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Why Leaves Change Color

The Fall Color Report compiled by the Wisconsin Department of Tourism estimated that the fall colors in Door County would peak this week. While most of northern Wisconsin is past its peak and southern counties are just beginning to turn, we looked at what makes the leaves change color and why we have had a good fall this year.

There are three pigments in leaves that create the spectrum of colors on trees. Chlorophyll keeps leaves looking green through the summer as it converts sunlight from long summer days into food for the tree. Although weather plays a big part in the intensity and duration of the fall colors, leaves stop producing chlorophyll at nearly the exact same time every year since the process is linked to the length of the days and the intensity of sunlight.

Without chlorophyll in the picture, carotenoids (yellows, oranges and browns) and anthocyanins (reds, purples and blues) begin showing their true colors.

Carotenoids are present in leaves throughout the summer, but are dominated by chlorophyll. When chlorophyll dies off, carotenoids are the first fall pigment to show off. Yellows and oranges begin to dominate the landscape while deep red color is more uncertain and dependent on the weather.

Fall colors in Door County. Photo by Len Villano.

Fall colors in Door County. Photo by Len Villano.

When warm and sunny autumn days are paired with nights that do not dip below freezing, anthocyanins take the stage. Leaves continue producing sugars during those warm, sunny days while cooler nighttime temperatures prevent the sugars from running off of the leaf. This trapped food source kicks off the production of anthocyanin and helps it thrive.

A series of freezing nights, whether they be early in the fall or late, will zap this sugar production and leave us with brown leaves void of any pigments.

There are a series of circumstances that have left Door County with nearly a month of intense fall colors. A dry spring delayed the onset of fall colors to the beginning of October while the mostly sunny summer and early fall caused the breakdown of chlorophyll to reveal the golden hues sooner. Additionally, rain in late September and early October stave off the shutdown of sugar production, leaving the colors on the branches for a longer period of time. Unless strong winds and heavy storms prematurely rip the weakening leaves from their branches, expect the fall colors to stick around until the final week of October.

Sources: The Farmer’s Almanac, Wisconsin Department of Natural Resources, United States Forest Service

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