with Tr, as SIF and Tr are linked through photosynthetic physiology and stomatal behavior [18]. Therefore, the covariation relationship between fluorescence emission and photochemistry (light energy distribution), as well as the degree of coupling between photosynthesis and transpiration (stomatal control), will affect the relationship between SIF and Tr. Consistent with previous research, the correlation between SIF and Tr strengthens as the temporal scale is aggregated [18]. Temporal aggregation reduces response differences between SIF and Tr to environmental variations at the half-hour scale, along with mitigating random observation noise effects, thereby enhancing their correlation. Furthermore, the scale-dependent relationship between SIF and Tr is largely attributable to their co-variation with environmental factors. Temporal aggregation reduces the range of variation in SIF, Tr, and environmental factors. Notably, SIF and photosynthesis exhibit a strong linear relationship under moderate radiation levels, while photosynthesis and transpiration are strongly linked under favorable environmental conditions. These mechanisms collectively enhance the indirect linkage between SIF and Tr. At midday, short-term stressors diminish the correlation. Research on SIF and GPP within forest ecosystems has revealed that GPP reaches saturation at midday, a phenomenon not observed with SIF. This disparity is the primary driver of their nonlinear correlation [48]. SIF is related to the photoreaction phase of photosynthesis and is independent of the dark reaction phase. The main reasons for GPP saturation are twofold: firstly, due to the limitation of the photosynthetic substrates in the chloroplasts, the rate of the dark reaction becomes saturated when the input of photosynthetically active radiation reaches a threshold value; secondly, under the stress of water, heat, and radiation, vegetation adopts measures to mitigate the harm of short-term stress, including closing stomata to protect hydraulic integrity, regulating the efficiency of PSII in absorbing light, and upregulating NPQ to prevent photochemical damage to the leaves [54; 55]. Stomatal closure will decouple the processes of photosynthesis and transpiration, and the relationship between SIF and Tr will be diminished. The gradual increase in Tr/SIF may be related to the diurnal dynamic changes in SIF and Tr. In the afternoon, the slope of Tr is less steep than that in the morning, while the trend for SIF shows the opposite pattern (Figure S1). The time of satellite observation of SIF at a specific location is fixed. Differences in diurnal observation time can affect the relationship and slope between SIF and Tr. Therefore, temporal effects should be considered when using satellite SIF. During the period of canopy stability, environmental factors predominantly influence changes in SIF and Tr. Correlation analysis showed that SIF and Tr were more sensitive to environmental factors at the half-hour scale. Therefore, high temporal resolution observation data are better suited for analyzing vegetation responses to environmental factors. Covariance between factors may obscure their actual effects, but partial correlation analysis indicates that SIF and Tr consistently show a correlation (Figure S3). SIF is the light emitted after photon transitions, during which the light-harvesting antenna absorbs light and transfers the energy to PSII. Driven by incoming radiation, Fs shows a strong correlation with PAR [56]. Stomata are the main channels for plant transpiration. Stomatal response to radiation involves both circadian rhythms and guard cell photoreceptors that activate proton pumps under blue light, driving ion uptake and osmotic changes to open stomata [57]. Therefore, radiation also drives Tr.