Why do Trees Change Color in Fall?

Think about what might be going on in the minds of elementary school children in fall (besides going trick or treating and eating caramel apples).  They might just be thinking about trees – at their school, their home, the park nearby) – and the changes going on with them.  Adults often wonder about the changes in fall…why do they change color after all?  What’s going on inside them?  Are they magic?  Does Mother Nature flip a switch?  Why do some turn a pretty yellow or red and others seemingly just go from green to drab, then fall off the tree?

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Figuring out the why and what requires a step back to spring and summer.  In the early and mid-part of the growing season, the trees have produced leaves that make food for all the tree parts, a key component of which is chlorophyll, which has a naturally green pigment.  The process is highlighted by absorption of sunlight energy and conversion of carbon dioxide into carbohydrates such as sugars and starches.  One of the key insights to the “color change” in the fall is that other pigments are produced at the same time as the green ones.  The yellow to orange pigments are called carotenoids and xyanthophylls– similar to the ones that make carrots appear orange; the red and purple ones are named anthocyanins.

 

All of these pigments are part of the reveal in the fall, initiated slowly as the chlorophyll pigments begin to break down in response to shorter daylengths and cooler temperatures.  The effect is one of the green color being slowly drained from the leaves, however, it’s really just the green pigments are giving way to the visibility of the other colors that we all like so much.

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The intensity of fall colors can vary from year to year.  Temperature, light and soil moisture all influence how bright or subdued the colors may be.  Cool temperatures above freezing tend to bring out bright colors, especially amongst those destined to express red and purple hues.  Trees growing in dry soils tend to produce weaker coloration than those in moist soils, which is another reason to check soil moisture during the growing season with a screwdriver and watering the landscape with the goal in mind of keeping the soil moist, not soggy or dry.

 

 

John Fech
Horticulture Extension Educator at Nebraska Extension
John Fech is a horticulturist with the University of Nebraska-Lincoln and certified arborist with the International Society of Arboriculture. The author of 2 books and over 200 popular and trade journal articles, he focuses his time on teaching effective landscape maintenance techniques, water conservation, diagnosing turf and ornamental problems and encouraging effective bilingual communication in the green industry. He works extensively with the media to extend the message of landscape sustainability, making over 100 television and radio appearances each year.
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