## 3 Results
### Seasonal and Diurnal Dynamics of SIF  
Just as the vegetation index represented the greenness and chlorophyll content, SIF showed an overall trend of first increasing and then decreasing in the growing season, with a range of 0.0225-0.8715 mW\(\cdot\)m\({}^{-2}\).sr\({}^{-1}\).nm\({}^{-1}\). Moreover, the maximum value appeared at DOY = 115, which was earlier than the NDVI and MTCI (Figure 2). The seasonal dynamic of SIF showed acute fluctuations, with unapparent daily variation characteristics, which are easily affected by environmental factors, especially PAR. As a part of the SIF retrieval, the seasonal dynamics of APAR and SIF were consistent. fPAR determined APAR, and its trend was similar to the NDVI and MTCI, i.e., a combination of the two features. In other words, it was affected by the canopy structure and the chlorophyll content (Figure 1). The seasonal dynamics of the f\({}_{\text{esc}}\) were opposite to those of the NDVI and MTCI.  
SIF is the energy released by vegetation after absorbing photosynthetically active radiation; therefore, it is affected by canopy structure, the vegetation's physiological situation and the environmental conditions. Environmental factors, especially PAR, affected SIF, which showed a violent fluctuation in 2020, without an obvious day-by-day pattern. To reveal the diurnal dynamics of SIF in different seasons under different weather conditions, we selected 3 contemporaneous days in each season, which were sunny, cloudy and rainy days, respectively. The results showed that SIF and PAR had relatively consistent diurnal variation under different weather conditions and seasons (Figure 3). SIF showed a single-peak curve on the sunny days; however, the SIF value fluctuated greatly on cloudy and rainy days, and its trend was basically consistent with PAR. The extent of the SIF value was driven by PAR, and its dynamic was earlier than PAR. In general, the SIF in the autumn was lower than that in the spring and summer, under all weather conditions. Under sunny conditions, the peak value of SIF appeared at 13:00 (spring), 12:30 (summer) and 11:30 (autumn), while the peak value of PAR in different seasons appeared at 13:00. With the progress of the growing season, the peak value of SIF gradually advanced. On cloudy days and rainy days, the time difference in the peak SIF and PAR was the same (both half an hour), and there was no change during the different seasons. To further analyze the  
Figure 2: Seasonal dynamics of solar-induced chlorophyll fluorescence (SIF) and intermediate variables (absorbed photosynthetic active radiation, APAR; the fraction of emitted SIF that escapes from the canopy, f\({}_{\text{esc}}\); the fraction of photosynthetically active radiation, fPAR; fluorescence emission yield, \(\phi\)F) (**(a)**: SIF and \(\phi\)F; **(b)**: APAR, f\({}_{\text{esc}}\) and fPAR). The shadows on either side of the point are the range of standard deviations of the value. The shaded area on both sides of the axis is the non-growing season.  
reasons for the difference of SIF during the different seasons and weather conditions, we analyzed the diurnal variation characteristics of APAR, \(\phi\)F and the f\({}_{\text{esc}}\) in our calculation of SIF. \(\phi\)F was generally high in the morning, followed by a gradual decline with significant fluctuations (Figure 4). On sunny days, the diurnal dynamics of the f\({}_{\text{esc}}\) were more severe, and, generally, in the afternoon, they were higher than in the morning. The fluctuation range of the intra-day f\({}_{\text{esc}}\) in the spring was greater than that in the summer and autumn. The APAR diurnal dynamics were the same as SIF, but the rate of decline was higher in the afternoon, which may have been related to the higher f\({}_{\text{esc}}\).