Increasing stand age accelerates ecosystem carbon density while climate extremes reduce soil organic carbon density in 12-44 year-old Sonneratia apetala mangroves in Bangladesh

Assessing the effectiveness of past mangrove afforestation projects is vital for guiding future initiatives and enhancing success in areas eligible for restoration. This can be assessed by analyzing ecosystem carbon (EC) density, which includes both vegetation carbon (VC) density and soil organic carbon (SOC) density. Using 72 representative plots across five stand ages (12-44 years), we evaluated EC density in Sonneratia apetala-planted mangroves within the planted mangrove reserves in Bangladesh. Results revealed significant differences in VC density, SOC density, and EC density across varying stand ages, soil depths, and mangrove locations. The increased EC density observed over 44 years mangrove reserves (10.06 t/ha/year) with advancing stand ages provides empirical evidence for the effectiveness of afforestation in enhancing EC density levels in mangroves. Notably, the higher SOC density in the upper (0-20cm) soil layer (46 t/ha in 44-year-old stands) compared to the lower (40-60cm) soil layer (21 t/ha in 32-year-old stands) across mangroves indicates that most soil carbon is concentrated in the top 20 cm of the forest floor. The inverse relationship between climate extremes and SOC concentration and SOC density across mangrove reserves suggests that regardless of variations in stand ages and tree density, warming accelerates SOC decomposition and leads to a decline in SOC density. These findings underscore the critical role of afforestation in improving carbon density in mangroves, highlighting the need for continued investment in restoration efforts to maximize their ecosystem services.