Emerald ash borer. Photo courtesy of National Park Service
A recent University of Vermont study found that natural causes, including invasive insects, are killing more trees than logging in the Northeast.
by Erika Tally, Community News Service
Across Northeastern forests, natural causes now account for more tree deaths than harvesting, including in Vermont, according to a recent study from the University of Vermont.
But these natural causes are still cause for concern. Insects, disease and extreme weather are now to blame for the majority of tree deaths in the region, a significant shift from just 15 years ago, the study found.
This shift comes at a time when Vermont’s warming climate is already posing dangers to the health of the forest, including worsening the threat of introducing an invasive species. Now, forest health experts are determining what strategies they can use to preserve Vermont’s woods.
“Our forests are really resilient,” said Savannah Ferreira, a forest health specialist with the Vermont Department of Forests, Parks & Recreation. “We’re doing a lot of great forest management that really helps increase the health and vigor of our trees.”
In 2009, logging killed 5% more trees in the Northeastern forests than natural causes. By 2024, however, natural causes killed 39% more trees than logging, the study says.
Map of US counties infested with emerald ash borer as of July 2024. Map courtesy USDA – APHIS
Across the 18 states included in the study, natural mortality surpassed harvesting as a cause of tree loss in all except Virginia and Maine. In four of the states, insect-related mortality alone exceeded harvesting as the primary cause of death.
While about half of the trees studied between 2018 and 2024 died for unknown reasons, 22% were killed by insects, the study shows. Weather caused 13% of tree deaths, disease 9%, and the remainder were killed by vegetation, animals or fire.
The changing climate has created a more hospitable habitat for insects such as the emerald ash borer, which first appeared in Vermont in 2018. Higher temperatures also increase the frequency of natural disturbances, such as flooding and drought.
“This all puts pressure on the tree species, which makes them more vulnerable to being infected or infested with pathogens or insects,” Ferreira said. “So when trees are declining or they’re really stressed out from some abiotic or non-living environmental condition, they can become a dinner bell for a lot of our pests to become established.”
Emerald ash borers damage ash trees by laying their eggs on their bark. When the eggs hatch, the larvae burrow into the tree, leaving S-shaped markings on the inner bark they feed on. When the insect emerges as an adult, it leaves exit wounds in the bark. The damage kills the tree within three to five years and is responsible for the death of millions of ash trees in North America.
Despite this, Alexandra Kosiba, a forest ecophysiologist who leads UVM’s Extension Forestry Program, said Vermont ash trees aren’t likely to disappear.
Emerald ash borer damage. Photo by Judy Gallagher/Flickr
“They will still exist, but they’re gonna exist in very different ways because essentially (emerald ash borer) kills big mature trees,” she said.
Kosiba noted that there will still be smaller ash trees scattered and regenerating through the forest, but that the loss of larger ash trees will change forest dynamics “considerably.”
A 2025 study published by the Journal of Plant Ecology found that global warming is worsening the threat of emerald ash borer infestations. As such, Kosiba said Vermont foresters are adapting their management strategies to align with those in states that have been harder hit.
Many Vermont communities have proactively removed ash trees and replanted other species. But forest officials have found that a small number of ash trees have been able to survive the infestation. Officials recommend that forest managers and landowners not cut every ash tree, Kosiba said.
“We understand that you might wanna cut some of them to recoup some of the lumber and the value in those trees, but leave some of them,” she said. “We know there’s gonna be some individual ash trees that show some resistance.”
Foresters are also using strategies that don’t solely rely on mitigating the loss of ash trees, but instead focus on promoting their regrowth.
One approach involves determining the sex of the ash tree, as ash trees can be male or female, but only females produce seeds, Kosiba said.
“Ash trees need light to regenerate. So people have been going out and actually creating gaps, creating openings in trees. Cutting some trees next to (a female) ash tree so that you can get some seeds that fall into that opening. Those little ash trees need full sun to grow and develop,” she said.
These small cohorts of ash trees can play a large role in preserving the species, even if they can’t directly stop the damage inflicted by the emerald ash borer. The insects prefer larger trees, in part because they struggle to lay eggs in smaller trees’ thin bark, Kosiba said.
“If you have some little trees there, they can survive that infestation,” she said.
A Vermont forest. Photo via Kurtkaiser/Wikimedia Commons
Joshua Halman, forest health program manager at the Vermont Department of Forests, Parks & Recreation, said human activity is a large reason that invasive species can spread. Emerald ash borers can be found in firewood or even hitch rides on vehicles.
Halman suggested that residents help track the insect by using the “Report It!” tool on the Vermont Invasives website. From there, experts review each submission and determine if further action needs to be taken.
Kosiba said the tool can help experts get ahead of the infestation. She encouraged people to report sightings even if they’re not positive it’s the emerald ash borer.
“It’s better for folks to err on the side of caution,” she said. “The key is early detection, and we can then act really quickly.”
Scientists are already on the lookout for another invasive insect: the Asian longhorn beetle. Introduced to the U.S. in 1996, warming climates have extended the insect’s range. The beetles, which feed on hardwoods like maples, have been found in the Northeast but not yet in Vermont.
“When you have a pest or a disease that’s able to affect much more of our ecosystem, that’s when I feel like it becomes more concerning,” Ferreira, the forest health specialist, said.
It’s unclear to what degree climate change is encouraging invasives to spread to Vermont. Experts say they’ll rely on both novel and trusted methods to actively manage the threat.
For Halman, of the forest health program, that includes research and collaboration with the public. He said it’s important to remember that, despite the concerning tree deaths, Vermont’s forests are adaptable.
“Introduced or invasive forest pests are not new to Vermont — some species have been here for more than a century,” he said. “Through education and observation, we can find areas and methods to focus our efforts on and mitigate the impacts they cause to Vermont forests.”
Community News Service is a University of Vermont journalism internship.
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