## 3 Results
### Relationships between SIF and Meteorological Data  
The relationship between SIF and temperature (daily mean temperature, daily maximum temperature, and daily minimum temperature) observed in cropland, forest, and grassland areas is shown in Figure 6. We found that plants were sensitive to temperature, particularly to daily minimum temperature values. SIF showed a closer relationship with temperature in the forest than in the other two land cover types, which may be because the forest region considered in this study was situated at higher latitudes and in a cooler climatic region. In the case of cropland and grassland, the contribution of daily mean temperature to SIF was more than 30%, and the contribution of daily minimum temperature to SIF was more than 40%. This indicated that in cropland and grassland areas, the daily minimum temperature significantly affects the strength of plant photosynthetic processes.  
To understand the hysteresis effect of temperature on photosynthesis, we measured the relationship between SIF and the temperature of the previous 16 days (T\({}_{\rm P16d}\)) (SIF-T\({}_{\rm P16d}\)) (Figure 7). The correlation coefficient between SIF and T\({}_{\rm F16d}\) showed a pronounced increase in both cropland and grassland areas compared to temperature recorded in those areas during the same period (T\({}_{\rm 16d}\)). However, the opposite was observed in forested areas. Compared with the previous 16-day mean and maximum temperature values, higher correlation coefficients were found between SIF and the previous 16-day daily minimum temperature for both cropland and grassland.  
No significant correlation was found between SIF and precipitation in either cropland or forest areas, which indicates that precipitation is unlikely to be the limiting factor for crop and tree growth in northeastern China. In the case of grassland, a weak significant correlation was found between SIF and precipitation (R\({}^{2}\) = 0.16, \(p\) < 0.001), and a slightly higher correlation between SIF and the previous 16-day precipitation (R\({}^{2}\) = 0.29, \(p\) < 0.001) (not shown here). The grassland area considered in this study is located in the semi-arid region of northeastern China, where the presence or absence of precipitation can have a profound impact on plant growth.  
\begin{table}
\begin{tabular}{c c c c} \hline  & **SIF–EVI** & **SIF–FPAR** & **SIF–LAI** \\ \hline Cropland & R\({}^{2}\) = 0.39 & R\({}^{2}\) = 0.26 & R\({}^{2}\) = 0.06 \\ Forest & R\({}^{2}\) = 0.21 & R\({}^{2}\) = 0.07 & R\({}^{2}\) = 0.08 \\ Grassland & R\({}^{2}\) = 0.34 & R\({}^{2}\) = 0.29 & R\({}^{2}\) = 0.09 \\ \hline \end{tabular}
\end{table}
Table 2: R\({}^{2}\) values between SIF and MODIS products for cropland, forest, and grassland at footprint level (\(p\) < 0.001 for all the R\({}^{2}\) values).  
Figure 6: Relationships between SIF and daily mean temperature, daily maximum temperature, and daily minimum temperature for cropland, forest, and grassland areas (\(p\) < 0.001).  
Figure 7: Relationships between SIF and the previous 16-day daily mean temperature, daily maximum temperature, and daily minimum temperature for cropland, forest, and grassland areas.