The Pacific Northwest’s once-prevalent tidal forests are mostly gone. But what remains stores an important secret
New world: A manmade “beaver dam analog” (foreground) and dead Sitka spruce trees are features of a restored tributary of Washington’s Grays River. Columbia Land Trust breached a nearby dike to reconnect the land, leading to tidal inundation. Photo: Nathan Gilles
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By Nathan Gilles. November 7, 2024. On a foggy October morning, Columbia Land Trust Stewardship Director Ian Sinks steers the old, donated aluminum workboat to a stop along the banks of the Grays River in southwest Washington. His coworker, wetlands ecologist Amy Borde, sits near the bow of the boat.
Dressed in a pair of chest-high waders, Borde hops off and into the shallow water, pulling the boat onto the shore of a muddy bank made up of dark, almost black, and, as it happens, very carbon-dense soil. I follow her.
The influence of the tide is huge here. By the end of the day, much of where Borde and I now walk heavy-footed—our boots making that characteristic squishy sound that walking in thick mud creates—will be underwater.
The Grays River feeds into the Columbia River just upstream from Astoria, Oregon, where the Columbia drains into the Pacific Ocean. Right now, the tide is out.
The tideline is clearly visible from our spot, marked by a thin green line of ferns and shrubs, which starts about four to five feet higher than the mud we’re walking in. The ferns sit atop the dark soil like green frosting on a slice of chocolate cake.
However, the ecosystem’s most important plant is at home right where we stand. Growing in the dark, muddy soil, well below the tideline, are 200-year-old Sitka spruce trees.
“This is a good example of a spruce that is getting frequently inundated,” Borde tells me, pointing at the thick trunk of a spruce.
The lowest three feet of the spruce’s trunk is the same color as the soil. From there, the tree’s characteristic grayish-brown, scaly bark starts climbing its sides before disappearing into a maze of branches and green needles overhead.
This is a West Coast tidal forest or swamp, an ecosystem few have visited. Only a handful of sites like this one can be found intact. And until recently, they had all but faded from our regional memory.
In recent years, Sitka spruce-dominated tidal forests have gained interest among conservationists and scientists alike.
New efforts to map the Pacific Northwest’s coasts have found that only a handful of these tidal forests remain. For this reason alone, conservation groups have sought to find, conserve and restore these forests.
One of those conservation groups is Columbia Land Trust.
Sinks and Borde both work for the Vancouver, Washington-based environmental nonprofit, and both wear trucker hats with their organization’s logo on them. Columbia Land Trust acquires land and places it in trusts to conserve and restore native ecosystems.
But rarity isn’t the only reason tidal forests have gained attention.
The other reason is a phenomenon that has captured the scientific community’s imagination—a phenomenon tied to the tide, the dark soil underfoot and the trees overhead—a phenomenon called “blue carbon.”
Hot topic
Blue carbon refers to carbon stored in marine ecosystems, including tidal marshlands and forests. Blue carbon ecosystems are now considered some of the most carbon dense on Earth, including the Pacific Northwest’s Sitka spruce-dominated tidal forests.
Blue carbon has become a hot topic among scientists hoping to study the phenomenon and environmentalists hoping to use it to cool the climate.
The spruce tidal forests along the Grays River lie at the nexus of both goals.
Wetlands ecologist Amy Borde on Grays River. Photo: Nathan Gilles
Columbia Land Trust first acquired land along the Grays in the early 2000s. It now owns and manages multiple tidal forest and marshland sites along the river, many of which have appeared in recent scientific studies.
This includes sites with intriguing names like “Secret River,” where some of the last mature and old-growth Sitka spruce tidal forests remain and where the full carbon storage potential of our region’s blue carbon has been calculated, one muddy bank at a time.
Just as important as these older “reference sites,” Columbia Land Trust has also opened its newer restoration sites to scientific scrutiny.
Over the years, the group has converted former pastureland along the river into tidal marshes, a process involving breaching old dikes to create new wetlands and reconnecting those lands to the river and the tide.
The group has also planted new spruce forests in the tidal zone.
If and when these restored ecosystems will match the older, naturally occurring reference forests is the topic of ongoing research.
