What causes leaves on a tree to turn red or yellow?

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Question What causes leaves on a tree to turn red or yellow?

Answer During the summer, tree leaves produce sugar from carbon dioxide and water by the action of light on chlorophyll. This process is called photosynthesis. Chlorophyll also causes the leaves to appear green. Water and nutrients flow from the roots, through the branches, and into the leaves. The sugars produced by photosynthesis flow from the leaves to other parts of the tree. Some of this chemical energy is used for growth and some is stored.

The shortening days and cool nights of autumn trigger changes in the tree. One of these changes is the growth of a corky membrane between the branch and the leaf stem (petiole). This membrane blocks the flow of nutrients into and out the leaf. Because the nutrient flow is interrupted, the production of chlorophyll in the leaf declines. This causes the green color of the leaf to fade.

If the leaf contains carotene, as do the leaves of ginkgo, birch and hickory, it will change from green to bright yellow as the chlorophyll disappears. In some trees, as the concentration of sugar in the leaf increases, the sugar reacts to form anthocyanins. These pigments cause the fading leaves to turn red. Red maples, sassafras, and sumac produce anthocyanins in abundance, and display the brightest reds and purples in the fall landscape.

The range and intensity of fall colors are greatly influenced by the weather. Low temperatures destroy chlorophyll, and if they stay above freezing, promote the formation of anthocyanins. Cool nights also prevent the movement of sugars from the leaves. Bright sunshine destroys chlorophyll and enhances anthocyanin production. Dry summer weather increases the amount of anthocyanin by increasing the concentration of sugar in the sap. Getting a few fall showers will help the trees hold on to their leaves longer, so you can enjoy the color longer.

In 1665, Sir Isaac Newton proved that light is the source of all color and that all colors in the spectrum are present in white light. Newton also discovered that the color of an object is determined by selective reflection of light rays. When light strikes an object, some light rays are absorbed and lost. Visible color is the light rays which are reflected. In other words, chlorophyll reflects green rays, carotene reflects yellow rays and anthocyanin reflects red rays.

Like the talents of a child, the beautiful fall colors are inside the leaves waiting to be visible. For more info on leaf color, visit http://www.clemson.edu/extfor/publications/forlf25/ For a list of trees and shrubs which have great fall color, visit http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-new/treelists/fallcolor.html

If you don’t have internet access then call me at 910-893-7533 or email me at gpierce@harnett.org I once told a lady that my favorite season was fall. She told me that her favorite season was vinegar, but mustard was a close second.

Gary L. Pierce

Horticulture Extension Agent

Harnett County