It's Fall Y'all: The Science Behind Fall Colors

ANDREA ZANI

Andrea Zani is managing editor of Wisconsin Natural Resources magazine.

When it comes to fall colors, no two years are the same and exactly how a given year will shape up remains unpredictable. That’s because two of the main factors that influence fall color outcomes — temperature and rainfall — can vary widely from year to year.

“The intensity of Wisconsin's fall color season is influenced by the weather the state receives during September and October,” said Colleen Matula, DNR forest ecologist and silviculturist. “To have the most brilliant and vibrant fall color display, a series of fall days filled with bright sunshine and cool but frostfree evenings are required.”

As for the colors themselves, the bright yellows, glowing oranges and flashy reds we see are all about leaf pigments — chlorophyll, anthocyanins and carotenoids, including xanthophylls. These account for trees’ individual colors, which can vary by species.

Let’s look at the process: During a typical growing season, trees produce chlorophyll, which gives leaves their green color. Asdays in the Northern Hemisphere get shorter in fall, trees respond by producing less chlorophyll until stopping altogether.

When that happens, other pigments already in tree leaves, carotenoids, show through. This gives leaves their yellow, orange and brown colors.

While these colors are fairly constant from year to year, they still can vary in brightness and timing. Yellow and orange hues may be more muted or take longer to show through, for example, if fall temperatures stay warmer for longer. That means more chlorophyll remains in leaves, keeping them greener longer at the start of fall.

The red colors in trees come from anthocyanins. Not all trees make these pigments, which mostly are produced only in autumn and only under certain conditions. Anthocyanins are produced by trees as a form of protection, research has shown, allowing the plant to recover nutrients in the leaves before they fall off, thus preparing for the next growing season.

Photo Illustration by Rachel Hershberger/Travel Wisconsin

Perhaps you’ve noticed years when the red, purple and crimson colors of fall seem brighter and more ubiquitous. “Cooler nighttime temperatures tend to amplify the brightness of reds and purple in leaves,” Matula said, “while warmer nights will mute this color change.”

Rainfall, or lack of it, affects autumn leaf color in other ways. Wet weather and drought can lower color intensity, while drought can affect the timing of fall colors. An early arriving severe frost will kill the leaves, causing them to turn brown and drop prematurely.

Despite all the weather uncertainties, it’s possible to generalize about the colors of fall.

“Fall color predictions by the National Oceanic and Atmospheric Administration are based on mathematical algorithms that factor in historical leaf peak, temperatures, precipitation, leaf volume, health and day length,” Matula said.

In far northern Wisconsin, peak color usually occurs in the last week of September and first week of October. Central Wisconsin generally sees peak color during mid-October, and the southern part of the state mostly reaches peak at the end of October.

No matter where you are, you can get out and enjoy the autumn display.

“With a 17-million-acre forest resource in Wisconsin that is increasing in volume each year,” Matula said, “fall color is one show that must go on.”

FALL COLOR REPORT

Looking for the best places and right timing to take in the vibrant beauty of the season? Check out Travel Wisconsin’s Fall Color Report for in-depth information on color status and estimated peak weeks all around the state — travelwisconsin.com/ fall-color-report.