## 4 Discussion
### Seasonality of GHG Fluxes and the Impact of Tree Species
According to this study, fluxes from stems exceed those from soil in upland wetland habitats, highlighting the importance of tree stems as significant GHG sources [9]. In a study carried out in Finland in a forested fan, where fluxes were measured from April to June, the median CH\({}_{4}\) emissions from downy hich stems ranged from 0.11 to 100 \(\mu\)g m\({}^{-2}\) h\({}^{-1}\), depending on the tree height [44]. Our results fall within the previously reported estimates. During a study of peatlands in Finland, the measured CO\({}_{2}\) fluxes from soil ranged from \(-\)0.01 to 2.14 g m\({}^{-2}\) h\({}^{-1}\), CH\({}_{4}\) fluxes varied between \(-\)0.53 and 24.5 mg m\({}^{-2}\) h\({}^{-1}\), and N\({}_{2}\)O fluxes ranged from \(-\)0.07 to 0.45 mg m\({}^{-2}\) h\({}^{-1}\). These emissions and removals are higher than those of stems in our study [45]. In a study carried out in black alder stands growing in rewetted peatland in north-eastern Germany the highest stem CH\({}_{4}\) flux was recorded in May--4.0 mg m\({}^{-2}\) h\({}^{-1}\), whereas the average stem emissions were 0.1 \(\pm\) 0.3 mg m\({}^{-2}\) h\({}^{-1}\). The highest soil CH\({}_{4}\) flux was 132.4 mg\({}^{-1}\) m\({}^{-2}\) h\({}^{-1}\), but the average soil CH\({}_{4}\) emissions were 4.8 \(\pm\) 18.8 mg\({}^{-1}\) m\({}^{-2}\) h\({}^{-1}\)[46]. In a study carried out in Latvia, the mean CO\({}_{2}\) emissions from drainage ditches in peatland forests was 136.6 \(\pm\) 28.7 mg CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\), the mean CH\({}_{4}\) emissions were 0.085 \(\pm\) 0.034 mg CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) and the mean N\({}_{2}\)O emissions were 0.009 \(\pm\) 0.003 mg N\({}_{2}\)O-N m\({}^{-2}\) h\({}^{-1}\)[47]. In a study carried out in the volcanic Reunion Island, it was found that mosses, lichens, algae and bacteria found on tree stems consume CH\({}_{4}\) and make trees a major CH\({}_{4}\) sink (8.3 \(\pm\) 3.0 \(\mu\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\)) [48]. The CH\({}_{4}\) and N\({}_{2}\)O removals in our study could also be explained by the activity of microorganisms on tree bark. In the case of N\({}_{2}\)O, it is assumed that these organisms reduce N\({}_{2}\)O to N\({}_{2}\) in the final denitrification process. Species morphological and physiological differences could be the reason why birch stands emit more CO\({}_{2}\) than black alder stands. Stem CO\({}_{2}\) emissions are the main indicator of tree respiration [49]. In a study focusing on differences of CO\({}_{2}\) fluxes between deciduous tree stems, it was found that CO\({}_{2}\) efflux differences are related to differences in the concentration of non-structural carbohydrates and the proportion of living parenchyma in sapwood [50]. Species-specific differences could also explain why birch stands emit more CH4 than black alder stands when moisture conditions are similar. Studies have shown that black alder is more tolerant to elevated groundwater levels than birch [51; 52]. Another study carried out in a temperate wetland forest showed that stem CH4 emissions varied significantly between black alder and downy birch (_Betula pubescens_ Ehrh.). The seasonal variations for black alder were minimal, while substantial variations were observed for downy birch [14]. It is well known that there are species-specific differences in CH4 transport mechanisms in plants [53; 54]. It is possible that black alder and birch use different CH4 transport mechanisms or a combination of convective transport and passive diffusion [14]. [PERSON] et al. suggested that stem CH4 transport in black alder trees is mainly driven by passive diffusion [55]. The significant differences in N2O emissions between the species could be explained by the fact that black alder grow in symbiosis with actinomycetes _Frankia_ sp. that are capable of fixing N2[28; 29], whereas birch trees do not have such ability. It is known that black alder increases N availability in soil and mediate N2O from soil.