All during spring and summer the leaves have served as factories where most of the foods necessary for the trees' growth are manufactured. This food-making process takes place in the leaf in numerous cells containing the pigment chlorophyll, which gives the leaf its green color. Along with the green pigment leaves also contain yellow or orange carotenoids which, for example, give the carrot its familiar color. Most of the year these yellowish colors are masked by the greater amount of green coloring. But in the fall, partly because of changes in the period of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down,. the green color disappears, and the yellowish colors become visible and give the leaves part of their fall splendor.
At the same time other chemical changes may occur and cause the formation of additional pigments that vary from yellow to red to blue. Some of them give rise to the reddish and purplish fall colors of leaves of trees such as dogwoods and sumacs. Others give the sugar maple its brilliant orange or fiery red and yellow. The autumn foliage of some trees, such as quaking aspen, birch, and hickory, shows only yellow colors. Many oaks and others are mostly brownish, while beech turns golden bronze. These colors are due to the mixing of varying amounts of the chlorophyll and other pigments in the leaf during the fall season.
Fall weather conditions favoring formation of brilliant red autumn color are warm sunny days followed by cool, nights with temperatures below 45o F. Much sugar is made in the leaves during the daytime, but cool nights prevent movement of, sugar from the leaves. From the sugars trapped in the leaves the red pigment called anthocyanin is formed. The degree of color may vary from tree to tree. For example, leaves directly exposed to the sun may turn red, while those on the shady side of the same tree or on other trees in the shade may be yellow. The foliage of some tree species just turns dull brown from death and decay and never shows bright colors.
Through fallen leaves, Nature has provided for a fertile forest floor. Fallen leaves contain relatively large amounts of valuable elements, particularly calcium and potassium, which were originally a part of the soil. Decomposition of the leaves enriches the top layers of the soil by returning part of the elements borrowed by the tree and at the same time provides for more water-absorbing humus.
North Carolina leads the parade for leaf lookers, and depending upon the season, the species of trees involved, and the relative proportion of the three pigments, just about every imaginable color combination may be seen.
Prepared by Dr. Robert Bardon Extension Specialist