LAUSR

Abstract Land ecosystems can play a crucial role in climate-change mitigation by acting as sinks for carbon. Legacy effects of past land use, including land conversion and changes in land-use intensity, influence the capacity for future ecosystem carbon sequestration. These effects are hard to quantify, however, and the influence of changes in land-use intensity are still largely overlooked. This study assessed the long term dynamics (1850–2015) of the terrestrial carbon budget in France. We developed a new dynamic model: LUCCA (Land Use Change & Conversion Accounting) that robustly quantifies the dynamics of carbon stocks and fluxes. It allows disentanglement of land conversion from land management effects following a socio-ecological perspective. Carbon stocks in terrestrial ecosystems in France increased from 3.5 GtC in 1850 to 4.8 GtC in 2015 as a result of contrasting regional land-use trajectories. Based on five counterfactual scenarios, we unravel that changes in land-use intensity explained 46 % of the observed carbon stock changes since 1850, while land conversion was responsible for 30 %, and the rest can be attributed to changes in forest growth rates induced by both environmental and management changes. The effects of land conversion and changes in land-use intensity on carbon stock accumulation, however, would have been hardly possible in the absence of agricultural intensification and release of harvest pressure following wood-fuel substitution by fossil fuel. In fact, carbon emissions induced by fossil fuel consumption in France from 1850 to 2015 were 9 times the carbon sink in terrestrial ecosystems. These figures highlight the importance of considering long-term trajectories of ecosystem carbon fluxes and their relationship with emissions from other processes for climate action strategies.