# Investigating the Performance of Red and Far-Red SIF for Monitoring GPP of Alpine Meadow Ecosystems
Several studies on evergreen coniferous [26], evergreen broadleaf [34], and deciduous broadleaf forests [35] have shown that, under normal growth conditions, SIF\({}_{\text{Far-red}}\) exhibited a positive linear relationship with GPP at different time scales, although the coefficient of determination (R\({}^{2}\)) of this linear relationship varied obviously depending on the observed time scale. More recently, a study on coastal salt marsh ecosystems also reported that SIF\({}_{\text{Far-red}}\) had an obvious linear correlation with GPP in coastal salt marsh ecosystems, and the linearity strengthened with increasing time scale [29]. However, other studies have also revealed that SIF\({}_{\text{Far-red}}\) and GPP have a nonlinear relationship. For example, [PERSON] et al. [23] found a hyperbolic relationship between maize SIF\({}_{\text{Far-red}}\) and GPP. The results from a study on evergreen coniferous forests showed a strong nonlinear relationship between SIF\({}_{\text{Far-red}}\) and GPP on both diurnal and seasonal scales, mainly because the GPP was saturated with increasing absorbed photosynthetically active radiation (APAR), while SIF\({}_{\text{Far-red}}\) was not [36]. Another study demonstrated that the linear correlation for SIF\({}_{\text{Far-red}}\) with GPP shifted from positive to negative when the trees were subjected to extreme weather conditions (e.g., heat waves) in evergreen broadleaf forests, thus showing an overall nonlinear correlation [34]. This is mainly due to a change in internal plant energy allocation as an evergreen broadleaf forest experiences nonphotochemical quenching (NPQ) saturation and continuous photoinhibition during heat waves [34]. Notably, researchers prefer to use SIF\({}_{\text{Far-red}}\) to study the dynamics of GPP in various ecosystems [14], owing to the fact that fluorescence in the red band is strongly influenced by reabsorption, and that accurate retrieval of SIF is easier in the far-red than the red band. Benefiting from the improvement of signal-to-noise ratio and spectral resolution of spectral measurement instruments, as well as the development of fluorescence inversion algorithms, researchers have also explored the relationship between canopy red SIF (SIF\({}_{\text{Red}}\)) and GPP [37]. For instance, through downscaling from the observed canopy level SIF to the photosystem level in maize, [PERSON] et al. [38] investigated the relationship between photosystem level SIF and GPP in maize. As compared to canopy-level red SIF, the correlation between photosystem-level red SIF and GPP was found to be greatly enhanced and even better than that between photosystem-level far-red SIF and GPP. During the wheat canopy growth stage, the correlation between \(\text{SIF}_{\text{Far-red}}\) and GPP was found to be stronger than that between \(\text{SIF}_{\text{Red}}\) and GPP [14]. When the growing stage reached a stable state, \(\text{SIF}_{\text{Far-red}}\) was slightly weakened in relation to GPP, while \(\text{SIF}_{\text{Red}}\) was strengthened in relation to GPP, thereby causing the correlations between \(\text{SIF}_{\text{Red}}\) and GPP and between \(\text{SIF}_{\text{Far-red}}\) and GPP to be comparable [14]. [PERSON] et al. [26] studied on an evergreeniferous forest and stated that the expected increase in \(\text{SIF}_{\text{Red}}\) did not occur as a consequence of the absence of a significant increase in chlorophyll concentration, and that \(\text{SIF}_{\text{Red}}\) was capable of closely tracking the seasonal changes in GPP in this evergreeniferous forest. Across various environmental conditions, the relationship is by no means invariant between SIF and GPP. Previously, an investigation on mangroves showed that VPD negatively affected the strength of the SIF-GPP relationship in mangroves, and that the SIF-GPP correlation did not consistently get stronger as PAR and air temperature increased. The strength of the SIF-GPP relationship in mangroves was most strongly correlated at moderate PAR and air temperature.