Mangroves Excel at Storing Carbon

Monday, 4 April 2011

Deborah Zabarenko Reuters

The amount of land mangroves grow on has declined by up to 50 per cent over the last 50 years

Tropical mangrove trees are better at storing carbon dioxide than most other forests, and cutting them down unleashes more greenhouse gas than deforestation elsewhere, say scientists.

Mangroves are so efficient at keeping carbon dioxide out of the atmosphere that when they are destroyed, they release as much as 10 per cent of all emissions worldwide attributable to deforestation - even though mangroves account for just 0.7 per cent of the tropical forest area, says researchers.

Daniel Donato, of the US Agriculture Department's Forest Service and lead author of a study published in the journal Nature Geoscience, says mangroves store two to four times the carbon that tropical rainforests do.

"Mangroves store a lot of carbon, much more so than most forests on Earth, on a per hectare basis," says Donato. "Since they store so much carbon, there's probably a lot being released from all the mangrove deforestation that's going on."

Mangroves live where many people want to live, along ocean coastlines in the tropics, and their areal extent - the amount of land they grow on - has declined by 30 per cent to 50 per cent over the last 50 years, the study found.

Coastal development, aquaculture and over-harvesting have all contributed to mangrove deforestation. Rising sea levels expected this century are also a threat, the study says.

Besides storing carbon, mangrove forests act as fisheries, keep sediment in place, produce fibre and protect inhabited areas against storms and tsunamis, the researchers say.

Coastal mangrove forests and the ecological services they provide could be gone in as little as 100 years, according to the researchers.

Part of the reason for mangroves' efficiency in keeping carbon locked away lies in their location in tidal zones, where their roots are often covered with sea water.

They need complex root systems to keep them breathing even when the tides come in, says Donato. This same complexity traps sediment that comes in from rivers and it builds up.

Slow decay means carbon storage

In most forests, this kind of sediment and litter would decay rather quickly, but because much of it gets submerged in a coastal mangrove forest, there is not enough oxygen to break it down, so the breakdown of materials is much slower.

A slower decay means more carbon dioxide gets stored.

Scientists have previously studied the rate at which mangroves could sequester carbon, but this latest research looked at how much of a pool of stored carbon was locked away in the trees, in their root systems and in the soil decomposing slowly around them.

To figure this out, Donato and his colleagues went to 25 mangrove forests stretching from the Ganges Delta in Bangladesh to Micronesia in the western Pacific, and from the Malay Peninsula in southeast Asia to northern Australia.

Mangroves grow in 118 countries, but the region the scientists chose has the greatest mangrove area and diversity.

They figured out how much total carbon these trees kept out of the air by measuring the trees' size, the tree litter on the forest floor, the amount of carbon in the soil around them and the depth of the soil.

"Mangroves store about two to four times what (tropical rainforests) store, and it's mostly in that thick organic muck layer in the soil," says Donato. "That's really what sets mangroves apart in terms of carbon storage."

Source: ABC Science News