# Investigating the Performance of Red and Far-Red SIF for Monitoring GPP of Alpine Meadow Ecosystems
## 4 Discussion
[58] also observed that fluorescence in the red band was more sensitive to GPP than fluorescence in the far-red band under drought stress. Hence, from a plant physiological perspective, the strong correlation between PSII and \(\text{SIF}_{\text{Red}}\) may be one of the reasons for the performance of \(\text{SIF}_{\text{Red}}\) being better than that of \(\text{SIF}_{\text{Far-red}}\) in monitoring GPP in this alpine meadow ecosystem. Last but not least, the red SIF showed a linear correlation with GPP in this alpine meadow ecosystem. That linear relationship between red SIF and GPP is more advantageous for GPP tracking than the nonlinear relationship between far-red SIF and GPP. Alpine meadow ecosystems are dominated by grasses with relatively low leaf area index values, and the resorption effect within the canopy is likely to be relatively small. However, during a period of vigorous vegetation growth, the SIF in the red band is probably inevitably affected by the reabsorption effect. For example, following the downscaling of grass and wheat canopy SIF to the photosystem level using the random forest method, [PERSON] et al. [63] found that the correlation between red SIF at the photosystem level and chlorophyll absorption of photosynthetically active radiation was remarkably enhanced compared to red SIF at the canopy level. This also implied that the red SIF is mainly influenced by the internal reabsorption effect of leaves. Therefore, it is reasonable to speculate that the red SIF observed in the alpine meadow canopy was saturated due to the reabsorption effect. Moreover, many studies have confirmed that the relationship between GPP and absorption of photosynthetically active radiation is nonlinear and that GPP will saturate when the APAR reaches a certain level [40; 56]. Therefore, the saturation phenomenon that occurred for both GPP and \(\text{SIF}_{\text{Red}}\) resulted in a linear relationship between the two. Whilethe far-red SIF is mainly affected by canopy scattering, and the effect of reabsorption on it can be basically ignored [43; 63], thus the far-red SIF does not exist saturation phenomenon. When PAR reached a certain level, GPP no longer increased, while far-red SIF was still increasing, thus far-red SIF showed a nonlinear correlation with GPP in this alpine meadow ecosystem. As the time resolution of the data was reduced from half-hourly average to daily average, there was smoothing of the data noise and variance, which led to a reduction in the difference between the results of fitted \(\text{SIF}_{\text{Far-red}}\) to GPP with nonlinear and linear functions under the daily averaged data. Nevertheless, from the trend of data distribution, whether half-hourly or daily averaged data, GPP showed a more linear relationship with red SIF than with far-red SIF. Differences in the Response of \(SIF_{\text{Red}}\)/GPP and \(SIF_{\text{Far-red}}\)/GPP to Changes in Environmental Factors  
How well synchronized the SIF and GPP are in response to environmental changes affects the accuracy of SIF estimation of GPP. Alpine meadow ecosystems are sensitive to environmental changes [3; 5; 6; 7]. For the alpine meadow ecosystem at the AR site, the average annual Ta is low. Even in summer, frosts may still occur in the early morning [64]. The AR station is at high altitude and has high levels of harmful ultraviolet-B radiation, it also has sufficient radiation and sufficient hours of sunshine to allow plants to perform normal photosynthesis to maintain their organic growth. VPD also has a range of fluctuations in this alpine meadow ecosystem. The level of VPD has a direct effect on the stomatal conductance of plant leaves. Excessive VPD leads to decrease in stomatal conductance to prevent excessive water loss in plants, though this inevitably reduces the CO\({}_{2}\) uptake of photosynthesis and affects the photosynthetic process. In this research, the ratio of \(\text{SIF}_{\text{Red}}\)/GPP was found more stable than \(\text{SIF}_{\text{Far-red}}\)/GPP as environmental factors varied at the seasonal scale.