Cold Nights Make Beautiful Days: Why Leaves Change Colors in the Fall

It's the moon if the changing leaves when we catch the Kodachrome beauty of the season. Though the leaves turn about the time of the first frost, it's just a coincidence - frost doesn't make the leaves change colors. In fact, a hard frost will kill the leaves quickly, resulting in drabness or ugly black leaves. Longer nights and shorter days, along with a sliding temperature cause cells at the juncture of twig and leaf stem to thicken and stiffen. Water can't get into the leaf and sugar can't get out, so photosynthesis (the process by which chlorophyll- containing cells use the energy of light to synthesize carbohydrates from carbon dioxide and water) comes slowly to a habit. Chlorophyll, the green stuff that keeps the process going, disappears from the scene, leaving the under pigments to show off their stuff. Trees that have a lot of Xantophyll pigments flash a lot of yellow. Carotenoid pigments, for instance, in birch and many maples, show yellow as well as orange. Bright days and cold nights make for the perfect conditions under which Anthocyanin is produced.

Trees with an acid sap, such as red or sugar maple, produce red leaves. Trees with alkaline sap, such as ash trees, produce purple leaves. The degree of color may vary from one tree to its next door neighbor, and colors on the same tree may vary - look carefully and see. Sometimes you can see a green ash with yellow leaves closer to the crown and purple leaves where it gets more sunshine, or a service berry tree with yellow leaves nearer the ground and red leaves near the top of the tree. When it is clear by day and chilly, if not chatter-bone chilly, by night, the color display will be more vibrant, when it is warm, cloudy and rainy, the color display will be less vibrant. That's because the sugar made in the leaves escapes out of the leaves during warm nights. If the sugar isn't trapped in the leaves, it can't act to form pigments. The more sugar is trapped in the leaves, the more golden, crimson, mahogany, russet, amber, apricot, cinnamon-red, and other breath-taking colors you may see. The best time to plant trees for fall color is fall, when you can see what kind of color to expect. Visit a nursery and tag a tree.

Sources:

Bell, Anne H., and Lindsey C. Ritchie Fall Color and Woodland Harvests: A Guide to the More Colorful Fall Leaves and Fruits of the Eastern Forests 2007

Cook, Anthony E., Art Wolfe and Ann Zwinger Fall Colors Across North America 2001