What is clear is that very few Sitka spruce tidal forests are left.
Mapping the lost past
The growing range of Sitka spruce (Picea sitchensis) stretches some 2,000 miles from northern California to southern British Columbia.
While the tree’s range is long, it’s also incredibly narrow. Sitka spruces don’t typically grow more than a few dozen miles from the coast.
The tree also has a talent that few other large conifers have: it can withstand having its roots periodically submerged in water, even partially saline water, a talent revealed in the tree’s nickname, “tideland spruce.”
Yet despite the telling moniker, until recently, few knew that the tideland spruce once dominated the tideland forests of the Pacific Northwest.
That all changed with a 2019 report led by Laura Brophy, director of the Estuary Technical Group at the Institute for Applied Ecology in Corvallis, Oregon.
Tenacious T: Sitka spruce trees stand as part of an intertidal forest on Grays River. Photo: Nathan Gilles
The report estimates that over half of all tidal wetland ecosystems on the Oregon coast were once Sitka spruce-dominated tidal forests and that nearly 95% of these forests have been lost since Euro-American colonization began in the 1800s.
“The fact that [tidal forests] once occupied a larger area than tidal marsh, very few people knew [that]. And I think, still, very few people know that. They were just gone from the public imagination,” says Brophy.
In the Columbia River Basin, where Sitka spruce tidal forests stretch as far upriver as Puget Island, some 30 miles from Astoria, losses are estimated to be high.
Using data from a 2017 study in the Journal of Coastal Conservation, Borde estimates the Columbia River once contained 11,089 acres of Sitka spruce swamps. Only 1,529 acres remain.
Similar estimates of Sitka spruce tidal forest losses haven’t yet been made for the coasts of Washington and California, something Brophy plans to remedy with a new coast-wide analysis she’s working on. But based on data already available, Brophy says losses along the coasts of Washington and California are also likely high.
A 2019 study led by Brophy in the journal PLOS ONE revealed that the West Coast once had around 735,000 hectares (roughly 2,838 square miles) of tidal wetlands prior to colonization. About 85% of this has been lost.
Brophy’s 2019 studies began with research she first conducted in the 1990s involving the arduous task of mapping the Oregon coast’s 15 major tidal estuaries.
This work led Brophy to follow the tide, determining how far it would have reached had dikes and other obstructions not existed.
This, in turn, led her upriver to the farthest reaches of the tide, where she would see Sitka spruce tidal forests and where she first began to suspect the forests were much more widespread in the past than previously estimated.
It was a revelation. At the time, official maps of tidal wetlands didn’t even include tidal forests as a wetland category, let alone map them.
Years later, when she started putting numbers to her intuition, Brophy says many people were surprised at how widespread tidal forests had once been.
She credits the recent interest in blue carbon with raising awareness of tidal forests.
Blue carbon in the tropical Pacific and Pacific Northwest
Along with determining what has been lost, Brophy and others also helped find the remaining tidal forests.
This handful of “reference sites” would become key for scientists wanting to understand how much blue carbon these forests contain.
One of those scientists was Boone Kauffman, a professor in the Department of Fisheries, Wildlife, and Conservation Sciences at Oregon State University.
“The old-growth Sitka spruce tidal wetlands are probably the largest carbon stock of any forest type in the Pacific Northwest,” says Kauffmann.While many scientists have contributed to the literature around blue carbon in the Pacific Northwest, Kauffman is notable due to the connection he made between blue carbon in the tropical Pacific and the temperate Pacific Northwest.
From 2003 to 2011, Kauffman worked at the U.S. Forest Service-run Institute of Pacific Islands Forestry, where he became one of the first scientists to study the carbon storage potential of tidal mangrove forests, work that would lead Kauffman to venture throughout the Pacific, including to Indonesia, Micronesia and the Republic of Palau.
Mangrove forests are found in 105 countries across five continents. Like Sitka spruce, mangroves grow in tidal wetlands.
Branching out: Young Sitka spruce planted in a restoration effort on Grays River. Photo: Nathan Gilles
In 2011, Kauffman was the coauthor of a paper in the journal Nature Geosciences that concluded that mangrove forests were among the most carbon-rich forests in the tropics.
