## 3 Results  
### Seasonality of GHG Fluxes and the Impact of Tree Species  
The CO\({}_{2}\) emissions from tree stems varied seasonally for both silver birch and black alder stands. Birch stands on average emitted 21,588 \(\pm\) 3624 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) during spring months, 54,255 \(\pm\) 5669 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) during summer and 12,423 \(\pm\) 1337 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) during autumn, whereas black alder stands emitted 9241 \(\pm\) 2751 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) during spring months, 40,053 \(\pm\) 5920 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) during summer and 9247 \(\pm\) 826 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) during autumn. The highest CO\({}_{2}\) emissions were detected in June, which coincides with the highest air and soil temperatures. In silver birch stands, 78,165 \(\pm\) 3842 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\) were emitted during this month, and in black alder stands--53,871 \(\pm\) 2086 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\). The lowest CO\({}_{2}\) fluxes in all the study sites were measured in March. Data from the winter months were not included in the calculations, as the emissions were insignificant. The CO\({}_{2}\) fluxes were higher in birch stands compared to black alder stands. On average, during the measurement cycle, birch stands emitted 30,782 \(\pm\) 3197 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\), while black alder stands emitted 20,830 \(\pm\) 3119 ug CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\).The difference between both species during the whole measurement period is statistically significant (\(p=0.01\)). Evaluating the differences between both of the tree species in spring, summer and autumn months separately, the difference in spring is statistically significant (\(p=0.008\)). An insignificant, weak, negative correlation was found between CO\({}_{2}\) fluxes and stand age (Spearman correlation coefficient \(\rho=-0.1338924\), \(p>0.05\)). Monthly GHG fluxes for each of the tree species are shown in Figure 4.  
The seasonality was less pronounced for CH\({}_{4}\) fluxes. Birch stands on average emitted 7.41 \(\pm\) 4.29 \(\mathrm{\SIUnitSymbolMicro}\) CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) during spring months, 32.51 \(\pm\) 11.54 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) during summer and 17.36 \(\pm\) 10.13 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) during autumn. Black alder stands emitted 7.90 \(\pm\) 8.16 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) during spring months, whereas in summer and autumn months on average, slight removals were detected (\(-\)0.66 \(\pm\) 0.20 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) and \(-\)0.15 \(\pm\) 0.43 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) for summer and autumn, respectively). The CH\({}_{4}\) fluxes in birch stands were the highest in June\(-\)40.60 \(\pm\) 8.27 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\), while in black alder stands they were--in May--24.17 \(\pm\) 9.25 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\). On average during the measurement period, birch stands emitted 17.63 \(\pm\) 5.54 \(\mathrm{\SIUnitSymbolMicro}\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\). As the groundwater level stabilized, CH\({}_{4}\) fluxes were negligible. Differences between both of the species were statistically significant for all of the study period (\(p\) = 0.03) and for summer months (\(p\) = 0.0058) when evaluating the seasons separately, but no statistically significant differences (\(p\) > 0.05) between species were found when evaluating each month separately (Figure 5). An insignificant, moderate, positive correlation was found between CH\({}_{4}\) fluxes and stand age (\(p\) = 0.32, \(p\) > 0.05). Large data variation was observed for N\({}_{2}\)O fluxes.