Dead trees and trees with holes in them may look ominous, but they provide valuable habitat for wildlife. | Beth Botts/The Morton Arboretum
By Beth Botts | Posted to the Chicago Tribune
Dark against the autumn sky, its branches broken and twisted, with mysterious black holes and cracks in its bark, a dead tree might seem spooky. But to many animals, it’s home.
Bats, screech owls, woodpeckers, bluebirds, squirrels and many other animals live in holes in dead or living trees, or overwinter nestled deep in the furrows of their bark. “It’s almost like an apartment building for wildlife,” said Spencer Campbell, Plant Clinic manager at The Morton Arboretum in Lisle.
Holes, or cavities, occur in living trees where the trunk or branches have cracked, or where decay has created a soft spot that a woodpecker or other animals can excavate and enlarge. Once a tree has died, its gradual deterioration creates many more cavities, and expands the opportunities for animal apartment hunting.
Of course, a tree that has died or one with conspicuous holes or cracks can be a cause for concern near a home, sidewalk or roadway. “If you see cavities in a tree in your yard, you should have it inspected by a certified arborist to determine whether it poses any risk,” Campbell said. “Sometimes, a decayed branch or tree may need to be removed.”
But as long as the tree isn’t a threat to people, its cavities are one of the big ways it contributes to the ecosystem of which it is a part.
A tree doesn’t have to be dead to be housing. A hole that doesn’t undermine a living tree’s structure is usually not a problem and may even be a benefit, according to Lyn Myers, lead education program guide in the Arboretum’s school field trip program. “If woodpeckers and other animals have a cavity to live in, they may be helping it by eating the insect larvae that would be eating the wood,” she said.
Many holes have a whole succession of tenants, Myers said. First, a woodpecker may find a soft spot and use its jackhammer beak to dig out a hole large enough for nesting. Then a squirrel or bat may move in. “The mammals are opportunists,” she said. “They take advantage of a cavity that’s already there.”
Some animals don’t even need a hole; craggy bark will do. Butterflies such as the mourning cloak overwinter in the deep bark furrows of bur oak or the peeling bark of shagbark hickory. They find a spot high enough to be safe from foraging predators such as raccoons and opossums, Myers said. Bark can even shelter bats, swaddled snugly in their folded wings and hanging upside down on the sunny, warm south side of the trunk. “It’s amazing how small they are when they’re folded up,” she said.
Once a tree dies, “it keeps giving back in a different way,” Campbell said, as it gradually decomposes over years or decades. Decay is actually a process of feeding, as fungi, insects and other organisms gradually consume the wood and recycle the nutrients that were stored in it during the tree’s lifetime.
In time, the tree weakens enough to fall. As the wood continues to break down and the trunk hollows out, its decay creates more nooks where animals such as raccoons and chipmunks can shelter. Eventually, the tree’s wood dissolves to become part of healthier, richer soil that supports a wide range of life, including other plants.
That’s why many dead trees are not removed from the Arboretum’s East Woods and other natural areas, such as forest preserves. “A dead tree that isn’t at risk of falling on people or property can actually be more valuable left standing,” Campbell said.
When you spot a dead tree, take a moment to wonder: Who lives there?
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
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