In the intervening years, multiple studies have demonstrated that mangrove forests are some of the most carbon-dense ecosystems on the planet. In fact, the term “blue carbon” was first coined—most likely around 2009—in part to highlight the carbon storage capacity of mangrove forests.
Kauffman would go on to study mangrove forests in South America, where he and his colleagues came to the surprising conclusion that mangrove forests store more carbon per area than trees on land in the Amazon rainforest.
In 2011, Kauffman moved back to Oregon, where he turned his interest in blue carbon to the region’s Sitka spruce swamps.
Springboarding off of Brophy’s mapping effort, Kauffman and colleagues traveled to the region’s remaining intact Sitka spruce tidal forests to measure their carbon stocks.
One of these “reference” sites was on the Secret River, owned and managed by Columbia Land Trust.
This work culminated in a 2020 paper led by Kauffman in the journal Global Change Biology that concluded that tidal Sitka spruce forests in Oregon, Washington and California were comparable to mangrove forests in their ability to store carbon.
Significantly, Kauffman and his coauthors—including Borde and Brophy—found that Sitka spruce tidal forests were similar to old-growth coastal forests outside the tidal zone in their ability to store carbon.
Only there was a catch.
Due to the widespread loss of Sitka spruce tidal forests, Kauffmann and colleagues had to make do with reference sites made up of both mature and old-growth forests.
This meant that the younger tidal forests stored comparable amounts of carbon per area but weren’t as old as the old-growth forests on land.
This implies, says Kauffmann, that if left to age, old-growth Sitka spruce tidal forests—like mangroves and the Amazon—will eventually surpass terrestrial old-growth forests in their ability to store carbon.
However, here, too, there is a catch.
All the talk about carbon storage in tidal forests comes with a big caveat for efforts to restore them: methane emissions.
The methane elephant in the room
On my trip to the Grays River, Borde showed me just how leaky methane can be.
Underneath a stand of Sitka spruce, buried in the mud, are sensors to measure the gas’s release from the ground. The gas is so leaky, in fact, that wood planks have been placed around the sensors to prevent scientists’ footsteps from accidentally releasing the gas from the ground.
“A lot of our work is pretty clearly beneficial to the climate,” Sinks tells me. “But we’re less certain about these tidal wetlands because of the methane.”
For an ecosystem to have a net cooling effect on the climate, it must store and actively take in more carbon than it emits. Figuring this out is called finding a carbon balance.
Columbia Land Trust Stewardship Director Ian Sinks on Grays River. Photo: Nathan Gilles
Getting a carbon balance out of the red can be challenging for wetlands because they typically emit the greenhouse gas methane.
Methane, or CH4, is second only to carbon dioxide as the leading cause of human-made climate change.
While there are many human sources of methane—including rice paddies and the dairy and meat industries—methane is naturally emitted from wetlands as anaerobic bacteria break down organic matter.
The sensor Borde shows me is one of many on Columbia Land Trust’s Grays River sites that have provided data for a study currently in review with the journal Ecological Applications.
Study coauthor Scott Bridgham, professor at the Institute of Ecology and Evolution at the University of Oregon, shared the most recent draft of the study with me prior to our phone interview.
The study’s findings suggest that methane emissions pose a potential climate hurdle when restoring marshlands in the Pacific Northwest. Borde and Brophy are coauthors.
“Generally, when these sites are restored, they simply break the dikes,” says Bridgham. “When they do that, they tend to be quite low in the tidal frame. And so, they will emit methane for quite some time.”
According to the study, some of the most egregious emitters of methane are sites that have only recently been restored to wetlands. Older reference site marshlands, by contrast, emit less methane; older, naturally occurring Sitka spruce tidal forests emit the least methane of all.
The problem is this: When a dike is breached, and tidal water is allowed to flood a former pasture or field, bacteria consume some of the organic matter in that pasture, emitting methane as waste. When the site becomes a tidal wetlands again, it also emits methane, but over time, the amount of carbon from methane tends to decrease.
