Limited future carbon sink in China as forests become mature

China has experienced large land-use and land-cover changes (LULCC) over recent decades, resulting in a complex, mostly young, forest age structure. However, the impact of forest age dynamics on China’s terrestrial ecosystem carbon sink remains unclear. Here, using a process-based ecosystem model with an explicit representation of forest age cohorts, forced by satellite- and inventory-based maps of LULCC, we estimate China’s terrestrial carbon sink as 198 ± 54 Tg C yr^-1 in the 2010s. The forest carbon sink represents 124 ± 25 Tg C yr^-1, being predominantly (71.7%) contributed by middle-aged (16~50 year-old) forests. Following the national re/afforestation target of reaching 30% forest coverage by 2060 and assuming constant wood harvest rates in the future equal to present-day levels, the forest carbon sink is projected to be 181~217 Tg C yr^-1 during 2041-2060 but to decrease to 142~212 Tg C yr^-1 during 2081-2100 under Representative Concentration Pathway (RCP) 2.6, 4.5, 6.0 and 8.5. The carbon sink in established forests that were planted or existed before 2020 is the largest contributor to the future total carbon sink, but this contribution will decrease significantly (p<0.05) each year by -1.1 ~ -0.35 Tg C yr^-1 until 2100 due to forest aging and the slowdown of CO₂ concentration growth. New re/afforestation after 2020 will enhance the carbon sink in China by increasing forest area and rejuvenating forest demography. Our study emphasizes the importance of forest age dynamics on the carbon sink and implies that realizing China’s carbon neutrality target should not rely excessively on the ecosystem carbon sink.