Deb Babcock: Why aspen leaves turn yellow

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Deb Babcock

Deb Babcock's gardening column appears Thursdays in Steamboat Today.

Find more gardening columns here.

— On a hike along Independence Creek last Thursday, I noticed aspens just starting to change color. I bet they will peak in the next week and a half.

Unlike last year, when many of the spent aspen leaves were mottled and spotted, these ones were free of flaws in beautiful shades of gold, yellow, peach and orange-red.

Why do aspen leaves turn color in the fall? Why aren't they bright red like the leaves of maple, dogwood and oak trees?

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.

The intensity of the pigment depends on the amount of iron, magnesium, phosphorus or sodium present in the tree, as well as the acidity of material in the leaves. Additionally, different kinds of trees have different levels of pigment in their leaves, which gives us the multi-colored variety of oranges, golds and yellows of an autumn forest.

The red, purple and bronze autumn leaves of area oaks, maples 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 the deepest red pigments occur because of environmental issues such as drought, nutrient 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 the 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.

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