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

<div><p>Globally, it is recognized that blue carbon ecosystems, especially mangroves, often sequester 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.</p></div

Study area and sampling locations across three West African countries—Liberia, Senegal and Gabon.

<p>Mangroves were sampled along the Cess and Mechlin (St Johns) Rivers in Liberia, in the Saloum Delta, Senegal, Mondah Bay, Akanda National Park, (Gabon North) and the Ndougou Lagoon (Gabon South).</p

Mangrove tree basal area (m² ha^-1) in Liberia (i), Senegal (ii), Gabon South (iii) and Gabon North (iv).

<p>Mangrove tree basal area (m² ha^-1) in Liberia (i), Senegal (ii), Gabon South (iii) and Gabon North (iv).</p

Mangrove tree density (trees ha^-1) in Liberia (A), Senegal (B), Gabon North (C) and Gabon South (D).

<p>Mangrove tree density (trees ha^-1) in Liberia (A), Senegal (B), Gabon North (C) and Gabon South (D).</p

Ecosystem carbon stocks (Mg C ha^-1) across the Western African landscape.

<p>Data are means ± one standard error.</p

Ecosystem carbon stocks (Mg C ha^-1) of sampled mangroves of West-Central Africa.

<p>Horizontal lines are the location means (± SE) of tall, medium or low mangroves.</p

Case studies of (semi)constructed wetlands treating point and non-point pollutant loads to protect downstream natural ecosystems

Aim: This chapter provides an insight into the diversity of constructed wetlands across large geographic range and their functional role in treating both point and non-point sources of pollutants while simultaneously providing other services such as habitats for biodiversity. Main concepts covered: The four case studies included in this chapter highlights the role of constructed wetlands as natural treatment systems functioning successfully at both landscape level as well as at small scale to effectively reduce excessive nutrients which originated from point and non-point sources. The range of operation of these constructed wetlands (from semi-tropics to temperate climates) indicates the functional plasticity and flexibility of this natural technology to treat runoff and waste-water at various quality and quantity dimensions. Main methods covered: The predominant methods employed in the analysis of treatment effectiveness and efficiency of these constructed wetlands includes calculations of water and nutrient budgets taking into consideration inflows and outflows, and various transformations and modifications that occur within these systems mediated by biologically active microbes or spontaneously in form of chemical reactions following the laws of thermodynamics, kinetics and electrochemistry. Conclusion/outlook: An overview is provided on the future possibility of continued usage and expansion of constructed wetlands as ‘natural solutions’ to the existing challenges of water quality and possible ways to reduce pollution in water bodies. The ease of building these constructed wetlands and relative simplicity of operating such systems provide an opportunity for even wider use and adoption of this natural technology to treat poor quality in surface waters

Advances in Amazonian Peatland Discrimination With Multi-Temporal PALSAR Refines Estimates of Peatland Distribution, C Stocks and Deforestation

There is a data gap in our current knowledge of the geospatial distribution, type and extent of C rich peatlands across the globe. The Pastaza Marañón Foreland Basin (PMFB), within the Peruvian Amazon, is known to store large amounts of peat, but the remoteness of the region makes field data collection and mapping the distribution of peatland ecotypes challenging. Here we review methods for developing high accuracy peatland maps for the PMFB using a combination of multi-temporal synthetic aperture radar (SAR) and optical remote sensing in a machine learning classifier. The new map produced has 95% overall accuracy with low errors of commission (1–6%) and errors of omission (0–15%) for individual peatland classes. We attribute this improvement in map accuracy over previous maps of the region to the inclusion of high and low water season SAR images which provides information about seasonal hydrological dynamics. The new multi-date map showed an increase in area of more than 200% for pole forest peatland (6% error) compared to previous maps, which had high errors for that ecotype (20–36%). Likewise, estimates of C stocks were 35% greater than previously reported (3.238 Pg in Draper et al. (2014) to 4.360 Pg in our study). Most of the increase is attributed to pole forest peatland which contributed 58% (2.551 Pg) of total C, followed by palm swamp (34%, 1.476 Pg). In an assessment of deforestation from 2010 to 2018 in the PMFB, we found 89% of the deforestation was in seasonally flooded forest and 43% of deforestation was occurring within 1 km of a river or road. Peatlands were found the least affected by deforestation and there was not a noticeable trend over time. With development of improved transportation routes and population pressures, future land use change is likely to put South American tropical peatlands at risk, making continued monitoring a necessity. Accurate mapping of peatland ecotypes with high resolution (\u3c30 m) sensors linked with field data are needed to reduce uncertainties in estimates of the distribution of C stocks, and to aid in deforestation monitoring