Declining mangroves shield against global warming

Mangrove forests in tropical regions of the Indian and Pacific Oceans store more carbon than previously recognized, according to a study published April 3, 2011 in Nature Geoscience. The findings indicate that much of the carbon in such forests is found in the surrounding soil, which is rich in organic material. Destruction of these tropical coastal woodlands accounts for about 10 percent of carbon dioxide emissions from deforestation, the second largest source of CO2 after fossil fuel combustion, the study found.

Although carbon reserves in other types of tropical wetland forest have been assessed, the amount of carbon in mangroves has been largely ignored, even though they are present in more than 100 countries. For example, it is estimated that clearing of tropical peat lands, which also contain carbon-rich soils, produces about a quarter of all deforestation emissions. The extent of mangrove forests has declined by as much as 50% over the past half century because of development, over-harvesting and aquaculture, so estimating their carbon reserves will be important for future strategies to reduce climate change.

Mangroves -- whose twisted, exposed roots grace coastlines in more than 100 countries -- confer many benefits on humans living in their midst in addition to their storage of carbon.

The brackish tidal waters in which the trees thrive are a natural nursery for dozens of species of fish and shrimp essential to commercial fisheries around the world. Mangroves, also, offer protection from deadly storm surges from hurricanes and cyclones. Cyclone Nargis, which killed 138,000 people in Myanmar in 2008, would have been less deadly, experts say, if half the country's mangroves had not been ripped up for wood or to make way for shrimp farms. Overall, mangroves benefit the ecosystem by about 1.6 billion dollars a year.

Mangroves can sequester more carbon than an average tree in a tropical rainforest because of the soil they grow in. Mangroves grow in deep soils that are on average five times larger than other forests soils in the tropics, as well as in temperate and boreal regions. Tidal water buries organic and inorganic material in this soil, which due to low-oxygen conditions stores carbon more efficiently than other forest soils.

To estimate the abundance of carbon in mangroves, lead investigator J. Boone Kauffman, an ecologist at the Northern Research Station of the US Forest Service in Durham, New Hampshire, and his team sampled 25 mangrove sites across a broad territory of the Indo-Pacific region that included Micronesia, Indonesia and Bangladesh. This area spans 30 degrees of latitude and 73 degrees of longitude and represents about 40% of the global area covered by these trees. Mangroves grow in 118 countries, but the region the scientists chose has the greatest mangrove area and diversity.

Sludge stores

Kauffman and his team assessed above-ground and below-ground carbon pools in mangrove sites occupying estuaries and oceanic settings, such as island coasts. They found that these forests hold much more carbon than do boreal, temperate or tropical upland forests — especially in an organic-rich 'muck layer' of soil more than 30 centimeters below the surface. The trees stored atmospheric CO2 just as well as land-based tropical forests, and below the water line, they were even more efficient, hoarding five times more carbon over the same surface area.

The team found that this underground layer is thicker in mangrove forests in estuaries than in those near the ocean, accounting for more than 70% of total carbon stores in estuarine mangroves and upwards of 50% in those in oceanic zones.

By combining their findings with global data, the researchers predict that worldwide carbon reserves in mangrove forests may be as high as 25% of those in tropical peat lands, and at the current rate of annual clearance, emissions from mangrove destruction could reach 40% of those from the clearing of peat lands.

"Mangroves are among the most carbon-rich forests in the tropics," Donato and his colleagues said in the study, published in Nature Geoscience. "Our data show that discussion of the key role of tropical wetland forests in climate change could be broadened significantly to include mangroves."

Branching out

"This paper represents an important step forward in quantifying and understanding the significant pool of carbon in mangrove ecosystems," says Shimon Anisfeld, an expert in coastal ecology at Yale University in New Haven, Connecticut.

However, the numbers still only represent rough estimates, owing to a lack of information about geographic variation in soil depth, the relative area of mangrove forests in estuaries compared with those near oceans, and the effect of land-use changes on carbon release from soils. They may even be overestimates, because "the authors seem to have sampled some of the largest, most robust stands around," says Thomas Smith, an ecologist at the US Geological Survey in St Petersburg, Florida.

Still, the study could have a substantial impact on conservation efforts around the world, says Gail Chmura, an expert in coastal ecosystems at McGill University in Montreal, Canada. "Hopefully, it will help arguments to extend REDD+ to mangroves," she says, referring to an international plan to pay developing countries to preserve forests in a bid to help reduce global carbon emissions.

Robert Jackson, an ecologist at Duke University in Durham, North Carolina, agrees with Chmura, adding: "Mangrove forests are important for diversity, for coastal stability and for carbon, based on this paper. It gives another justification for preserving mangrove forests."

Daniel Murdiyarso, Senior Scientist at The Center for International Forestry Research (CIFOR), a co-author of the paper on mangroves and carbon sequestration, said, "There is a lack of awareness of the full implications of mangrove loss for humankind. " Murdiyarso adds "There is an urgent need for governments to acknowledge their importance and develop better policies to ensure their protection." (At left: roseate spoonbills roosting in mangroves)

Currently, less than 7% of the world's mangroves are under legal protection.


Source:
AFP,"Declining mangroves shield against global warming", accessed April 7, 2011
Nature,"Carbon-rich mangroves ripe for conservation",by Janelle Weaver, accessed April 7, 2011
Mongobay.com,"Vanishing mangroves are carbon sequestration powerhouses", accessed April 7, 2011
IOL Scitech,"Mangroves store climate-warming carbon", accessed April 7, 2011