## 4 Discussion
### Implications of the Local Soil Moisture Regimes for the CO\({}_{2}\) Exchange of Spruce Forests
The GPP at both of the sites was higher in 2018 than in 2017 (in the memorose spruce forest, M-W U test, U = 14,534, Z = \(-\)2.18, \(p\) = 0.029; in the paludified spruce forest, M-W U test, U = 12,400, Z = (\(-\)4.29, \(p\) < 0.001). Both ecosystems showed increased primary production in the growing season in response to the high radiation and air temperatures, but the effect was less pronounced for the paludified spruce forest due to its relatively low productivity [51]. The GPP/TER ratio of the memorose spruce forest was less sensitive to the air temperature anomaly that occurred in the winter of 2019/2020 than in the paludified spruce forest (Figure 9). We suppose that this difference may be related to the greater portion of spruce in the tree vegetation composition in the paludified forest or the higher fraction of heterotrophic respiration in TER in the memorose spruce forest. Additional observational data considering the components of TER in winter in the two forests are needed to test these hypotheses. Figure 8: Boxplots of the mean daily (**a**) total ecosystem respiration (TER) and (**b**) gross primary production (GPP) in the growing seasons (12 April–11 October) of 2017 and 2018 in the memorose spruce forest (NF, this study) and the paludified spruce forest (PF) [51]. This study experimentally demonstrated that local hydrology and soil moisture regimes are important environmental factors that determine the differences in the response reactions of the ecosystem CO\({}_{2}\) exchange in forest ecosystems to climate extremes and anomalies and should be considered in future projections of forest atmosphere exchange [26; 28]. Soil moisture controls water and nutrient availability to plants as well as influencing the vegetation composition, structure, and productivity of forest ecosystems, determining the ecosystem-atmosphere feedback. Depending on the severity and duration of extreme events, the reaction of NEE may vary [27]. Considering the current climate trend seen in the Valday Hills--an increase in the winter temperatures and a shift in the start of the growing season to an earlier date [75]--we expect to see an increase in the CO\({}_{2}\) uptake of memorose and paludified spruce forests in the winter to spring transition in the future. The response of an ecosystem to heatwaves occurring in the growing season can differ depending on the moisture regime at the particular site: if GPP is not limited by the available ground water or high VPD, an increase in the CO\({}_{2}\) uptake of the forest can be expected [27], but the loss of CO\({}_{2}\) at sites which usually have a high water level can cancel out the effect of the increased GPP and lead to smaller changes in NEE than would be seen at well-drained sites. A high frequency of summer heatwaves or lack of thaw water can also lead to the accumulation of water stress and negatively influence CO\({}_{2}\) uptake [28]. In the case of a higher frequency of severe and long droughts, as observed in 2010 and predicted for the region in the future [6], a decrease in GPP, as well as TER in paludified and well-drained spruce forests, is possible, as well as a decrease in CO\({}_{2}\) uptake.