why leaves change
Fall Colors
Why leaves change color
Many people suppose that Jack Frost is responsible for the color change, but he is not. According to an Indian legend, the leaves were believed to change because celestial hunters slew the Great Bear in the autumn. There are many legends, but we know that the change in coloring is the result of chemical processes which take place in the tree as the season changes from summer to winter.
All during spring and summer the leaves have served as factories, manufacturing most of the foods necessary for the trees' growth. 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 the shorter days and cooler nights, 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.
Red leaves appear when warm sunny days are followed by cool nights below 45º F. Much sugar is made in the leaves during the daytime, but cool nights prevent movement of sugar from the leaves. These trapped sugars result in bright red leaves, highlighting the area with bright red huckleberry and vine maple leaves.
The degree of color may vary from tree to tree. For example, leaves directly exposed the the sun may turn red, while those on the shady of the same tree or on other trees in the shade my be yellow. The foliage of some tree species just decay and never show bright colors.
Also, the colors on the same tree may vary from year to year, depending upon the combination of weather conditions. When there is much warm, cloudy, rainy weather in the fall, the leaves may have less red coloration.
Before long the leaves will flutter away from their summer home and become a part of the rich carpet that covers the forest floor. Fallen leaves contain relatively large amount of valuable elements, particularly calcium and potassium, which were originally a part of the soil. Decompositions 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.
MAKING LEAF PRINTS
It is easy to copy brightly colored leaves with crayons or colored pencils. Place a leaf lower side up, because the veins on the lower side are usually raised. Then put a sheet of thin paper or writing paper not thick drawing paper) on top of the leaf. Next, holding the paper and leaf so that they do not move, color the paper on top of the leaf. Use fast, slanting strokes as in shading. The shape and markings will be copied exactly. The veins and leaf border will show as heavier lines. Different colors can be used to match the shades or markings. After you have colored over all the leaf, cut out the paper leaf with scissors. Of course green leaves can be copied at any time in the same way.
Leaf prints can be made also with a stamp pad. Press the leaf lower surface down against the stamp pad, with a piece of paper on top to avoid soiling the fingers. Then place the leaf, inked side down, on a sheet of white paper with another sheet of paper on top. Hold the leaf firmly and rub hard over it. When the upper sheet of paper and the leaf are removed, a printed copy of the leaf will remain. A scrapbook of leaf prints with names of the trees is an interesting project for any boy or girl.
Story provided by the Mt. Hood National Forest, USDA-Forest Service, Revised September 1996.
September/October 2006