The Colors of Fall

In the summer, the leaves of trees are a dark green color. But as the summer nears its end, and the nights grow longer and become cooler, these dark green leaves begin to turn to yellows, browns and brilliant reds. Fall has arrived.

Why are leaves green?
Leaves are green due to pigment molecules present in the leaf structure. When white light, which contains all the colors in the visible spectrum, hits clorophyll a and chlorophyll b pigments present in the leaf, blue, yellow, and red light is absorbed while green light is reflected. Our eyes process the reflected green color and leaves look green.

Why do leaves change color in autumn?
Chlorophylls are not the only pigment molecule present in leaves, its just the most abundant. Carotenoids (yellow, orange, brown) and anthocyanins (reds) are also present. When autumn arrives, leaves stop producing chlorophyll pigment molecules and the colors of fall emerges.

Some autumns may appear to be more colorful than others. This is due to the presence of anthocyanin pigment molecules and the weather the tree experiences. If the fall daily temperatures are warm and sunny, leaves will produce alot of sugars. Cool nights will cause the leaf veins, which are responsible for transporting those sugars away from the leaves to other parts of the tree, to close. This causes the sugars to remain trapped in the leaf. These sugar molecules will react with proteins in the leaf sap to form anthocyanins. The color of the anthocyanins has been found to differ based on the pH of the sap. A lower pH will cause a brilliant red color to emerge whereas a higher pH will cause a more purple color to form.

So you can believe it when someone says, this fall has been exceptionally beautiful. The amount and intensity of the red autumn colors of leaves do vary from year to year.

USDA Forest Service
Department of Natural Resources

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