# Predicted Future Changes in the Mean Seasonal Carbon Cycle Due to Climate Change
## 2 Materials and Methods  
### Model Description (3D-CMCC-FEM 'v.5.6')  
The 'Three Dimensional-Courle Model Carbon Cycle-Forest Ecosystem Module' (hereafter '3D-CMCC-FEM') is a biogeochemical, biophysical, process-based forest ecosystem model (see [10; 11; 36; 37; 38; 39; 40; 41; 42; 43; 44] and reference therein). The model is designed to simulate carbon, nitrogen, and water cycles in forest ecosystems at commonly 1-hectare spatial resolution and the main eco-physiological processes (e.g., photosynthesis) at daily temporal resolution. The most recent code versions since [PERSON] et al. [17] adopt the biogeochemical photosynthesis model of [PERSON], [PERSON], and [PERSON] [45] to compute gross primary productivity (GPP). The biogeochemical photosynthesis model is parameterized as in [PERSON] et al. [46; 47] and temperature acclimation for leaves as in [PERSON] and [PERSON] [48]. The 3D-CMCC-FEM considers, as in [PERSON] and [PERSON] [49], light interception, reflection, transmission, and assimilation (and leaf respiration) for both sun and shaded leaves. Autotrophic respiration (R\({}_{\rm A}\)) is computed mechanistically following the 'Growth and Maintenance Respiration Paradigm' (GMRP; [50]), which is divided into the metabolic costs for synthesizing new tissues (growth respiration, R\({}_{\rm G}\)) and the metabolic costs for maintaining the existing ones (maintenance respiration, R\({}_{\rm M}\)). In 3-D-CMCC-FEM, the maintenance respiration is based on Nitrogen amount (a fixed fraction of carbon mass varying between the six tree compartments) and is temperature-controlled by a standard Arrhenius relationship [36]. 'Type I' and 'Type II' acclimation of respiration to temperature (i.e., short- and long-term acclimation; [10; 39; 51]) are also accounted for. Any imbalance between carbon assimilation and carbon losses because of plants' respiration is buffered by a seventh pool, the Non-Structural Carbon pool (NSC; starch and sugars undistinguished), which has priority in the carbon allocation all over the year. The net primary production (NPP) is the GPP minus R\({}_{\rm A}\). Biomass production (BP) is the amount of NPP not used for replenishing the NSC pool. Indeed, other forms of non-structural carbon losses (e.g., biogenic volatile organic compounds, BVOCs, or root exudates to mycorrhizas) are not accounted for by the model. The phenological scheme, as well as the carbon partitioning/allocation scheme, distinguished for deciduous and evergreen tree species, is in-depth described in [PERSON] et al. [10; 36; 39; 52]. Heterotrophic respiration follows a BIOME-BGC-like approach (which follows the CENTURY model; [53; 54]) distinguishing decomposition for litter and soil pools with each of the four different conceptual pools characterized by different decomposability degrees (i.e., fast, medium, slow, and a recalcitrant carbon pool) [39; 55]. Altogether, litter and soil decomposition emissions form heterotrophic respiration (R\({}_{\rm H}\)), which, summed up to R\({}_{\rm A}\), constitutes ecosystem respiration (R\({}_{\rm eco}\)). Net ecosystem exchange (NEE) is equal to R\({}_{\rm eco}\) minus GPP. Therefore, negative values indicate carbon uptake from the atmosphere (i.e., the system acts as a C-sink, NEE \(<\) 0), and positive values indicate carbon release (i.e., the system acts as a C-source, NEE \(>\) 0). The 3D-CMCC-FEM's sensitivity to its 54 species-specific parameters and how it varies along the forest development and under climate change is described in [PERSON] et al. [17].  
In the present study, we used version 5.6 [41], which slightly differs from v.5.5, described in [PERSON] et al. [8] and [PERSON] et al. [11], for a new scheme (and relative parameterization) for sapwood and live wood turnover and dynamics and some additional new forest management schemes (not used here) described in [PERSON] et al. [43].