Why do the leaves change color in the fall?

GREENVILLE, N.C. (WNCT) – October is the time of year when leaves that have been green all spring and summer turn to gorgeous shades of yellow, orange, and red. This change in color is one that happens for a very specific reason.

Through a process called photosynthesis, which means “putting together with light,” plants use sunlight to turn water and carbon dioxide into oxygen and sugar. Oxygen is released as a gas into the air but glucose, a type of sugar, is a plant’s food source. A chemical called chlorophyll, which gives plants their green color, helps make photosynthesis happen.

As the days get shorter and shorter during fall, trees know it is time to get ready for winter. There is not enough sunlight or water during the winter for photosynthesis to happen so trees shutdown food production and rely on their food stores to get through the winter. As this shutdown happens, chlorophyll disappears from the leaves. Chlorophyll’s green pigment is dominant, so as the green fades away, other natural pigments in the leaves come to the forefront. So basically, these colors have been there all along but have been hidden all spring and summer by the green chlorophyll. This is why leaves turn yellow, orange, and red in the fall.

Trees break down the green pigments and nutrients stored in the leaves. The nutrients are shuttled into the roots for reuse in the spring.

In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red color.

The red color is actively made in leaves by bright light and cold. The crisp, cold nights in the fall combine with bright, sunny days to spur production of red in leaves, especially in sugar maple and red maple trees. Fall seasons with a lot of sunny days and cold nights will have the brightest colors.

In some cases, about half of a tree’s leaves are red/orange and the other half green. This can happen due to micro-environmental factors, like only half the tree being exposed to sunlight or cold.

Hardwoods in the Midwest and on the East Coast are famous for good color selections. Some of the more reliably colorful trees are liquid amber trees (also called sweet gum) that turn a variety of colors on the same tree, and sometimes the same leaf. Ash tree leaves often turn a deep burgundy color. Ginkgo trees will feature an intense yellow, almost golden, color.

The brown color of trees like oaks is made from wastes left in the leaves.

There are some theories on why the color pigments are there in the first place. One theory is that some trees use the pigments to filter out sunlight. Some leaves can get sun burnt, stress out, and die. Another theory is that the color of a plant’s leaves is often related to the ability to warn off pests or attract pollinators.

Here’s a look at when fall color typically peaks in western NC.

Courtesy: Appalachian State University


The Foliage Network and Explore Asheville offer current fall color reports through the season.



– Meteorologist Pierce Legeion

Pierce Legeion is a meteorologist and digital journalist for WNCT 9 First Alert Weather.

Courtesy: earthsky.org and sciencemadesimple.com

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