The effects of seaward distance on above and below ground carbon stocks in estuarine mangrove ecosystems

Abstract

Background: Mangrove forests have gained recognition for their potential role in climate change mitigation due to the carbon sequestration of the productive ecosystems and storage in the sediments that are trapped by the mangrove tree roots and pneumatophores. Africa hosts about 19% of the world’s mangroves, yet there are relatively few studies that have examined the carbon stocks of African mangroves, and the studies available report great differences among sites and amongst the different pools of carbon stocks, particularly between the above ground carbon stored in trees (AGC) and organic carbon stored within the sediment - ‘soil organic carbon (SOC)’ and none considered the effects of seaward distance. We investigate if AGC and SOC carbon stocks differ with increasing seaward distance for estuarine mangroves in Tanzania and, how our results compare to those reported elsewhere across Africa.Results: AGC ranged between 10.9-54.9 Mg C ha-1 , but was not significantly affected by seaward distance. SOC for 0-1m depth ranged from 153.7-483.6, with a mean of 301.7 Mg C ha-1. SOC was significantly negatively correlated with seaward distance, which is different from studies in Micronesia and the Indo-Pacific. Mangrove type (estuarine/oceanic), soil erosion and soil depth may explain these differences We note important methodological differences in previous studies on carbon stocks in mangroves in Africa. Conclusion:This study indicates that seaward distance has an important effect on SOC stocks in the Lindi region of Tanzania. There is the need to standardise methodological approaches for mangroves in Africa, to report seaward distance and to describe the type of mangrove stand to make results easily comparable across sites and to assess the true value of Blue Carbon in Africa. We recommend focusing on trees >10cm diameter for AGC, and sampling soils to greater than 1m depth for SOC, which would provide a more complete assessment of the potentially considerable mangrove carbon store.