## Discussion
### Sensitivity Analysis of Peatland Plant Communities
Whereas, CLa, characterized by a higher abundance of vascular plants, provided high SIF signals. We did not observe any trend of similarity between SIF\({}_{687}\) and SIF\({}_{760}\) signalsfrom different plant communities. Firstly, this might be due to less sensitivity for re-absorption of red-fluorescence at microscale plant community level ([PERSON] and [PERSON], 2016). Secondly, the probability of a mixture of signals from heterogeneous surfaces may also be a reason for lower sensitivity at a lower spatial scale ([PERSON] and [PERSON], 2016). In this study, we also showed the relationships between SIF and VIs for homogeneous and heterogeneous categories of peatland vegetation (fen, peatland rush, and meadows). From Figure 10, it is clear that for homogeneous meadows the relationships between SIF and VIs were positive and comparable to those determined based on a larger spatial scale of vegetation groups identified for different ecosystems. No such relation was observed, or they were just very poor and non-significant, for heterogeneous rush and peat vegetation communities. The impact of different vegetation species on such relationships may be strongly related to the type of peatland, such as the homogeneous or heterogeneous nature of the peatland ([PERSON] and [PERSON], 2016). The uniqueness of our study was that the Rzecin peatland consisted of both kind of surfaces, which were located in different parts of the peatland area. The homogeneous meadows were located in the periphery of the peatland, whereas heterogeneous rush and peatland vegetation was the most dominating vegetation group located inside of the peatland. Studies have shown that differences among vegetation types or the character of the peatland are influential enough to affect the vegetation relationship with spectral data (e.g., ([PERSON] and [PERSON], 2016)). Thus, the chances of mixed pixel (in terms of the mixture of signals) obtained from peatland plant communities is higher in heterogeneous patches than a homogeneous ones. [PERSON] et al. ([PERSON] et al., 2016) showed that mixed pixels might influence statistical classification methods because most algorithms are based on the assumption of spectral homogeneity at pixel-scale within a particular class of land cover. Thus, we can assume that due to the spectral homogeneity of meadows, the emitted SIF signals were not much disturbed, whereas in heterogonous fen and peatland rush vegetation groups the SIF signals were influenced by mixing, which ultimately impacted the correlation of VIs and SIF. Our results indicate that SIF values did not increase with an increase in species diversity and species richness within the plant communities identified on the peatland. Higher values of SIF\({}_{760}\) were associated with a higher biomass of vascular plants with higher LAI (Figure 6). On the other hand, SIF\({}_{687}\) was less sensitive to changes in species richness and biomass of vascular plants. We can state, that SIF, especially SIF\({}_{760}\), was strongly associated with the functional diversity of peatland vegetation. Therefore, it was possible to identify the plant communities of high perennials, showing higher values of SIF\({}_{760}\) (associated with LAI \(>\) 5.0 m\({}^{2}\cdot\)m\({}^{-2}\)). Whereas, lower values of SIF\({}_{760}\) indicated peatland patches dominated by _Sphagnum_ spp., which are known to have lower productivity and lower coverage and LAI (\(<\)0.8 m\({}^{2}\cdot\)m\({}^{-2}\)) of vascular plants (Table 2, Figure 6). In general, we can state that SIF reflects very well the productivity gradient on the fen peatland, from _Sphagnum_-dominated patches with the lowest SIF and the lowest fAPAR and productivity to the _Carex_-dominated patches with the highest SIF and higher fAPAR and productivity ([PERSON] and [PERSON], 2016; [PERSON], 2016; [PERSON], 2016).