Ghost Forests are examples of the advance of Climate Change
08 October 2019 | Adaptation
Matt Kirwan, the Virginia Institute of Marine Science ecologist study what is known as a “ghost forest.” These swaths of dead, white, trees are created when salty water moves into forested areas, first slowing, and eventually halting, the growth of new trees. That means that when old trees die, there aren’t replacements.
The first descriptions of ghost forests date back to about 1910. The phenomenon can now be seen all along the Atlantic seaboard of North America, from Louisiana up to southern Canada.
Kirwan likens ghost forests to other drastic markers of environmental change. Similar to how a receding glacier leaves signs of where ice used to be, ghost forests represent where dry land used to be. “You can touch it and see it,” he says. “It’s just as real as a melting glacier.”
And, according to Kirwan’s latest research, ghost forests are forming faster than ever. Rates of forest retreat, he found, are an average of about three times higher than before the Industrial Revolution—when human activity started generating the vast amounts of greenhouse-gas emissions that scientists link with the current climate crisis.
“Ghost forests are the most striking indicator of climate change anywhere on the Atlantic coast,” says Kirwan.
One of the primary mechanisms for ghost-forest creation is sea-level rise, says Keryn Gedan, a professor at George Washington University in D.C., and Kirwan’s co-author on a recent article about the phenomena in the journal Nature Climate Change. Higher waters reduce the elevation between land and sea, making it easier for salt-water to seep in to shoreline soils. And, trees, she says, “don’t like salt,” which ultimately leads to ghost-forests.
Climate change also exacerbates events such as hurricanes, which bring storm surges that drive salt-water landward, explains Emily Bernhardt, a professor at Duke University. Droughts, which have been similarly linked to climate change, mean less rain to wash out the salt that enters the ecosystem, “compounding the effects of sea level rise,” she says.
“When the trees die out, the marsh is going to take over,” said Matt Hurd, a regional forester with the Maryland department of natural resources. “We just don’t know how to slow it down.
According to Kirwan, the transition from forest to marsh isn’t, in itself, detrimental. He sees it as a sign of natural resilience and says that marshland can actually be beneficial in many ways. They are, for instance, very biologically diverse, and can even sequester more carbon than the forest they replace. The problem is that the ghost-forest process doesn’t necessarily stop at marsh. Over time, the marsh too can end up submerged, resulting in open water. That, says Bernhardt, would mean more carbon in the atmosphere and, unless replaced inland, represent a net loss of land or wetland surface.
The more immediate issue is what ghost-forests have meant for coastal communities, many of whom depend on dry land. Kirwan, whose ancestors lived near the Blackwater refuge for centuries, says, “for private landowners and people like a lot of my family it is a bad thing. It comes at a cost.” Driving near the refuge, Kirwan points to lawns with salt patches where grass won’t grow, abandoned houses, and long-deserted buildings where phragmites grows rampantly. The salt-tolerant grass abuts the church his family used to attend. Ultimately, Kirwan says, these climatic factors, along with economic pressures, have led many in the area to move out.
For those who remain, the salt water continues to creep higher. “In addition to the change in the forests, there is a whole lot of farmland [that] was also lost,” Gedan says. Her analysis of one coastal plain county in Maryland found that, between 2009 and 2017, it lost some 2% of farmland. That trend seems set to continue, she says, which could mean tougher times for farmers in the area.
“It is an eerie connection between sort of their visual beauty,” she says about ghost forests, “and the implications of how humans are changing the landscape.”
Research on ghost forests is relatively nascent, despite decades of awareness of their existence. “[They’ve] always been thought of as an exception not a rule,” says Kirwan, “something that occurs in a few places here and there.” As the phenomenon has become more widespread, with greater human impacts, scientific interest has picked up.
Among the questions researchers are now trying to answer are how much area ghost forests cover, and how rapid the transition occurs. According to Marcelo Ardon, an ecologist at North Carolina State University who studies ghost forests, better understanding them could result in more effective conservation policies. “ If we know the changes that are going to happen,” he says, “maybe we can be more proactive in how we manage the transition.”
Dressed in a red shirt, with a camouflage dry bag on his hip, Kirwan, along with his colleagues, disperses out along the border between healthy forest and ghost-forest, trying to better understand the process that turns the former into the latter.
Some take tree cores to get an insight into tree rings. Others use a higher-tech machine called a resistograph, which pushes a thin needle into the tree to measure the strength of the wood, which Kirwan says is an indicator of tree health. At one end of the study site is a patch of forest the researchers had intentionally killed off so they can monitor the ghost-forest transformation in a controlled setting.
The trees, says Kirwan, tell a story.
“The trees are showing us what the land used to look like,” he says. “If you either don’t know your history or don’t pay close attention, you might never know that a forest was ever here.”