Steady Forest Sink of 10% of Carbon Emissions Resulting from a Tenfold Increase in Biomass Stock over 150 Years in France

European forests have been providing a persisting carbon (C) sink of a magnitude of ca. 10% of current carbon emissions over recent decades. While this sink is often viewed as natural, the dynamic conditions of its existence have remained unexplored, questioning its steadiness or transiency over the longer term. The cumulated intensity of this sink since the onset of C emissions also remains critically undocumented, in the absence of a historical monitoring of forest growing stock (GS) course since European forest transitions. Both questions were addressed for the temperate forests of France that underwent the forest transition in the mid-19th century. We developed a forest densification model, guided by a contemporary 40-year time series of forest GS established from the national forest inventory (NFI), and recurrent century-old areal statistics. Simulations identified a singular convex GS densification model over 150 years, and an initial GS density of 25 m3/ha, supporting historical forest depletion reports. GS as estimated for three historical statistics required plausible low-density (15-30%) contemporary forest analogues to match simulated trajectories and figured out a 5-fold centennial GS increase, and a minimum 10-fold since 1850. The roles of continuous forest extension, management extensification and environmental growth changes in this dramatic biomass increase were inquired. Rescaled to aerial wood carbon, this in situ accumulation represented 10.5% of the cumulated historical country C emissions, questioning the viability of the forest dynamic to be maintained, were forests to remain a prioritized option for climate change mitigation in the future.