Passive versus active afforestation of agricultural land: Two decades of impacts on tree biomass and ecosystem carbon storage

• Mature forests stored the highest total wood biomass, reflecting long-term carbon accumulation. • Plantation forests accumulated biomass 15 × faster than naturally colonized forests after ∼21 years. • Naturally colonized forests developed distinct and heterogeneous tree communities compared to managed forests. • Ecosystem carbon stocks were highest in plantations, while naturally colonized sites remained similar to agriculture. • Topsoil SOC showed no short-term response to afforestation, underscoring slow soil carbon recovery.