Why Do Leaves Change Color?
The University of Maine Cooperative Extension shares science with Down East.
In a Native American legend, spirit hunters in the sky slew the Great Bear (a constellation) in autumn. The bear's blood, dripping on the forests, changed many leaves to red. Some trees were turned to yellow by the fat that splattered out of the kettle as the hunters cooked the meat. Other native cultures also had folklore that explained the fall colors. We now know that the color changes are caused by chemical changes that take place in the leaves as summer turns to winter.
A Chemical Process
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 starch.
In addition to chlorophyll, leaves also contain yellow and orange pigment, such as carotene: the same one 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 changes in daylight and temperature, the leaves stop producing food. As the chlorophyll breaks down, the green color disappears and the yellowish colors become visible.
At the same time, other chemical changes are taking place. These may cause yellow or red or even blue pigments to form. Some of them give the reddish and purplish fall colors to dogwood and sumac leaves. Others give the sugar maple its brilliant orange, fiery red or yellow colors. Some trees, such as aspen, birch and hickory, show only 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.
Fall weather conditions do play a part in the formation of autumn colors. Warm sunny days, with nighttime temperatures below 45 degrees F, raise the level of red coloration. Sugar, made in the leaves during the day, cannot move out of the leaves when the nights are cool. The trapped sugars change into the pigment anthocyanin. This pigment is usually red, but may range to violet or blue.
Trees with red or scarlet leaves in autumn include red and silver maple, flowering dogwood, sweetgum, black gum, red and scarlet oak, and sassafras.
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 can move out of the leaves during the warm nights. Then, no excess sugar remains in the leaves to form the red pigments.
As the leaves change color, other things are happening to them. At the base of the leafstalk (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.
Fall Colors for a Fortunate Few
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.
Fallen leaves help fertilize the forest soil. Leaves contain nutrients, particularly calcium and potassium, which the trees' roots took 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.
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.
This column was excerpted with permission from "Why Leaves Change Color," bulletin #7078, from University of Maine Cooperative Extension. Originally adapted from USDA Forest Service FS 12. Revised by James Philp, Extension forestry specialist, July 2001. For more information and additional publications, visit UMaine Extension online.
For more than 90 years, University of Maine Cooperative Extension has supported UMaine's land-grant public education role by conducting community-driven, research-based programs in every Maine county.
- By: Kyle McCaskill