Mangroves among the most carbon-rich forests in the tropics; Coastal trees key to lowering greenhouse gases

Their findings are published online in the journal Nature Geoscience.

A research team from the U.S. Forest Service's Pacific Southwest and Northern research stations, University of Helsinki and the Center for International Forestry Research examined the carbon content of 25 mangrove forests across the Indo-Pacific region and found that per hectare mangrove forests store up to four times more carbon than most other tropical forests around the world.

"Mangroves have long been known as extremely productive ecosystems that cycle carbon quickly, but until now there had been no estimate of how much carbon resides in these systems. That's essential information because when land-use change occurs, much of that standing carbon stock can be released to the atmosphere," says Daniel Donato, a postdoctoral research ecologist at the Pacific Southwest Research Station in Hilo, Hawaii.

The mangrove forest's ability to store such large amounts of carbon can be attributed, in part, to the deep organic-rich soils in which it thrives. Mangrove-sediment carbon stores were on average five times larger than those typically observed in temperate, boreal and tropical terrestrial forests, on a per-unit-area basis. The mangrove forest's complex root systems, which anchor the plants into underwater sediment, slow down incoming tidal waters allowing organic and inorganic material to settle into the sediment surface. Low oxygen conditions slow decay rates, resulting in much of the 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.

This high-carbon storage suggests mangroves may play an important role in climate change management. Aside from the main greenhouse gas contributor of fossil-fuel burning, the forestry sector can play a part -- especially carbon-rich forests that are being cleared rapidly on a global scale, such as mangroves.

"When we did the math, we were surprised to see just how much carbon is likely being released from mangrove clearing," says Donato. This suggests, says Donato, that where consistent with local management objectives, mangroves may be strong candidates for programs aiming to mitigate climate change by reducing deforestation rates.

Recently, mangroves have experienced rapid deforestation worldwide -- a 30-50 percent decline in the past 50 years. Mangrove deforestation generates greenhouse gas emissions of 0.02-0.12 petagrams of carbon per year, which is equivalent to up to 10 percent of carbon emissions from global deforestation, according to the research team's findings.