## 3 Results  
### Seasonal Variation in GPP and SIF Under Changing Conditions  
In the analysis of meteorological factors, GPP, SIF, PAR, and evapotranspiration (ET) in the study area, we observed distinct seasonal variation patterns in all of these factors. The daily average of \(R_{\text{g}}\) closely followed changes in PAR, but, unlike other factors, both \(R_{\text{g}}\) and PAR exhibited a steady decline throughout the observation period, reaching approximately half of their values in winter compared to other seasons. Combined with precipitation data, this suggests a higher frequency of cloudy and rainy days during late summer and early autumn (Figure 2b,e), contributing to greater fluctuations prior to the onset of winter. The daily average \(T_{\text{air}}\) ranged from \(-13.8~{}^{\circ}\)C to \(30.0~{}^{\circ}\)C and dropped below \(0~{}^{\circ}\)C on Julian day (DOY) 322 (where DOY represents the day of the year starting from January 1 st as DOY 1) (Figure 2a). Similarly, the soil water content (SWC) at a depth of 10 cm exhibited seasonal trends similar to those of radiation and temperature, with significant fluctuations in late summer and autumn, and it remained low and stable during winter (Figure 2h). Distinct from other meteorological variables, VPD showed substantial fluctuations until approximately DOY 320, after which it rapidly decreased and stabilized (Figure 2g). Overall, GPP, SIF, and all meteorological factors exhibited a declining trend from DOY 230 to DOY 360 (Figure 2). During late summer and autumn (DOY 230-270), GPP and SIF fluctuated sharply. Further analysis of rainfall, radiation, soil moisture, and VPD indicated similar fluctuations during the same period. Particularly noteworthy is the transitional period between autumn and winter (DOY 290-310), during which temperature dropped below \(0~{}^{\circ}\)C and VPD plummeted from around 10 kPa to near 0, accompanied by sharp declines in SIF, GPP, and ET. Compared to other meteorological factors, ET and SIF exhibited the most synchronized temporal response to seasonal changes (Figure 2d,f), both approaching zero around DOY 293. Notably, SIF declined more rapidly than GPP, leading by approximately 20 days (DOY 285-310), and it approached zero earlier.  
In addition to overall seasonal patterns, the diel dynamics of CO\({}_{2}\) fluxes revealed distinct seasonal differences (Figure 3). Because plant respiration emits CO\({}_{2}\), CO\({}_{2}\) fluxes are positive during the day, indicating a carbon source, and negative at night, indicating a carbon sink. From August to December, NEE in the study area was 100.875 gC\(\cdot\)m\({}^{-2}\), suggesting that the ecosystem functioned predominantly as a carbon sink. The only exception occurred in November, where it acted as a carbon source, releasing 0.989 gC\(\cdot\)m\({}^{-2}\).  
Trends in GPP and Reco exhibited similar temporal trends, characterized by an initial increase followed by a subsequent decline. The diurnal variation in NEE exhibited a \"U-shaped\" pattern (Figure 3), with negative values during daylight hours and positive values at night. In August, the minimum NEE reached \(-7.0~{}\upmu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}\) at approximately 12:00, while in December the minimum dropped only to \(-3.62~{}\upmu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}\) around 10:00, indicating a significant seasonal decline in photosynthetic capacity.  
From August to December, the timing of the minimum (most negative) NEE advanced from around 12:00 to 10:00, indicating a seasonal decline in photosynthetic capacity. Similarly, the transition from a carbon sink back to a source occurred between 16:30 and 17:30, with later transitions observed closer to the peak of the growing season (Figure 3).  
Figure 3: Diurnal variation of CO\({}_{2}\) fluxes by month in ENF from August to December.  
Figure 2: Seasonal variation in ENF. T\({}_{\rm air}\) (**a**), R\({}_{\rm g}\) (**b**), GPP (**c**), SIF (**d**), PAR (**e**), ET (**f**), VPD (**g**) and SWC (**h**) from August to December.