Mangrove forests

Mangroves are coastal forests that lie on the crossroad where oceans, freshwater, and land realms meet. They are among the most productive and complex ecosystems on the planet, thriving in salty and brackish conditions that would just kill ordinary plants very quickly. Their capacity to protect against storms and even sea level rise make them indispensablefor coastal communities in their fight against climate change.

Distribution

Mangrove forests are particularly found in tropical and subtropical regions within the 30” of the equator. These tidal areas, such as estuaries and marine shorelines, are frequently inundated with salt water. Strongly in decline, mangrove forests occupy about 15.2 million hectares of tropical coast worldwide (Spalding et al. 2010). Africa represents about 19% of this mangrove cover, totaling some 3.2 million ha. The African country with the largest mangrove area is Nigeria with 1mln ha.

Biodiversity

The continent holds 17 species of mangroves, equally divided between the east and west. African mangroves are home to very diverse fauna. Aquatic mammals include monkeys, antelopes and manatees. Its roots and mud are home to molluscs, such as bivalves and oysters, and crustaceans. Live and decaying mangrove leaves and roots provide nutrients that nourish plankton, which in turn are food for many of these species.
 
With this abundance of food, mangroves function as nurseries for many fish species; many of commercially caught fish have spent part of their lives in mangroves. Mangroves are also home to terrestrial fauna, including mammals, reptiles and avian species; especially waterbirds.

Livelihoods

Humans benefit from such a productive environment. Fish, shellfish and mollusks are mangrove delicacies. Another is mangrove honey. The mangrove trees provide wood for construction, firewood and carbon for cooking or tanning production. The forests’ abundant plant life provide for traditional medicine.
 
Research by Barbier (2007) concluded that the economic annual value of one (!) hectare of mangrove forest (by adding the values of collected wood and non-wood forest products, fishery, nursery and coastal protection against storms) is $12,392. Next to economic value, mangroves also bear great cultural significance for communities

Coastal Resilience

Mangrove forests also provide protection and shelter against extreme weather events, such as storm winds and floods, as well as tsunamis. Mangroves absorb and disperse tidal surges associated with these events. As indicated by Hirashi and Harada (2003), a mangrove stand of 30 trees per 0.01 hectare with a depth of 100 m can reduce the destructive force of a tsunami by up to 90%. Recent research by TNC and Wetlands International proves that mangroves reduce wave height by as much as 66% over 100 metres of forest.

Climate change

Storage of carbon in mangroves takes place through accumulation in living biomass and through burial in sediment deposits. With living biomass typically ranging between 100-400 tonnes/ha, and significant quantities of organic matter being stored in the sediments, mangroves rival the sequestration potential of rainforests.

Mangroves under threat

Globally, half of all mangrove forests have been lost since the mid-twentieth century, with one-fifth since 1980 (Spalding /et al./ 2010). Conversion into shrimp farms causes 25% of the total destruction, according to UNEP (Botkin and Keller, 2003), happening mostly in Southeast Asia and Latin America, but this threat is expanding to east Africa, although this region has suffered deforestation by 8% in the pas 25 years (FAO, 2005).
 
In west and central Africa some 20-30% of the mangroves have been lost in the past 25 years. Threats vary per country, but the largest are man-made: urbanisation and urban infrastructure development, salt and sand extraction, industrial pollution, but also mangrove cutting for firewood (Ajonina et Usongo, 2001; Ajonina et al 2005). This wood is used for fish smoking and salt water cooking as part of salt production. Read more on the sustainable production techniques we introduce in West Africa. Natural causes include salt intrusion and drought.

 

Vidéo (FR)

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