They look like the skeletons of giants reaching for the sky. Tall and ashen, stripped of their leaves and most of their branches, the trees stand in uneven rows that extend as far as the eye can see, grave markers of a forest that once thrived near this North Carolina coastline.
“These ghost forests are the leading edge of climate change,” Emily Bernhardt, an ecosystem ecologist and biogeochemist at Duke University, tells The Scientist. They succumbed in slow motion to salty water making its way inland as sea levels rose, and serve as a reminder that global warming isn’t something just affecting polar bears in remote locations, or something that will happen in the distant future, she says. “It is already happening, and happening really fast on the coastal plain of North Carolina.”
In the decade that Bernhardt has worked on the Albemarle-Pamlico Peninsula, a few dozen miles west of the Outer Banks barrier islands, she’s watched “acres and acres” of the state’s wetland forest die. Cedars, maples, pines, and elms are drying up, becoming too stressed by salt to produce the chlorophyll they need to survive. While the loss is devastating, Bernhardt says, she and her colleagues are acutely aware of the opportunity they now have to study what’s going on. They want to identify exactly how salt kills, understand what happens to the ecosystem as the trees die, and determine whether there’s any way to intervene and preserve the habitat.
Inland forests aren’t the only eco-systems threatened by the rising seas. All along the Atlantic coast, from Canada to northern Mexico, saltwater is inundating freshwater marshes, cypress swamps, and even farm fields. “Ghost forests are perhaps the most iconic and easy-to-photograph example of a much larger problem,” Bernhardt says. “Basically, if you’re on the coast and on flat land, saltwater intrusion is going to be an issue.”
Ghost forests are perhaps the most iconic and easy-to-photograph example of a much larger problem.—Emily Bernhardt, Duke University
As salty water floods and then recedes from these freshwater environments after storm surges or record-breaking high tides, the periodic inundation may increase the emission of greenhouse gases such as methane from the soil. Because of this, scientists are concerned about the environmental effect of saltwater’s inland intrusion, while farmers and residents, who are starting to see their fields and yards affected by salt and flooding, are bracing for the likelihood that they may soon be forced to move.
Such concerns have led researchers to get creative in their search for solutions, including envisioning how ecosystems and land use will change as sea levels rise. The goal of this predictive exercise, says Matt Whitbeck, a wildlife biologist with the US Fish and Wildlife Service working at a station on the Chesapeake Bay, is to figure out how to shape the land into a system that is not only resilient to climate change but also best supports biodiversity, minimizes greenhouse gas emissions, and provides for human needs. And it’s a safe bet, he says, that such an ideally adapted habitat won’t be farmland or forest, but probably something more like marsh.
Droughts and storm surges bring the salt
On the peninsula known as Maryland’s Eastern Shore, off historic Route 50, thousands of acres of forest and marsh sit relatively untouched. The land is part of the Blackwater National Wildlife Refuge, a 28,000-acre sanctuary established in 1933 for birds migrating along what’s called the Atlantic Flyway. Despite nearly 90 years of preservation, the area hasn’t escaped the scars of human activity. From an observation deck at the edge of Blackwater Pond, which sits at the center of the refuge, tree skeletons shoot from the ground in every direction. Only in the distance can one see leafy trees dotting the landscape.
“Blackwater is really a place where people can see the impacts of sea level rise with their own eyes,” Whitbeck says.
Some of the earliest aerial images of the refuge date to 1938. Several years ago, Whitbeck and his colleagues compared those pictures to photos taken in 2006 and estimated that more than 5,000 acres of tidal marsh in the refuge had turned to open water, and that some 3,000 acres of forest had turned into marsh. The forests and marshes are moving, retreating farther and farther inland as sea levels rise. “That [migration] was something that really didn’t come to our attention until 2010 or 2011,” Whitbeck says.
It was around that time that Bernhardt and her colleagues started tracking a habitat that is similarly becoming much wetter—in this case, partly as a result of human intervention. It’s the Timberlake Observatory for Wetland Restoration, a stretch of former farmland that sits off Highway 64 on North Carolina’s Albemarle-Pamlico Peninsula. Developers started converting it back to its original wetland ecosystem in the early 2000s, lowering the fields, filling in the drainage ditches, and planting 750,000 live saplings of wetland tree species including bald cypress (Taxodium distichum), cottongum (Nyssa aquatica), and water oak (Quercus nigra). A few years later, the developers cut off the pumps that drained the land, flooding the area with freshwater, and they asked Bernhardt and Marcelo Ardón, then a postdoc in Bernhardt’s lab, to help monitor how residual compounds from farming fertilizer affected the soil and plants growing there.
