Don’t Drain the Swamps: Tidal Wetlands Store Huge Amounts of Carbon
Research published by the American Geophysical Union digs deeper into the carbon-storing power of wetlands along North America’s Pacific coast.
Forested tidal swamps like the upper reaches of the Sayward Estuary on northern Vancouver Island, British Columbia, store vast amounts of carbon in their soils.
A new paper featuring Hakai Institute researchers and led by Oregon State University reveals that the forested tidal swamps of the Pacific Northwest, alongside mangroves in Mexico, stash away the largest amounts of carbon on the Pacific coast of North America.
Analyzing transboundary datasets from Canada, the United States, and Mexico, the study contributes vital regional estimates for the carbon-storing potential of different coastal ecosystems, including seagrass meadows, unvegetated mudflats, marshes, mangroves, and tidal swamps—which are all threatened by development.
While mangroves have become known for their carbon-rich soils—due to the trees and plants that deposit carbon locally in their soils, and the low-oxygen conditions that lock it away—the new research shines a light on forested tidal swamps. These wetlands are defined by large tide swings and woody shrubs and trees, including northern species such as willows and Sitka spruce. The study finds that forested tidal swamps in the Pacific Northwest can bury around three million tonnes of organic carbon (or around 10 million tonnes of carbon dioxide) in the top meter of sediment alone. That’s equivalent to around what two million gas cars burn every year, and is more carbon storage per area than all the terrestrial ecosystems the authors analyzed save for Canadian peatlands.
Woody plants and trees in tidal wetlands deposit large quantities of carbon into low-oxygen soils.
“Our northwest tidal swamps are some of the best tidal swamps in the world in terms of carbon storage,” says Margot Hessing-Lewis, one of the Hakai Institute researchers involved in the study. “It’s a unique but very understudied ecosystem that’s quite threatened. They’re like the temperate version of mangroves.”
The high carbon storage of these swamps backs up recent evidence from the Pacific Northwest Blue Carbon Working Group—a US and Canadian network of scientists, practitioners, conservationists, carbon market experts, and government representatives working to understand and restore these ecosystems. The group found relatively low methane emissions from swampy soils, despite their low salinity. “This suggests that swamps may be one of the best natural climate solutions,” says Chris Janousek, the lead author of the new paper.
While previous research has quantified the carbon stocks of coastal habitats, often referred to as “blue carbon” ecosystems, their dynamic ocean influence makes calculations challenging, and few studies have looked at the variability of soil carbon across broad climatic and geographic ranges.
In response, the new study analyzes 1,284 sediment cores collected along more than 6,500 kilometers of the Pacific coast of North America to fill in knowledge gaps around certain habitat types as well as understudied regions, including northern British Columbia and Alaska.
Sonja Panozzo, a field technician with the Nature Trust of British Columbia, monitors the Sayward Estuary on northern Vancouver Island, British Columbia, for climate change impacts to estuaries.
“The cool thing about the dataset is that it brings together a huge number of cores taken by many different research groups,” says Carolyn Prentice, a Hakai Institute researcher responsible for some of the seagrass samples in the paper. “The power of the study is the sheer amount of data compiled to compare different habitat types. This paper solidifies what everyone was thinking we knew was happening in a robust way.”
One surprising finding was that even mudflats without trees and shrubs store significant amounts of carbon, on par with seagrass meadows—a major focus of blue carbon studies worldwide. While these two ecosystems have the lowest carbon stocks of the five habitats in the new research, they have larger footprints than mangroves and tidal swamps, and seagrass meadows in particular offer many additional benefits, from sheltering juvenile salmon to stabilizing sediment to improving water quality.
Overall, the new study helps cement the carbon-storage powers of these ecosystems, Hessing-Lewis says, and could help guide local and regional decision-making about what areas to prioritize for conservation. “For example, restoring one tidal swamp would be good bang for your carbon buck,” she says.
Global studies have shown that, collectively, blue-carbon ecosystems on a yearly basis could store roughly the equivalent of three percent of global greenhouse gas emissions—more than the annual emissions of the aviation industry.
Coastal habitats—including seagrass meadows, mudflats, marshes, mangroves, and forested tidal swamps—collectively store more carbon on an annual basis than the aviation industry emits.
While carbon storage suggests the ability to help offset greenhouse gas emissions, this study focuses on static carbon stocks and doesn’t directly measure new carbon removal from the atmosphere, Janousek says. The power of these ecosystems to sequester carbon is an area of active research.
“That is indeed something the Pacific Northwest Blue Carbon Working Group is addressing, but it is not the focus of this particular paper,” Janousek says.
But the new estimates for carbon storage are valuable in their own right. They’ll soon be deployed in climate mitigation modeling in Oregon, Washington, and British Columbia, and will inform British Columbia’s new Coastal Marine Strategy. They’ll also help guide Indigenous organizations and governments working to protect and restore coastal ecosystems—for their carbon stocks and other important ecosystem services.
