The trees are sick.
Epidemics are escalating, across numerous species, as more and more pests and pathogens invade forests. Their entryway is global trade — in live plants, and in the countless products that are shipped in wooden pallets, crates and other packaging. As trade has boomed, so have the opportunities for tiny stowaways to wreak havoc.
“We’re moving insects and microbes around the world through trade very rapidly, and basically running an experiment” to see which ones will take hold, said Bernard Slippers, a forest pathologist at the University of Pretoria in South Africa.
The attackers have killed billions of trees already, with countless more under threat. In some severe cases, forests are transforming into bleak spaces of brambles and vines.
And the impact extends far beyond the woods. Trees filter water, stabilize the land and clean the air. They absorb and store planet-warming carbon dioxide, fighting climate change. They create the three-dimensional habitat that countless species use for shelter and shade. And they offer something even more foundational.
“Plants are the only thing that take sunlight, carbon dioxide, water, and turn it into biological energy,” said Anurag Agrawal, an ecologist at Cornell University. “Plants, trees are the ultimate source for all of the energy, all of the food on the planet.”
Different tree species offer sustenance at different times each season: Nuts and berries for mammals and birds, pollen for pollinators. Some insects can feed only on certain trees, and when those trees disappear, so do they. Fallen leaves affect the acidity of the forest floor and the water running through it.
Each tree species that drops out leaves a hole. Some are bigger than others.
“You really have this whole cascade of effects that grows exponentially as you lose more and more species,” said Kathleen Knight, a research ecologist with the United States Forest Service.
Scientists are doing what they can to help a growing number of species withstand the onslaught, even as they struggle to understand what it will mean.
Tiny attackers felling mighty trees
While tree species are getting hit all over the world, the northeastern United States has emerged as an epicenter. Scientists attribute that to both the volume of imports and the rich variety of tree species in the region.
The beetle responsible for some of the worst recent damage is tiny, bright green and shimmers like a jewel: the emerald ash borer.
Scientists don’t know exactly when it entered the United States, but by 2002 in the Detroit area, something was terribly wrong with the ash trees. Their crowns died back and their outer bark split apart, revealing strange, serpentine marks underneath. The bright green beetles were found among dying trees.
Less than two decades later, the borer had so devastated North American ash trees that all six of the most prominent species were classified as endangered or critically endangered. Death rates for mature trees are sky high.
Because ashes were common in cities and because they are dangerously prone to falling when heavily infested, local governments have spent billions of dollars removing them. Cities and towns have lost much of their canopy, just as climate change makes urban trees more important than ever.
Each invasion, as scientists call them, brings its own consequences for forests.
At the Mohonk Preserve, conserved land in New York’s Hudson Valley, a stretch of serene forest leads to a sudden starkness. Fallen branches litter the ground. Naked trunks of dead trees stand like giant toothpicks.
The victims here are hemlocks, an evergreen notable for creating dense, year-round shade, cooling the ground in the summer and insulating it in the winter. Hemlocks can grow on rocky slopes that challenge other trees, stabilizing banks. And they cool streams, protecting aquatic life.
Radka Wildova, a senior scientist at the Ecological Research Institute, walked among the grove of dead trees on a recent day. “In 2023 at this plot, the mortality was around 80 percent,” she said.
A tiny, aphid-like insect called the hemlock woolly adelgid is behind this massacre. First detected in Virginia in 1951, it probably arrived on plants imported from Japan. The insect spread slowly in the beginning but has now decimated hemlocks across the Southeast.
In New York, winter cold snaps have helped hemlocks survive longer by knocking back the adelgid populations.
But this hemlock graveyard shows that where conditions aren’t perfect — where there’s less water, more heat, maybe thinner soils — it’s harder for the trees to survive the attack. A warming planet is expected to compound the problem by both stressing the trees and helping adelgids survive.
An hour’s drive west into the Catskill Mountains shows the high stakes.
The forests there are famous for filtering New York City’s drinking water. Instead of spending billions of dollars on water filtration plants, government officials agreed in 1997 to conserve enough land for the forests to do the job.
But near the Neversink Reservoir, one of six that collectively deliver roughly 90 percent of the city’s water, the overlapping plagues affecting hemlock, ash and beech are evident.
Ailing beech trees are victims of one of the most recent outbreaks, which first showed up in Ohio in 2012. Beeches were already suffering from beech bark disease, facilitated by a small insect. Now a microscopic worm native to Japan is attacking their leaves, causing the tissue to thicken and the leaves to drop.
When one species of tree disappears, others can partly fill the ecological gap. The American chestnut, a grand tree that once anchored many eastern forests, has been functionally extinct since around 1950. It was felled by a fungus that was accidentally introduced from East Asia.
When the chestnuts vanished, oaks and hickories broadly took their place. But now they, too, face threats from invasive insects and pathogens.
Another malady, Dutch elm disease, probably arrived with beetles in logs imported for furniture veneers. By 1980, stately American elms had largely vanished from forests and city streets alike.
