Differences in ecosystem organic carbon stocks due to species selection and site elevation of restored mangrove forests

•A. marina should be planted at the site below local mean sea level altitude.•K. obovata should be planted at the site above local mean sea level altitude.•Planting pattern could provide guidance for future mangrove restoration projects. Mangrove forests are important blue carbon ecosystems. Site elevation and species selection are very important for successful restoration of mangrove forests. It is vital for calculating carbon sequestration of mangrove forests to know the combined influences of species selection and site elevation on the ecosystem organic carbon stock (EOCS). In this study, the combined effects of site elevation (above or below local mean sea level) and species selection (Kandelia obovata vs Avicennia marina) on the EOCS including vegetation organic carbon stock (VOCS) and soil organic carbon stock (SOCS) were quantified through an in situ planting experiment. The total vegetation biomass decreased approximately 48.79% in the K. obovata forest and 19.71% in the A. marina forest because of changes in each component (leaf, stem, branch and root) biomass with the decreases in site elevation. Due to changes in SOCS and VOCS with the decreases in site elevation, EOCS of the A. marina forest decreased approximately 12.3% and EOCS of the K. obovata forest decreased approximately 40.3%. At the site below local mean sea level, the A. marina forest had higher EOCS than the K. obovata forest. These indicated that in order to maximize carbon sequestration in restored mangrove forests, A. marina should be selected for the projects with site elevation below local mean sea level, while K. obovata should be selected for those with site elevation above local mean sea level.