# Investigating the Performance of Red and Far-Red SIF for Monitoring GPP of Alpine Meadow Ecosystems
## 4 Discussion
We investigated the relationship between \(\text{SIF}_{\text{Red}}\)-GPP and \(\text{SIF}_{\text{Far-red}}\)-GPP with seasonal changes in PAR, Ta, and VPD at seasonal scales using the ratio of SIF to GPP. As Ta, VPD levels varied along the \(x\)-axis, \(\text{SIF}_{\text{Red}}\)/GPP was able to remain relatively stable, and the boxes at different levels in Figure 4c were roughly on the same level, indicating that the sensitivity of \(\text{SIF}_{\text{Red}}\) and GPP to changes in Ta and VPD were similar. Therefore, the magnitude of \(\text{SIF}_{\text{Red}}\)/GPP values did not change obviously on the seasonal scale. Compared to the stability of \(\text{SIF}_{\text{Red}}\)/GPP, the value of \(\text{SIF}_{\text{Far-red}}\)/GPP showed a more obvious fluctuation with the variation of Ta and VPD and showed an increasing trend. Both Ta and VPD can cause physiological activity alterations in vegetation as Ta variations cause changes in photosynthetic enzyme activity, while VPD variations change the degree of opening of vegetation stomatal conductance. Red SIF is more closely correlated with changes in vegetation physiological activity and may provide more timely feedback than far-red SIF on physiological changes such as photosynthetic carbon assimilation due to environmental changes [11], and this may explain why \(\text{SIF}_{\text{Red}}\)/GPP was more stable than \(\text{SIF}_{\text{Far-red}}\)/GPP. As shown in Figure 4, when PAR levels varied along the \(x\)-axis, \(\text{SIF}_{\text{Red}}\)/GPP also showed slight fluctuations after PAR reached a certain level, but obviously not as much as \(\text{SIF}_{\text{Far-red}}\)/GPP. As discussed in Section 4.1, both the red SIF and GPP were probably saturated, but likely the degree of saturation was different for both, resulting in slight fluctuations in \(\text{SIF}_{\text{Red}}\)/GPP. While saturation occurred in GPP, the far-red SIF value was still increasing, thus leading to large fluctuations with an increasing trend in the value of \(\text{SIF}_{\text{Far-red}}\)/GPP. These results are consistent with the conclusions obtained in Figure 2.  
PAR dominated the variation in the slope of the linear fitted line with zero intercept for SIF-GPP at the diurnal scale among the three environmental factors investigated in this research. At the diurnal scale, when PAR experienced an obvious drop, the slope of the linear fitted line between SIF and GPP changed by 107.14% on average, while when Ta and VPD had an obvious drop, the slope of the linear fitted line between SIF and GPP changed by only 13.68% on average. Thus, combining Figures 5 and 6, it is evident that the slope variation of the linear fitted line of SIF versus GPP depended more on PAR than on Ta and VPD. The slope of the linear fitted line between \(\text{SIF}_{\text{Red}}\) and GPP varied obviously less than that between \(\text{SIF}_{\text{Far-red}}\) and GPP. This also indicated that the red SIF was more similar than the far-red SIF in response to environmental changes with GPP. Both red SIF and far-red SIF correlated stronger with GPP on the cloudy day than on the sunny day. This result is consistent with previous results in crops [31] and forests ecosystems [35]. On cloudy days, the proportion of scattered light increases, which penetrates into the lower leaves of the plant and the light-use efficiency of the plants gets improved [16; 23], leading to a larger slope of the linear fitted of SIF to GPP. Against the background of global climate change, alpine meadow ecosystems will face a very uncertain growth environment, in which changes in meteorological environmental factors will be unpredictable. The estimation of GPP in alpine meadow ecosystems urgently needs a stable indicator, and the results of this paper exactly demonstrated the potential of red SIF to track GPP in alpine meadow ecosystems when environmental changes occur.