## 3 Results
### Variations in the Correlation Between GPP and SIF Under Seasonal Transitions  
At the half-hourly scale, both SIF and GPP exhibited strong temporal correspondence across different months (Figure 4). The hourly \(R^{2}\) values for each month were August--0.698; September--0.748; October--0.884; November--0.452; and December--0.293. Both SIF and GPP exhibited diurnal patterns with values increasing from morning to midday, peaking around 12:00, and declining in the afternoon. However, GPP exhibited a more stable temporal pattern compared to SIF. From August to December, the SIF value at 12:00 decreased from 0.42 to 0.003 \(\upmu\)W\(\cdot\)cm\({}^{-2}\cdot\)nm\({}^{-1}\cdot\)sr\({}^{-1}\), while GPP decreased from 14.35 gC\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) to 2.84 gC\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\). Notably, SIF responded earlier than GPP in terms of both its daily onset and decline. At sunrise (around 6:30), while GPP remained minimal, SIF had already responded to early light conditions. In the afternoon (around 17:00), SIF had nearly dropped to zero, while GPP continued to decline. These observations suggest that weak photosynthetic activity persisted in the ENF during the winter months of November and December. At the daily scale, our results indicated a significant linear relationship between GPP and SIF in the _Pinus sylvestris_ var. _mongolica_ plantation from August to December, with an  
Figure 4: Diurnal variation of SIF and GPP by month in ENF from August to December. overall coefficient of determination (\(R^{2}\)) of 0.6784 (Figure 5). This relationship was consistently strong at both daily and sub-daily scales (Figure 5). Specifically, the monthly correlations (\(R^{2}\)) between GPP and SIF were August--0.626; September--0.249; October--0.619; November--0.301; and December--0.086. During August and September, the scatter points were densely distributed along the regression line, with daily GPP values ranging from 1.65 to 12.25 gC\(\cdot\)m\({}^{-2}\)-day\({}^{-1}\) and SIF values from 0.003 to 0.045 \(\upmu\)W\(\cdot\)cm\({}^{-2}\)\(\cdot\)nm\({}^{-1}\)\(\cdot\)sr\({}^{-1}\), indicating both high photosynthetic activity and strong SIF sensitivity. In contrast, October and November were characterized by low solar radiation and frequent cloudy days, leading to larger scatter in the data and a weakened correlation, with SIF below 0.004 \(\upmu\)W\(\cdot\)cm\({}^{-2}\)\(\cdot\)nm\({}^{-1}\)\(\cdot\)sr\({}^{-1}\) and GPP rarely exceeding 2.0 gC\(\cdot\)m\({}^{-2}\)\(\cdot\)day\({}^{-1}\). The correlation peaked in October during the seasonal transition from autumn to winter (DOY 290-310), a period of the most pronounced changes in both variables. In contrast, during September--characterized by low radiation and frequent cloudy or rainy days--the SIF-GPP correlation weakened. As winter approached and photosynthetic activity nearly ceased, the relationship became negligible by December. We analyzed the sensitivity of SIF and GPP to key meteorological factors, including PAR, VPD, ET, R\({}_{\rm g}\), T\({}_{\rm air}\), and SWC. Although both SIF and GPP were significantly influenced by environmental factors, their correlation patterns exhibited notable differences (Figure 6). Both SIF and GPP showed strong positive correlations with most environmental factors, especially with ET, PAR, and SWC. However, there were differences in the strength of these correlations. For instance, the correlation between SIF and ET was 0.78, whereas GPP and ET showed a higher correlation of 0.84, indicating that ET has a more significant impact on GPP. Conversely, SIF was slightly more sensitive to PAR (\(R^{2}\) = 0.79) compared to GPP (\(R^{2}\) = 0.72). Figure 5: Relationship between SIF and GPP in ENF from August to December. (Gray points represent half-hourly values; colored points indicate daily means. The red regression line is fitted to the daily mean values). Furthermore, the two indicators responded differently to temperature and radiation.