## 2 Materials and Methods
### Closed Chamber Design and Gas Flux Exchange Measurements from Tree Stems  
Measurements were carried out for 12 months (from November 2020 till October 2021). GHG flux measurements were performed using static closed non-transparent chambers of fixed area and volume and Gasmet DX4040 FTIR analyser. Depending on tree diameter, different chambers were used (Table 3). Before a measurement, the area of the stem bark where the chamber was to be attached was treated with silicone to ensure airtightness after the chamber was installed. Before starting the measurements, the gas composition of the ambient air was determined. Upon installation of the chamber on the tree stem, a chronometer was started to facilitate and accurately record the time stamps at which the \"Gasmet DX4040\" measurement program must be turned off. When the chamber was installed, the first measurement was taken (at 0 min), followed by a 4 min pause. In the sixth minute, the device was turned on and three measurements were taken, each lasting two minutes. After the third measurement, the \"Gasmet DX4040\" was turned off again for 4 min, and turned on again at the 16 th minute, when three more measurements were taken, each lasting 2 min. After the third measurement, the device was turned off again for 4 min and turned on at the 26 th minute when the final three measurements were taken. Identical measurements were repeated on the two other trees in each sample plot. For each of the measurements with the \"Gasmet DX4040\", information about humidity (H\({}_{2}\)O\(\%\)) and the concentrations of carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)) and nitrous oxide (N\({}_{2}\)O) in the camera at specific time intervals was recorded. A schematic representation of the gas measurement chamber and its installation is shown in Figure 2, and a photo of the chamber is shown in Figure 3.  
Simultaneously to the gas flux measurements, soil and air temperature as well as the groundwater level were determined. The soil temperature was measured at a depth of 10 cm using a \"Comet\" thermometer by inserting its probe at the corresponding depth. The air temperature was measured with the same thermometer, but the probe was placed at approximately 1 m from the ground. The groundwater level was measured in a previously drug groundwater well using a ruler by measuring the internal part of the well up to the water surface and the external part up to the soil surface. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Tree Species**} & \multicolumn{2}{c}{**Diameter of Trees at Breast (1.3 m) Height, cm**} & \multicolumn{2}{c}{**Tree Height, m**} \\ \cline{2-7}  & **Tree 1** & **Tree 2** & **Tree 3** & **Tree 1** & **Tree 2** & **Tree 3** \\ \hline Silver birch & 20.5 & 16.4 & 11.5 & 18.5 & 17.8 & 16.0 \\ Silver birch & 20.3 & 14.7 & 12.7 & 21.4 & 18.2 & 19.4 \\ Silver birch & 28.6 & 25.9 & 19.5 & 22.7 & 22.4 & 18.2 \\ Silver birch & 14.5 & 10.6 & 9.2 & 12.6 & 12.2 & 12.0 \\ Silver birch & 20.5 & 18.0 & 12.9 & 22.4 & 21.3 & 20.2 \\ Silver birch & 29.9 & 21.0 & 14.0 & 25.4 & 23.6 & 20.8 \\ Black alder & 11.3 & 11.1 & 8.7 & 11.4 & 11.4 & 9.7 \\ Black alder & 24.3 & 19.3 & 14.2 & 23.7 & 21.3 & 19.8 \\ Black alder & 36.9 & 23.9 & 21.2 & 30.2 & 27.1 & 26.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of individual trees of the studied forest stands. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Chamber ID**} & **Height, cm** & **Width, cm** & **Thickness, cm** & **Volume, m3** & **Surface Area, m2** \\ \hline
1 & 20.1 & 25.0 & 2.2 & 0.0011 & 0.0503 \\
2 & 20.5 & 42.0 & 2.3 & 0.0020 & 0.0861 \\
3 & 20.0 & 56.5 & 2.5 & 0.0028 & 0.1130 \\
4 & 19.0 & 73.0 & 2.8 & 0.0039 & 0.1387 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of gas flux chambers. GHG fluxes (\(\upmu\)g m\({}^{-2}\) h\({}^{-1}\)) were calculated according to Equation (1). The quality of GHG flux measurements was checked by plotting concentration values over time to ensure that changes in concentrations had remained linear.