By Victoria Witke
Every fall for years, Deborah McCullough and her husband fished salmon from Western Michigan’s Betsie River.
“It’s a little river, and it was shaded, and you knew there was going to be big salmon in the deep spot over there under the shade,” she said.
But all that shade disappeared. The water became hot, and the fish no longer came to their spot.
The river used to be lined with ash, but an invasive insect called emerald ash borer wiped the trees out in four years.
The insect lays eggs on ash. Once hatched, larvae tunnel into the bark, which kills the tree because it can’t circulate water or nutrients.
Since 2002, emerald ash borers have killed anywhere from 80% to 99% of mature ash in Southeast Michigan and Northeast Ohio.
The death toll: tens of millions of trees in Michigan alone.
However, a new study shows white ash are more resilient than previously thought. Despite so many dying, trees continue to survive and reproduce in invaded forests.
“I talk to people from other states periodically, and there’s always this shock that there’s a single ash left alive in the state,” McCullough said.
She’s a forest entomology professor at Michigan State University and took an advisory role in the study.
The study said most ash today are little trees and saplings rather than big trees in the overstory – the top layer of the forest where the tallest trees meet – like they were before the insects invaded in the 1990s.
That’s because these young trees were too small to be infested during peak invasion.
Ash is a common tree genus, said Caleb Wilson of the University of Kentucky, the lead researcher of the study published in the journal Forest Ecology and Management.
The trees likely took up significant swaths of the overstory and were so big you couldn’t wrap your arms around their trunks, like mature oaks and maples now, he said.
In areas where the study took place in Clinton, Ingham and Eaton counties, mature ash now have significantly smaller trunk diameters – around 10 centimeters, or the size of a drink coaster.
And about half of those mature trees are dead, Wilson said.
Dead trees don’t grow leaves. In the abandoned space, other trees like maple, oak and black cherry grow outward to fill the canopy, the study said.
Ash need sunlight to mature, said McCullough.
“If you walk into a forest, there’s a boatload of little ash seedlings,” McCullough said.
“They’ll hang out and be seedlings for quite a while. But until a tree dies or blows over and a canopy gap opens up, they don’t have enough sun to really grow and move into the overstory,” she said.
So small trees stay small in the understory – and don’t make many seeds, slowing reproduction, the study said.
Those that can grow are more likely to be killed by borers, the study said. Mortality rates increase with size.
One-third of ash recruits surveyed in the study were infested.
That’s a smaller number of borers than when they first invaded. The number decreased after the peak because there weren’t enough ash to support the population.
But, the study said, numbers could rise once again as more ash grow.
McCullough said regeneration will depend on whether young trees can survive and coexist with emerald ash borer without the beetle overpopulating and wiping them out again.
The loss of overstory ash has cascading consequences for forest health, according to the study.
Swallowtail caterpillars and other native bugs eat ash. Birds use the tree as a habitat.
In the fall, leaves drop and provide nutrients for decomposer microorganisms, McCullough said.
Trees along rivers and streams provide nutrients for aquatic invertebrates and filter pollutants from the water.
“I would not say that we have a great handle on all the impacts and how far-reaching they are and all the different levels of organisms that are affected,” McCullough said.
James Wieferich is a forest health specialist at the Department of Natural Resources.
“A lot of our wildlife species, our fish species, streams, lakes and insect populations are all tied to the current and native ecosystem and won’t do as well if it’s changed,” Wieferich said.
He said the wood is also used widely in the timber industry to make things like baseball bats.
Wieferich said ash conservation efforts are important for biodiversity.
Beech bark disease wipes out beech, oak wilt kills oak and emerald ash borers destroy ash, for example. If few species exist in a forest, overstories can be quickly ruined.
Wieferich said conservation groups have found resistant genetics within different ash species that are less susceptible to borers.
The DNR and other organizations collaborate to find and propagate resistant seeds.
In 2019, they established a green ash plantation of resistant trees, though they’re too young to make seeds right now.
“Once they do produce seed, we’ll have resistant stock to do restoration efforts to bring ash back into some of these forests that were decimated,” Wieferich said.
He said the goal is to reintroduce large ash to forests.
Currently, woodpeckers are the best tool for fighting emerald ash borers, Weiferich said, and “do a notoriously good job” at finding and eating them.
Wilson, the lead researcher, said woodpeckers killed about a third of borers in the study.
The study was an offshoot of a larger research project to see what methods work to manage emerald ash borer.
Scientists are studying parasitic wasps that feed on borer larvae and eggs to control borer populations
Forest managers and landscapers inject insecticides into ash, and that’s effective for three years.
However, it’s an expensive and labor-intensive practice. Wilson said there is no silver bullet to controlling borers, but the study shows ash aren’t a lost cause.
McCullough said, “I think if we have the will and some resources, we can learn to live with the emerald ash borer. It won’t be like it was before, though. Things have changed.”