# Predicted Future Changes in the Mean Seasonal Carbon Cycle Due to Climate Change
## 3 Results
### Mean Seasonal NEE Cycle under Climate Change Scenario  
Figures 3 and 4 display the 10-year average NEE seasonal cycle under climate scenarios (i.e., RCP 2.6 and RCP 6.0) for the five case studies. Overall, across all the sites and scenarios considered, there is a consistent reduction in the absolute NEE over time (i.e., NEE is 'less negative' showing a reduction in the sink capacity) with changes in the source/sink (NEE becomes negative and the site turns into a C-sink) and sink/source (NEE becomes positive and the site turns into a C-source) switch over the year(s). However, RCP 2.6 generally exhibits lower reductions in annual and mean seasonal NEE when compared to RCP 6.0 across most study sites and time intervals. The loss in the modeled sink capacity is because R\({}_{\text{eco}}\) increases more than GPP, and the differences between the scenarios are related to an increase in R\({}_{\text{eco}}\) higher than that for GPP under RCP 6.0.  
Figure 3: Ten-year average NEE seasonal cycle under the RCP 2.6 climate scenario for 5 case studies selected, i.e., (1) Colleongo—IT-Col, (2) Le Bray—FR-LBr, (3) Hyytiala—FI-Hyy, (4) Billy Kriz—CZ-Bk1, and (5) Sora—DK-Sor. The histograms (**a**) represent the annual NEE variation (%) from the first decade taken as a benchmark of simulation (1999–2009). The xy plots (**b**) show the Mean Seasonal NEE Cycle of daily values (gC m\({}^{-2}\) day\({}^{-1}\)).  
The rate of decrease in the annual NEE varies among locations and forest species. For example, the beech forests at IT-Col and DK-Sor show a reduction in the NEE (i.e., the site is less C-sink) of about 68%, and 43% (Figures 3 and 4--panel 1 and panel 5) at the end of the century, generally exhibiting a more moderate decrease compared to evergreen sites. The Scots pine forest shows, at the end of the century, a reduction in the sink capacity of 95%, standing out as the most significant decrease in NEE (Figures 3 and 4--panel 3). Over time, the rate of decline shows a tendency to speed up, hinting at a reduction in the capacity of the carbon sink because of, despite an overall increase in the GPP (Figures A2 and A3), the increased ecosystem respiration due to increased temperatures (Figures A4 and A5). By the end of the century, substantial reductions in NEE, i.e., the sites become less C-sink, across all locations and scenarios were simulated, with some locations experiencing over 70-90% reduction compared to the 1999-2009 decade (Table A4).  
The GPP, similar to NEE, generally increases across forest types and scenarios over time, albeit with varying degrees (Figures A2 and A3). Specifically, RCP 6.0 shows higher growth rates, particularly in later years (2059-2099), with increases ranging from 28 to 51% across all studied forests, compared to 10-26% for RCP 2.6 (Table A5). Analysis of long-term trends suggests saturation and subsequent slight decreases in GPP growth  
Figure 4: Ten-year average NEE seasonal cycle under the RCP 6.0 climate scenario for 5 case studies selected, i.e., (1) Colellongo—IT-Col, (2) Le Bray—FR-LBr, (3) Hyytiala—FI-Hyy, (4) Bilý Kriz—CZ-Bk1, and (5) Sore—DK-Sor. The histograms (**a**) represent the annual NEE variation (%) from the first decade taken as a benchmark of simulation (1999–2009). The xy plots (**b**) show the Mean Seasonal NEE Cycle of daily values (gC m\({}^{-2}\) day\({}^{-1}\)).  
rates under RCP 2.6, with this phenomenon being most noticeable in the forests of FI-Hyy. RCP 6.0 consistently shows a higher percentage of change in R\({}_{\rm eco}\) compared to RCP 2.6. By the end of the century (2089-2099), R\({}_{\rm eco}\) changes from 37 to 106% for RCP 2.6 and 60 to 121% for RCP 6.0 across all forests examined (Table A6). Boreal and maritime pine forests experienced higher changes compared to deciduous forests. For example, by 2089-2099, under RCP 6.0, changes reach 142% and 121% at FI-Hyy and FR-Lbr forests, while deciduous forests like European beech in IT-Col and DK-Sor experience lower changes at 67% and 60% respectively.