## 4 Discussion
### The Impact of Groundwater Level  
Groundwater level shows a seasonal pattern in both forests with drained and naturally wet organic soils. This pattern can be used to predict the potential timing at which GHG emissions from tree stems are elevated. The results show that the groundwater level is lower in stands with drained peat soil, indicating that the drainage system is functioning effectively. This ensures a shorter period of time during which tree roots are exposed to excessive moisture and reduces the time during which tree stems are a source of GHG emissions. In our study, there was no relationship between the groundwater level and CO2 fluxes. The effect of groundwater level on CO2 emissions from tree stems is evident in stands with drained soil after thinning, where water availability was previously limited. As the number of trees decreases, competition for the necessary resources decreases, so physiological processes in the remaining trees improve, leading to an increase in the amount of CO2 released [62]. The main reason for the seasonal variation in CH4 emissions from tree stems is the groundwater level. Our results show that as the groundwater level decreases in both birch and black alder stands, the amount of CH\({}_{4}\) emitted from tree stems decreases. In the birch stands, the highest amount of CH\({}_{4}\) was emitted in June, which could be related to the high groundwater level in May, as emissions decrease in the following month when the water level drops. A similar tendency is observed in black alder stands, where the amount of CH\({}_{4}\) released is the highest in May when the groundwater level is 10 cm. As the groundwater level stabilizes, few or no fluxes are detected. In general, CH\({}_{4}\) emissions have been studied in waterlogged or flooded areas, where soil is usually the source of GHGs. The results of a study carried out in a temperate, spring-fed, forested peatland in the United Kingdom show that more CH\({}_{4}\) is emitted during the high water season (from April to July) [14]. In a study in mature riparian grey alder forest stands in Estonia, CH\({}_{4}\) emissions from tree stems at 170 cm height before flooding did not differ significantly between the experimental site and the control site. When the experimental area was flooded, the amount of CH\({}_{4}\) released from both soil and tree stems increased significantly, and this effect persisted even after the experiment. Similar observations were made for N\({}_{2}\)O emissions. In most cases there were no differences between N\({}_{2}\)O emissions from the control and treatment site before flooding, but as the groundwater level rose, the amount of N\({}_{2}\)O released increased [15]. In a study carried out later on the same study site in Estonia, 86% of all the CH\({}_{4}\) emissions were found to be due to excess humidity. The results of this study showed a strong positive correlation between emissions from tree stems and soil water levels. The study concluded that the amount of CH\({}_{4}\) emitted from tree stems differs significantly during dry and flooded periods [16]. Other studies carried out in wetland forests also show a positive relationship between tree stem GHG fluxes and water table depth [13; 14; 63; 64; 65]. These findings suggest that CH\({}_{4}\) produced under anoxic conditions in groundwater enters plant tissues, where it is further transported and emitted to the atmosphere. To limit CH\({}_{4}\) emissions in forests with wet and drained nutrient-rich peat soils, the groundwater level should be at least 15-20 cm deep during the vegetation season. In drained forests, this can be achieved by maintaining and restoring drainage systems and restoring natural water flows damaged by logging. In forests with wet soil, the moisture regime can be improved by creating a network of deep furrows during reforestation, which drains excess water from the top layer of the soil and reduces the area of waterlogged depressions.