The leaves being raked into curbside piles up and down the tree-lined streets in town share certain common elements. Within the mix, there are plenty of leaves from American elms, box elders, green ash, European buckthorn and black cherry trees.
A recent inventory of the trees in northeast Illinois found those species to be the most prevalent in Cook, DuPage, Kane, Kendall, Lake, McHenry and Will counties. The subsequent 114-page report, titled “Urban Trees and Forests of the Chicago Region” and published by the Morton Arboretum and the U.S. Forest Service, provides the first big-picture view ever compiled of the Chicago region’s forest community.
The researchers’ analysis, culled from data gathered by six crews who fanned out over the seven urban counties, provides a new view of the number and variety of trees in the metropolitan area. The information, and the presence of current challenges to a functional urban forest, enable foresters and other experts to understand more fully such things as how trees help humans, and what their prospective nemeses look like.
One primary finding of the study: variety is key.
“We know that virtually every urban area has difficulties getting good species, diversity and such, and that’s what our results show,” said Gary Watson, the arboretum’s research chief and one of the study’s authors.
Lydia Scott, community trees program manager for the arboretum, concurs that the broader the collection of tree types, the healthier the system.
“There are a lot of things that people can choose from,” she said.
The facility is devising lists of preferred species to plant, and to avoid.
“We would recommend that people look at their growing location,” said Scott, suggesting that homeowners take into account such factors as spray from road salt, drainage and light exposure. “As we look at our weather events that we’ve been seeing, in terms of heavy downpours and standing water that takes some time to drain, those are some things that people may want to take into consideration.”
Still, certain species are perennially favored by homeowners.
“American elms will grow just about anywhere (and) you get that cathedral look of that canopy down the street,” Watson said. “Everybody loves it.”
Much of the American elm population was decimated by Dutch elm disease in recent decades, and new cultivars that are resistant to the destructive elm bark beetle are showing promise. Among the most encouraging varieties, Watson said, is the Asian species developed by the late George Ware, who was his predecessor at the arboretum.
The primary current menace in the region, the emerald ash borer, is taking a steep toll as the bug feeds on the trees’ inner bark, steadily cutting off their access to nutrients and water.
“Some diseases and insects only attack weak trees. This one is not like that,” said Watson, who sees a grim future for the well-liked trees. “The population of ash trees as we know it is not going to be there. It’ll take a while, but they’re falling fast.”
However, an aggressive approach can save at least some of the area’s ash trees. Scott said many communities in the area have been slow to act against the borer, not realizing its effects can be countered through proactive strategies.
“If they’re treating their ash trees and they get to them early enough, there’s no reason they won’t have them moving forward,” Scott said.
New ash cultivars that can replace those beyond rescue are under development, but they’re not to be found in area nurseries yet.
“It’s going to be a while, but it will be done,” Watson said. “It takes many years with trees.”
Non-native trees that are plentiful in the region also usurp an undue share of the water and other tree nourishment that indigenous species need in order to thrive.
“I think the population of invasive species is higher than we might have expected, especially if you look on a county-by-county basis,” Scott said.
The European buckthorn is among those that came uninvited and have long overstayed their welcome. It represents more than one-quarter of the total tree population in DuPage County.
“It’s invasive for sure. It’s a non-native invasive species. I’m not even sure off the top of my head how it got introduced, though there are some interesting stories,” Watson said. “It’s one of those really tough species that can grow anywhere, and it does.”
The fundamentals of supply and demand can be put to use in a mindful process of broadening the predominance of trees that belong here.
“There’s a variety of species out there that’s available, and the more people ask for the less prominent species, the more the nurseries will carry them,” Watson said. “For decades we’ve been trying to break that cycle and get growers to grow more varieties, and get people to ask for them.”