Old-growth mixed beech-dominated forests continue accumulating carbon with advancing age

Old-growth forests uniquely support biodiversity while serving as some of the planet's most important carbon stocks. The influence of tree and stand age on carbon flux dynamics remains debated—an urgent question as climate-driven disturbances may reshape forest age structures and in situ carbon storage. To clarify these relationships in Fagus sylvatica, systems, we examined a unique dataset of 3,503 tree ring series from 190 plots across some of the best preserved old-growth forests from five southern European countries. By employing a dendrochronological approach and integrating key environmental variables, including elevation, slope, temperature, and the presence of large-diameter trees (≥60 cm), we analyzed the complex relationships between tree/stand age within a plot (represented by plot-level mean values, hereafter “stand age”) and aboveground carbon stock across live, standing, and lying deadwood pools. The average stand age was 220 years, with 230 tC·ha^-1 of carbon stored in aboveground biomass and necromass. We found a positive correlation between age and carbon storage at both the individual tree and plot levels. Notably, the presence of large-diameter trees was the strongest indicator of carbon stock, with carbon accumulation peaking at about 30% large-tree stems proportion before stabilising, while younger beech trees (below 100 years old) had a smaller contribution to carbon storage. We found no evidence of a decline in carbon stock with advancing stand age across the studied sites. Despite the ecological importance of old-growth forests, many of them remain unprotected and are disappearing across Europe. Our findings highlight the importance of preserving old-growth forests to maximize their role as long-term ecosystem carbon reservoirs.