## 4 Discussion
### Implications of the Heatwaves for the CO\({}_{2}\) Exchange of the Spruce Forests  
The most substantial increase in CO\({}_{2}\) uptake at the study site in the 2015-2020 period was detected in 2018 when a significant part of Europe experienced a heatwave. Several modeling experiments and in situ observations showed a decrease in the CO\({}_{2}\) uptake in summer 2018 for many ecosystem types, including forests, peatlands, grasslands, and agricultural lands, which were primarily associated with decreased GPP limited by high VPD and soil moisture stress values [25; 30; 32; 67]. For example, the comparison of the flux data obtained in four ecosystems in Estonia--mixed spruce forest, deciduous forest, coniferous (spruce, pine) forest, and clear cut in 2017 and 2018 [68]--showed that the heatwave of 2018 decreased the net CO\({}_{2}\) uptake of the ecosystems. This was caused by the decrease in GPP due to water stress, which canceled out the effect of the decrease in TER and also increased the annual CO\({}_{2}\) released by the ecosystems. The difference in the ecosystems' response to the heatwave is possibly related to the soil water availability and VPD dynamics at the sites. SWC in a mixed forest in Estonia in the summer of 2018 dropped to the values of less than 0.10 m\({}^{3}\). m\({}^{-3}\)[68], while at the study site in the Valdai Hills, it was higher than 0.35 m\({}^{3}\). m\({}^{-3}\). The VPD at the study site in the summer of 2018 was lower than reported by [PERSON] et al. [68]. On some days, the midday VPD of the memorose spruce forest in the Valdai Hills (this study) exceeded 20 hPa, but the mean summer value at the site was 5.3 \(\pm\) 5.1 hPa (\(\pm\)SD). A positive GPP anomaly in summer 2018 detected at the study site corresponded to the results of the modeling study conducted by [PERSON] et al. [25], who showed a slight positive GPP anomaly for the region during the heatwave of 2018. The important feature of the CO\({}_{2}\) flux dynamics in many boreal forests is that the maximum CO\({}_{2}\) uptake usually is observed in spring due to the fast increase in GPP at the beginning of the growing season, while the maximal TER lags behind, occurring in mid-summer. Thus, a high GPP rate in the growing season of 2018 at the memorose spruce forest was provided by the significant Rg due to the high frequency of clear sky days and the air temperatures being favorable for photosynthesis. Yet, it is possible that the thawing water supply provided enough moisture to the soil in spring when the maximum CO\({}_{2}\) uptake was observed in spite of the negative precipitation anomalies. Otherwise, the decrease in the SWC of the upper soil layer provided a decreased TER in summer, which correspondedto previous in situ estimates of TER and its components at the paludified spruce forest located 1.5 km from the study site during the 2002-2012 period and covering a drought that occurred in 2010 in West Russia [69]. However, unlike in 2010, the summer precipitation that occurred in 2018 was enough to prevent the deep soil layers from experiencing drought. Additionally, it is possible that the winter precipitation and thawing water supply are important factors that can affect the annual and growing season NEE of the memorose spruce forest as well as other boreal forest ecosystems [18; 70; 71] and potentially mitigate the effect of the decreased precipitation occurring in spring and summer. Unlike the relatively warm, sunny, and dry growing season of 2018, the anomalously warm winter with a thin snow cover of 2019/2020 led to the substantial positive GPP anomalies seen in late winter and early spring but did not alter the annual sums of NEE too significantly. As the winter and early spring GPP of forests located at high latitudes are limited by low temperatures and radiation, the temperature anomaly may have led to the early beginning of the CO\({}_{2}\) uptake [21; 72]. The modeling study conducted by [PERSON] et al.