## 2 Material and Methods
### Identification of Vegetation Groups and Peatland Plant Communities  
Regions of interest (ROIs) were defined to facilitate analyzing obtained spectral vegetation products and SIF signals for diverse vegetation groups representing peatland, forest, and grassland ecosystems, as well as peatland plant communities found in and around the peatland area. ROIs calculation was performed using ENVI 5.3 (Exelis VIS, Boulder, CO, USA) and ArcGIS 10.4 (Esri, Redlands, CA, USA). Selection of ROIs was based on the detailed vegetation survey. Vegetation surveys counted the dominant vegetation species and other plant species in the Rzcin peatland between 2014-2017. Areas without any vegetation survey background were not analyzed in the study. Two different approaches were applied to assess RS information from the area of interest. In the first approach, the big area (forest, peatland, and grassland) of vegetation plant community in Rzcin (Poland) was considered. In the second approach, the small areas described as plots inside the heterogeneous peatland were considered. For the first approach, we defined 158 ROIs with different shapes and sizes that were distributed over the whole site and covered with HyPlant. These 158 ROIs were subsequently categorized into 19 vegetation groups, considering three major ecosystem types: (i) Forest, (ii) grassland, and (iii) peatland. The spatial distribution and detailed names of the 19 vegetation groups are shown in Figure 3a. For the second approach (i.e., the assessment of VIs and SIF for highly complex plant community patches inside the peatland area), we chose 52 individual plots. For these plots, a detailed botanical survey was done after the SWAMP campaign in the years of 2015-2017. The plots were selected by botanists to ensure that the most important plant communities of this peatland were covered. Each plot had an area of 25 square meters (5 m \(\times\) 5 m). Vegetation cover (%) was recorded for the plots and later transformed according to the Van [PERSON] scale ([PERSON], 1959) to categorize the most dominant species for that particular plot (1 = 0.5%, 2 = 0.5-1.5%, 3 = 1.5-3%, 4 = 3-5%, 5 = 5-12.5%, 6 = 12.5-25%, 7 = 25-50%, 8 = 50-75% and 9 = 75-100%). The plant communities were identified for each location according to the Braun-Blanquet method ([PERSON], 1960). To reduce redundancy of a single plant community among the 52 locations, we integrated them into 20 unique plant communities, categorized into three major groups: (i) Meadows (ME), (ii) peatland rush (PR), and (iii) fen vegetation (FE). It must be noted that the lake area located inside the peatland was not considered in this study, and corresponding VIs and SIF values were masked and considered as non-value. Also, the floating vegetation and macrophytes over the lake area (plots 26-28, 31-32) were excluded from the analysis. Besides, there were two plots (38-39) located outside the image boundary. Hence the total number of plots used for the analysis of VIs and SIF was reduced to 46. A detailed description of the 52 individual plant communities with their associated locational IDs in Figure 3b is given in Table S1 (Supplementary Materials). Legend a): (**i) Forest vegetation groups**: Herbaceous vegetation of forest clearings (HV); wooded dunes with _Pinus sylvestris_ (WDPS); semi-natural forests with _Pinus sylvestris_ (SeFPS); secondary forest communities with _Pinus sylvestris_ (SFPS); _Betula pendula_-secondary forest communities (BPFS); riparian forests (RF); secondary forest communities with _Almus glutinosa_ (SFAG); deciduous forest (DF); (ii) Grassland vegetation groups**: post-agriculture land (PG); pioneer vegetation of sandy and shallow soils (PVS3); moved meadows and mesic pastures (CM); meadows and mesic pastures (MMP);  
Figure 3: Boundaries of 158 ROIs identified within the HyPlant image and categorized into 19 unique vegetation groups (**a**); and location of 52 plots inside the peatland and its boundaries, categorized into 20 unique plant communities (**b**).