## 3 Results
### CO\({}_{2}\) Fluxes  
Eddy covariance flux measurements conducted in the 2015-2020 period showed that the memorose spruce forest was a CO\({}_{2}\) source or sink, depending on the year of the  
Figure 2: Environmental variables in the 2015–2016 period: (**a**) mean daily air temperature (T\({}_{\rm a}\)) at meteorological station “Toropets” and 10-day precipitation sums (Pr) at meteorological station “Lesnoy Zapovednik”; (**b**) anomalies of the monthly mean air temperature (\(\Delta\)Ta) at meteorological station “Toropets” and anomalies of the monthly sums of precipitation (\(\Delta\)Pr) at meteorological station “Lesnoy Zapovednik” (all anomalies were calculated relative to the monthly means in the 1991–2020 period); and (**c**) global radiation (Rg), (**d**) soil water content (SWC), and (**e**) soil temperature (T\({}_{\rm s}\)) obtained at the study site. measurements. The annual sums of NEE were primarily positive, with the maximum occurring in 2016 (Table 3). The memorose forest was a source of atmospheric CO\({}_{2}\) in all the years of measurements, excepting 2018, when under the relatively high temperature and global radiation as well as low precipitation, the GPP reached its maximal values, and the TER was minimal. The mean daily GPP/TER was significantly lower in 2016 and higher in 2018 than in the other years (K-W test, H = 119.040, n = 1827, \(p<0.001\); [PERSON]'s post hoc test \(p<0.044\) for 2016 and \(p<0.001\) for 2018). In 2018, the mean annual GPP/TER ratio reached 1.47, while it was between 0.87 and 0.97 in the other years. The annual sums of NEE, TER, and GPP were mostly determined by the CO\({}_{2}\) fluxes seen in the growing season. Thus, the growing season sums calculated for the mean long-term growing season period (12 April-11 October) determined 85-91% of the annual TER and 93-97% of the annual GPP. Unlike the annual sums in the growing season, the memorose spruce forest was found to primarily be a sink of atmospheric CO\({}_{2}\). The growing season NEE was positive (TER > GPP) in 2016 and 2020 and negative in 2017-2019 (GPP > TER). However, considering the aggregated uncertainty of the growing season sums of the CO\({}_{2}\) fluxes, it is difficult to identify the status of the memorose spruce forest as a CO\({}_{2}\) sink or source in the growing seasons of 2017, 2019, and 2020. The strongest CO\({}_{2}\) source was detected in the growing season of 2016 (GPP/TER = 0.91), and the strongest sink was detected in the growing season of 2018 (GPP/TER = 1.63). The negative (2016) and positive (2018) anomalies in the mean daily GPP/TER in the growing seasons were significant (K-W test, H = 202.314, n = 915, \(p<0.001\); [PERSON]'s post hoc test \(p<0.003\)). Increased CO\({}_{2}\) uptake in the growing season of 2018 was determined by the decreased TER and increased GPP. In all winters (1 November-31 December) of the period, the memorose forest acted as a slight CO\({}_{2}\) source (Table 4). Under the strong positive temperature anomaly and sparse snow cover observed in the winter of 2019/2020 increases in both daily TER and GPP were detected (K-W test, H = 141.092, n = 757, \(p<0.001\) for TER and H = 116.257, n = 757, \(p<0.001\) for GPP; [PERSON]'s post hoc test \(p<0.001\)). Despite the mean GPP/TER ratio in the winter of 2019/2020 being the highest among the winters of the period and significantly higher than the mean GPP/TER values for the other years (M-U test U = 40,617, \(p=0.026\)), the winter NEE was also higher (M-U test U = 37,685, \(p<0.001\)) and reached the maximal value. The seasonal variability of the daily NEE, TER, and GPP in the study period had the following pattern: In winter, the forest was a slight source of atmospheric CO\({}_{2}\), with small NEE (0-2 gC\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\)), TER, and GPP values (Figure 3). In winter, the NEE slowly decreased between November and March due to the decrease in TER and slight growth of the GPP from February.