Japanese barberry is taking over areas of Pennsylvania’s forests and woodlands.
“Here’s some barberry right here. Some bigger patches up there. It’s everywhere,” said Ryan Utz, assistant professor of water resources at Chatham University’s Falk School of Sustainability & Environment.
LISTEN to Ryan Utz discuss his research with The Allegheny Front’s Kara Holsopple
On a hot July morning, Utz heads into the woods at Chatham’s Eden Hall Campus, north of Pittsburgh. It’s one of three places where Utz and collaborators most recently set up study plots to better understand how barberry is affecting the growth of native trees.
Japanese barberry is a bush that’s commonly used in landscaping. People like it because it grows bright red berries. But it’s escaped into forests and suburban woodlands, where it causes problems.
“It spreads; it sprawls,” Utz said. “It makes a canopy that is pretty contiguous, and it gets really, really heavy.”
In some places the thorny barberry is so thick, a person can’t walk through it, and little else can grow. That’s what Utz has been studying.
“What we found was a lot of young trees coming up outside of the barberry, but very, very few underneath the barberry,” he said. “We didn’t even detect many red maple, which is a very tough native tree that grows almost anywhere in our state.”
Killer leaves or Hungry Mice?
Utz and his students thought the growth of young trees could be inhibited by a process called allelopathy. Invasive plants drop their leaves in the fall, which release a chemical that kills other plants or the fungi that the plants depend upon. So they planted young oak and black cherry trees under the barberry canopy, expecting them to die off.
“In fact, what happened is we planted those young trees, and they did just fine, at least for their first year,” said Utz.
When Utz presented his findings at a seminar, another plant biologist, David Ward from Kent State University, had an idea about what could be preventing the growth of tree seedlings under barberry: small mammals.
Tree seeds and seedlings are food for small mammals like chipmunks or white-footed mice. A thick cover of barberry would make it harder for predators like hawks and foxes to catch the mammals while they forage.
“In a few decades time, where barberry is very heavily invaded, as the older trees die, there may not be younger trees to replace them.”
The relationship had already been demonstrated with another invasive plant. To test the theory for barberry, Utz and collaborators set out live traps and feeding trays underneath barberry bushes and in control plots where there was no barberry.
Indeed, small mammals foraged more under the barberry. The study was published in the journal NeoBiota.
“We have evidence now that when barberry invades a forest, it can change either the foraging behavior or the population density of small mammals,” Utz said.
Bad News for Future Forests
The mammals are emboldened because of the protective cover of barberry, and that could be bad news for the future of Pennsylvania woodlands.
“In a few decades time, where barberry is very heavily invaded, as the older trees die, there may not be younger trees to replace them,” Utz said.
He says another concern is that white-footed mice, which take cover under the barberry, are a vector for the pathogen which causes Lyme disease in humans.
Utz says at this stage, invasive Japanese barberry is here to stay in forested areas. Landowners can try to manage it with a weed torch or cut it down to the ground and apply an herbicide to keep it at bay.
Native forests are under threat from invasives in part because deer don’t like to browse them, Utz says. That gives invasives an advantage over native shrubs like blueberry and viburnum.
“I don’t really know what the answer is — how to deal with the deer problem — but I know it is going to be a problem when it comes to a holistic management approach for our forests,” Utz said.
A native understory, with the kind of biodiversity that is muscled out by invasive shrubs like barberry, makes forests more resilient to drought, Utz says. Healthy forests also provide services to people like timber, crop pollination, and water filtration.
This story is part of our series, Wild Pennsylvania. Check out all of the other stories in the series here.