Human-driven degradation impacts on mangroves in southern Sierra Leone

Abstract

Mangrove ecosystems remove carbon from the atmosphere and store it predominantly in soils constituting an important carbon sink. Additionally, they foster biodiversity and provide ecosystem services supporting local livelihoods and enhancing coastal protection. However, mangroves are declining in extent and experiencing degradation due to human activities as well as climate change impacts like sea-level rise and increasing frequency of extreme events. This study investigated how different levels of human-driven degradation affect forest structure, above- and belowground carbon stocks, tree species composition, and the population structures of dominant tree species. We investigated 19 transects of different degradation levels (pristine, moderately degraded, heavily degraded) in various locations (oceanic, riverine, interior) in the Sherbro river estuary in Sierra Leone. Results showed that total carbon stocks of pristine mangroves amounted to 707 Mg C ha−1 including soils to 1 m depth, which is higher than carbon stocks reported from many other regions in Africa. Degradation resulted in declining basal area, decreasing density of large trees (≥ 30 cm diameter) but increasing stem density of small trees (5 - 10 cm diameter). All carbon pools declined with increasing degradation across all locations. While above- and belowground carbon decreased by 96 %, soil carbon (up to 1 m depth) decreased by 44 %. Heavy degradation resulted in a change in the dominating species from Rhizophora racemosa to Rhizophora mangle across all locations. Overall, we found major effects of degradation on mangrove characteristics and minor effects of location. We urge conservation of pristine forests, restoration of degraded mangroves, and the establishment of management systems that are ecologically informed and based on a comprehensive understanding of human-driven degradation impacts.