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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 it’s only a passing pleasure. Before long, leaves will fall and become part of the rich carpet that covers the forest floor, providing nutrition for new growth. Many people suppose frost causes the color change, but it does not. Some leaves begin to change color before any frost.


During spring and summer, leaves serve as food factories for tree growth. Photosynthesis, the food-making process, takes place in the numerous leaf cells containing the green pigment, chlorophyll. Chlorophyll absorbs energy from the sun and changes carbon dioxide and water to sugars and starches important to tree growth.


In addition to chlorophyll, leaves also contain yellow and orange pigments, such as xanthophyll and carotene. Carotene gives carrots their familiar color. The yellow and orange colors are hidden by much greater amounts of green pigment. In the fall, because of decreasing day-length and temperature, the leaves stop producing chlorophyll. The remaining chlorophyll breaks down and the green color disappears, making the yellow and orange visible. Some trees – silver maple, aspen, birch – show only yellow.


At the same time, other chemical changes are taking place. 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. Oaks are mostly brownish, while beech turns golden bronze. Mixtures of pigments cause various colors during the fall season. Trees with red or scarlet leaves in autumn include red and sugar maple, flowering dogwood, and red oak.


Fall weather conditions play a part. Warm, sunny days, with nighttime temperatures below 45 degrees F., but above freezing, raise the level of red. Sugars, made during the day, are used to produce the anthocyanin. Color may vary among the same species because of genetic differences, and even on the same tree. Leaves exposed to direct sun may turn red, while those on the shady side may be yellow. Some leaves may just die and turn brown, never showing any bright color.


Colors on the same tree may also vary year to year. When there’s a lot of warm, rainy weather, less red can be expected. Sugar produced in limited sunlight is insufficient to form red pigments.  Color change depends on weather, latitude, elevation, and microclimate. Warm but not excessively hot temperatures, and sufficient spring and summer rainfall, will usually produce peak color. 


As leaves turn, other things are happening. Leaf veins close and stop carrying liquids. At the base of the leafstalk or petiole, where it’s attached to the twig, a layer of special cells forms and gradually separates leaf from twig; it’s called the abscission layer. It’s the reason leaves fall. Scar tissue forms on the twig to seal the old pathway. Leaf scars are so unique they can often be used to identify trees after 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. Some Western forests, especially in the mountains, also have bright coloration. Eastern Asia and southwestern Europe have colorful foliage.


Most northern broad-leaved trees shed their leaves in the fall. Some oaks, and a few other species, may keep their dead brown leaves until growth resumes in the spring. In the south, where winters are milder, some broad-leaved trees are evergreen. Most conifers — pine, spruce, fir, hemlock, cedar — are evergreen both north and south. The leaves, needle- or scale-like, remain green year-round. Individual leaves may remain two or more years.


Fallen leaves fertilize soil. Leaves contain nutrients, particularly calcium and potassium, which the trees’ roots took up. Decaying leaves recycle nutrients back into the top layers. Humus produced by decaying leaves is also important for conserving water. 


(Adapted from UMaine Cooperative Extension Service, “Facts About Leaf Color in Maine.”)
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