“We were very interested in this interplay—what happens when you have nutrients coming from upstream and salt coming from downstream where they mix, which seems to happen a lot in these areas,” says Ardón, now an ecologist at North Carolina State University.
The newly flooded area didn’t stay wet for long. Severe droughts across the state in the summers of 2007 and 2008 dried out the soil, which Ardón and Bernhardt noticed was becoming extremely salty. The wetland sits more than 30 miles from the Atlantic coast, but the Albemarle-Pamlico Peninsula is flat—so flat that the water that surrounds the land can be pushed upstream by the wind. In drought conditions, lots of water evaporates, leaving behind saltier water to get blown inland. “When we have these long, extended, two- or three- or four-month droughts like we did in 2007 and 2008, we saw the water getting saltier and saltier and saltier,” Bernhardt says. And when it did rain, the salt didn’t go away. The soil was still caked in salt when it dried out after the rain.
Blackwater is really a place where people can see the impacts of sea level rise with their own eyes.—Matt Whitbeck, US Fish and Wildlife Service
Land managers at Goose Creek State Park, which sits on the southern side of the Albemarle-Pamlico Peninsula, ran into a similar problem in 2011 as Hurricane Irene blasted the coast, creating a storm surge that pushed tons of salty water into the sounds and rivers, flooding coastal towns. After the waters receded, the land managers noticed that the water at Goose Creek was still salty. The cause in this case—identified by NC State hydrologist Ryan Emanuel—was a raised dirt road that cut across the forest and prevented drainage of a pool of salty water that had formed in the forest during the hurricane.
“I got interested in ghost forests through this very specific problem that was happening at Goose Creek State Park,” Emanuel says. “And then, as I drove around eastern North Carolina, I became attuned to just how widespread [the problem] was.” And the saltwater wasn’t just affecting forests, but farms, too.
When Emanuel chatted with Ardón, Bernhardt, and other researchers in North Carolina about what he’d seen, they all agreed that saltwater intrusion would become a major issue all along the North Carolina coast. Once inland, salt begins to blight farmland and to damage trees, creating the now-iconic ghost forests. With both droughts and storms expected to worsen and increase in frequency as Earth’s climate warms, the researchers hope to understand exactly how salt kills and to identify early signs of salt stress that could flag at-risk trees and crops before they become ghosts.
“What we’d love to know is which forests are vulnerable, which forests are likely to become ghost forests in the near future,” says Bernhardt. “That might help us actually think about how to manage the [forests], and perhaps protect them.”
While some plants and animals living in ever-saltier landscapes seem to be capable of adapting, it’s the people there who are going to be displaced, notes Erin Seekamp, who develops models of climate adaptation planning at North Carolina State University. Near Taylors Island, west of the Blackwater National Wildlife Refuge in eastern Maryland, saltwater marshes have already begun to infiltrate residential backyards and are encroaching on a historic graveyard. South of the refuge, in Somerset County, residents are selling their homes as the invasive saltmarsh reed Phragmites australis advances deeper into their lots.
Part of the land being left fallow is farmland. Monitoring abandoned fields as salty water periodically makes its way inland, coastal marine ecologist Keryn Gedan of George Washington University in Washington, DC, and biologist Eduardo Fernández-Pascual of the University of Oviedo in Spain found that these regions are producing a new and diverse set of plant communities not seen in traditional wetlands, suggesting the fields may respond to salty conditions differently than natural marshes. They may be dominated by marsh shrub species, rather than reed species, and have a greater resilience to P. australis (J Veg Sci, 30:1007–16, 2019).
That transition of farmland and residential land to marsh is happening predominantly in low-lying, rural communities that are already economically disadvantaged. These communities typically lack the funds to pay for infrastructure—such as seawalls seen in China, dikes in the Netherlands, or even elaborate pumping systems used on large-scale farms—that could help to mitigate saltwater intrusion and later sea level rise. On the Albemarle-Pamlico Peninsula in North Carolina, for example, it took nearly 20 years for the town of Swanquarter to raise the funds to build a dike system to prevent storm flooding. The cost of that project means that strategy is now likely “off the table” for other communities that have begun to be affected by saltwater intrusion, Seekamp says.
“We’re not really talking about how human communities are affected by ghost forests,” she says. “But as ghost forests form and marshes migrate, we’re going to see loss of land. . . . We’re going to be faced with really hard decisions about what people should do, and how our current federal policies and state level policies can help these communities.”
