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The development of appropriate tools to quantify long‐term carbon (C) budgets following forest transitions, that is, shifts from deforestation to afforestation, and to identify their drivers are key issues for forging sustainable land‐based climate‐change mitigation strategies. Here, we develop a new modeling approach, CRAFT (CaRbon Accumulation in ForesTs) based on widely available input data to study the C dynamics in French forests at the regional scale from 1850 to 2015. The model is composed of two interconnected modules which integrate biomass stocks and flows (Module 1) with litter and soil organic C (Module 2) and build upon previously established coupled climate‐vegetation models. Our model allows to develop a comprehensive understanding of forest C dynamics by systematically depicting the integrated impact of environmental changes and land use. Model outputs were compared to empirical data of C stocks in forest biomass and soils, available for recent decades from inventories, and to a long‐term simulation using a bookkeeping model. The CRAFT model reliably simulates the C dynamics during France's forest transition and reproduces C‐fluxes and stocks reported in the forest and soil inventories, in contrast to a widely used bookkeeping model which strictly only depicts C‐fluxes due to wood extraction. Model results show that like in several other industrialized countries, a sharp increase in forest biomass and SOC stocks resulted from forest area expansion and, especially after 1960, from tree growth resulting in vegetation thickening (on average 7.8 Mt C/year over the whole period). The difference between the bookkeeping model, 0.3 Mt C/year in 1850 and 21 Mt C/year in 2015, can be attributed to environmental and land management changes. The CRAFT model opens new grounds for better quantifying long‐term forest C dynamics and investigating the relative effects of land use, land management, and environmental change.

The consistent and robust assessment of ecosystem carbon stocks remains central to developing and monitoring climate change mitigation strategies. Here, we investigate the dynamics of forest ecosystem carbon stocks in the conterminous United States between 1907 and 2012 at national and regional levels. We build upon timber volume records from historical forest inventories and use a modelling approach to include all relevant pools, e.g. soil carbon, to derive a comprehensive long-term dataset. We find a consistent increase in forest carbon stocks across the country, from 27 PgC in 1907 to 39 PgC in 2012, with persistent regional variations between western and eastern United States, signalling pronounced land use and land management legacy effects. We identify additional potential to increase forest C sinks in both west and east, on diverging levels. Extended forest C stocks stem from forest biomass thickening i.e. increases in biomass C densities, rather than forest area expansion. Our study reflects the first such effort to collectively understand the effects of environmental change and land management on contemporary biomass C stocks at the national level, and critically engages with ongoing initiatives towards assessing the potential for carbon sequestration in forest ecosystems.

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