Turning points of subtropical forest carbon sinks in southern China: spatiotemporal dynamics and management insights

Abstract

Forest carbon sequestration faces the risk of critical transitions under ongoing climate change; however, identifying turning points remains challenging because existing early-warning signal approaches are largely limited to retrospective historical analysis. This study examines whether and when the carbon sink function of subtropical forests in China’s Guangdong-Hong Kong-Macao Greater Bay Area may approach turning point, defined as the statistical threshold at which aboveground biomass (AGB) accumulation rates shift from positive to negative growth. Using the process-based LANDIS-II landscape model, we simulated forest dynamics from 2020 to 2100 under four Shared Socioeconomic Pathways (SSP) climate scenarios and a current climate baseline. Our results show that: (i) forest types exhibit divergent responses: evergreen broad-leaved forests maintain positive AGB growth throughout the century, whereas evergreen needle-leaved forests are projected to experience carbon sink turning points after 2070 across all scenarios, with the earliest onset under SSP3-7.0; (ii) based on a piecewise slope detection approach, turning point zones remain spatially limited (< 0.5% of total forest area), and are concentrated in ecologically fragile zones of Zhaoqing characterized by overlapping forest fragmentation and human disturbances; (iii) notably, the moderate-emission SSP2-4.5 scenario yields the highest regional AGB accumulation yet exhibits the most rapid post-peak biomass decline. Our findings suggest that carbon sink turning points mark the breakdown of stable carbon budget equilibria and serve as early warning proxies for broader shifts in ecosystem functional status. Derived from multi‑scenario, spatially explicit process modelling, these outputs provide policy‑relevant insights for managing coupled carbon‑climate‑human systems in urban-affected subtropical forests. We recommend prioritizing adaptive forest management for needle-leaved forests, especially within identified turning point zones, to improve ecosystem resilience and sustain regional carbon sequestration capacity before critical growth reversal thresholds are reached.