Deb Babcock: Why leaves change color
August 26, 2012
On a couple of hikes this past week, I noticed some aspens just starting to change color and many of the mountain ash leaves already had turned to brilliant shades of red and orange.
Why do leaves turn color in the fall? Why are most of the colors more muted here in the West than the bright reds, oranges and peach colors of the East?
Once our mountain temperatures start dropping at night and the days become shorter as we move into the autumn of the year, the trees begin preparing for winter. The chlorophyll in their leaves — which makes them green and helps trees absorb nutrients through a process called photosynthesis — begins to recede from the leaves into the tree trunk and roots. What is left behind is the pigment that always is present in the leaves. 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.
Our quaking aspen trees don’t have much of the red pigment in their leaves, instead giving us a beautiful golden hue each fall. The maples of the East tend to have a lot of red pigment and sugar in their leaves — thus the more brilliant colors in Eastern forests.
The red, purple and bronze autumn leaves of area oaks, maples, mountain ash and dogwood are created through a process that uses the sugar trapped in leaves rather than the iron, sodium, etc. 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 it 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. If we have a wet, windy autumn, leaves may drop prematurely and could stress the tree throughout the winter.
But dropping its leaves is necessary for the health of the tree. By dropping leaves, trees protect themselves and conserve water for the long winter ahead. The large surface area of leaves is great for trapping sunlight necessary to photosynthesize, but it is susceptible to freezing and desiccation as temperatures decrease. The excellent design of a leaf causes the cells of the stem that attaches to the branch (the petiole) to become brittle in autumn. 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 gift 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 CSU Extension Routt County. Call 970-879-0825 with questions.
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