## 3 Results
### Nee  
Mean diurnal variation of \(NEE\) by months is shown in Figure 4. On a daily basis, a net carbon uptake (i.e., negative values of \(NEE\)) during the vegetation season usually started soon after sunrise, while the net release of CO\({}_{2}\) started about 30 min before the nightfall. The exchange of CO2 showed a clear seasonal pattern (Figures 4 and 5). During dormant season NEE was moving around zero. Some smaller uptake could be occasionally noticed at midday also during the winter months, probably due to mosses, grasses and other herbaceous vegetation. Carbon uptake started in early spring with the development of grasses and the budding of leaves on trees. Noticeable CO2 uptake started usually at the end of March. During May, carbon uptake increased strongly and reached a peak in June. However, there were also days when the forest lost carbon to the atmosphere during the vegetation season, which indicates that respiration was larger than the CO2 uptake due to unfavourable meteorological conditions. With the start of leaf senescence in early  
Figure 4: Mean diurnal variation of \(NEE\) by months during the years 2008 to 2017; blue dashed lines represent standard deviation among years; vertical dashed lines mark the time of sunrise and sunset on the 15 day in a month. Figure 3: Top panel: Average daily air (_Ta_) and soil (_Ts_) temperatures; bottom panel: daily precipitation (P, left y-axis) and soil water content (_SWC_, right y-axis) in the top 30 cm soil layer. Horizontal lines indicate the values of _SWC_ at wilting point (dotted line), at field capacity (short-dashed line) and at saturation point (long-dashed line) for the location of the EC site [37]. autumn carbon uptake rapidly declined. During winter, when vegetation was in a state of dormancy, the absolute values of \(NEE\) reached their minimal values and ecosystem was a net source of carbon, which indicates active respiration even during the cold winter days. Growing season lasted mainly until the middle of October. The average (\(\pm\)SD) length of the growing season (\(GSL\)) was 195 (\(\pm\)13) days, where \(GSL\) was defined as the number of days between the first and the last day of the period when integrated three-day \(NEE\) was negative [40] (Table 1). Ten years of EC measurements showed the high inter-annual variability of net ecosystem CO\({}_{2}\) exchange (Table 1, Figure 5). Annual sums of \(NEE\) (\(\pm\)95% confidence interval) range from \(-147\pm 13\) gC m\({}^{-2}\) year\({}^{-1}\) in 2017 to \(-496\pm 15\) gC m\({}^{-2}\) year\({}^{-1}\) in the year 2009, with an average of \(-319\) gC m\({}^{-2}\) year\({}^{-1}\). Linear regression of annual \(NEE\) with years reveals a small positive, even though it was not significant (\(p\) = 0.106), trend in \(NEE\) (16.8 \(\pm\) 18.3 gC m\({}^{-2}\) year\({}^{-1}\)). Net carbon balance of the vegetation is controlled, to a great extent, by the environmental variables. However, annual means of the environmental variables are rather poor predictors of annual \(NEE\) at Jastrebsarko forest. The matrix of cross correlation coefficients between main annual fluxes and the mean annual values of the environmental variables is given in Table 10. Figure 6 provides an overview of the relationships (or lack thereof) between the \(NEE\) and the annual means of the main meteorological variables as well as soil water content. Correlations between \(NEE\) and average annual global radiation (\(\overline{R_{g}}\)), air temperature (\(\overline{T_{a}}\)), and precipitation (\(\overline{P}\)) are not statistically significant (Figure 6A-C). On the other hand, \(NEE\) showed strong negative correlation (r = \(-\)0.71, \(p\) = 0.02) with the difference (i.e., the decline) in soil water content from May to September (\(\Delta SWC_{May-Sep}\), Figure 6D). Here, we introduced a \(\Delta SWC_{May-Sep}\) as a proxy variable that describes the dynamic of soil moisture. In the case of excess, or of a shortage of water in the soil during the growing season, the \(\Delta SWC_{May-Sep}\) will be smaller.