Why Ecologists Are Haunted by the Rapid Growth of Ghost Forests

For years, Emily Ury traversed North Carolina’s coastal roads, studying patches of skeletal trees slain by rising seas that scientists call "ghost forests." Killed by intruding saltwater along the Atlantic Coast, they are previews of the dire fate other forests face worldwide.

Ury knew that ghost forests were expanding in the region, but only when she began looking down from above using Google Earth did she realize how extensive they were.

“I found so many dead forests,” says Ury, an ecologist at Duke University and co-author of a paper on the rapid deforestation of the North Carolina coast published last month in the journal Ecological Applications. “They were everywhere.”

As the ocean intrudes and saltwater rises, it kills trees and creates these ghost forests—bare trunks, and stumps, ashen tombstones marking a once-thriving coastal ecosystem. In North Carolina, pine, red maple, sweetgum and bald cypress forests are being replaced by saltmarsh. Eventually, that saltmarsh will be replaced by open water, a shift that leads to significant and complex costs to the environment and the local economy. The loss of forests will reduce carbon storage, further fueling climate change, and the agriculture industry, timber interests will suffer as saltwater moves inland.

To look at the extent of the spread of ghosts forests, Ury turned from Google Earth, which provides only a few years of data, to satellite images taken from 430 miles above the Earth going back more than thirty years. Her team created an algorithm that searched thousands of images for changes in forest coverage focusing on the Alligator National Wildlife Refuge on the Albemarle-Pamlico Peninsula, an area of 152,000 acres untouched by logging or development.

The results shocked her. ”I was surprised not just by the amount of ghost forest, but the amount of area that was forest in the ‘80s and no longer is,” she says. Between 1985 and 2019, more than 10 percent of the area’s tree cover became ghost forest.

The accelerating transformation of healthy forests along North Carolina and the rest of the Atlantic and Gulf coasts are an ominous sign as saltwater marches inland in low-lying places like Vietnam, Mexico, Bangladesh and even Italy, fouling fresh water and destroying rice paddies, farmland and forests.

“What we see here today will happen in other places around the U.S. and around the world in the decades to come,” says Matthew Kirwan, an associate professor at the Virginia Institute of Marine Sciences, who has been studying the transition from forests to marshes on the Chesapeake Bay for two decades.

While saltwater intrusion killing forests has been studied for nearly two decades, the Duke team’s work revealed the process isn’t always slow and inexorable. Rapid sea-level rise and extreme events fueled mass deaths across the area.

A five-year drought that began in 2007 weakened an already-stressed forest. Hurricane Irene’s storm surge followed, sweeping more than a mile inland and coming before a massive forest fire, both in 2011. Those events, Ury says, created a tipping point that accelerated the rise of ghost forests in the following years. In the year following the hurricane, more than 11,000 acres of ghost forest was created. Over the 35 years, her team studied, more than half the 46,950 acres of forest the refuge occurred in the interior, half a mile from the coast.

Emily Ury measures soil salinity in a ghost forest. (Emily Bernhardt, Duke University)

The Atlantic Coast is a hotspot for ghost forest research because of its particular geography. The land is sinking so the relative sea-level rise is faster. The low coastal plain means sea level rise intrudes further, and a lack of levees allows saltwater intrusion. Humans have changed the landscape as well by building canals and ditches, which carry saltwater inland during storms and high tides.

Coastal forests giving way to saltmarsh as water intrudes has long been known as a natural process. Dead trees along the margins of estuaries were documented as far back as 1910. Kirwan’s research using historical photos and sediment cores reveals that in the Chesapeake Bay region alone more than 150 square miles of forests have converted to marsh since the mid-1800s. Over the past century, he says, the death march of coastal forests retreating inland has accelerated every decade and is now three times faster.

Kirwan grew up walking the coastal forests of the eastern Maryland shore and has been studying saltwater intrusion and marsh migration on the Chesapeake Bay for two decades. Residents there complain about the forest loss, but it happens just slowly enough that they don’t realize the changes are accelerating.

Both ecosystems offer benefits. Coastal forests provide timber, agriculture, places to live and some storm protection. Saltwater marshes provide marine and bird habitat, recreation, water filtration and shoreline protection during storm surges.

“From an ecological perspective, I always argue that [migration] is a sign of ecosystem resilience. It’s a win or at least a tradeoff, Kirwan says. “But from a human perspective, we’re giving up usable farmland, usable forest land and places where people lived 50 years ago—dry land that’s no longer inhabitable.”

This animation shows the loss of healthy forest (in green) from 1985 to 2019 on North Carolina’s Albemarle-Pamlico Peninsula. (NASA / USGS)

Scientists are still investigating how the loss of coastal forests will affect the climate. Lindsey Smart, a research associate at the North Carolina State University Center for Geospatial Analytics, turned to satellite images and LIDAR (light detection and ranging), which uses laser pulses to map the land, to examine the rise of North Carolina’s ghost forests and the effect on carbon storage, a key buffer against global warming. “A really great benefit of healthy coastal forests is that they can sequester and store carbon above ground and below ground,” she says. “As coastal forests transition to marsh, we lose this above-ground carbon.”

Some carbon is released into the atmosphere and some shifts to other places that store carbon like soils and wetlands. “How much remains uncertain,” she adds. “We need more research on that, but we think that leads to a loss in carbon, at least in the near term.”

The vast majority of forest land along coastlines in the eastern half of the United States is privately owned. Understanding where, how extensively, and how fast ghost forests are forming and transitioning to saltmarshes is important as localities look to adapt to climate change and preserve the natural resilience they provide.

In many places, coastal forests and saltmarshes bump up against development—seawalls, levees, bulkheads and even lawns—that stops the natural migration inland. When rising seas come up against those hard barriers, the saltwater quickly turns the trapped ecosystems from coastal forest to saltmarsh to open water. Open water doesn’t provide the same ecosystem services, including carbon storage and storm protection. Smart says we need to think about providing resources for landowners who want to create adaptations allowing the migration of forests and saltmarshes inland while understanding the negative economic impacts—lost farm and timber lands. In one federal program, for instance, farmers are paid to remove land from production as saltwater intrudes.

“We have this very complex social and ecological challenge,” she says. “We need to think creatively about providing appropriate resources for landowners who want to implement adaptation.”

The demand for coastal living and the accelerating rate of ecosystem loss has Ury worried that coastal forests and marshes along the Atlantic Coast face a bleak future. Increasingly, she says, fewer and fewer places exist for them to migrate as nature intended.

“There just isn’t room because of development and agriculture,” Ury says. “These ecosystems are being squeezed out probably to the point of extinction.”

from Hacker News https://ift.tt/3yeYfEt