## 4 Discussion  
### Variability of CO\({}_{2}\) Fluxes from EC  
The observed seasonal variability of carbon fluxes at Jastrebarsko forest shows a pattern typical for the temperate broadleaved forest in the northern hemisphere [5; 40]. Annual sums of carbon fluxes are within the range of values for temperate deciduous broadleaved forests published in a recent review by [PERSON] et al. [7] (Table 3). If we make a comparison at a site-to-site level, we notice differences in fluxes. The site Denmark, Sore [72] has smaller _NEE_ but larger _GPP_ and _RECO_, while the site France, Hesse [6] has similar _NEE_ but smaller _GPP_ and _RECO_. Both sites are beech forests with distinction that Sore was \(\sim\)80 years old, while France, Hesse is \(\sim\)40 years old, similar to Jastrebarsko. A higher _NEE_ at Hainich site [73], with respect to Jastrebarsko site, might be related to the difference in forest structure. Hainich forest is characterised as an old-growth forest (up to 250 years) with a highly diverse horizontal and vertical structure [73]. Also, while average _GPP_s are similar, higher _NEE_ at Hainich might be due to lower _RECO_ as a result of significantly lower average air temperature at Hainich site (8 \({}^{\circ}\)C) as compared to Jastrebarsko (11.2 \({}^{\circ}\)C). There are two sites (the UK, Straits Inclosure [40] and US, Duke Forest [74]) where all fluxes are noticeably higher than the ones that were observed at Jastrebarsko site. The UK, Straits Inclosure site [40] is an 80-years old oak plantation in a climate that typically does not exhibit summer drought, which might contribute to the higher annual carbon accumulation than our natural forest in a climate with hot and, comparatively, dry summers. Nevertheless, relatively large fluxes observed at UK, Straits Inclosure site are still interesting and more thorough comparison would be needed. On the other hand, fluxes observed at the US, Duke Forest might not be directly comparable due to some differences in methodology, namely gap-filling and partitioning [74; 75]. Finally, all fluxes at US, Harvard Forests [26] are somewhat smaller but not far from fluxes at Jastrebarsko forest. The aim of this simple comparison with other sites was not to explain the reported differences in fluxes, but to emphasize the multitude of possible issues and the very limited number of comparable sites with long time-series, which might help in explaining the observed variability. Carbon fluxes are strongly driven by environmental variables [76; 77]. Their inter-annual variability is found to be influenced by meteorological conditions, e.g., light [21; 22; 25; 40; 78; 79], temperature [78; 79; 80], length of growing season [77; 79], soil water status [6], this study, as well as with stand characteristics, e.g., successional change in forest composition [26]. However, our results indicate that, at the Jastrebarsko site, the average annual values are poor predictors of annual carbon fluxes (Figure 6A-C). In the present study, we found a significant linear relationship only between _NEE_ and \(\Delta SWC_{May-Sep}\) (Figure 6D). The low _NEE_ values that were observed in 2012 and 2017 were clearly the result of prolonged dry conditions from previous 2011 and 2016, represented by low \(\Delta SWC_{May-Sep}\) values (Figure 6D). On the contrary, the low _NEE_ that was observed in 2014 was probably due to cloudy weather with low mean annual _Rg_ (Figure 6A). In 2014, the low \(\Delta SWC_{May-Sep}\) did not imply dry, but extremely wet conditions. In fact, _SWC_ did not change much from May to September due to excessive rain, which caused _SWC_ to be high throughout the year (Figure 3 bottom, and Figure 6C).