Mapping North Carolina’s ghost forests from 430 miles up

Mapping North Carolina's ghost forests from 430 miles up

Emily Ury remembers the first time she saw them. It was heading east from Columbia, North Carolina, on the flat, low stretch of US Highway 64 toward the Outer Banks. Coming out of the swamp at the side of the road was not one, but hundreds of dead trees and stumps, the relic of a once healthy forest that had been invaded by the dragging of sea water inland.

“I was like, ‘Whoa’. No leaves, no branches. The trees were literally just logs. As far as the eye could see,” said Ury, who recently earned a Ph.D. in biology. at Duke University working with Professors Emily Bernhardt and Justin Wright.

In the lowlands of the east coast of the US, trees are dying as rising sea levels and higher storm surges push salt water inland, poisoning soils far offshore.

While these “ghost forests” are becoming more common on the North Carolina coastal plain, scientists only had a rough idea of ​​their extent. Now, satellite images are providing new answers.

In a study published April 4 in the journal Ecological Applications, a team led by Duke extracted 35 years of satellite imagery from a 245,000-acre area on the state’s Albemarle-Pamlico peninsula.

The images show that, between 1985 and 2019, 11% of the tree cover of the area was occupied by ghost forests. Rather than reflecting the gradual pace of sea level rise, most of this spread occurred abruptly as a result of extreme weather events, such as hurricanes and droughts, which can concentrate salts or send them into the region.

The study focused on the Alligator River National Wildlife Refuge, which was established in 1984 to protect the area’s unique forested wetlands and the red wolves, red-crested woodpeckers and other endangered wildlife that live there. .

Here, the Duke team is monitoring what Bernhardt and other researchers call “the cutting edge of climate change.”

From 1900 to 2000, sea levels rose about a foot in this part of the North Carolina coast, faster than the world average. By the end of this century, it could rise two to five feet higher.

Shrinking shores dominates most discussions of sea level rise, as oceans submerge shores and eat up beachfront properties. However, less is said about what is happening inland.

Long before the beaches shrink and disappear under the rising sea, seawater begins to infiltrate the lowlands.

Most of the Alligator River National Wildlife Refuge is less than two feet above sea level, “which makes it even more vulnerable to rising sea levels,” Ury said.

Add to that the hundreds of miles of ditches and canals that run through the region. Built in the mid-1900s to drain water, they now act as a conduit for seawater to flow, which is about 400 times saltier than fresh water.

With no barriers in the way, seawater is pushed inland through these channels, leaving its salty fingerprints on the ground. As the salt enters, it draws water out of the plant cells and takes moisture away from the seeds, making it difficult for new tree seedlings to sprout. Salt-sensitive tree species do not first reproduce and eventually die, as freshwater forests turn into salt marshes.

Using images taken by 430-mile-high Landsat satellites, the team was able to map the spread of ghost forests in the refuge over time.

Each pixel in satellite images represents the wavelengths of light bouncing off the Earth below, in an area of ​​the ground roughly the size of a baseball diamond.

The team fed the satellite images to a computer algorithm, which in turn analyzed each pixel and determined whether it was dominated by pine trees, hardwoods, shrubs, grassy swamps, open water, or dead trees. Any pixel with between 20 and 40 visibly dead trees present at one time was labeled a ghost forest.

The view from space changed during the 35 years of the study.

More than three-quarters of the study area was covered by trees in 1985. Since then, even without any logging or development, the refuge has lost more than 46,950 acres of forest, or a quarter of its 1985 tree cover.

More than half of these losses occurred inside the refuge, more than a kilometer from any coast, the study revealed.

“It’s not just the strip that is getting wet,” Ury said.

Of the more than 21,000 acres of ghost forest that formed between 1985 and 2019, the most notable extinction was in 2012. The area had just endured a five-year drought and then a powerful hit from Hurricane Irene in 2011, when a 6 – The foot wall of seawater was pushed ashore. The storm surge passed through the refuge, reaching Highway 264, more than 1.2 miles inland from shore. Within months, entire groups of dead and fallen trees were visible from space.

What’s happening in eastern North Carolina is happening elsewhere as well, researchers say. In coastal regions around the world, saltwater is beginning to reach areas that have not seen it before, even reducing crop yields and endangering the freshwater aquifers that people depend on for drinking.

Duke’s team is collaborating with other researchers to expand their study to other parts of the Atlantic and Gulf coastal plains, from Cape Cod to Texas.

“Because of its geologic location, North Carolina is a little ahead of other coastal areas in terms of how much sea level rise has progressed,” Ury said. “Lessons learned here could help manage similar transitions elsewhere” or identify areas that are likely to be vulnerable in the future.

Investigation report: “Rapid deforestation of a coastal landscape driven by rising sea levels and extreme events”

related links

Duke university

Forest news: global and local news, science and applications



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