The current paper in review focusses on methane emissions. But Bridgham says he and his colleagues are now working on a paper that calculates what these emissions might mean for the carbon balance of restored tidal ecosystems.
Their conclusion: many restored sites are likely to have a net warming effect on the climate in the short term. But given enough time, restored tidal wetlands will likely have a net cooling effect on the climate.
This process could take a long time to play out due to the way carbon builds up in wetlands and where it’s stored.
Sediment and soil carbon
Sitka spruce tidal forests can store as much carbon as they do because much of their carbon isn’t contained in the trees. It’s contained in the soil. This is what Kauffman and colleagues found in their 2020 paper.
The scientists determined that Sitka spruce forests in the tidal zone contained more carbon above ground (locked in the trees and other vegetation) than nearby marshlands. But they also contained more carbon below ground, locked in the dark soil the trees grow in.
Standing by: Sitka spruce in an intertidal forest. Photo: Nathan Gilles
It’s now known that this soil carbon is built up from sediment carried by water from the tide and other sources.
This process, called accretion, effectively buries plant matter and other ecosystem carbon.
It also helps build up the land, leading to a phenomenon observed in multiple studies: the higher in elevation within a tidal zone one travels, the larger and deeper the carbon stocks are likely to be.
Columbia Land Trust has conserved about 3% of the Sitka spruce tidal forest’s pre-colonial area along the river.
This process will eventually lead to Sitka spruce tidal forests in the Pacific Northwest, where large trees and deep soils equal lots of stored carbon.
Given enough time, this process will flip the carbon balance sheet until the amount of carbon stored and taken in by the ecosystem exceeds the methane it emits.
This is the potential that has so many people interested in blue carbon.
But this buildup of carbon takes time. This means restored sites are unlikely to be used in carbon markets intended to offset emissions, according to study coauthor Christopher Janousek, an assistant professor at Oregon State University’s College of Agriculture Sciences and coauthor on Kauffman’s 2011 paper.
“Based on estimates of accretion rate, carbon soil probably took at least several hundred years, if not a millennium or two, to sort of develop into these forests. So, my guess is that their long-term carbon sequestration and storage potential is high, but their short-term capacity maybe is not,” says Janousek.
Bridgham agrees with Janousek, adding that methane emissions pose a potential obstacle for groups trying to restore wetlands.
“There’s a group of practitioners who rightly are very involved in wetland restoration where they don’t always like the answer [related to methane], but they want the answer because they want to do the objective right thing,” says Bridgham.
Beyond the climate lens
Sinks agrees that methane emissions are a concern in restored wetlands, but he says the interest in blue carbon and concerns about methane can create a myopic view of tidal ecosystems as “anaerobic deserts,” a term he heard recently from a Grays River community member while discussing his organization’s proposal to break a dike, flood a pasture and turn it back into a tidal marsh.
“The soils might be anaerobic, but the surface water certainly isn’t. There’s plenty of life above ground,” says Sinks. “I think it harkens back to the days when swamps were regarded as wastelands and not useful to society. We know much better these days how valuable these areas are for flood mitigation, for wildlife habitat, for water quality, all of these functions.”
Bridgham and Janousek make similar statements, suggesting that seeing all ecological issues through a climate lens could distract people from seeing the bigger ecological picture and the value of restoring wetlands.
Borde says the methane issue needs to be faced squarely.
“It’s been a question for a while about these tidal wetlands and what the carbon balance is, because it has been known that methane is potentially an issue,” says Borde. “So, it may not be the answer that we want, but to figure out how we can work within that and alleviate some of that to make these systems as beneficial as possible, it’s really the goal.”
Borde and her colleagues are also experimenting with ways to speed up the restoration of Sitka spruce forests by building elevation, including by burying logs.
“We see these as important restoration projects to ultimately reach that goal of restoring these systems that have been lost,” says Borde. “And along the way, there’s a lot of co-benefits, from habitat to flood control. And they are on that trajectory to storing carbon. It may take a while, but they’re on the right path.”
Borde estimates Columbia Land Trust has conserved roughly 312 acres of Sitka spruce tidal forests in the Columbia River Basin, roughly 20% of what remains and just under 3% of the forest’s pre-colonial area along the river.