Their replacement? The ash trees that are dying now.
Hiding away on cargo ships
To survive in a new land, an insect must encounter a climate that meets its needs. Much of northeast Asia aligns perfectly with the northeastern United States. (Infestations happen in both directions. For example, the pinewood nematode, native to North America, is attacking East Asia and Europe.)
The number of highly destructive pests identified by scientists has increased since 1990, as global trade ramped up, said Juliann Aukema, a scientist who helped identify the surge and is now an assistant director of research at the Forest Service.
The direct costs are formidable. In 2011, researchers estimated that bark- and wood-boring insects would cost local governments in the United States around $1.7 billion a year in tree removal, replacement and treatment.
Over the decades, authorities have stepped up measures to prevent outbreaks. In 2002, countries agreed to a global standard that required packaging materials from raw wood to be treated with heat or fumigation and stamped to show that they were safe.
A study later found that infestation rates declined by around a third to half after the agreement was implemented. But compliance is inconsistent, scientists said.
“Material is still coming in infested,” said Deborah McCullough, who studies invasive forest insects at Michigan State University.
And while regulations governing international trade in live plants have also been tightened, plants can still be sick or infested even if they look healthy, scientists said.
At the border, federal inspectors examine shipments determined to be highest risk while also conducting random checks, said Dudley Hoskins, an under secretary at the Agriculture Department. “There will never be a scenario where there’s a zero risk profile,” Mr. Hoskins said. “We do interdict shipments of concern, if not daily, then almost every other day.”
How to rescue a tree
To save individual trees, insecticides can be injected or applied to the soil around the roots. The tree becomes poisonous to insects that try to feed on it, whether or not they’re beneficial.
While effective, the costs add up: One tree might run $150 every few years, for example. Property owners often choose to treat a small number of beloved trees, but protecting a forest can be impractical.
Scientists are racing to find more sustainable solutions.
Standing among the cluster of sick hemlocks near the Neversink Reservoir in the Catskills this June, Nicholas Dietschler, who works with Cornell University’s New York State Hemlock Initiative, popped off the top of a small plastic bottle filled with tiny flies that prey on the adelgid. These are Leucotaraxis argenticollis, a type of silver fly so obscure it doesn’t have a common name.
The flies quickly climbed to the mouth of the bottle, flew out and disappeared into the forest.
“Be free,” Mr. Dietschler murmured.
With oversight from the federal government, several universities and state agencies are trying to fight hemlock woolly adelgid with what’s known as biocontrol, bringing in predators to drive down their numbers. Cornell has released hundreds of thousands of flies and beetles in recent years.
The irony of introducing a nonnative species to combat another nonnative species is not lost on scientists. They acknowledge the risks of unintended consequences.
But scientific protocols have tightened over the years, and many scientists think insect predators could give embattled tree species a chance. Four species of stingless wasps from Asia have been enlisted to fight emerald ash borer, and an imported fungus has helped tamp down the spongy moth, which defoliates oaks and other hardwood trees.
Other scientists have brought breeding to the battlefront. They are focused on finding trees with natural genetic resistance and helping them to multiply.
In 2006, Kathleen Knight was a young Forest Service research ecologist tasked with monitoring the spread of emerald ash borer. Ahead of the insect’s arrival in Ohio, she set up plots of healthy forests and watched the invader rip through, killing the trees in just a few years.
One day, she and her team recorded “hundreds and hundreds” of dead trees in one plot, she said. But on the way back to the car, they stumbled across something stunning.
“We found this really healthy, amazing-looking ash tree,” Dr. Knight said. “And we were just dumbfounded. How did this tree survive?”
As more strangely healthy ash trees were found, she teamed up with Jennifer Koch, a colleague at the Forest Service who had been looking for natural resistance to beech bark disease. The survivors were given a name: “lingering trees.”
When grafts of the lingering trees were exposed to emerald ash borer, a significant number of the larvae died. And when the scientists bred lingering trees with each other, the offspring were also resistant, some even more so than the parents.
“Trees are similar to humans in that they have a lot of genetic variability,” Dr. Koch said. But in the wild, the survivors are so few and far between that breeding is improbable. Their pollen and blossoms are unlikely to find each other.
“We’re helping Mother Nature along,” Dr. Koch said.
She has partnered with groups like the Ecological Research Institute in New York, which has recruited citizen scientists and conservation professionals to help detect lingering trees.
This kind of work can give people hope, said Jonathan Rosenthal, the institute’s director.
When asked what the future holds for forests in the Northeast, scientists struggled to answer. Ecosystems are made up of a dizzying number of variables, all interacting in ways that can be impossible to predict. Climate change makes the forecasting even harder.
“In the coming decades, the eastern forests might look quite different,” Dr. Agrawal said, “with a lot of death and destruction that not only saddens me as a biologist and a lover of nature, but makes me worry about all the things that nature does for us.”
Douglas Main contributed reporting.
The post The Overlapping Plagues Quietly Devastating American Forests
