## 4 Discussion  
### Seasonality of GHG Fluxes and the Impact of Tree Species  
In our study, seasonality of CO\({}_{2}\) fluxes was observed as well as positive correlations with soil and air temperature; however, for CH\({}_{4}\) and N\({}_{2}\)O, seasonality was either less pronounced or there were only removals detected. A significant impact of stand age on GHG fluxes was not found in this study. Results from a study carried out in a boreal forest in Finland show a distinct seasonal pattern for both CO\({}_{2}\) and N\({}_{2}\)O fluxes from tree stems, and the level of emissions remains significant throughout the year. The emissions reach the maximum during the  
Figure 12: N\({}_{2}\)O fluxes depending on organic C content in soil in silver birch and black alder stands without outliers. Figure 13: N\({}_{2}\)O fluxes depending on total N content in soil at 0–10 cm, 10–20 cm and 20–30 cm depth in silver birch and black alder stands without outliers. vegetation season, decrease in October, and remain low during the dormant period in winter [33]. Results from a study carried out in a deciduous upland temperate forest in the Mid Atlantic region of the USA show the seasonal pattern for CO\({}_{2}\) and CH\({}_{4}\) stem emissions, whereas no such pattern was observed for N\({}_{2}\)O, which is similar to our study [41]. Additionally, in a study carried out in Estonia in a riparian grey alder forest, a seasonal variability for CH\({}_{4}\) and N\({}_{2}\)O fluxes was observed. The measured tree stems were net emitters of CH\({}_{4}\) with flux rates of 4.63 \(\pm\) 1.42 \(\mu\)g C m\({}^{-2}\) h\({}^{-1}\) during daytime and 2.96 \(\pm\) 0.88 \(\mu\)g C m\({}^{-2}\) h\({}^{-1}\) during nighttime in summer 2017, which were substantially higher in the following spring in May 2018 (168.47 \(\pm\) 33.59 \(\mu\)g C m\({}^{-2}\) h\({}^{-1}\), nighttime). Our estimates are higher than the fluxes measured in summer in the Estonian Grey alder forest, but lower than those of the following spring in this study. In the same study site in Estonia, tree stems were also net N\({}_{2}\)O emitters: in summer, 14.93 \(\pm\) 4.90 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\) in the day and 11.17 \(\pm\) 3.56 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\) at night; in spring, 3.30 \(\pm\) 0.83 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\) in the daytime and 2.39 \(\pm\) 0.56 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\) at nighttime; summer 2018, 0.11 \(\pm\) 0.08 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\) in the daytime and \(-\)0.04 \(\pm\) 0.22 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\) at nighttime [42]. In our study in silver birch stands, only removals were detected during the whole study period, and in black alder stands, 1.44 \(\pm\) 0.71 \(\mu\)g N\({}_{2}\)O-N m\({}^{-2}\) h\({}^{-1}\) was measured in June, whereas in spring and summer, on average, only removals were detected. In a study carried out in Agali Drained Peatland Forest Research Station in Estonia, birch trees were a net annual source of both CH\({}_{4}\) (0.38 \(\pm\) 0.09 \(\mu\)g C m\({}^{-2}\) h\({}^{-1}\)) and N\({}_{2}\)O (0.94 \(\pm\) 0.32 \(\mu\)g N m\({}^{-2}\) h\({}^{-1}\)). Birch stem CH\({}_{4}\) emissions peaked significantly in November and June. Birch stem N\({}_{2}\)O emissions were negligible for most of the year, increasing slightly during March and the autumn months [43]. [PERSON] et al. measured CH\({}_{4}\) fluxes from tree stems and soils in different habitats along a moisture gradient. The study was carried out in the USA, at Chesapeake Bay in Maryland. The mean CH\({}_{4}\) emissions from tree stems were 68.8 \(\pm\) 13.0, 567.9 \(\pm\) 174.5 and 180.7 \(\pm\) 55.2 \(\mu\)g CH\({}_{4}\)-C m\({}^{-2}\) h\({}^{-1}\) for upland, wetland and transitional habitats, respectively, whereas the mean CH\({}_{4}\) emissions from soil for those habitats were 64.8 \(\pm\) 6.2, 190.0 \(\pm\) 123.0 and 7.4 \(\pm\) 25.0 \(\mu\)g m\({}^{-2}\) h\({}^{-1}\), respectively.