Michael Flynn was surveying the beach in Rodanthe, N.C., after a nor’easter swept through. Storm surges are common in the state’s thin strip of barrier islands known as the Outer Banks. Flynn, a physical scientist with the National Park Service who lives nearby, is often first on the scene to survey any damage.
Something odd caught his eye. A log of weathered wood protruded straight up from the beach, close to the waterline. Most buildings in the Outer Banks are built on stilts, so he first assumed it was an old piling. But as he got closer, he realized it looked like the trunk of an eastern red cedar, a species that typically grows farther inland.
Curious, Flynn posted photos to Twitter, where they reached Emily Elliott, a coastal geologist at the University of Alabama. The two have been collaborating ever since.
Over the past 2 years, the team has documented 48 such stumps emerging from the sand, indicating the location of former forests devoured by the sea. The tides have been uncovering and reburying such stumps for centuries. But now, Flynn, Elliott, and their team of collaborators are working to understand the history of the islands’ movement, and they are hoping these long-gone forests can help modern land managers plan for the future. They will present their preliminary research at AGU’s Annual Meeting 2024 in Washington, D.C., on 11 December.
Sunken Trunks
“This has real implications in terms of the way we work with the coast and where we decide to continue to build.”
The Outer Banks are an ever-shifting ecosystem. As sea level rises, the shoreline moves toward higher ground in a geologic process called transgression. It’s likely that creeping seawater killed the roots of these trees, so understanding when the trees died could mean understanding the timeline of past transgression.
If the present rate of transgression matches the rate from recent centuries, builders will have a better idea about the future shape of the Outer Banks. The research “has real implications in terms of the way we work with the coast and where we decide to continue to build,” said Elliott.
Following a storm, Flynn and his colleagues now scan the beaches for stumps. They’ve identified trees across roughly 138 kilometers (86 miles) of the island chain’s coastlines. Flynn documents each trunk’s coordinates and elevation and, when possible, chips off a sample before the tide reburies the evidence. Initial radiocarbon dating of these samples shows trees on Hatteras Island, for example, died between 1650 and 1800. The stumps on Currituck Banks date between 1450 and 1500.
Reading the Trees
The team plans to use alternate methods to further narrow the timeline of when transgression occurred. For instance, ground-penetrating radar can map the subsurface and will hopefully allow researchers to tie the tree stumps to specific layers of deposition, Elliott said.
They also hope to use dendrochronology, or tree ring dating, to pinpoint the precise year various trees died. But there are complications. For one, dendrochronology works by comparing tree ring samples from trees with unknown ages to samples with known ages from the same area. But that local chronology doesn’t yet exist, meaning the team will need to build a new century-spanning timeline to identify the death dates of the stumps.
That information “would be critical for really understanding how rapidly that barrier island is rolling over,” explained Joshua Bregy, a paleotempestologist at Clemson University who is collaborating on the project.
“It’s a great place to have extra data.”
But accurate dating also depends on the condition of the stumps, which have been deteriorating for centuries. “I would be really pleasantly surprised if they were able to get traditional tree ring chronology information,” said Erika Wise, a dendroclimatologist at the University of North Carolina at Chapel Hill who was not involved in the research. Beachy areas don’t produce particularly long-lived trees, and the stumps might be too weathered to provide useful cross sections, she said.
Despite the drawbacks, she said, “it’s always useful to have more than one line of evidence.” That’s especially true in the Outer Banks, where coastal erosion and collapsing homes frequently make the national news.
“It’s a great place to have extra data,” Wise said.
Rerouting for Erosion
Transgression affects decisions about where to build. In 2023, for example, the National Park Service bought and destroyed two threatened homes in Rodanthe, the easternmost point of North Carolina, with private funding. In 2022, the North Carolina Department of Transportation opened a 4-kilometer (2.5-mile) bridge connecting the inland side of Rodanthe to the Pea Island National Wildlife Refuge to protect the existing transportation corridor from ongoing erosion.
These are just two adaptations among many. The islands are ephemeral and will continue to change. Any information on the rates of transgression could help island communities adapt to the ongoing erosion. Each new stump that emerges can help pinpoint the historic position and shape of the coastline, aiding planners as they move homes and infrastructure along with the ever-moving islands.
—J. Besl (@J_Besl), Science Writer