# Predicted Future Changes in the Mean Seasonal Carbon Cycle Due to Climate Change
## 4 Discussion
### Mean Seasonal NEE Cycle under Climate Change Scenario  
The sensitivity of forest ecosystems to changes in environmental factors such as climate change, seasonal variations, and atmospheric CO\({}_{2}\) levels has been thoroughly evidenced in the literature. Indeed, the interaction among these variables plays a crucial role in shaping the carbon exchange dynamics [6; 12; 17; 31; 76]. The analysis of the mean NEE seasonal cycle under different climate change scenarios presents several fascinating findings. The first key observation is the consistent reduction in forest stand capabilities to act as carbon sinks from the atmosphere in the coming years, across all study sites and climate change scenarios. Despite the overall lack of understanding regarding how forest ecosystems will respond to climate change, several new studies concur regarding the diminishing forest carbon uptake capabilities, thus confirming our statement [26; 31; 77; 78; 79]. In broader terms, climate change affects forest carbon balance by influencing key processes, which can respond differently due to their sensitivity to various environmental drivers [80]. Indeed, the reduction in NEE, within the forests observed in this study, has a disparity in the intensity of climate impacts across these scenarios: RCP 2.6 exhibits less intense NEE reduction compared to RCP 6.0 scenarios. The warming accelerates both autotrophic and heterotrophic respiration meaning that increased temperature may lead forests to an increase in R\({}_{\text{eco}}\)[27; 58; 81; 82]. On the other hand, the increasing atmospheric CO\({}_{2}\) concentration intensifies the GPP through the 'carbon fertilization effect' (i.e., reported to be the cause of 44% of the GPP increase since the 2000s) [18; 19]. Under the RCP 2.6 scenario, R\({}_{\text{eco}}\) exhibits a steady linear increase until the century's end, while GPP follows a bell-shaped curve, reaching saturation around the mid-century. Consequently, as GPP saturates, its compensatory capacity reduces, while R\({}_{\text{eco}}\) continues to rise due to further temperature increases in the latter half of the century resulting in a further decline in NEE. Conversely, for the RCP 6.0 scenario, GPP saturation occurs only towards the end of the century, and while one might anticipate a reversal in NEE trends considering this factor, it does not materialize. This is because, under the RCP 6.0 scenario, a higher temperature increase is predicted compared to RCP 2.6 (i.e., [58]), leading to a more pronounced rise in ecosystem respiration relative to photosynthesis. As a consequence, there is a greater decrease in the stand forest's carbon sink capacity. The result refers to all forests studied in this work, but in the forest of FI-Hyy, the phenomena look more pronounced (Figures A2 and A3 in Appendix A). The fate of ecosystem carbon flux depends not only on atmospheric and climate conditions but also on the type of forests analyzed. The reduction in C-sink capabilities is particularly notable in evergreen forests, which exhibit a higher decrease in NEE compared to evergreen sites. The boreal Scots pine forest of FI-Hyy stands out with the most significant reduction in NEE, indicating a heightened vulnerability to climate change effects. This finding is supported by the study of [PERSON] and [PERSON] [26] who found that, over a 17-year period, the forest ecosystem in a boreal forest stand in northern Sweden transitioned from being a carbon sink to a carbon source. This could mean that past efforts to validate the neutrality hypothesis [83] with climate change impact show limitations, and we need new research directions and new perspectives to better capture changes in the carbon fluxes between the ecosystem and atmosphere [84]. Indeed, a long-standing debate around the C-neutrality of old-growth forests (and some of the sites become old-growth at the end of simulations) raises concerns, and debates will increase about the reduction of the sink capacity of aging forests, as assumed by [PERSON]'s theory.