Restoration of secondary forest in the Greater Mekong Subregion struggles to offset primary forest carbon losses

The Greater Mekong Subregion (GMS) is experiencing significant changes in forest area, prompting an urgent investigation into whether the alterations in carbon stock from forest loss and restoration can meet the region's need for increased carbon sequestration. Therefore, utilizing remote sensing data such as Landsat and machine learning methods, we established distribution maps of primary and secondary forests and forest carbon density maps for the GMS from 2000 to 2020. By analyzing the gradient effect of forest carbon density across four altitude zones, we investigated the altitude asymmetry of the compensatory effect of secondary forests in the GMS, and predicted the carbon potential of the regional forests. The results indicate that, influenced by human activities, forests in the GMS have transitioned from the loss of primary forests in the 2000s to the recovery of secondary forests in the 2010s. While the rates of area change for loss (−2.22 × 10⁵ ha yr⁻¹) and recovery (1.97 × 10⁵ ha yr⁻¹) were similar, an altitude asymmetry caused a regional forest carbon imbalance. The low and mid-altitude regions, with higher carbon density and significant forest loss, can only compensate for 31.50 % of the carbon loss in low-altitude (122.28 TgC) and 47.57 % in mid-altitude (76.25 TgC) through secondary forest recovery. In contrast, the high-altitude region, with lower carbon loss (12.93 TgC) and larger recovery area, results in a forest net carbon sink of 10.66 TgC. Over the next decade, if primary forest loss continues at the current pace, existing secondary forest growth will absorb only 50.41 % of carbon emissions. Therefore, collaboration among GMS countries is essential to protect primary forests and promote secondary forest planting in low to mid-altitude areas for sustainable regional forest carbon development.