The 3D-CMCC-FEM Hybrid and BioGeoChemical Model
The 3D-CMCC-FEM (Three Dimensional - Coupled Model Carbon Cycle - Forest Ecosystem Model) is a hybrid or bio-geochemical forest ecosystem model (it depends on the version) that simulates storage and fluxes of Carbon, Water, and Nitrogen including the dynamics occurring in forests. The model is able to reproduce forests with a complex canopy structure constituted by cohorts competing with each other for light and water. The model simulates carbon fluxes, in terms of gross and net primary production (GPP and NPP, respectively), partitioning and allocation in the main plant compartments (stem, branch, leaf, fruit, fine and coarse root, including non-structural carbon compounds). In the recent versions, nitrogen fluxes and allocation, in the same carbon pools, are also reproduced. The 3D-CMCC-FEM also takes into account management practices, as thinning and harvest, to predict their effects on forest growth and carbon sequestration.
The 3D-CMCC-FEM is a command-line program code written in C-programming language and divided into several subroutines. To run the model, some input data are required. The meteorological forcing variables, on a daily time step, are represented by average, minimum and maximum air temperature, shortwave solar radiation, precipitation, vapour pressure deficit (or relative humidity). The model also needs some basic information about soil, such as soil depth and texture (clay, silt and sand fractions), as well as the forest stand information referred to plant species, ages, diameters, heights and stand density. An additional input is represented by species-specific eco-physiological data for the model parameterization.
The 3D-CMCC-FEM has been primarily implemented by Alessio Collalti at both the Foundation Euro-Mediterranean Centre on Climate Change (CMCC), Division Impacts on Agriculture, Forest and Ecosystem Services, Viterbo Division (CMCC-IAFES) and at the Tuscia University (UNITUS), Department for Innovation in Biological, Agro-food and Forest systems (UNITUS-DIBAF); and subsequently at the National Research Council (CNR), Institute for Agricultural and Forestry Systems in the Mediterranean (CNR-ISAFOM). During the years, several improved versions of the model have been developed. In the early version of 2014 only carbon cycle has been simulated, at monthly time step, reproducing fluxes and allocation in 3 main compartments (stem, leaf and root) subsequently increased to the 6 described above, while plant respiration was considered as a fixed fraction of GPP. The 5.1 version of 2016 has been characterised by the addition of nitrogen dynamics, non-structural carbon (NSC) and simulation of the processes at daily time scale and the explicit simulation of autotrophic respiration. In the 5.3.3-ISIMIP version of 2018, the reproduction of atmospheric CO2 effects on the simulated processes and the dynamics related to plant mortality have been added explicitly as the effect on the 5.4-BGC (and subsequent) version of the model, acclimation of leaf photosynthesis to increasing temperature is accounted following Kattge & Knorr (2007). In both versions, FEM and BGC, acclimation of autotrophic respiration is based on Smith & Dukes (2013). In the current version (v.5.4 and subsequent), to simulate plant photosynthesis, the Light Use Efficiency (LUE) approach has been substituted by the Farquhar, von Caemmerer and Berry (FvCB, Farquhar et al., 1980) approach as implemented in the DePury and Farquhar method (however, both versions can still be used depending on the choice of the user). The photosynthesis rate depends on nitrogen content in the leaves and RuBisCO, the temperature leading enzyme kinetics, the maintenance respiration and the difference between internal and external partial pressure of CO2. The version which uses LUE approach is commonly denoted as the 'FEM' version, while the one which uses FvCB is denoted as BGC version.Recently, Carlo Trotta, within the EU Life Project OLIVE4CLIMATE, has developed a model version for Olive (Olea europea) orchards, namely 3D-CMCC-OLIVE.