## 2 Materials and Methods
### Model Description  
The 3D-CMCC FEM is a stand-scale process-based model (PBM) designed to simulate C and water cycle in natural and managed forest ecosystems (for a full description see [36; 37]). Several eco-physiological processes were modeled at species-specific level, and at a variable temporal scale (from daily to annual) depending on the process to simulate (Figure 2). Model outputs were generally represented at hectare scale, while processes were simulated at different spatial scales from cellular (e.g., stomatal conductance), to canopy (e.g., transpiration), to individual tree, up to stand level.  
Carbon assimilation was modeled for sun and shaded leaves using the LUE approach [43]: potential C assimilation was constrained by environmental and stand structural (e.g., tree age) scalars [44]. Autotrophic respiration (R\({}_{\text{A}}\)) was explicitly modeled as the sum of growth and maintenance respiration (R\({}_{\text{G}}\) and R\({}_{\text{M}}\), respectively). The first was computed as a fixed ratio of new growth tissues (30%) and the latter was based on nitrogen content in stems, branches, leaves, fine and coarse roots, non-structural carbon (NSC), and fruit tree pools. carbon allocation among these pools was controlled by species-specific parameters, phenology, light and water availability. Water cycle was modeled calculating the daily balance between precipitation, canopy transpiration, evaporation, soil evaporation, and runoff. Meteorological variables used to force the model were: global solar radiation (MJ m\({}^{-2}\) day\({}^{-1}\)), maximum and minimum air temperature (\({}^{\circ}\)C), relative humidity (%), and daily cumulated precipitation (mm day\({}^{-1}\)). To be initialized, the model required knowing stand structural characteristics such as: species composition, stand density, diameter at breast height (DBH), tree height and age. Soil initialization required the estimation of total organic carbon (TOC) in the different SOC pools, as described in the following section.  
Figure 1: Location of the 6 Fluxnet sites used to evaluate 3D-CMCC-PSM.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Site Name** & \begin{tabular}{c} **Coords** \\ **(Lat”/Lon”)** \\ \end{tabular} & **Climate** & **Species Composition** & \begin{tabular}{c} **Mean Annual** \\ **T (\({}^{\circ}\)C)** \\ \end{tabular} & \begin{tabular}{c} **Mean Annual Prec.** \\ **(mm year\({}^{-1}\))** \\ \end{tabular} & \begin{tabular}{c} **Mean** \\ **DBH (cm)** \\ \end{tabular} & \begin{tabular}{c} **Tree** \\ **Height (m)** \\ \end{tabular} & \
\begin{tabular}{c} **Stand Age** \\ **(years)** \\ \end{tabular} &
\begin{tabular}{c} **Stand Density** \\ **(trees ha\({}^{-1}\))** \\ \end{tabular} \\ \hline \hline Colleongo (ITCo) & 41.8/13.5 & Temperate & _Fagus sylatica_ L. (DBF) & 6.3 & 1180 & 20.27 & 19.84 & 100 & 900 \\ \hline Hainich (DEHai) & 51.0/10.4 & Temperate & _Fagus sylatica_ L. (DBF) & 8.3 & 720 & 30.8 & 23.1 & 120 & 334 \\ \hline Hy Hytsilla (FIHy) & 61.8/24.2 & Boreal & _Fagus sylostra_ L. (EBF) & 3.8 & 709 & 10.3 & 6.5 & 39 & 1796 \\ \hline Renon (ITRen) & 46.5/11.4 & Temperate & _Fican also_ L. (EBF) & 4.7 & 809 & 16.98 & 11.32 & 50 & 767 \\ \hline Castelporziano (ITCrp2) & 41.2/12.3 & Mediterraneanen & _Quercus lac_ L. (EBF) & 15.6 & 780 & 16 & 12.5 & 45 & 488 \\ \hline Purchabon (FRPro) & 43.7/3.6 & Mediterranean & _Quercus lac_ L. (EBF) & 13.5 & 883 & 7 & 6 & 59 & 6149 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Site descriptions and stand initialization data. Mean temperature (T) and Precipitation (prec.) are annual averages collected at site.