Biology behind signs of fall

Why leaves change color as weather cools

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The first signs of fall can be seen in some of the aspen groves around Steamboat Springs. Some leaves have lost their green color and are starting to turn peach and yellow.

The lower temperatures -- finally -- and shorter days of September are a signal to the trees to start breaking down the chlorophyll in their leaves. During the summer, chlorophyll captures the energy from the sun to help the trees absorb nutrients in a process called photosynthesis. Chlorophyll also gives the leaves their green coloring.

When chlorophyll moves from the leaves to the trunk and roots of the tree, the yellow and orange pigments that were always present in the leaves start showing through. Different kinds of trees have varying amounts of pigment in their leaves, which gives us the variety of oranges, reds and yellows of an autumn forest. The richness of the color depends on how much iron, magnesium, phosphorus or sodium is in the tree and the acidity of the chemicals in the leaves.

The red, purple and bronze autumn leaves of area oaks, maples and dogwood are created through a different process that uses the sugar trapped in leaves by warm days and cold fall nights. The more sugar, the brighter the reds and purples. It is the same process that gives us the red skin on apples and the purple of ripe grapes. This sugar-producing process requires sunlight, which explains why some tree leaves are two-toned: red where the sun reaches them and yellow-green where there is less sun, and why some trees have more color on one side than the other.

Some scientists think that the deepest red pigments occur because of environmental issues such as drought, nutritional deficiencies, wounds and exposure to ozone rays.

While the colors are forming, the tree is recovering the last valuable nutrients from its leaves. The red pigment in leaves works as a sunscreen that protects the leaf during this brief period of nutrient recovery in the fall. Should we have a wet, windy autumn, leaves may drop prematurely and could stress the tree through the winter.

But dropping leaves is necessary for the health of a tree. By dropping leaves, trees protect themselves and conserve water for the long winter ahead. The large surface area of leaves is great for photosynthesis, but is susceptible to freezing and desiccation as temperatures drop. The perfect design of a leaf causes the cells of the stem, which attaches to the branch (its petiole), to become brittle in the fall. Wind and gravity do the rest of the job to help leaves fall to the ground.

For gardeners, the dropping of autumn leaves is a final boon for the compost pile. Drop them in now, and use the rich, organic soil in your garden next summer.

Deb Babcock is a master gardener through the Colorado State University Cooperative Extension office in Routt County. Questions? Call 879-0825 or e-mail: gardeners@co.routt.co.us

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