Ecosystem carbon stocks of mangroves across broad environmental gradients in West-Central Africa: Global and regional comparisons

Study Number

96

Author

J. Boone Kauffman, Rupesh K. Bhomia

Abstract

Globally, it is recognized that blue carbon ecosystems, especially mangroves, often seques-
ter large quantities of carbon and are of interest for inclusion in climate change mitigation strategies. While 19% of the world’s mangroves are in Africa, they are among the least investigated of all blue carbon ecosystems. We quantified total ecosystem carbon stocks in 33 different mangrove stands along the Atlantic coast of West-Central Africa from Senegal to Southern Gabon spanning large gradients of latitude, soil properties, porewater salinity, and precipitation. Mangrove structure ranged from low and dense stands that were <1m in height and >35,000 trees ha-1 to tall and open stands >40m in height and <100 ha-1. Tremendous variation in ecosystem carbon (C) stocks was measured ranging from 154 to 1,484 Mg C ha-1. The mean total ecosystem carbon stock for all mangroves of West-Central Africa was 799 Mg C ha-1. Soils comprised an average of 86% of the total carbon stock. The greatest carbon stocks were found in the tall mangroves of Liberia and Gabon North with a mean >1,000 Mg C ha-1. The lowest carbon stocks were found in the low mangroves of the semiarid region of Senegal (463 Mg C ha-1) and in mangroves on coarse-textured soils in Gabon South (541 Mg C ha-1). At the scale of the entirety of West-Central Africa, total ecosystem carbon stocks were poorly correlated to aboveground ecosystem carbon pools, precipitation, latitude and soil salinity (r2 = 0.07 for all parameters). Based upon a sample of 158 sites from Africa, Asia and Latin America that were sampled in a similar manner to this study, the global mean of carbon stocks for mangroves is 885 Mg C ha-1. The ecosystem carbon stocks of mangroves for West-Central Africa are slightly lower than those of Latin America (940 Mg C ha-1) and Asia (1049 Mg C ha-1) but substantially higher than the default Intergovernmental Panel on Climate Change (IPCC) values for mangroves (511 Mg C ha-1). This study provides an improved estimation of default estimates (Tier 1 values) of mangroves for Asia, Latin America, and West Central Africa.

Main Results and Conclusions:

• Africa is one of the regions of the world with the greatest abundance and diversity of mangroves, which makes conservation of the areas essential.
  • “Africa hosts above 19% of the world’s mangroves, of which 59% are located in West-Central Africa.” (2)
  • “These mangroves are important to many people along the Atlantic Coast of Africa. For example, about five million people are dependent on small-scale fisheries for their livelihoods and fish are a major source of dietary protein.” (2)
• The areas studied are along the Atlantic coast of West-Central Africa.
  • The four regions included: “..(1) the Saloum Delta, Senegal… (2) the Cess and Mechlin Rivers, Liberia, (3) , (3) near or within Akanda National Park, Mondah Bay, Northern Gabon, (4) the Ndougou Lagoon, Southern Gabon…” (2)
• There were several mangrove types encountered within the study of the Atlantic coast area of West-Central Africa.
  • “The dominant mangrove trees encountered during our studies included Rhizophora mangle L, (Rhizophoraceae), Rhizophora racemosa L. (Rhizophoraceae) and Avicennia germinans (L.) Stearn (Acanthaceae). In addition, we also encountered Laguncularia racemosa (L.) C. F. Gaertn. (Combretaceae), Conocarpus erectus L. (Combretaceae), and Avicennia africana Palisot de Beauvois (Acanthaceae).” (3-4)
• Different areas consisted of different types of mangroves as well as differences in heights and density of the mangroves.
  • “The mangroves of the Soloume Delta, Senegal consisted of medium stands along the estuary fringe while low mangroves dominated in the interior and upland edge of mangrove environments. Medium mangroves were dominated by R. racemosa while low mangroves were dominated by R. mangle, A. germinans and A. africana.” (4)
  • “In Liberia, tall mangroves occurred on estuarine margins and medium mangroves in the interior and upland margins. All were dominated by R. racemosa.” (4)
  • “In both Northern and Southern Gabon, the majority of mangroves were tall in stature and dominated by R. racemosa except for 3 of the Southern Gabon sites that were dominated or co-dominated by A. germinans. Of the 17 sampled Gabon sites, only three were medium in stature (and dominated by R. racemosa).” (4)
• There was large variation in carbon stocks within mangroves.
  • “…ecosystem carbon stocks were significantly greater in tall mangroves than medium mangroves at Liberia and Gabon South (p<0.05) and ecosystem carbon stocks were significantly greater in medium mangroves compared to low mangroves in Senegal” (11)
  • “…there was great variation in the carbon stocks of tall mangroves across regions. The mean ecosystem carbon stock of tall mangroves at Gabon South (615 Mg C ha-1) was <56% of those of Gabon North or Liberia (>1093 Mg C ha-1; Fig 5).” (9)
  • Soil salinity had little effect on the carbon stocks that mangroves store.
  • “Within sampled regions, there were no significant differences in salinity when comparing between tall and medium, or medium and low mangroves.” (13)
  • “Given effects of soil salinity on carbon cycling and allocation, we would expect that soil salinity would affect ecosystem carbon stocks. However, we found a weak relationship between salinity and both aboveground (r2 = 0.07) and total ecosystem carbon stocks (r2 = 0.10; Fig 6).” (12)
• There were differences between West-Central Africa carbon stocks and east African carbon stocks.
  • “At the regional level, mean carbon concentrations were 8.5% in Senegal, 10.1% in Liberia, 11.8% in Gabon North and 4.8% in Gabon South.” (13)
  • “In contrast, east African mangroves have been reported to have very low concentrations of carbon [37, 39]. Mean soil carbon content of mangrove soils in the Zambezi River Delta was 1.8% [36] and was 3.4% for mangroves soils of northern Madagascar [38].” (13)
• African mangroves can be compared to other mangrove sites around the world.
  • “This is a similar structural range encountered in the Latin American mangrove studies (e.g., refs [22, 33, 34]). Sites from the Asia Pacific in this example were all tall mangroves and were limited to areas of relatively high rainfall.” (14)