# Investigating the Performance of Red and Far-Red SIF for Monitoring GPP of Alpine Meadow Ecosystems
## 4 Discussion  
Reasons Why Red SIF Shows More Potential Than Far-Red SIF for Monitoring GPP in an Alpine Meadow Ecosystem  
At present, researchers have not reached a definite conclusion as to which has greater potential for monitoring GPP in various ecosystems, red SIF or far-red SIF. Most previous studies at the canopy level have shown that the potential of red SIF for monitoring GPP was limited by strong reabsorption effects within the canopy and relatively low retrieval accuracy [9; 14; 27]. However, the results in this study show that red SIF had even more  
Figure 6: Scatter plot of the fitted relationship between (**a**) \(\mathrm{SIF_{Red}}\). (**b**) \(\mathrm{SIF_{Far-red}}\), and GPP at the diurnal scale and daily variations of (**c**) air temperature (Ta), (**d**) vapor pressure deficit (VPD), and (**e**) photosynthetically active radiation (PAR) for a typical sunny day and cloudy day at AR site. The scatter points represent half-hourly averaged data, the solid line represents the fitted line of the SIF-GPP relationship, and the shaded area indicates the 95% confidence level of the prediction, where red represents sunny day and blue represents cloudy day. potential than far-red SIF in monitoring GPP in an alpine meadow ecosystem. Here are several possible main reasons for this result. First of all, we obtained a relatively high precision SIF inversion result for the red and far-red bands. The specifications of the instruments used to measure the spectra and the spectral properties of the irradiance and the reflectance spectra at the absorption bands affect the accuracy of SIF retrieval [57]. In this study, measurements of the continuous spectrum were performed using the SIFSpec system, the core of which is a customized Ocean Optics QE65 Pro spectrometer having high signal-to-noise ratio (>1000) and high spectral resolution (~0.34 nm), ensuring the quality and accuracy of the collected spectral data. Meanwhile, the SFM algorithm was used for the inversion of canopy SIF. The SFM algorithm uses mathematical functions to, first, model the reflectance and fluorescence spectral curves and then invert the chlorophyll fluorescence intensity. In a narrow spectral range, relatively simple fluorescence and reflectance spectral curves can be fitted relatively accurately using polynomial functions. In this study, a quadratic polynomial function was used to fit the spectral curves in both absorption bands of O\({}_{2}\)-A and O\({}_{2}\)-B, therefore, high-precision canopy SIF inversion results were obtained for both bands. For the second, the theoretical basis for red SIF to be sensitive to physiological changes in vegetation. SIF contains APAR information, and it also contains, to some extent, energy distribution information at the photosystem level [11; 17; 20]. There are two photosynthetic reaction systems in the mechanism of photosynthesis in higher plants, photosystem I (PS I) and photosystem II (PS II) [58; 59]. Variable plant fluorescence is observed for PS II due to changes in plant physiological conditions, but this variable fluorescence is not observed for PS I [11; 60; 61]. SIF emission in the far-red band is performed by PS I and PS II, while SIF emission in the red band is mainly performed by PS II [11; 62]. Therefore, theoretically, the SIF in the red contributed mostly by PS II shows greater responsiveness to changes in the physiological condition of vegetation [11]. Previously, several studies have reported the potential of red SIF in monitoring GPP. For example, [PERSON] et al. [38] showed that after downscaling the red SIF of the maize canopy to the photosystem level, an obvious improvement in the red SIF and GPP correlation was achieved. Their findings also showed that at the photosystem level, red SIF monitored GPP equally well or even better than far-red SIF, which indicated that after removing the effect of canopy reabsorption, red SIF could provide more effective information on plant photosynthesis and physiology and, thus, red SIF was more related to GPP than far-red SIF. [PERSON] et al.