Divergent long‐term trends in vegetation carbon turnover between mature and young forests

The terrestrial carbon sink depends not only on how much biomass forests accumulate, but also on how long that carbon remains stored in vegetation. This is known as vegetation carbon turnover time ( τ veg ), which varies with forest age, structure and climate. Furthermore, τ veg is widely expected to decline under climate change as warming and disturbance accelerate biomass loss. However, most evidence comes from mature forests, even as global forest demographics shift due to reforestation, disturbance and land‐use change. Younger forests differ in growth rates, mortality patterns and stand structure, which may create distinct turnover trajectories with rapid biomass accumulation potentially slowing τ veg . Here, we evaluated this possibility using 17 years of in situ data from a network of temperate, subtropical and tropical forest plots across China. We found that τ veg declined over time in mature forests but increased in younger ones. These opposing trends were shaped by age‐specific trade‐offs between growth and mortality, mediated by stand structure and environmental conditions. Synthesis . Our results demonstrate that forest age strongly influences vegetation carbon turnover and must be integrated into models to improve predictions of terrestrial carbon sink dynamics.