Importance of mangroves and its role as carbon sink and in mitigation and adaptation of climate change impacts

--Mohammad Sayedur Rashid Deputy Conservator of Forest Coastal Circle, Barisal

20. Shidur Rahman

Mangroves are highly productive, biodiversity-rich, inter-tidal forest ecosystems adapted to survive in the harsh interface between land and sea. Mangrove plants are mostly trees and large shrubs, but also include ferns and a palm species. According to the 2010 World Atlas of Mangroves, there are a total of 73 species and hybrids, which are considered to be true mangroves – those that have adapted to mangrove environments and are rarely found elsewhere. Thirty-eight of these species might be considered ‘core species’ that typify mangroves and dominate in most locations. The remainder are either not so abundant or are found mostly on the fringes of mangrove habitats.

Mangroves are found in 123 countries, covering an area of 152,360 km2.¬† However, despite wide distribution, over two-thirds of the world’s mangroves are found in just 12 countries, with Indonesia alone accounting for over 20% of the total mangrove cover of the world. On a regional scale, Southeast Asia has one-third (51,049 km2) of the world’s mangroves – more than any other region in the world. This can be attributed to the region’s highly conducive environment for the growth of mangrove forests, characterized by such qualities as a humid climate, high rainfall and a number of rivers with large deltas supplying freshwater and sediments. The important ones include the Ayeyarwady delta in Myanmar (Burma), the Mekong in Vietnam, and the extensive deltaic coastline along southern Papua in Indonesia. Moreover, the region is also known as the global center of mangrove diversity, with 51 species, which is 71% of the total mangrove species found all over the world.

Other large areas of mangrove forests are found along the coastlines of South and Central America and West and Central Africa, northeast India and northern Australia. The best known area for mangroves in the whole world is the Sundarbans, a vast mangrove forest that spreads across the boundary between India and Bangladesh and covers an area of 6,500 km2, extending up to 85 km inland. Besides, Bangladesh has raised more than 1900 sq.km. of mangrove plantations along its coastal belt.

Mangroves are one of the most productive ecosystems on the earth. They perform a variety of useful ecological, bio-physical, and socio-economic functions, and are the source of a multitude of benefits to coastal populations.

The timber from mangrove forests is used for a variety of purposes, including for making houses, boats, and fish traps. In many countries, mangrove wood is used to produce charcoal, and as firewood. There are a number of other non-timber benefits extracted from the range of mangrove forest species, including honey, tannin from bark, thatch material, edible fruits, fodder and medicinal properties of certain species with potential commercial applications and recreational values.

The entangled roots of mangrove forests help to stabilize coastal areas through sediment capture and bio-filtration of nutrients and some pollutants from the water, and reducing coastal erosion. The aerial roots of mangroves hold back sediments and reduce pollutants from sewage and aquaculture in estuaries and coastal waters. Coastal protection is another important function of mangrove forests, serving as a natural barrier against storms, typhoons, and tsunami, and thus protecting lives and properties of coastal inhabitants. Recent  experiences  of tsunami and major storms in Southeast Asia and other parts of the world have shown that mangroves can and have played important roles in absorbing and weakening wave energy as well as preventing damage caused by debris movement.

Mangrove forests play an important role in providing breeding grounds and habitats to a variety of fishes and other marine species of high commercial value, including mud crabs, mollusks, and prawns. Mangroves have been estimated to support 30% of the fish catch and almost 100% of the shrimp catch in Southeast Asia. The valuation of mangroves to fisheries alone has been estimated at US$ 1,700 per hectare per year in Matang, Malaysia.

Role of Mangroves as Carbon Sink & in Climate Change Mitigation and Adaptation

The capacity of mangroves, sea grasses, and salt marshes to sequester carbon dioxide from the atmosphere is becoming increasingly recognized at an international level. Of all the biological carbon, also termed as ‘green carbon’, captured in the world, over half (55%) is captured by mangroves, sea grasses, salt marshes, and other marine living organisms, which are also known more specifically as ‘blue carbon’.

Mangroves, salt marshes, and sea grasses form much of the earth’s blue carbon sinks. These coastal vegetations sequester carbon far more effectively (up to 100 times faster) and more permanently than terrestrial forests. Further, studies have shown that per hectare, mangrove forests store up to five times more carbon than most other tropical forests around the world. This ability of mangroves and other coastal vegetation to store such large amounts of carbon is, in part, due to the deep, organic rich soils in which they thrive. The entangled root systems of mangroves, which anchor the plants into underwater sediment, slow down incoming tidal waters, allowing organic and inorganic material to settle into the sediment surface. The sediments beneath these habitats are characterized by typically low oxygen conditions, slowing down the decay process and rates, resulting in much greater amounts of carbon accumulating in the soil. In fact, mangroves have more carbon in their soil alone than most tropical forests have in all their biomass and soil combined.

Carbon offsets based on the protection and restoration of coastal vegetation could therefore be far more cost effective than current approaches focused on terrestrial and peat forests, even before taking into consideration the enormous additional benefits to fisheries, coastal protection, and the livelihoods of coastal inhabitants. Therefore, cutting down mangroves means releasing larger amounts of carbon into the atmosphere. This in turn causes the wet soil to dry up, leading to the release of even more stored carbon into the atmosphere. Estimates suggest a range of between 150 million to 1 billion tones of CO2 that is emitted annually due to the destruction of mangrove forests globally. Thus, at the global scale, coastal wetland destruction could account for 1-3% of industrial emissions; a number that is on the rise as more and more coastal wetlands are destroyed every year around the world.

Thus, mangrove forests offer a unique and highly efficient approach as carbon sink and thus to climate change mitigation and adaptation.

Challenges Facing Mangrove Ecosystems

However, despite such diversity in the roles of mangroves, these ecosystems are still often seen as valueless wastelands available for other uses. Such negligence toward protecting mangroves is leading to a faster rate of destruction for mangroves all over the world than for tropical forests. The most substantial loss of the world’s mangrove cover is due to their conversion to other land uses, such as urban area expansion, industrial development, aquaculture, agricultural development, and making salt pan and ship breaking yards. Among these, shrimp aquaculture has been the single biggest driver of mangrove destruction, particularly in Southeast Asia. In Bangladesh, more than 8000 ha. of rich mangrove reserved forests have been officially leased out to influentials for aquaculture. This initiates total destruction of ‘Chokoria Sundarbans’ reserved forests in Cox’s Bazar District.

Since 1970s, aquaculture development has decimated vast areas of mangrove forests in the Gulf of Thailand, Vietnam, Java and Kalimantan in Indonesia, and the Philippines. Every year thousands of tons of shrimps from aquaculture farms from Southeast Asia are exported to western markets.

One of the key challenges facing mangrove conservation is inadequate understanding of their multiple roles due to poor research, particularly in the areas of climate change mitigation and adaptation. As compared to terrestrial ecosystems, the research focus on coastal and marine systems is about a decade behind. There are isolated examples of a few very useful research studies, but a comprehensive account of the various ecological, economic, and bio-physical roles played by mangrove forests is still lacking. In order to raise awareness of the multiple benefits of mangrove ecosystems, there is a need to conduct more research and also focus more on expanding mangrove areas in participation with local communities and other key stakeholders.