Forest Area and Biomass Density Drive Regional Carbon–Water Service Trade-offs in Subtropical Forests of China

Forests provide critical ecosystem services (ESs), and there is growing emphasis on enhancing multiple services simultaneously to ensure long-term sustainability and multifunctional benefits. However, trade-offs among different forest ESs are often overlooked, especially when carbon storage is prioritized because of its high economic value. This study quantified 4 key forest ESs, namely, carbon sequestration, flood mitigation, soil retention, and water retention, based on economic value across a representative subtropical forest region of China, the Dongting Lake watershed. We explored the relative importance of the biophysical and management drivers shaping these 4 forest ESs from 2000 to 2020 and their effects on interservice trade-offs. We found that both forest area and biomass density had a positive effect on forest ESs and their combined economic value. Biomass density exerted a stronger effect on carbon sequestration than forest area, whereas plantation area played a key role in mediating the trade-off between carbon sequestration and flood mitigation services under different environmental conditions. These results imply that maximizing the total value of forest ESs, under complex environmental conditions, requires targeted regulation for forest areas to effectively balance the trade-offs between different forest ESs. Species diversity exhibited a relatively limited influence on most ESs, with only a weak positive trade-off observed for flood mitigation and a weak negative trade-off with soil retention. Overall, our findings highlight the central roles of forest area and biomass density in shaping regional carbon–water service trade-offs and provide guidance for balancing climate mitigation, hydrological regulation, and ecosystem sustainability in subtropical forest management.