Carbon stocks in vegetated and disturbed mangrove stands

Mangroves are coastal vegetated habitats that have been recognized for their role in providing ecosystem services such as carbon storage and sequestration. In South Africa, mangroves cover a relatively small area (~2000 ha) with a fragmented distribution compared to forests along the East African coast. Long term changes in mangrove extent and population structure have been studied and with a good understanding of persistent pressures, but little is known about their carbon storage potential. This study investigated the mangrove population structure of 11 estuaries, measured sediment organic matter and organic carbon content along a depth profile for 88 sediment cores to compare vegetated mangrove stands with adjacent disturbed areas. Total mangrove area in 2021 for the 11 estuaries was 270 ha, with a net increase in cover of 4 ha since 2012. Aboveground biomass was significantly higher in Avicennia marina (3 to 35 Mg ha -1 ) mangrove stands compared to Bruguiera gymnorrhiza (0.02 to 6.3 Mg ha -1 ) stands. Significantly higher sediment organic matter was found in vegetated mangrove areas compared to adjacent disturbed sites for ten of the estuaries. This study highlighted the effects of anthropogenic pressures on sediment compaction and removed vegetation. Wood harvesting, livestock browsing, and trampling caused localised degradation of mangroves; sites with these pressures had a lower density of mangrove seedlings and saplings. The average carbon storage capacity was 50% greater in vegetated mangrove stands because of the higher sediment organic content and aboveground biomass. This study showed that an average of 29 Mg C ha -1 could be gained if removal of anthropogenic pressures took place and mangrove forests were restored or allowed to regenerate naturally in the disturbed areas. • Vegetated mangrove stands had 40 Mg C ha -1 more than disturbed areas. • Wood harvesting, trampling and livestock browsing caused disturbance. • Carbon storage capacity was 50% greater in intact mangrove stands. • Intact stands had higher sediment organic content and aboveground biomass. • Removal of pressures and mangrove regrowth will restore disturbed stands.