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New Arboretum Tree Census Reveals Ways To Protect Chicago Region’s Urban Forest

Jul 30, 2021 ● By Sheryl DeVore
2020 Chicago region tree census, Morton Arboretum

Two project team members measure the diameter of a tree during the 2020 Chicago region tree census. (Photo courtesy Morton Arboretum)

In the past decade, millions of ash trees have died in the Chicago region, with more likely to die in another 10 years. Meanwhile, the percentage of invasive species continues to rise, causing harm to the region’s ecosystems. Trees continue to provide numerous benefits for humans and wildlife, and there’s much individuals can do to improve and maintain a healthy urban forest.

The results of a Morton Arboretum 2020 census of trees in the Chicago region, which includes Cook, DuPage, Kane, Kendall, Lake, McHenry and Will counties, have spawned the new Plant Trees Campaign, which asks each of the 340 communities within the region to plant one new tree.

“The campaign draws attention to value of trees,” says Lydia Scott, director of the Chicago Region Trees Initiative, which started the new program. “Trees provide oxygen for us, they cool our air, they reduce our flooding and they improve our health,” she explains.

The 2020 census followed a 2010 tree census that was done on randomly selected plots by the U.S. Forest Service with the Morton Arboretum. In 2020, the Morton Arboretum re-measured trees in the same plots to gain a comparative snapshot of the regional forest and the benefits it provides. The snapshot showed red oak, on the list of 10 most common tree species in the region in 2010, has fallen off the list in 2020. In addition, there’s a greater percentage of invasive trees in the mix compared with a decade ago.

The census also revealed the functional value of trees, considering how they remove pollution, store carbon, reduce flooding and save energy. If all the trees were gone, they would cost $45 billion to replace and $416 million annually to reduce the ensuing flooding.

Chai-Shian Kua, Ph.D., urban tree science leader at the Morton Arboretum, led the 2020 census. The project team measured the diameter at breast height of each tree located within the randomly selected plots. They identified the species, assessed its condition and the canopy cover. “We also measured if the tree is next to a building, measured the distance next to a building and estimated energy-saving benefits provide by trees,” Kua says.

The trees were measured in residential areas, open spaces such as forest preserves, corporate and transportation areas, and agricultural lands. “In 2010, when the first study was conducted, the emerald ash borers were already here,” Kua says. “We expected to lose a lot of ash trees in 2020 due to the introduced pest, and we did.”

The emerald ash borer, introduced from Asia to the U.S. likely via wood packing materials, has destroyed tens of millions of native ash trees in 30 states, according to the U.S. Department of Agriculture. “In 2010, we estimated 13 million ash trees in the region,” Kua says. “In 2020, there were 7 million. But out of this 7 million, about 4 million are dead and still standing or in decline. So, we think there are about 3 million ash trees left.” One of the biggest lessons from the 2020 tree census is, “We should plant more diversity,” says Kua. “We should also plant the right tree in the right place and give it the right care.”

Greg Spyreas, a botanist with the Illinois Natural History Survey, agrees. Native ash trees were planted abundantly in the Chicago region neighborhoods over the years. “The issue is that when a pest or disease comes, the whole neighborhood’s trees are obliterated. And it will take 20 years until new trees planted bring back any kind of urban forest,” he says.

“Just by taking a simple step of planting a diverse assortment of native trees we will not only create beautiful landscapes and streetscapes, but also save money on maintenance and landscape, and improve habitat for all kinds of wildlife such as our native imperiled songbirds,” Spyreas says.

Another of the biggest concerns the census revealed was the increase in European buckthorn. Thirty-six percent of the species in the census consisted of this invasive plant, also called common buckthorn. It was brought to the U.S. from Europe in the early 1800s to serve as privacy hedges because it remains greener longer than other trees and shrubs. Since then, scientists have discovered that the buckthorn produces a chemical in the soil that inhibits the growth of other native species.

“Also, buckthorn leaf litter decomposes very quickly and adds high concentrations of nitrogen in the soil,” the Arboretum’s Scott says. “Our native oaks don’t like high-nitrogen soil. Our native plants aren’t designed to thrive in conditions that are high in nitrogen.”

Even though volunteers are working in forest preserves and other natural areas to remove buckthorn, “Seventy percent of what’s out there is on private property,” Scott says. “That’s why we developed the Healthy Hedges program.” The program helps people learn to remove buckthorn and other invasive plants like honeysuckle and replace them with native species that also can create a privacy hedge.

“You don’t have to get rid of it all at once,” she suggests. “Remove the females that produce the berries first.” Bird eat the berries and “plant” new buckthorn. Some suggestions on hedge replacement species include oak-leafed hydrangea, vernal witch hazel and serviceberry.

Kua suggests walking around the neighborhood to see what kind of trees are there, then planting something different to add diversity. If there are lots of maples, plant an oak, for example. Planting natives makes sense, but she thinks some non-native trees that aren’t invasive may also be planted. “One example is ginkgo and another is London planetree, which are tolerant to the urban environment.” See and for a list of trees recommended to plant in the Chicago region.

“Taking care of nature is part of life,” Scott says. “We should all adopt that kind of mentality. We want people to think about opportunity areas to plant trees to reduce the heat coming to their house in the summer and reduce their energy bills. Trees are more than something pretty to look at. Trees are a solution to a lot of problems.”

Sheryl DeVore has written six books on science, health and nature, as well as nature, health and environment stories for national and regional publications. Read more at

Trees to Plant

Greg Spyreas, a botanist with the Illinois Natural History Survey, suggests planting the trees listed below to create a more diverse landscape in the Chicago region urban forest. Before choosing a tree, know the conditions of soil, moisture and amount of sunlight where it will be planted, and also consider how large the tree may grow. Learn the type of care it needs and after planting, water it deeply, about an inch a week. Most of these listed are native to the Chicago region. The smaller trees listed grow more quickly, with sassafras the quickest. Also see and for a list of trees recommended to plant in the Chicago region.

Small Trees




Hawthorn species

Prairie crabapple

Eastern redbud

Blue beech


Larger Trees

White pine (evergreen)

Eastern hemlock (evergreen)

Shagbark hickory

American beech

Common hackberry

Ohio buckeye

Kentucky coffee tree