Trees provide more than $416 million in benefits to residents of the seven-county Chicago region, but widespread invasive species, the massive loss of ash trees, and the need for more mature and diverse trees are significant issues that The Morton Arboretum’s scientists say will impact the region for years to come.
Those are among the key findings in the Arboretum’s 2020 Chicago Region Tree Census, released on Arbor Day, April 30, a day set aside for planting and calling attention to trees. The report provides the first measure of change for the regional forest in Cook, DuPage, Kane, Kendall, Lake, McHenry, and Will counties since the 2010 tree census. The 2010 tree census was the first such assessment conducted at the regional scale and the largest of its kind in the U.S. The Arboretum remeasured 1,576 plots to gain a comparative snapshot of the regional forest and the benefits it provides.
According to Chai-Shian Kua, Ph.D., urban tree science leader at the Arboretum, the overall tree canopy cover increased in the region during the previous decade, but the canopy declined in the city of Chicago and McHenry County. Standing ash trees (Fraxinus spp.) decreased by 46% to 7 million due to an emerald ash borer (EAB) insect infestation, with approximately 4 million more either dead or in decline, leaving only 3 million healthy ash trees in the region. Because ash trees are so common in Chicago, the canopy in the city was impacted. Canopy refers to the upper layer of forests formed by mature tree crowns that shelter the ground below. Kua noted that canopy quality is ecologically important and that large, healthy trees are able to provide more benefits than small trees.
While the census reports that the region has more trees than it did in 2010 — growing from 157 million to 172 million today — it also identified invasive European buckthorn (Rhamnus cathartica), a medium-sized tree that spreads rapidly and degrades native ecosystems, as the most common species in the region, making up 36% of its trees. For context, the second most common tree is boxelder (Acer negundo), which accounts for 4% of trees. Kua explained that buckthorn and other invasive plant species reduce the diversity of the regional forest by out-competing native plant species and preventing the growth of young saplings.
The impact of buckthorn is of such concern that the Chicago Region Trees Initiative (CRTI), a partnership of more than 200 organizations founded by the Arboretum to address the kind of challenges highlighted in the 2010 tree census, developed the Healthy Hedges program to assist individuals and communities with buckthorn removal and replacement.
“One of the most urgent actions that need to be taken in the region is for homeowners and other landowners to remove invasive buckthorn and replace it with non-invasive trees and plants,” said CRTI Director Lydia Scott.
The regional forest needs a greater diversity of tree species to be resilient to threats posed by the changing climate, pests, and diseases. Significantly, Arboretum researchers determined that there are at least 194 different tree species in the region and 103 in the city of Chicago. However, this wide range of species is not broadly planted throughout the region.
“Concerted effort is needed to expand species diversity,” Kua said. “Residents and communities must focus on planting a wide variety of species now to ensure a healthy regional forest as a key defense against the uncertainties of the changing climate.”
Toward that effort, CRTI recently launched the Plant Trees for Communities campaign to support proper planting and care for trees, with the short-term goal of planting at least one tree in every community in the seven-county region, as well as all 50 Chicago wards in 2021. CRTI is seeking corporate partners to support the program to be able to provide trees at no cost for under-resourced communities that have the most urgent needs.
Another challenge, according to the census, is that three-fourths of the trees in the region are less than 6 inches in diameter, and the Arboretum’s scientists are concerned that many may not survive and grow to provide the benefits of large canopy trees without proper attention to their care.
“We need more people to plant the right trees in the right places and provide the right long-term care so they grow to maturity, or we risk losing the many critical benefits they could provide for decades to come,” Kua stressed. She noted that the Arboretum has tools available to help residents select trees suitable for the regional climate, including a searchable online database of trees and plants.
Scott said that even those who can’t plant their own tree can do their part for the regional forest by watering trees in city parkways or donating to and volunteering with organizations focused on proper tree planting and care.
Kua noted, “The Morton Arboretum will use the census results to inform efforts to improve the diversity and health of Chicago’s regional forest, and ensure that trees are equitably distributed to deliver benefits to all communities.”
For the 2020 Chicago Region Tree Census Report and more information about how to support tree planting and invasive species removal, visit mortonarb.org/tree-census.