## 3 Results
### The Impact of Groundwater Level on GHG Emissions  
In both forests with drained and naturally wet organic soils, the groundwater level is high during spring; it lowers as the summer approaches and rises again in autumn. The groundwater level was the highest from March till May when the average groundwater level in birch stands was 4.4 cm and in black alder stands--2.3 cm. The lowest groundwater level was detected in July; in birch stands it was 66.5 cm, but in black alder stands--78 cm (Figure 8). If we compare the emissions from tree stem surfaces in forest stands on drained and wet organic soils without considering tree species, emissions do not differ significantly--CO\({}_{2}\) (\(p\) = 0.72), CH\({}_{4}\) (\(p\) = 0.11) and N\({}_{2}\)O (\(p\) = 0. 32). If we compare the \(p\)-values, the ones for CO\({}_{2}\) and N\({}_{2}\)O are considerably higher than CH\({}_{4}\). The highest CO\({}_{2}\) emissions in birch stands (88,939.85 \(\pm\) 17,458.35 CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\)) were detected when the groundwater level was 61-80 cm, whereas in black alder stands--when it was 1-20 cm (31,655.63 \(\pm\) 7522.42 \(\upmu\)g CO\({}_{2}\)-C m\({}^{-2}\) h\({}^{-1}\)). There is no significant correlation between the groundwater level and the amount of CO\({}_{2}\) released through the stems for both of the studied tree species. Weak, insignificant, positive correlations were found both in birch (\(\rho\) = 0.27) and black alder stands (\(\rho\) = 0.14). Average CO\({}_{2}\) fluxes, depending on the groundwater level, are shown in Figure 9.  
More CH\({}_{4}\) is released during flooding and when the groundwater level is <0 and 1-10 cm (Figure 10). The CH\({}_{4}\) fluxes are higher in silver birch stands. A significant, moderate negative correlation (\(\rho\) = \(-\)0.48, \(p\) < 0.001) was found between the CH\({}_{4}\) flux from birch stems and the groundwater level. The influence of groundwater level up to 10 cm is very strong (\(\rho\) = 0.95), but, when the level is up to 20 cm, the impact decreases (\(\rho\) = 0.46), whereas up to 30 cm the impact of groundwater level is insignificant (\(\rho\) = 0.18). A significant moderate negative correlation (\(\rho\) = \(-\)0.38, \(p\) = 0.0013) was found also between the amount of CH\({}_{4}\) released from black alder stems and the groundwater level. Moreover, for black alder, CH\({}_{4}\) fluxes are higher during flooding and when the groundwater level is up to 10 cm (\(\rho\) = 0.47); up to 20 cm, the effect is moderately strong (\(\rho\) = 0.30), but up to 30 cm, the influence of groundwater is insignificant (\(\rho\) = 0.14). Figure 8: Monthly groundwater level from March till October in forest stands with nutrient-rich drained and wet peat soil. Different lowercase letters show statistically significant differences (\(p\) < 0.05, two-sample \(t\)-test) between soil moisture type within the same month. Similar to CH\({}_{4}\), the N\({}_{2}\)O flux also increases with groundwater levels. When the groundwater level remains at a depth of around 20 cm, higher N\({}_{2}\)O fluxes from tree stems in black alder stands is observed (\(\rho\) = 0.33), but when the groundwater level drops to 30 cm, the effect disappears (\(\rho\) = 0.07). In birch stands, a very strong effect on the N\({}_{2}\)O released from the surface of the stems is observed when the groundwater is up to 10 cm deep  
Figure 10: CH\({}_{4}\) fluxes and removals depending on the groundwater level in silver birch and black alder stands. Different lowercase letters show statistically significant differences (\(p<\) 0.05, two-sample \(t\)-test) between tree species within the same groundwater level group. Figure 9: CO\({}_{2}\) fluxes depending on the groundwater level in silver birch and black alder stands. Different lowercase letters show statistically significant differences (\(p<\) 0.05, two-sample \(t\)-test) between tree species within the same groundwater level group.