NORTHBROOK — Sun beams highlighted green patches of native witch hazel and bottlebrush grass as Anna Braum walked through Chipilly Woods one recent weekday afternoon.

Two years ago, the forest preserve was a “wall” of invasive buckthorn shrubs, said Braum, a northwest regional ecologist with the Forest Preserves of Cook County. Those dense buckthorn trees were cleared from the area last winter, but a new crop of waist-high shoots now stand in their place.

Forest preserves workers treated the shoots with herbicide last month, in continuation of a cycle that’s become emblematic of the modern-day task of looking after Chicago-area forests: A recent study found that nearly 80 percent of forests in and around the city are infested with invasive shrubs.

“You can’t just remove all the invasives and say ‘you’re done,’” Braum said. “It takes constant maintenance.”

Aided by a growing pool of volunteers and a 2022 referendum that helped secure key funding, the Forest Preserves of Cook County has been able to expand its invasive species removal work in recent years. Every year, restoration crews take on invasives with a careful combination of fire, brush mowers, hand saws, herbicide and native plantings.

Researchers behind a new study by Purdue University and the Morton Arboretum that mapped Chicagoland’s invasive plants hope the project will inspire land owners to take action as well.

Mapping The Problem

Yellow splotches symbolizing shrubby invasives fill an interactive map that Lindsay Darling and her study co-authors published this spring.

Darling, who recently earned her PhD in forestry and natural resources from Purdue, said she’s been working to map invasive species in the Chicago area for years.

While working as a contractor with the Lisle-based Morton Arboretum, Darling used LiDAR, a type of laser scanning technology, to create 3D maps of forests in Cook, DuPage, Kane, Kendall, Lake, McHenry and Will counties. The study found that about 77 percent of Chicago-area forests have been overtaken by invasive shrubs.

McHenry County had the most non-native shrub species invasion with 90 percent while Cook County had the third-highest with more than 77 percent.

“It’s both devastating and hopefully informative and inspires movement,” Darling said.

European buckthorn and Tatarian honeysuckle are two of the most common invasive shrubs in the Chicago area, according to the study. Buckthorn, which grows in dense thickets, was originally introduced to the area in the 1800s as an ornamental plant used for hedges. The shrub spreads quickly with help from birds who eat buckthorn berries. Because the berries are a diuretic, their seeds then get distributed far and wide via the waste of the birds that eat them.

Buckthorn shades the understory of the forest, restricting native plant growth and emits a toxin called emodin that kills surrounding plants, leaving the soil beneath buckthorn thickets bare and vulnerable to erosion.

When buckthorn takes over, it creates a monoculture that limits insect and animal diversity, Braum said.

Clearing buckthorn from Chipilly Woods has helped bring more light into the forest, allowing wildflowers and other native plants to fill the patches of empty soil, Braum said. Buckthorn removal has also exposed the forest’s gentle hills and streams, she said.

“When there was buckthorn, you couldn’t see the large-scale landscape, you could only see a few feet in front of you,” Braum said “We’ve kind of been able to rediscover this place.”

Kim Kalosky, assistant resource project manager with the Forest Preserves, said buckthorn removal also makes wooded areas more enjoyable for visitors because there is more light, wildlife and “it just feels healthier and more open.”

While an invasive species infestation rate of 77 percent may seem like a daunting number, there was a time when the Chicago region’s invasives problem was even worse.

A Decades-Long Battle

When Jim Vanderpoel’s family and a few other local volunteers first started restoring suburban Barrington’s Grigsby Prairie in 1987, the area was an old hay farm dotted with buckthorn and black locust trees.

The volunteers started by clearing the invasive shrubs and then began conducting what Vanderpoel calls “plant rescues” along the North Western Railway tracks.

Invasive plants had overrun most nearby natural areas. But the land surrounding the railroad tracks was filled with native prairie plants because the railroad company conducted regular prescribed burns to keep the tracks clear from brush, Vanderpoel said.

The Vanderpoels would take native plants from the tracks and transplant them at the Grigsby hay farm, gradually transforming the area into a thriving prairie. Now crews spend all summer collecting native seeds from Grigsby Prairie to be planted at sites throughout Cook County that are under restoration.

Similarly, conservationist Stephen Packard and volunteers with the North Branch Restoration Project began planting native seeds along the North Branch of the Chicago River in the late 1970s.

However, the general public was less aware of invasive species 30 years ago, said Benjamin Cox, executive director of Friends of the Forest Preserves. Some neighbors called the invasive removal work deforestation and, in 1996, a moratorium was placed on all restoration work. While the moratorium was partially lifted a year later, the ban wasn’t lifted on the Far Northwest Side of the Chicago until 2006.

“’Let nature take its course’ is what you would always hear people say,” Cox said, “But, people are part of nature and always have been.”

A study published by Friends of the Forest Preserves and Friends of the Parks in 2002 found that the quality of the land was considered poor in 68 percent of the Cook County Forest Preserves. The study also found that the most common small tree in the Forest Preserves was buckthorn.

“The land itself offers some reasons for hope but, on the whole, is in a sorry state and getting worse,” the study stated.

Friends of the Forest Preserves was formed in 1998 to help address the invasive plant problem. Now the nonprofit gets thousands of volunteers out into the forest preserves every year.

The Forest Preserves of Cook County created its own division of volunteer resources in the early 2000s. Kris DePra, the Forest Preserves director of volunteer resources, said they started offering formalized volunteer trainings and certifications around the same time.

Today, between 4,000 and 5,000 people volunteer with the group annually, she said.

“This idea of volunteers doing restoration work was born here in Cook County,” Cox said. “People are out doing it and it’s working, we’ve seen places go from giant weed patches to Illinois Nature Preserves.”

The LiDAR map showed that areas where there have been continued efforts to remove invasive species have been largely successful, Darling said. However, plants like buckthorn can easily spread from neighboring land parcels.

“All it takes is one bird, eating one seed,” she said. “Birds don’t know about property lines or county borders.”

Darling said she hopes the map helps inform private land owners about the invasives on their property. The Morton Arboretum also has a guide on how to identify and remove invasive plants and the Illinois Department of Natural Resources offers some tax incentives for large-scale invasive removal projects.

Jeanette Burger has been doing volunteer restoration work for the past eight years. When she moved onto three acres of land in suburban Long Grove in 2011, she knew nothing about invasive species. Through volunteering, she said, she’s since learned how to address the buckthorn problem in her own back yard.

In 2014, the Forest Preserves of Cook County set a goal of restoring 30,000 acres of land to good or excellent condition by 2039. Now, close to 17,000 acres are under active restoration.

“We’ve got a long way to go but we have a plan and we’re working it,” Cox said. “We want to get everybody out and involved because the forest preserves belong to all of us.”

You can learn more about volunteering with the Forest Preserves of Cook County here, and about volunteering with Friends of the Forest Preserves here.