# Predicted Future Changes in the Mean Seasonal Carbon Cycle Due to Climate Change
## 3 Results
### Changes in NEE Dynamics under Different Climate Scenarios  
We considered fluctuations in the length of sink/source and source/sink forest stand behaviors throughout the year. This calculation involved determining the total number of days in a year (\"N. days year\({}^{-1}\)\") where a forest stand exhibited either C-sink (NEE \(<\) 0) or C-source behavior (NEE \(>\) 0). The 10-year average number of days as a C-sink and as a C-source under the RCP 2.6 and 6.0 for the five case studies selected is presented in Figure 5a. The number of days identified as a C-sink in the evergreen forests for the scenario RCP 2.6 (i.e., FR-LBr, CZ-Bk1, and FI-Hyy) starts relatively low in the first decade (1999-2009) but increases significantly over time, showing a consistent upward trend (Figure 5(a,a,a,a). At the CZ-Bk1 site, the number of days considered as a C-sink starts at 47 N. days year\({}^{-1}\) in the 1999-2009 decade and increases steadily over the decades, peaking at 199 N. days year\({}^{-1}\) in 2059-2069 and remaining relatively high thereafter. For FI-Hyy forests, the number of days as a C-sink starts at 107 N. days year\({}^{-1}\) in 1999-2009 and rises to 217 days in 2089-2099, with a peak of 225 in 2079-2089. Lastly, at the forests of FR-LBr, there were no days as a C-sink in 1999-2009, but they increased steadily over time, reaching 114 days by 2079-2089. The number of days as a C-source varies inversely to those as a C-sink across decades, demonstrating a general reduction over time for evergreen forests. The number of days functioning as a C-source decreased, from \(\sim\)300 in 1999-2009 to \(\sim\)200 N. days year\({}^{-1}\) in 2089-2099 (Table A1), for evergreen forests. Deciduous forests on the contrary almost maintain a relatively stable number of days as a C-sink through the century (Figure 5(a,a,a). The number of days as a C-sink at IT-Col forests ranges from 212 to 224 N. days year\({}^{-1}\), while at DK-Sor from 214 to 222 N. days year\({}^{-1}\), with slight fluctuations observed across decades and no clear overall trend. The same trend for the capacity of the stand to act as a C-source, but with slightly different days in the range from 137 to 159 N. days year\({}^{-1}\) recorded in the forest of Sore, and from 140 to 149 N. days year\({}^{-1}\) for the IT-Col site. The discrepancies between the already described RCP 2.6 and RCP 6.0 scenarios are minimal as they exhibit a similar trend for evergreen and deciduous, respectively, with only a slight change in the number of days, mainly in the last decade, with a magnitude of \(\sim\)10 N. days year\({}^{-1}\).  
To assess the changes in MSC due to climate change scenarios, we focused on the shifting patterns of the DoY over decades, particularly examining when forest stands transitioned from a C-source to a C-sink and vice versa (see Figure 5b). Regardless of the scenario considered, the European beech forests revealed constant transition periods across decades. In IT-Col, the shift from being a C-source to a C-sink occurred between DoY 136 and 150, with the opposite transition from sink to source around DoY 285. Meanwhile, in DK-Sor, the same transitions happened between DoY 115 and 127, and reversed between DoY 272 and 280 (Figure 5(b,b,b)). Even for evergreen forests, there are no noticeable differences in the shift corresponding to RCP 2.6 and RCP 6.0 scenarios. At CK-BZ1, the shift to a C-sink occurs relatively early in the year, spanning from DoY 2 to 31 across the decades. At FI-Hyy, the transition timings vary widely, ranging from DoY 30 to 50 over the decades. Similarly, at FR-LBr, the C-sink transitions occur from DoY 1 to 16 across the decades, with some decades exhibiting earlier shifts. On the other hand, transitions to C-sources at CZ-Bk1 occur from late November to early December (DoY 201-329), displaying a decreasing trend over the years. In contrast, at FI-Hyy, transitions take place from mid-September to late September (DoY 197-285), while at FR-LBr, they occur from late September to early October (DoY 255-285) (Figure 5(2,a,3,a,4.a)).