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Saturday, September 29, 2012

Autumn Colors



As I pondered this view yesterday, I decided it was time to research what exactly causes the leaves to turn colors and why some years the colors are more brilliant than others.

What causes this phenomenon?
As for the primary cause, the Science Made Simple website posting Autumn Leaves and Fall Foliage: Why Do Leaves Fall Colors Change? explains "The bright reds and purples we see in leaves are made mostly in the fall. 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 brown color of trees like oaks is made from wastes left in the leaves"

Why are the colors more brilliant some years than others?
 From the USDA Forest Service's Why Leaves Change Colors, "A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions-lots of sugar and lots of light-spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year."  The exact opposite of what I had thought -- I had it in my head that the "good" years were caused by an early hard frost. According to this explanation, this should be a really good year for brilliant colors.

Additional Reading
The Science of Color in Autumn Leaves (The United States Arboretum)

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