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.

土地生态系统可以充当碳汇，从而在缓解气候变化中发挥关键作用。过去土地使用的遗留效应，包括土地转化和土地利用强度的变化，影响着未来生态系统碳固存的能力。这些影响很难量化，但是土地利用强度变化的影响仍然被大大忽略了。这项研究评估了法国陆地碳预算的长期动态（1850年至2015年）。我们开发了一个新的动态模型：LUCCA（土地使用变更和转换会计），可以可靠地量化碳储量和通量的动态。从社会生态学的角度看，它使土地转化与土地管理的影响脱节。法国陆地生态系统中的碳储量从1850年的3.5 GtC增加到4。由于区域土地使用轨迹的对比，2015年实现了8 GtC。根据五个反事实情景，我们得出结论，自1850年以来，土地利用强度的变化解释了46％的观测碳储量变化，而土地转化占了30％，其余原因可归因于由环境和管理方面的变化。然而，如果没有农业集约化和以化石燃料替代木材燃料后收获压力释放的情况，土地转化和土地利用强度变化对碳储量的影响几乎是不可能的。实际上，从1850年到2015年，法国因化石燃料消耗引起的碳排放量是陆地生态系统中碳汇的9倍。