## 3 Results  
### Trend of SIF in Different Ecosystems  
Evident periodical changes in forest, cropland, and grassland are shown in Figure 2. As observed, the wave crests corresponding to grassland are of a lower amplitude than those of forest or cropland. The highest values recorded and, consequently, the highest amplitude data peaks visible in Figure 2 corresponded to cropland. [PERSON] et al. [40] analyzed how climate change affects vegetation phenology (derived from maximum annual NDVI) in different ecosystems and found that forest and agriculture show different phenological trends. In the latter case, peak SIF values were recorded in late July or early August each year. With regard to the peak values of the three land cover types, the maximum SIF value occurred in croplands and the minimum value in grasslands. During the initial stages of photosynthesis, the forest ecosystem yielded higher SIF values than the cropland areas. However, by July, cropland SIF values had overtaken the forest SIF values. [PERSON] et al. [41] also obtained similar SIF trends of evergreen forest, cropland, and grassland to this study. Furthermore, they also found double peaks in the seasonality of SIF due to two ecosystems that are out of phase with each other. However, in this study, the selected forest, cropland, and grassland were nearly pure ecosystems and no obviously double peak of SIF was found.  
\begin{table}
\begin{tabular}{c c l} \hline \hline
**Land Cover Type** & **LC ID** & \multicolumn{2}{c}{**Classification System**} \\ \hline \multirow{4}{*}{Cropland} & 10 & Rainfed cropland \\  & 11 & Herbaceous cover \\  & 20 & Irrigated cropland \\ \hline \multirow{4}{*}{Forest} & 61 & Open deciduous broadleaved forest (0.15 \(<\) fc \(<\) 0.4) \\  & 62 & Closed deciduous broadleaved forest (fc \(>\) 0.4) \\ \cline{1-1}  & 71 & Open evergreen needle-leaved forest (0.15 \(<\) fc \(<\) 0.4) \\ \cline{1-1}  & 72 & Closed evergreen needle-leaved forest (fc \(>\) 0.4) \\ \cline{1-1}  & 81 & Open deciduous needle-leaved forest (0.15 \(<\) fc \(<\) 0.4) \\ \cline{1-1}  & 82 & Closed deciduous needle-leaved forest (fc \(>\) 0.4) \\ \hline \multirow{4}{*}{Grassland} & 130 & Grassland \\  & 150 & Sparse vegetation (fc \(<\) 0.15) \\ \cline{1-1}  & 152 & Sparse shrubland (fc \(<\) 0.15) \\ \cline{1-1}  & 153 & Sparse herbaceous (fc \(<\) 0.15) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Classification system of cropland, forest, and grassland used in this study.  
Figure 2: MSixteen-day SIF of cropland, forest, and grassland from 2015 to 2020. The missing data for SIF in May 2015 and August 2017 are because the OCO-2 orbit did not cover the study region.  
We also calculated the multi-year average SIF values for each 16-day period. These calculations notably depict the SIF changes that occur within a growing season (Figure 3). The SIF values for the forest ecosystem increase from May 8, reaching a peak at the beginning of July, and then subsequently decrease. During the intermediate period, i.e., from late June until the end of July, SIF values remained high. For cropland, SIF values reach a peak value on 27 July or 12 August. The grassland ecosystem exhibited comparatively gentle changes in SIF throughout the growing season. We also found that the peak SIF value of cropland was higher than that of forest, the former surpassing the latter in early July. These differences were primarily because of the different responses of SIF to the canopy structure of the different photosynthesis processes and mechanisms (especially for C3 and C4 plants) in different ecosystems [14].