Carbon sequestration following afforestation of agricultural soils: comparing oak/beech forest to short-rotation poplar coppice combining a process and a carbon accounting model

To compare the benefits for carbon (C) sequestration of afforestation with a multifunctional oak-beech forest vs. a poplar short-rotation coppice (SRC), model simulations were run through a serial linkage of a mechanistic model and an accounting model. The process model SECRETS (Stand to Ecosystem CaRbon and EvapoTranspiration Simulator) was used to predict growth, C allocation and soil C. The output from SECRETS was used as an input for the C accounting model GORCAM (Graz Oak Ridge Carbon Accounting Model) yielding data on C sequestration in wood products, substitution of wood fuel for fossil fuel and total CO2 emission reduction. Such C accounting based on a process model enables a more realistic calculation of forest growth, litter decomposition and soil processes. Moreover, it allows simulating the influence of climate change on the C budget. Net primary production of an oak-beech forest is low, a stable 2.5 t C ha-^1 yr-^1 after 150 years, compared to 6.2 t C ha-^1 yr1 for a SRC plantation. But while the yield from the SRC poplar is used as fuel and thus returns quickly to the atmosphere, the yield from the oak-beech forest is used in long-lasting wood products. The total C pool in the mixed forest (living biomass, wood products and soil) after 150 years amounts to 324 t C ha-^1 compared to 162 in the poplar coppice. However, when account is taken of the energy substitution, coppice culture reduces emissions with 24.3-29.3 t CO2 ha-^1 yr-^1 while the mixed forest reduces only 6.2-7.1 t CO2 ha-^1 yr-^1. These results demonstrate the added value of combining detailed process models with C-accounting models to improve the predictive capacity of model simulations