## Discussion
### Sensitivity Analysis of Vegetation Groups
[18; 19], SIF\({}_{760}\) is sensitive towards structural and functional properties of photosynthetic light conversion in canopies. We can further assume that SIF\({}_{760}\) may originate from the deeper layer of the canopies. This was because of foliage which was having a high transmission in that particular spectral region, as also previously reported by [PERSON] et al. [36]. For the peatland vegetation, we found clear and significant (\(p<0.007\)) relationship between SIF\({}_{760}\) and SIF\({}_{687}\) and fAPAR (Figure 6). Therefore, the results confirm that SIF can be considered as an excellent indicator of the amount of light absorbed by chlorophyll and used in photosynthesis (e.g., [36; 37; 74; 75]) for a heterogeneous ecosystem like peatland. High fluorescence values of the deciduous forest, secondary forest communities with _Alnus glutinosa_, meadows and mesic pastures, as well as alder forest, suggest that SIF reflects more than just greenness of the canopies. From the ground observation of vegetation during the time of the campaign, we confirmed that the deciduous trees were in their young stage, which was also shown by RS observation. Our finding was in agreement with [PERSON] et al. [16], where he showed a strong relationship between high SIF values and young age deciduous trees. In agreement with our study, [PERSON] et al. [37] have also observed a high SIF emission from densely covered grasslands. In our study, alder forest and low birch bush located inside the peatland area, as well as grasslands located at the edge of the peatland, were covered by the dense green canopies and were observed to be a source of high emission of both SIF\({}_{760}\) and SIF\({}_{687}\) signals during the campaign. However, low vegetative covers were observed to show a weak SIF signal, as well as low values of greenness indices. The result also showed a significant positive relationship between two SIF bands with greenness indices. Different studies have demonstrated that reflectance-based VIs like NDVI have successively improved our understanding about global ecosystems by providing estimates of potential photosynthesis from greenness estimates [76; 77; 78]. Sometimes, NDVI is used to track the development of vegetation phenology, whereas SIF is considered to be a prime parameter to estimate GPP (e.g., [25; 79]). Considering that we found significant relationships between SIF and NDVI at the vegetation group level, and positive relationships between SIFs and fAPAR (Figure 6), we can hypothesize that canopies with higher values of SIF and greenness VIs were more photosynthetically active during the campaign. Therefore, we can farther assume that SIF retrieved from Hyplant may reflect the spatial variability of GPP for these ecosystems (although not confirmed by measurements). The reflectance-based indices such as NDVI and SR are insufficient for the purpose of estimating the photosynthetic process, whereas SIF can be highly correlated with photosynthesis [80]. A study by [PERSON] et al. [30] showed that at the initial stage, SIF\({}_{760}\) increases with chlorophyll content, whereas SIF\({}_{760}\) saturates at with high chlorophyll content due to the saturation of light absorption by chlorophyll molecules with the high chlorophyll content [30; 81]. Our results showed that SIF\({}_{687}\), which is less sensitive to photosynthesis but does not easily get saturated, correlated highly with greenness indices, but also with EVI and PRI. These relationships were, however, weaker for SIF\({}_{760}\). Some other reasons behind the good agreement between SIF and VIs for large vegetation patches may be: (1) Less quantity of mixed pixels, (2) the absence of mixed signals in dominant vegetation groups, and (3) less influence of microclimatic and local environmental stress factors.