Why do trees fall in the forest? The answer is changing. New research suggests that in just 15 years, the causes of most tree loss have flipped from human hands to a handful of natural causes.
That’s what University of Vermont researchers found when they studied forests in 18 states: in 2009, human harvesting accounted for most tree loss, but by 2024, pests, diseases, and other “natural” causes activities were causing far more tree loss.
The researchers dug in, comparing nearly 324,000 records of tree mortality across 18 states and almost 62,000,000 hectares, from the federal Forest Inventory and Analysis dataset from 2009 to 2024. In 2009, human harvesting caused a bit more tree loss than natural causes. Fifteen years later, tree loss from natural causes was outpacing harvest-caused loss by nearly 40%, and overall tree loss also increased by nearly 16% during this period.
It wasn’t a change the researchers were looking for. Lead author Lucas Harris was poring over forest data to understand how seedlings were faring in Northeastern forests, when he noticed something about the other end of the trees’ lifespans: their causes of death were changing over time.
“In 2009, harvesting was the leading cause of tree biomass loss,” says Harris, a postdoctoral researcher in the Rubenstein School of Environment and Natural Resources, explaining that tree biomass is the amount of living material the tree contains. “But moving forward in time, we saw a really sharp rise in natural mortality and natural disturbance.” Cause of death could be determined for about half of the trees, which is typical, the researchers say.
“When people talk about trees dying from climate change, they picture big fires in the West,” says study coauthor and Rubenstein School professor Tony D’Amato. “But in the East, it’s more like death by a thousand cuts: this insect, that disease, this pathogen, that windstorm.”
Although tree harvests decreased slightly during the study period, the change was largely driven by increasing insect activity. Emerald ash borers, spongy moths, and a diminutive-but-dangerous insect, the hemlock woolly adelgid, contributed, as did storms and disease.
“Insect-driven mortality in particular really shifted things quite a bit,” Harris says. More frequent and severe climate disturbances, driven by global warming, work in concert with these new pests, Harris adds. “It's not just that a couple trees are being taken out here and there by storms. It's also that moderate- to high-severity disturbances are increasing in the region. You have these introduced pests and pathogens that seem to be running the show in many ways in terms of tree mortality, and interacting with climate change.”
Although climate change is human-caused, the researchers differentiated these causes from direct human harvesting, and the findings have implications for forest management policies, the researchers say. It’s one thing to manage a forest whose trees are dying from logging by humans who can be motivated by a range of objectives, values, and outcomes. It’s another to manage one where the main dangers are novel insects and microbes whose movement is initially difficult to detect, and where ecological outcomes are more challenging to predict.
The researchers hope their work will inform forest management going forward.
“There's a lot going on out there that threatens the values that forests provide us,” D’Amato says. “It’s a slow creep, but we’re now seeing bigger areas of impact that don’t bode well for the future values and services that forests provide.”
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The original research paper can be accessed here.
The research area included 18 states: Wisconsin, Illinois, Indiana, Michigan, Ohio, Pennsylvania, West Virginia, Virginia, Maryland, Delaware, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, Vermont, New Hampshire and Maine.
