OVERLAND PARK, Kansas — Each fall, as temperatures drop, people in this suburb can count on head-turning displays of richly hued maple leaves.
But maples make up about one-third of the city’s street trees, and Overland Park has learned the hard way that too much of a good thing can mean fragility.
So last year, the city put the kibosh on planting more maples.
The emerald ash borer — a tiny, shiny green hitchhiker from Asia with a voracious appetite for a different beloved street tree — taught this city and others across Kansas and Missouri a painful lesson.
Now some communities are hedging their arboreal bets and protecting local home values and tax bases by embracing variety.
Streets with mature trees command higher home prices, temper the dog days of summer, and draw more people outdoors for fresh air, walks and chats with neighbors.
“It is likely the emerald ash borer will kill all of Overland Park’s ash trees,” city spokeswoman Meg Ralph said in an email. “The more variety we have in our tree canopy, the more resilient it will be when the next invasive species or tree disease comes to our community.”
Ash made up about a fourth of the city’s street tree canopy when the insect arrived a decade ago.
In a thoroughly globalized world, the city has no guarantee that a bug with a taste for maple syrup won’t turn up next.
So last fall, its forester dropped maples from the list of trees that people can plant along residential and commercial streets in the public right-of-ways — such as that strip of turf between street and sidewalk.
For now, Overland Park won’t add more maples to any city property. It will steer developers away from those trees, too. Builders need city forester approval for their landscape plans.
“We are working with developers to ensure maples are not planted in new private development,” Ralph said.
The price of replacing 1,700 ash trees
The Winterset area of Lee’s Summit, Missouri, illustrates the stakes.
Ash trees are celebrated for their purple, red and yellow fall foliage. So streets lined with hundreds of the tall, graceful trees seemed a dream when the developers of Winterset broke ground in 1990 — before emerald ash borers turned up in Michigan.
“That ended up being a major mistake,” developer David Gale recalled, “because we did a monoculture.”
More than a decade later, Gale heard about the invasive beetles eating their way across U.S. cities and forests, decimating trees. Neighborhood leaders tracked the situation.
“You could see the radius just expanding and expanding,” he said. “You just have to face reality.”
By the time the invasive insects approached Lee’s Summit, the 900 homes in the growing Winterset area enjoyed 1,700 ashes. After two and a half decades in the ground, they stood an average of 40 feet tall.
“You can imagine how attractive they were,” Gale said.
The Winterset board voted to remove the trees and replace them. The work wrapped up in 2021 and cost $475,000.
This time around, the community planted a mixed street canopy.
Residents chose among nine native hardwood species, including oaks, maples, lindens and black gums.
The replacements are still relatively young, but once they mature, they’ll feel like an extension of the deciduous woodlands in and around Winterset.
Picking trees in a globalized world
Landscapers have long tapped into the appeal of visual consistency by planting scores or hundreds of identical trees at a time.
Elm. Red maple. Ash. Bradford pear.
“Whatever the favorite tree is at the time,” said Kim Bomberger, a forester who helps northeast Kansas cities take stock of their situations. “I’ve done inventories where I’ve gone to roll into a neighborhood, and as far ahead as I could see, it was all pin oaks.”
Bomberger works for the Kansas Forest Service.
“The national conversation now is to reduce those numbers” of single species or closely related trees overplanted in certain areas, she said. “We’re trying to better guard against catastrophic loss.”
But it’s hard to break the habit.
One woman called up Bomberger, frustrated that her neighborhood’s covenant had codified ash trees before the advent of the emerald ash borer. Homeowners who now faced the invasive beetles were bound by an old rule to replace their ill-fated trees with yet more of the same.
The National Park Service raised eyebrows among some arborists when it decided in 2013 to replace the 800 ashes lining paths near the St. Louis Gateway Arch with a new monoculture – 800 London plane trees. The agency said consistency was integral to the park’s historic character.
The risks that come with overdoing any one species grow year by year.
Global trade is rearranging life on the planet at an unprecedented pace.
It whisks living things to faraway places where they escape many or all of their natural predators and find a buffet of scrumptious plants unequipped with the evolutionary defenses that checked their appetites back home.
That’s when a beetle or a fungus can morph from a balanced part of one place’s food web into a devastating invader of another’s.
Dutch Elm Disease turned up on our continent less than a century ago, ravaging street canopies and continually battering elms that persist in natural areas.
A fungus from Asia blotted out billions of North American chestnut trees in just a few short decades.
