Net primary productivity and carbon sequestration rate in young and mature afforested mangrove stands of coastal Bangladesh

Abstract

Mangroves and coastal afforestation are important biodiversity conservation, shoreline stabilization, and carbon sequestration, essential components of climate change mitigation. However, limited studies have estimated the net primary productivity (NPP) of mangrove plantations across age classes, especially in Bangladesh, where afforestation plays a crucial role in climate resilience. This study evaluates the NPP and carbon sequestration rate for young (20 years) and mature (40 + years) mangrove stands along the Bishkhali River in Barguna district, central coastal Bangladesh. Above- and below-ground biomass increments were estimated using periodic tree measurements, allometric equations, and the root biomass from soil pit method. At the same time, litter production was measured using traps over three years. NPP was calculated by summing annual above- and below-ground biomass increments and litter production, then converted to carbon sequestration rate. Young stand exhibited significantly higher basal area increment (3.06 m2/ha/yr) and above-ground biomass accumulation (17.58 ± 0.72 t/ha/yr) than mature stand. Peak litter production occurred from September to November, ranging from 1.78 to 1.88 t/ha in the young stand and 2.27 to 2.32 t/ha for the mature stand. However, total annual litter production (12.27 ± 0.92 t/ha/yr in young stand vs. 15.94 ± 1.2 t/ha/yr in mature stand) showed no significant (p > 0.05) difference. NPP and carbon sequestration rates ranged from 39.67 ± 0.49 to 37.64 ± 0.57 t/ha/year and from 18.64 ± 0.23 to 17.69 ± 0.27 t/ha/year, respectively, showing significant (p(T ≥ = t) two-tail) differences between young and mature stands. These findings address important knowledge gaps in age-specific productivity and provide insights for optimizing mangrove-based climate adaptation strategies.