The Chemistry Behind Fall Foliage

By Laura Tran, C2ST Intern, Rush University

One of the highlights of autumn is watching the trees change from a sea of green to an explosive ocean of red, orange, yellow, and purple.

Although the effect seems like magic, there’s a perfectly reasonable scientific explanation for the change in color!

Imagine yourself walking between a grove of trees in the summer. The leaves are a brilliant shade of green. The green color, or pigment, comes from chlorophyll, an important component in helping plants produce food for themselves through a process called photosynthesis. Plants utilize photosynthesis to absorb light from the sun and to convert light energy into sugars.

The official start of autumn this year is September 22nd and since then, the days have become shorter and the nights have become cooler. With less sunlight available, the chlorophyll inevitably begins breaking down.

While a magician never reveals their secrets, the leaves reveal their hidden secret. The pigments that cause the vibrant red, orange, yellow, and purple (1)  have been there in the leaves all along! They were simply hidden by the green chlorophyll for most of the year.

Now, not all trees change their leaves’ colors with the changing seasons. Coniferous trees such as evergreen trees (e.g., pines, spruces, firs, etc) and their needle- or scale-like leaves stay green year-round. We see the characteristic fall colors in deciduous trees and their large, broad leaves.

When chlorophyll breaks down, the orange (carotene) and yellow (xanthophyll) pigments are revealed. With darker reds, their unique color is a result of a chemical change. Sometimes sugars are trapped in the leaves and produce new pigments (anthocyanins). The presence of tannins will turn leaves brownish or tan.

Depending on where you live, you might see a wide spectrum of colors from different types of trees (2). For instance, oaks, dogwood, and red maple trees are likely to produce red leaves. Sugar maples produce more orange leaves and hickory and aspen trees will become golden yellow.

There are also a number of factors that influence color intensity in fall foliage. Temperature and moisture are the main influences. Fall colors are best seen during dry, sunny, and cool weather. Leaves will produce lots of sugar on warm, sunny days and if the nightly temperature is above freezing, these sugars will produce brilliant anthocyanin pigments.

This year, the Fall Foliage Map (3) predicts that Illinois won’t have peak foliage until October. Partial fall foliage will be seen in mid-October and reach its peak during the last week of October.

Take a moment out of your day to step outside to enjoy the crisp air and nature’s dazzling display of color!



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