Land-building marsh plants are champions of carbon capture

A new study published May 6 in Science by a team of Dutch, American and German scientists shows that it's not too late to reverse the losses.

The key to success, the paper's authors say, is using innovative restoration practices -- identified in the new paper -- that replicate natural landscape-building processes and enhance the restored wetlands' carbon-storing potential.

And doing it on a large scale.

"About 1 percent of the world's wetlands are being lost each year to pollution or marsh draining for agriculture, development and other human activities," said Brian R. Silliman, Rachel Carson Distinguished Professor of Marine Conservation Biology at Duke University, who coauthored the study.

"Once disturbed, these wetlands release enormous amounts of CO₂ from their soils, accounting for about 5 percent of global CO₂ emissions annually," Silliman said. "Hundreds, even thousands of years of stored carbon are exposed to air and start to rapidly decompose and release greenhouse gases. The result is an invisible reverse waterfall of CO₂ draining into the atmosphere. The wetlands switch from being carbon sinks to sources."

"The good news is, we now know how to restore these wetlands at a scale that was never before possible and in a way that both stops this release of carbon and re-establishes the wetland's carbon storing capacity," he said.

What makes most wetlands so effective at carbon storage is that they are formed and held together by plants that grow close to each other, Silliman explained. Their dense above- and below-ground mats of stems and roots trap nutrient-rich debris and defend the soil against erosion or drying out -- all of which helps the plants to grow better and the soil layer to build up, locking in a lot more CO₂ in the process.

In the case of raised peat bogs, the process works a little differently, Silliman noted. Layers of living peat moss on the surface act as sponges, holding enormous amounts of rainwater that sustain its own growth and keeps a much thicker layer of dead peat moss below it permanently under water. This prevents the lower layer of peat, which can measure up to 10 meters thick, from drying out, decomposing, and releasing its stored carbon back into the atmosphere. As the living mosses gradually build up, the amount of carbon stored below ground continually grows.

Successful restorations must replicate these processes, he said.

"More than half of all wetland restorations fail because the landscape-forming properties of the plants are insufficiently taken into account," said study coauthor Tjisse van der Heide of the Royal Institute for Sea Research and the University of Groningen in the Netherlands. Planting seedlings and plugs in orderly rows equidistant from each other may seem logical, but it's counter-productive, he said.

"Restoration is much more successful when the plants are placed in large dense clumps, when their landscape-forming properties are mimicked, or simply when very large areas are restored in one go," van der Heide said.

"Following this guidance will allow us to restore lost wetlands at a much larger scale and increase the odds that they will thrive and continue to store carbon and perform other vital ecosystem services for years to come," Silliman said. "The plants win, the planet wins, we all win."

Silliman and van der Heide conducted the new study with scientists from the Netherlands' Royal Institute for Sea Research, Utrecht University, Radboud University, the University of Groningen, the University of Florida, Duke University, and Greifswald University.

By synthesizing data on carbon capture from recent scientific studies, they found that oceans and forests hold the most CO₂ globally, followed by wetlands.

"But when we looked at the amount of CO₂ stored per square meter, it turned out that wetlands store about five times more CO₂ than forests and as much as 500 times more than oceans," says Ralph Temmink, a researcher at Utrecht University, who was first author on the study.

Funding for the new study came from the Dutch Research Council, the Oak Foundation, Duke RESTORE, the Lenfest Ocean Program, the National Science Foundation, and Natuurmonumenten.

In addition to his faculty appointment at Duke's Nicholas School, Silliman is director of Duke RESTORE.