Children play in fallen leaves at Morton Arboretum in Lisle.
Summer is over, but the drought that wilted corn fields and scorched lawns hasn’t stopped working its botanical abuse on trees this fall. Northern Illinois and Wisconsin fared better than drought-hit Southern Illinois, where annual bursts of colorful fall foliage are being disrupted.
Morton Arboretum plant specialist Doris Taylor says unusually dry conditions caused concern at the arboretum this year and had the effect of “adding stress to the trees.” But the arboretum, located 25 miles west of Chicago in Lisle, avoided the worst of the drought. “It doesn’t seem to be affecting fall color,” Taylor says.
“Right now it’s spectacular.”
Changing leaves generate travel and tourism, so October is an economically important month at Morton Arboretum and throughout the region. To accommodate color-seeking leaf enthusiasts, the arboretum hosts a month-long Fall Color Festival. In an average October, the arboretum attracts 135,000 leaf-peeping visitors, or about 45,000 more visitors than in the busiest of summer months.
Beyond an economic impact, though, fall colors also signal the onset of important biological processes.
Throughout the growing season, leaves produce glucose via photosynthesis. That glucose is stored as starch. Normally, as temperatures cool and daylight hours dwindle, trees go through a sort of biological shut-down sequence. Before leaves drop they transfer nutrients to limbs for winter storage.
Autumnal reds, yellows, purples and oranges play a crucial part in this shutdown sequence that plant physiologist Jeffrey Dawson characterizes as “highly coordinated,” “specific,” and sensitive to environmental influences. As a result, the vibrancy and timing of leaf-change acts as an indicator of forest health, says Dawson, of University Illinois at Champagne-Urbana.
As outer leaves begin to drop, inner leaves are especially vulnerable to light damage, infestation, and infection. The pigments responsible for fall colors are what Dawson describes as “multi-tasking” agents that help protect still-active leaves from these threats.
Leafy yellows and oranges, for example, are produced by carotenoids such as lycopene and beta-carotene – familiar to many people as cancer-preventing anti-oxidants. Carotenoids assist in making glucose throughout the growing season, but during the fall they serve an additional function: protecting aging leaf cells against light damage and free radicals.
Purple and red anthrocyanins, unlike carotenoids, are found in leaves only during the fall and only in certain species, but they share the same light-protection and anti-oxidant properties, in addition to fortifying leaves against encroaching winter temperatures.
In southern parts of Illinois where drought was more severe, fall colors have been less vibrant. “It won’t be as spectacular as usual,” Dawson says. “Some trees have dropped their leaves early because of drought conditions.” Other trees are peaking prematurely, so that leaves that do turn will be “staggered instead of happening all at once.”
A less-than-spectacular fall has broader ecological significance. The trees hit hardest over the summer will continue to show the effects of it the following spring, Dawson says. When drought damage causes trees to drop their leaves early, trees lose nutrients they would have used to fuel budding after winter. A fall with less orange and yellow predicts a spring with less green.
Fortunately for Midwestern “leaf-peepers,” tree populations in northern regions fared better through the drought.
“Overall, the forests in Wisconsin came through it pretty well,” says Bob Manwell, spokesman for the Department of Natural Resources in Wisconsin.
And because of unusually uniform temperatures across the state in recent weeks – yet another factor that can impact fall foliage from year to year – foliage throughout Wisconsin is almost simultaneously in peak.
“Nothing is holding back right now,” says Manwell. “It’s a great time to get out and see some colors.”