## 4 Discussion  
### Ecosystem-Atmosphere CO\({}_{2}\) Exchange in the Spruce Forests  
Eddy covariance flux measurements in the 2015-2020 period showed that a southern taiga memorose spruce forest was a CO\({}_{2}\) sink or source for the atmosphere depending on the GPP/TER ratio, which was controlled by the environmental conditions of the particular year. Since the estimated uncertainty of the flux measurements was comparable to the annual NEE value, it is likely that the forest was generally a slight annual CO\({}_{2}\) source and was almost CO\({}_{2}\) neutral in the growing seasons. The annual NEE in the memorose spruce forest changed between \(-67\pm 34\) and \(196\pm 29\) gC\(\cdot\)m\({}^{-2}\). The annual NEE sums reported in this study correspond to the estimates obtained from old boreal spruce forests located in the other geographical regions of North America and Europe-e.g., [PERSON] et al. (2010); [PERSON] et al. (2011), and [PERSON] et al. (2012)--which were sources of the atmospheric carbon dioxide or CO\({}_{2}\)-neutral. However, many other mixed spruce stands have significant annual CO\({}_{2}\) uptake rates. For example, an old spruce forest near Valday Lake (Novgorodskaya oblast, Russia), located 170 km north of the study site, was found to be a CO\({}_{2}\) sink, with an annual NEE ranging between (\(-300\) and \(-95\) gC\(\cdot\)m\({}^{-2}\)([PERSON], 2015) in the 2010-2018 period. Mid-aged stands usually have higher CO\({}_{2}\) uptake rates ([PERSON], 2016). Flux measurements taken from a mid-aged hemiboreal forest in Estonia in 2015 showed an annual NEE of \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \(\varepsilon\) (\(\mu\)mol\(\cdot\)J\({}^{-1}\)) & \(\beta\) (\(\mu\)mol\(\cdot\)m\({}^{-2}\).s\({}^{-1}\)) & \(\mathbf{R}^{2}\) \\ \hline
**2016** & 0.026 & 1.361 & 0.689 \\
**2017** & 0.022 & 7.520 & 0.952 \\
**2018** & 0.161 & 1.263 & 0.569 \\
**2019** & 0.036 & 3.561 & 0.600 \\
**2020** & 0.106 & 3.572 & 0.695 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Light–response curves parameters \(\varepsilon\) and \(\beta\) (Equation (3) and Figure 6) as well as the coefficient of determination R\({}^{2}\) of the approximation (\(p<\) 0.001) for March 2016–2020. Figure 6: Light–response curves (Equation (3)) of the gross primary production (GPP) values calculated as mean 30 min GPP values for the 25 W\(\cdot\)m\({}^{-2}\) global radiation (Rg) bins in March 2016–2020. \(-586\pm 45\) gC\(\cdot\)m\({}^{-2}\)[63]. [PERSON] and [PERSON] [64] reported an NEE in an old mixed spruce forest in Tharandt (Germany) of between \(-395\) and \(-698\) gC\(\cdot\)m\({}^{-2}\). The annual NEE obtained by [PERSON] et al. [65] in a spruce forest with trees of different ages in Denmark during 6 years of measurements was between \(-553\) and \(-937\) gC\(\cdot\)m\({}^{-2}\). Twenty years of flux measurements taken from a mixed spruce forest in Ardennes (Belgium) showed an NEE of about \(-411\) gC\(\cdot\)m\({}^{-2}\) per year [36]. The interannual variability of NEE was dependent on the GPP/TER ratio. The flux measurements taken from an old southern taiga memorose spruce forest in the 2015-2020 period showed that the annual GPP/TER ratio at the study site varied between 0.87 and 1.47 and showed increased values in 2018 due to the relatively dry, sunny, and warm growing season conditions detected. The GPP/TER ratio obtained by [PERSON] et al. [63] in a mixed spruce stand in Estonia was about 1.84. [PERSON] et al. [62] reported a GPP/TER ratio of a mixed spruce forest in Sweden of about 0.75. The GPP/TER ratio obtained by [PERSON] et al. [65] in a spruce forest in Denmark was 1.49-1.88. [PERSON] et al. [66] reported that different post-harvest and post-fire 20-100-year-old stands in North America have a GPP/TER ratio of between 0.8 and 1.8. Therefore, the GPP/TER ratio obtained in this study is within the range of that of other boreal and temperate forests.