# Predicted Future Changes in the Mean Seasonal Carbon Cycle Due to Climate Change
## 6 Conclusions  
The MSC metric is an interesting and descriptive metric that associates phenology and carbon partitioning allocation within forest stands. Climate change impacts both the phenology, by changing the date for the beginning and the end of the growing season, and the ecosystem carbon allocation. We applied the process-based forest model 3D-CMCC-FEM to evaluate the potential modifications on the ability of different forest stands to absorb or emit carbon throughout the season and across the years up to 2100. Before that, we validated the model under current climate conditions and found a robust predictive ability of 3D-CMCC-FEM in estimating NEE, GPP, and R\({}_{\text{e}\infty}\) across different timeframes, forest species, and climates. The analysis of the mean NEE seasonal cycle under different climate change scenarios presents a consistent reduction in the forest stand capabilities to act as a carbon sink in the coming years, across all study sites, and climate change scenarios. The reduction in NEE ability has different intensities of climate impacts across these scenarios. The RCP 2.6 scenario demonstrates a less pronounced reduction in NEE compared to the RCP 6.0 scenario. This disparity primarily stems from variations in key variables, such as the differing rates of temperature increase between the two scenarios, as well as the CO\({}_{2}\) fertilization effect, while, in all sites, age effects depend on the age at the beginning of the simulations. The reduction in C-sink capabilities is mainly notable in evergreen forests, which exhibit a higher decrease in NEE compared to deciduous forest sites. Finally, we found that the number of days as a C-sink in evergreen forests increases over the years, indicating a consistent upward trend. Conversely, the number of days as a C-source decreases over the decades, showing a general reduction. This statement aligns with the forward shift of DoY to C-sink, and the backward shift of DoY to C-source. In contrast, deciduous forests maintain a relatively stable number of C-sink (and C-source) days throughout the century, reflecting a fixed DoY when the system becomes a C-sink (or a C-source). The DoY for deciduous forests remains constant as the earlier onset of the growing season, driven by warming temperatures, is offset by an earlier uptick in respiration. Decades pass with little change in the number of days as a C-sink (and a C-source), alongside a decrease in NEE. This indicates that deciduous forests, over the long haul, demonstrate greater efficiency in utilizing photosyntheates when compared to evergreen forests. M.M. : Data curation, Formal analysis, Investigation, Writing--original draft, Writing--review and editing; E.V. : Data curation, Writing--original draft, Writing--review and editing; A.C.: Conceptualization, Formal analysis, Investigation, Writing--original draft, Writing--review and editing. All authors have read and agreed to the published version of the manuscript. **Funding:** OptForEU Horizon Europe research and innovation programme under grant agreement No. 101060554; National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4--Call for tender No. 3138 of 16 December 2021, rectified by Decree n.3175 of 18 December 2021 of Italian Ministry of University and Research funded by the European Union--NextGenerationEU under award number: Project code CN_00000033, Concession Decree No. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, CUP B83C2002930006; Project title \"National Biodiversity Future Centre--NBFC\"; A.C. also acknowledges funding from the MIUR Project (PRIN 2020)--Research Projects of National Relevance funded by the Italian Ministry of University and Research entitled: \"Unraveling interactions between WATER and carbon cycles during drought and their impact on water resources and forest and grassland ecosySTEMS in the Mediterranean climate\" (WATERSTEM, project number: 20202 WF53Z), and \"WAFER\" at CNR (Consiglio Nazionale delle Ricerche); A.C. and E.V.