Ghost Forests Could Provide Respite for Marsh Birds as Coastal Habitat Disappears

At Blackwater National Wildlife Refuge in Maryland, acres upon acres of dead tree trunks stand sentinel, their bases partially covered by salt water. Back in 2003, Hurricane Isabel struck the refuge before it could recover from an unusually dry summer. The storm surges brought salty ocean water into the already parched landscape and essentially choked the salt-intolerant trees to death.

A 280-mile drive away, at Alligator River National Wildlife Refuge in North Carolina, it’s a similar story. A one-two punch of drought followed by a storm in 2011 killed massive swaths of trees on the refuge. And as climate change drives further sea-level rise, experts expect to see more of these haunting scenes called ghost forests. “It's a process that has been going on for quite some time, as the sea has been rising,” says refuge manager Scott Lanier. “It's just now, it seems more accelerated.”

People tend to frame these changes in morbid terms: Aside from “ghost forests,” those rows of dead trees have been called “wooden gravestones,” evoking a sense of somber finality. But they are not necessarily an ending. Ghost forests are markers of a habitat in transition, an intermediate phase between a forest and a marsh. The forest may die, but in its place a marsh is born.

To Mid-Atlantic marsh birds, these dead forests represent an opportunity. Some scientists and conservationists are working to ensure this land converts to quality marsh as the climate changes, providing a place for threatened coastal species to move inland as their current habitat is consumed by rising sea levels. At Blackwater, old tree stumps in present-day marshes are a sign of how the habitat has changed. “These areas used to be forests,” says Dave Curson, Audubon Mid-Atlantic's director of bird conservation. Now they have converted into quality marshland—and marsh birds have moved in. “We see Seaside Sparrows and Clapper Rails and Least Bitterns.” Even at protected areas like Blackwater refuge, the new habitat created by marsh migration inland will likely not be enough to compensate for all that’s lost, Curson says. But these marshes could still be a lifeline for threatened birds.

The process of a forest becoming a marsh starts with a seawater flood—often caused by a hurricane or, in the longer term, by sea-level rise. When salt water first encroaches on a forest, it might kill mature trees but it can prevent new ones from growing. With more frequent saltwater exposure, some trees may begin to die off. Then, as more trees die and become snags, more sunlight is able to reach the ground, allowing salt-tolerant shrubs to colonize what once was the forest floor. Finally the system will look like a marsh, with saltmarsh grasses and only stumps left behind from the ghost forest, like at Blackwater.

This change occurs at the expense of forest habitat. Coastal forests are important spaces for migrating birds, and many canopy-dwelling species will lose range as climate changes. But other birds benefit from their loss. From 2013 to 2015 ecologist Paul Taillie, now a postdoctoral researcher at the University of Florida, and others examined bird species on North Carolina’s Albemarle-Pamlico Peninsula, home to Alligator River National Wildlife Refuge. He found that although forest-canopy birds lose some habitat, ghost forests can support a range of other species, including many that are threatened. The dead snags appeal to cavity nesters such as woodpeckers and the Prothonotary Warbler, which has been declining in recent years due to habitat loss. And where scrub vegetation grows, Taillie found birds such as the vanishing Northern Bobwhite, which declined by 85 percent across its range from 1966 to 2014.

Taillie promotes helping ghost forests transition to marsh as a conservation strategy. He argues the forest species that will lose out have ample space elsewhere, while the birds that benefit from snags and marshes are more in need of habitat. “There's lots of reasons to be concerned about dying trees, and I share that concern,” he says. “But when it comes to animals, coastal marshes are actually super important. They support a lot of species that don't occur anywhere else.”

__________

Taillie promotes helping ghost forests transition to marsh as a conservation strategy. 

__________

A ghost forest doesn’t always convert to quality marsh naturally. At Blackwater, Curson has seen transitioning habitat get choked out by Phragmites, an invasive grass that displaces native plants and wildlife. In other cases, small depressions form around dead trees as their roots shrink. Salt water fills these areas and erodes more of the soil, eventually converting the forest to open water rather than marsh. When these less-than-ideal alternatives play out, the possibility for coastal birds to move into higher-elevation marshes is “severely compromised,” Curson says.

Curson and his colleagues have experimented with ways to drain the areas where the ground has subsided, such as digging channels to funnel the water away. So far, they haven’t found success. “But we’re going to keep working on the issue,” he says. In 2016, conservationists sprayed a mixture of dredged sediment and water across 40 acres of marsh at Blackwater. Over the following months, new marsh grasses were planted as the sediment settled into a layer of 4 to 6 inches, raising the marsh’s elevation. "This resulted in firmer ground across the project site and will very likely extend the life of this marsh in the face of sea-level rise," Curson says.

Lanier, for his part, is striking a balance between building shoreline resilience and supporting a healthy future marsh. He’s working with partners such as The Nature Conservancy to make Alligator River National Wildlife Refuge more resilient to rising waters. They’ve built some oyster reefs along the coast, which naturally break waves and slow erosion of the shoreline. They’ve reintroduced prescribed burns to promote marsh health. And recognizing that more land will become marsh, they’re looking at which plant species may be the most salt-tolerant for future mass plantings, when they have more funding. “We’re not ready to pull the plug on it,” he says. “It’s still good wildlife habitat, and you can’t give up on that.”

Realistically, more land that is now forested will become marsh due to climate change, even if the world stopped burning fossil fuels today. Taillie suggests that land managers identify the most vulnerable forested areas and work to begin their transition to marsh. With more time to develop before the oceans encroach, these marshes might have a better chance at building up sediment and becoming elevated enough to withstand sea-level rise, he says.

Ghost forests demonstrate that when making decisions about climate change, it might be best to acknowledge how the world will look different as the results of our policies play out. Taillie doesn’t want to lose forests, but he recognizes that we very likely will. “If we are going to lose it, I want to make sure that it's transitioning to something that is also going to contribute to biodiversity,” he says. “I like to think of it as an opportunity.”