Millions of trees seem to be dying in the US, both near the coasts and inland near lakes, thanks to rising water levels. The finding may force a re-evaluation of how much carbon is stored in forests, which are important carbon sinks for mitigating the more severe effects of climate change.
Rising sea levels and intense storms linked to climate change have driven widespread tree die-offs along the US Atlantic coast, creating armies of skeletal dead trees known as ghost forests. North Carolina, for example, is thought to have lost 21 per cent of its coastal forest between 1985 and 2021 because of waterlogged soil and higher salt levels resulting from rising seawater.
To see how widely this is happening, Henry Yeung at the University of Virginia and his colleagues used aerial imagery from the US National Agricultural Imagery Program. They analysed images captured between 2012 and 2023 to map dead trees within 10 kilometres of the Pacific, Gulf and Atlantic coasts of the US, as well as along the US shorelines of the Great Lakes, down to a resolution of about 1 square metre.
They have identified more than 260 million dead trees and found that more than half of the mortality was concentrated in just 10 per cent of the mapped forests. These hotspots weren’t only in low-lying coastal forests prone to flooding, but also in high-elevation inland wetlands, far from salty water.
Over the decade they studied, tree mortality tripled in low-lying coastal forests, where water inundation was combined with increased levels of salt.
But mortality rates also doubled around expanding freshwater lakes, including the Great Lakes, and in inland wetlands, which have experienced more precipitation.
“This is the first map of ghost forests that covers the entire US. It’s also perhaps the first comprehensive study that takes the concept of sea level rise killing trees and applies it to more inland locations,” says Matthew Kirwan at the Virginia Institute of Marine Science. “As a coastal scientist, I immediately think of salt as the culprit. These authors have extended it to water in general.”
Arya Iwantoro at the University of Plymouth, UK, says it is striking how widespread the pattern is. “Prolonged inundation and salinity are already known to stress and kill trees, but the study suggests that water-related forest mortality may be more widespread than is usually recognised,” he says. “That broadens the issue beyond sea-level rise alone.”
Yeung and his colleagues found that between 2013 and 2020, high levels of precipitation and water runoff drove record-high levels in the Great Lakes, and they suggest that this led to sustained waterlogging, increasing the chances of nearby trees dying.
“The study provides convincing evidence that changing hydrological conditions are an important contributor to the mortality patterns, but I would be cautious about attributing every tree death directly to inundation or salinity,” says Iwantoro, adding that tree mortality is often influenced by other interacting factors, including storms, drought, pests, local drainage and soil conditions.
He says the work adds to evidence that the water-related effects of climate change could accelerate ecosystem shifts.
As the climate warms, the melting of ice means global mean sea levels are rising at about 4.1 millimetres per year, and saltwater intrusion is expected to affect nearly 77 per cent of the world’s coastlines by 2100. What’s more, climate warming also fuels heavier rain and snow, because for every 1°C that atmospheric temperature rises, the amount of water vapor in the atmosphere can increase by about 7 per cent.
This means that in coastal and lakeside areas, forests may retreat as conditions become more suitable for wetland vegetation, with knock-on effects on carbon storage, says Iwantoro.
“Tree mortality and decomposition releases carbon into the atmosphere, and if the forest does not recover, then these areas may not regain their capacity to sequester carbon, and it could represent a more long-term carbon stock loss,” says Michelle Sims at the World Resources Institute, a global non-profit.
However, the ecosystem that replaces the forest may also accumulate carbon, says Iwantoro. “The important question is how carbon storage changes as the landscape transitions, rather than simply how much carbon is lost when trees die.”