Originally adapted from Why Leaves Change Color, USDA Forest Service FS 12, February 1967. Revised by James Philp, Extension forestry specialist – wood products, July 2001.
Revised by Kathryn Hopkins, Extension Professor, August 2014.
Reviewed by Dr. Abby van den Berg, Research Assistant Professor in Plant Biology, University of Vermont Proctor Maple Research Center, August 2014.
It doesn’t take much imagination to picture Nature going about on autumn days and painting the leaves of trees and other plants red, purple, orange, and yellow. Every fall, we delight in the beauty of the trees and shrubs, knowing that it is only a passing pleasure. Before long, the leaves will fall from the trees and become part of the rich carpet that covers the forest floor, providing nutrition for new forest growth. Many people suppose that frost causes the color change, but it does not. Some of the leaves begin to change color before we have had any frost.
During spring and summer leaves serve as food factories for trees’ growth. Photosynthesis, the food-making process, takes place in the numerous leaf cells that contain the green pigment, chlorophyll. Chlorophyll absorbs energy from the sun and uses it to change carbon dioxide and water to sugars and starches that are used for tree growth.
In addition to chlorophyll, leaves also contain yellow and orange pigments, such as xanthophyll and carotene. Carotene is the pigment that gives carrots their familiar color. For most of the year, the yellow and orange colors are hidden by the much greater amount of green pigment. In the fall, because of decreasing daylength and temperature, the leaves stop producing chlorophyll. The remaining chlorophyll breaks down and the green color disappears making the yellow and orange colors visible. Some trees, such as silver maple, aspen, birch, and hickory, show only yellow colors.
At the same time, other chemical changes are taking place in the leaf. In some species, anthocyanins are formed, giving leaves a crimson red, purplish or blue color. Anthocyanins give the reddish and purplish fall colors to dogwood and sumac leaves and give the sugar maple its brilliant orange, fiery red or yellow colors. Oaks are mostly brownish, while beech turns a golden bronze. Mixtures of pigments in the leaves cause the various colors during the fall season. Trees with red or scarlet leaves in autumn include red and sugar maple, flowering dogwood, sweetgum, black gum, red and scarlet oak, and sassafras.
It is believed that fall weather conditions may play a part in the formation of autumn colors. Warm sunny days, with nighttime temperatures below 45 degrees Fahrenheit but above freezing, raise the level of red coloration. Sugars, made in the leaves during the day, are used to produce the pigment, anthocyanin. The degree of color may vary between trees of the same species because of genetic differences. Colors can even vary on the same tree. For example, leaves directly exposed to the sun may turn red, while those on the shady side of the same tree may be yellow. The leaves of some trees may just die and turn brown, never showing any bright color.
The colors on the same tree may also vary from year to year, depending upon combinations of weather conditions. When there is a lot of warm, rainy weather in the fall, less red color can be expected. The small amount of sugar made in the limited sunlight is not sufficient to form the red pigments.
The timing of the color change on leaves depends on weather, latitude, elevation above sea level, and the local microclimate. Warm but not excessively hot temperatures and sufficient spring and summer rainfall will usually produce the best fall color. Many states have foliage maps and webcams to track foliage color changes as fall progresses. This makes it easier to plan a foliage-viewing trip for peak color season.
As the leaves change color, other things are happening to them. The leaf veins close off and stop carrying liquids. At the base of the leafstalk or petiole, where it is attached to the twig, a layer of special cells forms and gradually separates the leaf from the twig. This layer of special cells is called the abscission layer. It is the reason that the leaves fall from the tree. At the same time, scar tissue forms on the twig to seal the old pathway between the twig and the leaf. Leaf scars are so unique in appearance that they can often be used to identify trees after the leaves are gone.
Only a few regions of the world have showy fall displays. Eastern North America has large areas of forests with broad-leaved trees and favorable weather conditions for vivid fall colors. Some areas of western North America, especially in the mountains, also have bright coloration. Eastern Asia and southwestern Europe have colorful fall foliage, too.
Most broad-leaved trees in the north shed their leaves in the fall. Some of the oaks, and a few other species, may keep their dead brown leaves over winter until growth starts again in the spring. In the south, where the winters are milder, some broad-leaved trees are evergreen.
Most conifers — pine, spruce, fir, hemlock, cedar, etc. — are evergreen in both the north and in the south. The leaves, which are needle-like or scale-like, remain green or greenish year-round. The individual leaves may stay on the tree for two or more years.
Fallen leaves help fertilize the forest soil. Leaves contain nutrients, particularly calcium and potassium, which the trees’ roots took up from the soil. Decaying leaves recycle the nutrients back into the top layers of the soil. The humus produced by the decaying leaves is also important for conserving water in the forest soil.
It is easy to copy leaves with crayons or colored pencils. Place a leaf, upside down, on a table or desk. Then put a sheet of 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. The shape and markings will be copied exactly, with the veins and leaf border showing as darker lines. After coloring over the leaf, you can cut out the paper leaf with scissors. Green leaves can be copied at any time in this way.
You can use a color photocopier to make exact copies of colored leaves. Or, you can use a black and white photocopier, at a lighter setting, to make pictures to color with crayons. Use fresh leaves that have not begun to wilt or wrinkle. Cutouts of the paper leaves can be used in a wide variety of art projects.
Leaf prints can also be made with an inked stamp pad. Press the leaf’s lower surface against the stamp pad with a piece of paper on top to keep your fingers clean. 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 youngsters.
Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.
© 2001, 2014
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