Resettling communities farther inland is an option—one that’s being tried in Louisiana with residents of Isle de Jean Charles. The narrow ridge of land, inhabited mostly by people with American Indian ancestry, is being swallowed by the Gulf of Mexico, with the salty water infiltrating the remaining oak forests and transforming them into graveyards of tree skeletons. The state government has allocated funds to relocate residents, but many aren’t willing to leave. “There’s this really important aspect of people’s connections to place,” Seekamp says. “It’s hard to hold a community together if that place connection doesn’t exist.”
A series of salty experiments
With this goal in mind, Bernhardt, Ardón, and their collaborators decided to run an extreme experiment: carefully sprinkle large bags of aquarium salt into plots in the restored wetland forest at Timberlake. The team also used similar tactics in the greenhouse on Duke’s main campus in Durham. Added salt makes it harder for plants to take up water because excess sodium and chloride ions accumulate around the roots, keeping the water in the soil instead of letting it diffuse into the roots. In addition, the sodium and chloride ions that do move into plants damage their tissues and prevent the production of chlorophyll, the light-harvesting pigment essential for photosynthesis. Ultimately too much salt will kill the plants.
As the researchers watch the plants respond to the added salt, they are looking for changes in spectral traits—the light reflected from the plants—and other characteristics that could reveal that they are ill well before they are on the verge of death. “We really would like to be able to sense salt-related stress from [satellites in] space or from drones,” Bernhardt says, “but we don’t know what that looks like yet.”
Part of the reason the researchers want to protect the trees is because they take in greenhouse gases, such as carbon dioxide (CO2), and sequester carbon in their tissues and in the soil. But if forests die en masse, it’s not clear what happens to those greenhouse gases. To figure that out, Melinda Martinez is “measuring tree farts,” as Ardón likes to say of his student’s work. Specifically, Martinez uses a plastic sheet lined with neoprene and a portable gas analyzer to measure emissions from dead trees. The goal is to see whether the skeletal tree trunks serve as chimneys, piping methane and CO2 up into the atmosphere from the soil microbes, or as plugs, keeping the gases in the ground. So far, Ardón says, the preliminary data point to the dead trees as plugs that minimize the emission of greenhouse gases into the atmosphere.
Still, greenhouse gases can leak into the atmosphere directly from the soil. Under normal conditions, forested wetlands constantly give off methane due to the high numbers of methane-producing bacteria that thrive in the water-soaked habitats, says Martinez, and repeated inundation with salty water may affect the soil microbiome composition and emissions.
In 2011, the University of Connecticut’s Ashley Helton, then a postdoc in Bernhardt’s lab at Duke, collected soil samples in PVC pipes from Timberlake and brought them back to the lab, where she regularly measured their greenhouse gas emissions as they were subjected to different treatments: irreversible inundation with salt or fresh water, or being left to dry out between floodings with salt water once a week for 20 weeks. Separately, the team started dumping water with various salt concentrations into PVC pipes pushed down into the ground at different spots around Timberlake, then measuring the soil’s emissions in the field.
Based on past research suggesting that adding moisture to the soil could speed the decomposition of roots and stems of dead plants in forested wetlands and thereby accelerate the flux of greenhouse gases into the atmosphere, Bernhardt’s group had predicted that CO2 emissions would increase. But the opposite happened: in both the lab and field experiments, upping the salt reduced CO2 emissions by almost half. Methane emissions also dropped in permanently saltwater-flooded soils. But when the soils switched back and forth between being dry and wetted with salty water, they spouted methane, releasing double the amount exuded by untreated soils or soils intermittently flooded with freshwater. Bernhardt notes that similar studies have shown conflicting results, so it’s not exactly clear how flooding affects a forest’s soil-to-atmosphere greenhouse gas emissions.
Repeated influxes of saltwater are exactly what is expected as strong hurricanes and storm surges increase in frequency. “Sunshine flooding,” where higher tides, as a result of rising seas, leave standing water in inland regions independent of precipitation, is also expected to become more common. And as lower-lying lands become fully inundated with saltwater, the land farther inland will begin to experience this intermittent flooding. If this type of flooding does increase methane emissions, it could exacerbate climate change.
To complicate matters further, farmers in near-coastal areas can control how water flows to their fields and adjoining wetlands with pumps and floodgates designed to keep water off their land. In some places the intermittent flooding will advance, and in others it will stall until the sea level rises so much that it overwhelms that infrastructure. “And that’s a big part of the uncertainty about our future—how are those things going to be managed?” Bernhardt says. “It’s important that the water movement in this landscape is very much under human control.”
Salt Kills Trees from the Roots Up
Most trees are extremely sensitive to salt, from the roots, which struggle to take up water from salty soils, to the trunk, branches, and leaves, where high concentrations of salt ions hinder plants’ cellular processes.
