Forest Modelling Lab.

The ‘Three Dimensional - Coupled Model Carbon Cycle’ (3D-CMCC) with its ‘Forest Ecosystem Module’ (FEM), hereafter ‘3D-CMCC-FEM’, is an eco-physiological, biogeochemical, biophysical process-based model that simulates the dynamics of carbon (C), nitrogen (N), energy, and water (H2O) fluxes and the C- and N-allocation occurring both in homogeneous as in the heterogeneous forests with different plant species, for different ages, tree diameters and height classes. Indeed, the model is specifically designed to represent forest stands, from simple ones to those with complex structures, involving several cohorts competing for light and other resources in a prognostic way (i.e. it is not constrained by diagnostic observations over time but rather builds a given system from a series of first principles and initial data representing the initial conditions of a system). The 3D-CMCC-FEM is a hybrid (LUE version) or fully bio-geochemical (BGC version) forest module within the 3D-CMCC-platform that simulates storage and fluxes of Carbon, Water, Nitrogen, and Energy, including the population dynamics occurring in forest ecosystems. The model is able to reproduce dynamics occurring in homogeneous and heterogeneous forests with different plant species, for different ages, diameters and height classes competing with each other for resources. 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 past management practices - as e.g. thinning and harvest - and to predict in the future their effects on forest growth and carbon sequestration under climate change scenarios.

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.

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he 3D-CMCC-FEM has been primarily implemented by Alessio Collalti formerly at the Tuscia University (UNITUS), Department for Innovation in Biological, Agro-food and Forest systems (UNITUS-DIBAF)(Collalti 2011); and subsequently at the at both the Foundation Euro-Mediterranean Centre on Climate Change (CMCC) Impacts on Agriculture, Forests and Ecosystem Services Division (IAFENT-CMCC) and at the Forest Modelling Laboratory at the National Research Council (CNR), Institute for Agricultural and Forestry Systems in the Mediterranean (CNR-ISAFOM). The last remains the only developer and owner of the current model versions since 2018. During the years, several improved versions of the model have been developed. In the early version of 2011 and 2014 (Collalti et al. 2011, 2014) only carbon cycle has been simulated, at monthly time step, reproducing fluxes and allocation in three main compartments (stem, leaf and root) subsequently increased to the six described above, while plant respiration was considered as a fixed fraction of GPP (Collalti and Prentice 2019). The 5.1 version of 2016 (Collalti et al. 2016, Marconi et al. 2017) has been characterized by the addition of nitrogen dynamics, non-structural carbon (NSC, D'Andrea et al. 2019, 2020, 2021; Merganičová et al. 2019) and simulation of the processes at daily time scale and including the explicit simulation of autotrophic respiration. In the v.5.3.3-ISIMIP version of 2018 (Collalti et al. 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 v.5.4 (and subsequent) version of the model, acclimation of leaf photosynthesis to increasing temperature is accounted following Kattge & Knorr (2007). In both versions, LUE and BGC (which can be selected in the settings file), acclimation of autotrophic respiration is based on Smith & Dukes (2013). In the current versions (v.5.4 and subsequent; Collalti et al. 2019, 2020; Engel et al. 2021), to simulate plant photosynthesis, the Light Use Efficiency (LUE) approach can be changed with the Farquhar, von Caemmerer and Berry (FvCB, Farquhar et al., 1980) approach as implemented in the DePury and Farquhar (1997) method (however, both versions can still be used depending on the choice of the user). Recently, more advanced (and small bugs fixed) versions (v.5.5.ISIMIP and v.5.6) have been developed including simulations of coupled effects of climate change and forest management scenarios (Dalmonech et al. 2022, Testolin et al. 2023) and included in a broad model intercomparison (Mahnken et al., 2022) at European level.

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