How do fall leaves earn their vibrant color?

Story by Elise Sandlin, Contributing Writer

Photo by Bill Lickman, Photographer

Every autumn, deciduous trees shed their leaves to prepare for winter, but the biological process behind the pretty change of colors is more complicated than you think.

It’s a chemical process that begins as trees sense a change in the seasons. During the warm months of spring and the long days of summer, chlorophyll is the most prominent color in the leaves because of photosynthesis. Throughout the summer, trees are constantly making more chlorophyll, but as the days get shorter and there is less sunlight, the leaf is signaled to prepare for winter. The trees begin to make less chlorophyll, which leads to other pigments becoming more visible. As the chlorophyll begins to break down, yellow, orange, and red pigments become visible in trees of many species found in Tennessee, including maples, oaks, ginkgoes and hickories.

“With the passing of summer, days become shorter,” Wayne K. Clatterbuck, assistant professor in Agricultural Extension at the University of Tennessee, said. “The phytochrome, the light-sensing mechanisms in leaves, recognizes the shorter day lengths. The shorter days and lower temperatures arrest chlorophyll production. Chlorophyll breaks down faster than it is replaced, allowing the yellow and orange pigments to be unmasked.”

Some years the colors exposed by this process are more vibrant than others. This is due to a variety of reasons. Rainfall throughout the summer is an important contributor to the saturation of color. If a summer is too dry, the fall colors will not be as bright.

In addition, the temperature of fall weather is crucial to bright colors, Clatterbuck noted. The process can dramatically speed up if the weather gets too cold, resulting in the leaves immediately turning brown and dying. On the other hand, if there’s a balance of cool nights and warm days, the colors are more vibrant and noticeable. Such as Tennesseans are enjoying this fall.

“In the summer where it’s a really good growth, the plants will produce a lot more of the pigments. So, like this past summer was a pretty rainy summer, so I’m expecting the leaves to have pretty vibrant colors. If we have a summer where it’s hot and dry and we get very little rain, the plants won’t do quite as well. They’ll still have color. It just won’t be as vibrant,” according to John DuBois, who teaches biology at Middle Tennessee State University Biology.

“If we get a real early frost, that speeds up the whole process. At that point, the leaves get damaged, and the plant decides it doesn’t need them anymore. Then the leaves just start falling off the plant because the temperature zapped them,” DuBois added.

The fall of 2021 has been a good one and this is certainly good news for fall tourism locations. Leaf peeping is a common attraction in Tennessee as well as many other states. The brighter the colors are each year, the more tourists are attracted to areas like Great Smoky Mountains National Park.

The economic impact of autumn leaves is considerable. According to the Department of Biology at Appalachian State University, the gross income of leaf watching likely approaches a billion dollars a year in mountainous areas of the southeast. According to studies done by Appalachian State, for every million dollars spent on tourism, 38 jobs are created, and for every dollar spent on attracting tourism, an additional 69 cents in spending is generated.

MTSU Sophomore Sydney Roeper admires the autumn season and the changes of the leaves. “Fall touches my soul because the changing leaves correspond to my life,” she said. The changing tree colors “reminds me that you can go through many changes in one season, and it can be beautiful.”

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