# Investigating the Performance of Red and Far-Red SIF for Monitoring GPP of Alpine Meadow Ecosystems
## 3 Results
### Response of SIF and GPP to Environmental Factors  
The impact of environmental factors upon the SIF-to-GPP ratio was also analyzed. As shown in Figure 4, with increasing PAR values, \(\text{SIF}_{\text{Red}}\)/GPP values show a very slight increasing trend, while \(\text{SIF}_{\text{Far-red}}\)/GPP values show an obvious increasing trend, with obvious increases when PAR is higher than 400-600 \(\upmu\)mol/m\({}^{2}\)/s. \(\text{SIF}_{\text{Red}}\)/GPP values remain almost constant across different changes in Ta without obvious variations, while those of \(\text{SIF}_{\text{Far-red}}\)/GPP show an increasing trend, especially when the temperature increases to 15 \({}^{\circ}\)C, where \(\text{SIF}_{\text{Far-red}}\)/GPP shows a substantial increase. In addition, \(\text{SIF}_{\text{Red}}\)/GPP values did not show obvious variations across different changes in VPD, while \(\text{SIF}_{\text{Far-red}}\)/GPP values showed an obvious increasing trend with increases in VPD. In general, for the same value of a given environmental factor, the \(\text{SIF}_{\text{Red}}\)/GPP values tended to be concentrated and have smaller variance, while those of \(\text{SIF}_{\text{Far-red}}\)/GPP were dispersed and had larger variance. When there were changes in the value of the given environmental factor, the \(\text{SIF}_{\text{Red}}\)/GPP values tended to vary less in response and those of \(\text{SIF}_{\text{Far-red}}\)/GPP vary more. This indicates that \(\text{SIF}_{\text{Red}}\) and GPP tended to have a more consistent response to changes in environmental factors than \(\text{SIF}_{\text{Far-red}}\) and GPP. At the diurnal scale, first, observations from two days with similar PAR but different Ta and VPD were selected to analyze the impacts of environmental factors on SIF-GPP relationship at red and far-red band for alpine meadow. The diurnal variations and values of PAR are similar for DOY220 and DOY227 as shown in Figure 5, while changes in Ta and VPD are obvious, i.e., the values of Ta and VPD for DOY220 are lower than those for DOY227. Due to the decrease in Ta and VPD, the R\({}^{2}\) of the linear fitted with zero intercept of both \(\text{SIF}_{\text{Red}}\)-GPP and \(\text{SIF}_{\text{Far-red}}\)-GPP underwent serious decrease and the RMSE underwent dramatic increase in DOY220, with 29.63% and 40% decrease in R\({}^{2}\) and about one and three times increase in RMSE for \(\text{SIF}_{\text{Red}}\)-GPP and \(\text{SIF}_{\text{Far-red}}\)-GPP, respectively. Comparing the SIF-GPP relationship for these two days, we found that the slope of the linear fitted with zero intercept between \(\text{SIF}_{\text{Far-red}}\) and GPP decreased obviously when there was an obvious decrease in Ta and VPD values, i.e., of 24.35%, while the slope of the linear fit between \(\text{SIF}_{\text{Red}}\) and GPP did not show any obvious differences. Furthermore, we selected a typical sunny day and a cloudy day, between which there were obvious differences in the daily changes of PAR, Ta, and VPD. We analyzed the variations in the relationship of \(\text{SIF}_{\text{Red}}\) and \(\text{SIF}_{\text{Far-red}}\) with GPP separately for these two days. From the sunny day to the cloudy day, the R\({}^{2}\) of the linear fitted with zero intercept for \(\text{SIF}_{\text{Red}}\)-GPP increased by 7.69% and the RMSE decreased appreciably, by 64.78%, with the scattered distribution of the data on the cloudy day being closer to the fitted line compared to the sunny day, while the R\({}^{2}\) of \(\text{SIF}_{\text{Far-red}}\)-GPP decreased by 5.75% and the RMSE increased by 4.6%. As shown in Figure 6, the slopes of the linear fitted with zero intercept between \(\text{SIF}_{\text{Far-red}}\) and \(\text{SIF}_{\text{Red}}\) with GPP are both obviously steeper under cloudy weather than those under sunny weather, though the degree of changes in the slopes differ.