Mangrove forests in tropical regions of the Indian and Pacific Oceans store more carbon than previously recognised, according to a study published 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. Cutting down mangrove forests, which occupy less than one per cent of tropical forest area, could therefore contribute up to 10 per cent of global carbon emissions from deforestation.

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 peatlands, 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 per cent 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.

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, NH, and his team sampled 25 mangrove sites across a broad territory that included Micronesia, Indonesia and Bangladesh. This area spans 30 degrees of latitude and 73 degrees of longitude and represents about 40 per cent of the global area covered by these trees.

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 team found that this underground layer is thicker in mangrove forests in estuaries than in those near the ocean, accounting for more than 70 per cent of total carbon stores in estuarine mangroves and upwards of 50 per cent in those in oceanic zones. By combining their findings with global data, the researchers predict that worldwide carbon reserves in mangrove forests maybe as high as 25 per cent of those in tropical peatlands, and at the current rate of annual clearance, emissions from mangrove destruction could reach 40 per cent of those from the clearing of peatlands.

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, Conn.

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, Fla.

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.

“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, NC, 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.”

Nature News

Net absorber to net emitter

Last year’s drought in the Amazon raises concerns about the region’s capacity to continue absorbing carbon dioxide, scientists say. Researchers have reported in the journal Science that the 2010 drought was more widespread than in 2005 – the last big one – with more trees probably lost. The 2005 drought had been termed a “one-in-a-century” event.

In drought years, the Amazon region changes from being a net absorber of carbon dioxide into a net emitter. The scientists, from the UK and Brazil, suggest this is further evidence of the Amazon’s vulnerability to rising global temperatures. They also suggest the days of the Amazon forest curbing the impact of rising greenhouse gas emissions may be coming to an end.

The 2010 drought saw the Amazon river at its lowest levels for half a century, with several tributaries completely dry and more than 20 municipalities declaring a state of emergency.

Some computer models of climate change – in particular, the one developed at the UK’s Hadley Centre – project more droughts across the region as the planet warms, and a diminishing capacity to absorb CO2. There are several ways in which warming can turn greenhouse gas-absorbing forests into emitters. In the Amazon, the principal mechanism is simply that trees die and then rot; in addition, those trees are then not available to absorb CO2 from the air.

For this research, scientists used data from the Tropical Rainfall Measuring Mission (TRMM), a US/Japanese satellite that monitors rainfall in a belt extending either side of the Equator.

Its observation showed that whereas the 2005 drought covered an area of nearly two million sq km, in 2010 it stretched for three million sq km. Following the 2005 drought, scientists were able to study the impact on trees and work out the relationship between the rainfall loss and the release of carbon. In an average year, the basin absorbs about 1.5 billion tonnes of CO2 from the atmosphere.