Ghost forests are victims and harbingers of sea-level rise

Extracts from an article on the PNAS website by Amy McDermott

NEAR Taylor’s Island and Fishing Creek on Maryland’s Eastern Shore, hundreds of acres of dead trees stand upright, like toothpicks piercing the sky. They are among the mid-Atlantic’s largest “ghost forests,” woodlands rapidly converted to marsh because of sea-level rise.

All along the margins of the mid-Atlantic today, and other low-lying parts of the East Coast, rapid sea-level rise is driving a deadly sogginess inland. Frequent floods and higher high tides are pushing marsh into forests and drowning the cedar and pine trees that fringed the shore even a decade ago. Some habitats are changing so fast that the dead trees haven’t had time to fall over. They haunt the landscape, effigies of climate-driven change.

“Ghost forests are the most striking indicator of climate change on the East Coast,” says Matthew Kirwan, a leading authority. “The numbers are staggering,” he says. Some 40,000 acres of forest and farmland have converted into ghost forest in roughly the last 30 years.

Yet most people have never heard of these huge forest die-offs. Ghost forests have remained largely out of the public eye, along marshy back roads in rural areas. Farmers are often the first to notice that the land is slowly drowning, when saltwater affects their crops.

But mapping the die-offs reveals that ghost forests are growing. Their expansion raises questions about the amount of carbon dioxide and methane that decomposing trees could belch into the atmosphere. Field studies are tracking the precise causes of ghost forest formation and how much carbon they stand to lose.

The stakes are high. Storm surge and flooding are already pummelling many seaside towns, and communities are contemplating adaptation measures now. Dying forests will have an impact on economies, tourism, and housing. It may be too late to stop the die-offs. But predicting where they’ll spread next can help towns to proactively adapt, rather than waiting on the tide to wash in.

As recently as 2016, ghost forests were just a whisper in university halls. Lindsey Smart says she remembers hearing about them in college, in the 2010s, but only “in an anecdotal way.” Researchers had noticed progressive tree die-offs in coastal areas, but nobody had mapped the extent of the losses. What was clear from the botany literature is that forests have a limited tolerance for saltwater, she says.

Over several summers, Smart measured various traits of the vegetation in all three habitat types: healthy forest, marsh, and ghost forest.

She wrote down the plant species that she saw and wrapped her measuring tape around tree trunks at chest height to find differences in the average width-around (a proxy for biomass) of trees like pines versus marsh species like sweet gum and cypress. Smart found that healthy forests had tall, dense vegetation and were high in biomass. Marshlands had shorter plants, with lower biomass. And ghost forests were tall, with low biomass, and patchy vegetation.

In results that Smart published in 2020, some 41,300 forested acres, or 15% of the Albemarle-Pamlico Peninsula’s coastal unmanaged public land, had died in just 13 years.

That pattern has presumably continued or even sped up since 2014, Smart says, but the extent of Albemarle-Pamlico’s ghost forests has yet to be re-evaluated.

To predict where ghost forests may go next, ecologists, hydrologists, and others are pinpointing exactly how forests drown. They know that a combination of gradual sea-level rise over years, paired with punctuated floods during storms and high tides, are primary physical stressors for the trees.

Underlying Smart’s investigation (and others described in the original article) is a larger question with big implications: how much carbon will this many decomposing trees release into the atmosphere, perhaps accelerating climate change as part of a positive feedback loop?

The dead trunks of drowned trees not only release carbon dioxide as they rot, but can act like hollow straws, moving methane and other greenhouse gases from the soil and belching them into the atmosphere, via their dead, dry wood. Invading marshes, on the other hand, lock up carbon in their deep roots and soils. In fact, they may offset some of the greenhouse gas escape. As marshes develop, their dense root mats suck carbon from the water and create low-oxygen aquatic zones, where any fallen trees will decompose slower than in oxygen-rich environments.

Using data, maps and computer modelling analysis, Smart found a 42% net decline in carbon storage for areas that had made the transition from healthy forest to ghost forest.

The entire picture of how ghost forests will impact carbon emissions is still developing.

Read the full article here

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