Large pulses of greenhouse gases from mangroves not fully appreciated (or understood)

CR mangrove rootsOn climate change, a recent study warns that we are not accounting for the full cost of altering mangroves and other coastal ecosystems.

If you have ever visited a mangrove, you probably traveled by water.  Mangroves are a mixed-up tangle of roots and shallow, mucky organic sediments that are almost impossible to travel through by foot.  This tangle of roots, however, partly defines some of the unique ecosystem characteristics (and benefits) of mangroves.

Mangroves are found along the coastlines of tropical regions in waters that are shallow, salty, and oxygen poor.  The roots of mangrove trees have adaptations that allow them to survive in this harsh environment.  Pneumatophores, for example, are specialized roots that stick up out of the sediment and into the air, allowing gas exchange (especially oxygen) with the atmosphere.

Overall, one of the benefits of mangroves is that they protect shoreline from storms and ocean surges, while limiting the movement of pollutants from the surrounding landscape to the ocean.  Some estimate that 35% of worldwide mangroves have been removed for coastline development, shrimp farming, and other human activities.  The photo shows the edge of a mangrove (left) that has been altered to create salt ponds (right) in Costa Rica:

mangrove and salinas

Globally, the most important benefit of mangroves (and other vegetated coastal ecosystems) is probably their ability to remove carbon from the atmosphere and store it away.  In waterlogged ecosystems decomposition is limited by the lack of oxygen, which leads to a large accumulation of organic matter (also called a “pool” or “stock” of carbon).

A study published in PLoS by Pendleton and colleagues suggests that the total carbon content in these systems is huge and poorly understood.  Their study considered three types of vegetated coastal ecosystems — marshes, mangroves, and seagrasses – and found that mangroves contained the most carbon.

“Mangroves contain the largest per-hectare carbon stocks and contribute approximately half the estimated total blue carbon emissions.”

“Blue carbon” is a term that’s increasing being used to describe the carbon stored by coastal ecosystems, but we have yet to fully understand the dynamics of how this type of carbon is stored and lost.

When researchers estimate the impact of removing mangroves — or any ecosystem — on climate change, they usually just calculate what their loss means to carbon sequestration.  Carbon sequestration is the amount of carbon taken out of the atmosphere each year and “locked up” somewhere for a long time.  Most ecosystems accumulate carbon every year, because the amount of carbon that enters the system (through photosynthesis) is usually larger than the amount that leaves it (as organisms respire).

Ecosystems don’t accumulate carbon at the same rate, so we need to know how much carbon enters and exits each ecosystem separately.  In other words, you can’t study how this process works in tropical forests and apply it to mangroves.  Cut down tropical forests for pastures and we lose their ability to sequester carbon.  Clear away mangroves and replace them with buildings, fisheries, and salt ponds and they can no longer take carbon dioxide out the atmosphere and store it in their sediments.

The difference with mangroves is that they store a lot more carbon that tropical forests:

“Disturbance of the carbon stored in the biomass and top meter of sediment in a typical hectare of mangrove could contribute as much emissions as three to five hectares of tropical forest”

Their study demonstrates that we have been leaving out a large part of the carbon sequestration equation (literally)  — the large pulse of carbon that enters the atmosphere when a mangrove is removed.

“Indications are that such ‘pulse’ releases may have the largest and most immediate impact on greenhouse gas (GHG) emissions, possibly amounting to 50 times the annual net carbon sequestration rate.”

So, removing mangroves leads to a large pulse of carbon immediately into the atmosphere as well as the removal of an ecosystem that could take a lot of carbon from the atmosphere in the future.

Recently, the movement to quantify and document “blue carbon” has gained momentum in the conservation community.  Emerging global markets for carbon credits might offer a real economic incentive to protect mangroves … including the carbon that is already stored in the deep organic sediments, not just the potential for future storage, which makes mangrove carbon storage an economic alternative to ripping them out.  What we need is better information on the abundance of these types of coastal ecosystems on the planet and more detail on how much carbon is actually stored in them.

Pendleton L, Donato DC, Murray BC, Crooks S, Jenkins WA, et al. (2012)Estimating Global “Blue Carbon” Emissions from Conversion and Degradation of Vegetated Coastal Ecosystems.PLoS ONE 7(9):e43542.doi:10.1371/journal.pone.0043542