© KERRY HYNDMAN
Forests and marshes on the move
At Blackwater, as forested wetland is lost to rising sea waters, marsh habitat is appearing in its place. Because marshes support a unique ecosystem containing plant and animal life and may sequester greenhouse gases better than open water, marshlands might be better for the environment than letting the land become completely inundated. So land managers at the refuge are working to keep the marsh above water. One project, for example, involves taking layers of sediment from the Blackwater River, a feeder for the Blackwater Pond, and spraying it out over the marsh. So far, the team has spread 26,000 cubic feet of sediment across 40 acres of marsh to raise its surface four to five inches on average. The team then planted hundreds of thousands of marsh grass sprigs to keep the sediment in place and bolster the marshes’ resilience to sea level rise.
Around the globe, sea level has been rising since the peak of the last Ice Age, about 20,000 years ago, as a result of melting glaciers adding water to Earth’s oceans. “These marshes in the heart of the Blackwater River have been building elevation in response to [that] sea level rise for almost a thousand years,” Whitbeck says. But now the marshes can’t keep up. In addition to the continued melting of the world’s remaining glaciers, which are disappearing faster than ever, the mid-Atlantic region is sinking, causing it to experience sea level rise at a rate three to four times faster than the rest of the terrestrial world. Artificially raising the marshes “puts the plant community back in that sweet spot,” Whitbeck says, “so hopefully we can get a few more decades out of them rather than just a few years.” This strategy, he notes, could be applied to the countless marshes in the Chesapeake Bay and elsewhere that are “drowning in place.”
The issue is a pressing one. Mapping marsh and forest habitats within the refuge over time, Whitbeck and colleagues starkly revealed the thousands of acres of marsh that have turned into open water over the last few decades and the thousands more turned from forest to marsh. Models of sea level rise predict that by 2050 the refuge could lose thousands more acres of marsh. In other words, building up existing marshes won’t be enough; Whitbeck and his colleagues also want to predict where the marshes will move—where forest will turn to marsh—as sea levels continue to rise.
In a report published a few years ago, Whitbeck and his collaborators identified regions that might support marsh movement in the future, what they called marsh migration corridors. Two regions that the researchers have specifically focused on—the Nanticoke River at the east end of the Blackwater Refuge and Coursey Creek to the west, both of which are a mix of marsh and forest now—could maintain nearly 6,000 acres of marsh in the refuge by 2100. To encourage such marshes to grow, however, the team has to be mindful of obstacles, such as dead and dying trees and invasive reed species, that might impede migration. “Having those standing snags really precludes a lot of salt marsh species in the mid-Atlantic from using an otherwise suitable habitat,” says Whitbeck. His team has ripped out ghost forest tree stumps and controlled invasive plants to bolster the growth of native marsh plants that serve as high-quality habitat for bird species such as the black rail and the salt marsh sparrow. But despite the researchers’ best efforts, both bird species’ numbers are still declining, and the US Fish and Wildlife Service is now considering whether to list them as threatened, Whitbeck says.
Still, it’s not at all clear how the changing landscape is influencing local biodiversity overall. For example, North Carolina–based wildlife ecologist Paul Taillie, currently a University of Florida postdoc, and colleagues recently showed that while the transition from wetland forest to ghost forest in North Carolina detrimentally affected certain bird species that need heavily canopied forests to survive, it appeared to benefit a number of bird species that live in dense shrubs. Similarly uncertain are the effects of ghost forests on the landscape’s ecosystem services, as demonstrated by Martinez’s work on the corking of greenhouse gas emissions by dead tree trunks.
To get scientists who are studying these changes to talk with one another about their findings, Bernhardt is spearheading an effort to coordinate all of the research on saltwater intrusion in landscapes from the Gulf of Maine to the Gulf of Mexico. She and nearly 80 other researchers have started collaborating, using funds from the Environmental Research Institute at the University of Virginia, and the team has applied for a grant from the National Science Foundation to collate the “explosion of research . . . documenting the dramatic ecological change well inland as a result of salinization of surface waters, soils and ground-water,” according to the proposal. The scientists, together with local communities, want to help land managers decide what to do with land that’s becoming increasingly salty and therefore less useful for farming, logging, and other human uses. (See “Losing Land” on page 34.)
“Ghost forests will be a memory,” says Matt Kirwan, an ecologist at the Virginia Institute of Marine Science—only “a marker for the next fifty to a hundred years of what the land once was.”
Ashley Yeager is an associate editor at The Scientist. Email her at firstname.lastname@example.org.