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Leaves change color in the fall when deciduous trees and shrubs stop producing chlorophyll, the green pigment that lets plants obtain energy from sunlight. Shortly before leaves are shed, they put on a colorful show.
Change in day length is the most important factor regulating the onset of fall foliage. As days grow shorter and darkness increases, leaves stop producing chlorophyll and the green pigment remaining in the leaves breaks down.
This lets other colors take center stage. The yellow, orange, red, and brown tones that we associate with fall are always present in leaves; during the spring and summer, they are just masked by an abundance of chlorophyll.
Different tree species are associated with different colors. Hickory and sycamore leaves are golden-orange. Ash leaves tend to be yellow and purple. Sugar maples take on an orange or red tone.
You might notice that vibrant crimson canopies occur in sunny spots. They form when sugars and proteins in leaves react with sunlight.
The ecology of red fall foliage is cloaked in mystery. There is evidence that red leaves are more prevalent when days are warm, dry, and sunny, and nights are cool – but not freezing. Red foliage has also been linked to fungus and drought.
Is climate change influencing the autumn’s palette? During their lifetime, trees respond to a myriad of environmental influences, from airborne pollutants to insect pests, many of which can influence subsequent fall color. One year might yield more red, and another more gold.
A short warm spell is unlikely to result in a suppression of color. As a matter of fact, warm days and cool nights can actually increase the amount of red color that we see.
Photo, taken on October 14, 2010, courtesy of Kimberly Vardeman via Flickr.