The response of the carbon cycle in undisturbed forest ecosystems to climate change: A review of plant-soil models

We compared six plant-soil models from the literature which describe the C-dynamics in forests and include climatic forcing explicitly. Our selection included the two physiological models FOREST-BGC and TCX, the ecosystem/population model FORCLIM, two ecosystem/tissue models, MBL-GEM and CENTURY Forest and the global model TEM. The review revealed a multitude of differences with respect to the model structure, the incorporation of particular processes and the coupling with the abiotic environment; We made an assessment of what kind of questions the models can be best applied to and how well they are suited for studying the response of the C cycle under climate change. In this context organic C in litter and humus play a key role. The number of compartments and the pathways of C flows influence both the transient phase and equilibrium of the system, a fact that has been recognized before, but has not been investigated systematically for any of the models. Hence, the multitude of aggregation levels used to represent detritus and the variety of decomposition formulations used in the models may result in inconsistencies of the simulation results. Similarly, the control of ecoprocesses via abiotic factors differs among the models. They use distinct abiotic quantities and different mathematical parametrizations, thereby affecting the system's response in a changing environment substantially, even if this is not the case for present conditions. Given the differences in experimental frames of published simulations it was not possible to trace back behaviour deviations to particular model formulations. In order to make consistent projections of the C cycle's response in forests in a changing climate there remains an urgent need to analyse the models from a structural point of view based on quantitative model comparisons under well defined conditions.