The science behind fall foliage

Mother Nature's twists and turns can have a big impact on how the trees change in autumn

|  Story by Meghan McMahon |


Autumn has officially arrived, but one of the most anticipated events of fall — the leaves achieving peak fall form with hues of yellow, orange, red, purple and brown — is still a few weeks away.

On average, the leaves in Will County and northern Illinois start to change about the third week of September, but the peak is usually not until mid- to late October, said Bob Bryerton, an interpretive naturalist for the Forest Preserve District.

The weather is the most important factor in how colorful our trees and forests will be in the fall, and the weather right now is just one piece of the puzzle, Bryerton said. Nearly every weather factor during every season can affect when the leaves change and how long the season lasts. And abnormal weather conditions in the winter, spring and summer impact the fall color as well.

“Every season has variables, and they all affect the trees,” Bryerton said.

Why leaves change color

The big bridge along the Plum Creek Greenway Trail.

Plum Creek Greenway Trail (Photo courtesy of Bonnie Wydra)

Our annual fall color display is due to the presence of a pigment called chlorophyll in leaves. During the spring and summer, the chlorophyll makes the leaves green in color, but as the days get shorter and the angle of the sun changes in late summer and early fall, the leaves make less chlorophyll and their color begins to change.

When chlorophyll levels decrease, the presence of other pigments that develop from the sugars in the leaves become more apparent, producing the array of yellow, orange, red and purple hues that paint the fall landscape.

The more sugars a leaf produces, the brighter its fall color will be, according to the Illinois Department of Natural Resources. To maximize the amount of sugar in the leaves, we need a lot of bright sunlight in the fall.

Mother Nature’s role

A window view of a rainy, fall day.

Photo via Shutterstock

Like all years, the weather so far this year has had some unusual turns. And while the weather throughout the year affects the color of the leaves in the fall, weather conditions in the months leading up to fall have the biggest impact, said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum’s Plant Clinic.

“Adequate rainfall would be helpful, as well as sunny days followed by cool nights,” Yiesla explained.

Both heat and drought can be a showstopper for the fall color.

“If heat and drought cause leaves to be scorched, no amount of good fall weather will make up for that,” Yiesla said.

One thing that can help extend the fall color season is a nice frost in early October, Bryerton said. This is especially true if the weather then warms up again. The frost triggers the tree to prepare for winter, which will cause the leaves to change color. If it’s followed by a mild stretch, the brightly colored leaves will stay on the tree longer before dropping.

On the other hand, windy, rainy and stormy conditions can shorten the period of time when we can enjoy the color because the leaves will be knocked to the ground sooner, he said.

Dazzling displays in the Will County preserves

Fall color at Hickory Creek Preserve.

Hickory Creek Preserve (Photo courtesy of Michael Fagan)

Leaf-peeping, as it’s sometimes called, isn’t as big of a draw here in northern Illinois as it is in, say, New England or even in Door County, Wisconsin, but our trees do provide quite a show every autumn.

In Will County’s preserves, the forests are dominated by oak and hickory trees, which have leaves that typically turn shades of brown, purple and dull red in the fall, Bryerton said. Sugar maples are mixed in at the preserves as well, and these provide a pop of bright orange and yellow. Ash trees, too, turn a nice golden color, but their numbers are diminishing because of the emerald ash borer, an invasive beetle which is expected to kill most of the native ash trees in Illinois and the Midwest.

Bryerton said he’s partial to some of the fall vistas in and around Goodenow Grove Nature Preserve, because that’s where he spends most of his time.

“The view from the hill on a fall day can be really extraordinary,” he said.

Fall color at Goodenow Grove Nature Preserve.

Goodenow Grove Nature Preserve (Photo courtesy of Lorenzo Pesce)

Another good view from Goodenow Grove is behind Meadow Lark shelter during the morning hours, when the sun rising from behind provides a picturesque view.

Raccoon Grove Nature Preserve is another good place to check out fall color. A walk in the woods there will allow you to be surrounded by trees as they prepare for winter, and Pauling Road near the front of the preserve is a good place to see the golden color maple trees display each autumn. On the western side of the county, Hammel Woods and McKinley Woods both offer stunning, colorful fall vistas.

Bryerton said some of the best views within the District are in preserves where different landscapes combine to form a varied vista. At Hickory Creek Preserve, for example, the fall color mixes with the shades of brown in the surrounding fields. And at Monee Reservoir and Whalon Lake, the water adds to the backdrop.

“The combination of the water with the trees makes it stunning,” he said.

A lake view at Whalon Lake.

Whalon Lake (Photo courtesy of Greg DuBois)


(Lead image courtesy of Kevin Keyes)


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