Kaleidoscope of fall: The science behind the changing of the leaves
by Shane Harris
Special to The Star
Nov 04, 2012 | 2020 views |  0 comments | 12 12 recommendations | email to a friend | print
Photo: Trent Penny/The Anniston Star
Photo: Trent Penny/The Anniston Star
The cool, crisp mornings and the changing of the tree leaves means autumn has arrived.

The colorful fall leaves are a wonderful and welcomed sight. The array of reds, yellows, purples and browns, mixed in with the occasional greens, is good for the soul. The colorful show, unfortunately, lasts for only a little while.

But as with any show, very few people quite understand what goes on behind the scenes.

Many people suppose that frost causes the leaf color change, but it actually does not. Most tree leaves begin to change color before we have had any frost.

So, why do leaves change color? The fact is that the beautiful fall foliage is really just a chemical response to the changes in weather conditions.

During spring and summer, leaves serve as “food factories” for the tree. Through a process called photosynthesis, leaves combine sunlight, carbon dioxide and water to create food — glucose, or sugar. Glucose becomes a source of energy for the tree.

A special chemical called chlorophyll, found within leaf cells, helps the photosynthesis process take place. Chlorophyll is what gives the leaves their green color.

Also within the leaf are yellow or orange carotenoids pigments. However, throughout the spring and summer, the green pigment is in larger amounts, enabling it to mask the other color pigments.

Changes in daylight and temperature in early fall trigger trees to begin shutting down their “food factories.” During the winter, there is not enough light or water to continue making food.

As the tree begins to prepare for the winter, chlorophyll breaks down, causing the green color to disappear — allowing the yellow and orange pigments that were there all along to become visible.

Fall weather conditions do play a part in the formation of autumn colors. Warm sunny days, with nighttime temperatures below 45 degrees, raise the level of red coloration. Sugar, made in the leaves during the day, cannot move out of the leaves when the nights are cool. The trapped sugars change into the pigment anthocyanin. This pigment is usually red, but may range to violet or blue.

A year with vivid fall color occurs when we have had a warm, dry summer and early autumn rainfall. This also prevents the leaves from falling too early.

If there is a long period of wet weather in late fall, the fall leaf color tends to be less vivid.

Fall is here; go take a walk or a drive and enjoy the colorful show.

Shane Harris is an extension agent for the Alabama Cooperative Extension System.

For help on other home and garden questions, contact your local county Extension office or visit www.aces.edu.
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