Beetles are obliterating pines and thousand cankers disease is infecting black walnuts in Colorado. Aphid-like insects are wiping out mighty hemlocks on the East Coast and in the upper Midwest.
Scientists race to cultivate plants resistant to new invaders, but such efforts take decades without guaranteed success.
Nor do they prevent the staggering economic and environmental losses that happen in the meantime.
Kevin Boyle, an economist and the director of the Department of Real Estate at Virginia Tech, studies trees through the lens of property values.
He has examined the financial impact of pests killing trees on large scales — including emerald ash borer in the Milwaukee area.
“When trees are damaged and dying,” he said, “it reduces the value of properties.”
This monetary loss compounds the dollars that property owners and public agencies pour into taking down and replacing the doomed trees.
The three types of diversity
From a purely financial perspective, trees are an investment.
Boyle and his colleagues scoured existing research and published a meta-analysis. The findings suggest a house in a neighborhood with nary a tree sells for about $3,250 less than one in a neighborhood where a mature canopy covers about 30% of the ground.
To protect their investments, many cities take an increasingly nuanced tack.
Though they can’t generally control what you plant in your yard, they often set rules for trees in the public right-of-way.
Foresters recommend — and cities increasingly pursue — three kinds of diversity in these areas.
- Plant plenty of different species.
- Make sure those species are different enough. Imagine filling a town with a dozen oak species and nothing else. Those oaks are all so closely related – part of a single oak genus – that a single threat could spell devastation. So plant different genera.
- Make sure those genera are different enough. If you filled a subdivision with crabapples, hawthorns, cherries and serviceberries, you’d get four different genera, but all within a single family called Rosaceae. These more distant relatives can still share a lot of vulnerabilities.
An old rule of thumb is “10-20-30.” Make sure no more than 30% of a community’s tree plantings fall into any single family, no more than 20% fall into a single genus, and no more than 10% percent a single species.
But Bomberger says many experts now recommend cities push diversity beyond that, and she agrees.
Robert Whitman, a landscape architect at Multistudio firm in Kansas City, said neighborhoods can have the best of both worlds: tree diversity and visual consistency.
He helped the Mid-America Regional Council write model tree ordinances in 2020 that cities can use to protect the canopies that bolster their tax bases, clean their air and ease street flooding.
“Diversity is super important,” he said. “But what I will say is, if every street has a consistent mix of species going down it, it’s less unified. It looks a little bit less organized.”
Imagine instead switching species street by street. Swamp white oaks line one street, sycamores line the next, and sugar maples line a third. Some Prairie Village neighborhoods built in the 1940s took this approach.
“You still build in neighborhood diversity,” he said, “And there’s clear identity between each street.”
That approach makes some foresters nervous.
“We can plant very diverse streetscapes that are still appealing,” Bomberger said. “They just won’t be those allées of American elms that all of us have seen, that have that symmetry. Because in this day and age, why would we want to roll the dice of something coming in and being a catastrophe to that streetscape?”
Native vs. non-native
A second debate in the field: whether to stop planting trees from faraway places.
Perspectives vary because some experts worry more about maximizing diversity to hedge against pests, while others focus on ecological downsides linked to non-native species.
“I do prefer natives,” Bomberger said, but “the people that I work with, the cities that I work with, we will be planting some non-native trees because what we’re trying to do is guard against catastrophic loss.”
Many cities, such as Overland Park, Kansas City, Kansas, and Salina, promote a mix of native (bur oak, tulip poplar) and non-native species (zelkova, ginkgo).
But an increasingly vocal movement of scientists, environmentalists and impassioned homeowners promote native species.
Some non-native trees do not support native wildlife, such as birds and butterflies. Others spread from suburbia into natural areas, where they choke out native flora that these creatures need to reproduce. Bradford pear offers a prime example.
Whitman, at Multistudio, leans heavily on regional species in his designs.
“I’ll look at my species options,” he said, “and always select a native if a native will fit the bill.”
Whitman isn’t an absolutist and says he draws on non-native options if they better fit a site’s conditions and design.
But he argues that cities can plant sufficiently diverse canopies using native species – and warns that some non-native options, such as ginkgos and zelkovas, “have almost zero benefit to the overall ecosystem.”
“They look good in a neighborhood,” Whitman said. But “they’re not contributing (to the food web). Literally, they are like plastic trees.”
Celia Llopis-Jepsen is the environment reporter for the Kansas News Service. You can follow her on Twitter @celia_LJ or email her at celia (at) kcur (dot) org.
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