First report on carbon storage in warm-temperate mangroves of South Africa

Blue carbon is defined as the carbon sequestered and stored by coastal vegetated ecosystems including mangroves, seagrass and salt marsh. Research on blue carbon has rapidly escalated during the past decade following the 2009 United Nations Environment Program (UNEP) report, which highlighted the efficiency of these coastal habitats to act as carbon sinks. Mangrove forests in particular have been recognized for their significant role as carbon sinks and therefore have been prioritized in atmospheric carbon mitigation strategies. Despite global research efforts, there is still a notable knowledge gap on carbon storage potential of different mangrove tree species in underrepresented geographic regions. This study provides the first comprehensive assessment of blue carbon for a mangrove forest in a warm temperate estuary in South Africa. Carbon storage was quantified for Avicennia marina mangrove stands at the Nxaxo Estuary; using both elemental analysis and loss on ignition (LOI) techniques for sediment carbon pools and only elemental analysis for aboveground biomass (leaf litter, pneumatophores, wood).The environmental characteristics were the same at the five sites sampled and this was related to the estuary being well-mixed and permanently open to the Indian Ocean. Unexpectedly the carbon content was weakly correlated with that measured by elemental analysis. The cause of this is unknown and could potentially be an avenue for further investigation. The carbon content measured from elemental analysis was used in further calculations, as this method is reliable and meets global standards for comparisons. For the aboveground carbon pools, the pneumatophores (2.1 × 10-3 Mg C ha-1) stored significantly more carbon compared to the live trees (3.35 × 10-4 Mg C ha-1) and leaf litter (2.84 × 10-6 Mg C ha-1). The sediment carbon pool (228.05 Mg C ha-1) stored significantly more carbon than the aboveground carbon pools, which is in line with the findings of other studies from different regions. Sediment carbon storage was not related to mangrove tree population structure or sediment characteristics, such as moisture content and particle size. However, sediment carbon storage was highest at Site 2 which was located in a depositional area of the estuary. The total carbon storage is 1140.25 Mg C for the mangrove area of 9.5 the Nxaxo Estuary; this is considerably lower than what has been recorded for other regions. In South Africa, mangroves in warm--temperate regions occur at one of the southernmost limits globally and are restricted to occurring in sheltered estuaries due to the high--energy nature of the coastline. As a result, the growth and productivity of mangroves in these regions can be influenced by estuarine and climatic dynamics. Although total carbon storage was not variable between different sites at this estuary, this might not be the case on a regional scale as variability is expected to occur between estuaries within South Africa. Comparing the data collected from this study and others done globally, it is clear that the capacity for carbon storage by mangroves is variable. Therefore, regional scale blue carbon studies should be undertaken to provide estimates from direct quantification and allometric equation approaches. Carbon storage has been recognized as a valuable ecosystem service; therefore, going forward, accurate estimations of storage potential is essential, particularly for climate change mitigation and emission offset projects