Why do leaves change their color?
A GREEN LEAF IS GREEN because of the presence of a group of pigments known as chlorophylis. When they are abundent in the leaf's cells, as they are during the growning season, the chlorophylis' green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.
The chlorophylis have a vital function: they capture some of the sun's energy and utilize it in the manufacture of the plant's food - simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant's nourishment - the sole source of the carbohydrates needed for growth and development.
In their food-manufacturing process, the chlorophylis themselves break down and thus are being continually "used up". During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.
But as autumn approaches, certain influences both inside and outside the plant causes the chlorophylis to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf's life begin to show through. These are the carotenoids; they give us colorations of yellow, brown, orange, and the many hues in between.
The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. These pigments are not present in the leaf throughout the growing season as are the carotenoids. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences - both inside and outside the plant. Their formations depends on the breakdown of sugars in the presence of bright light as the level of certain chemical (phosphate) in the leaf is reduced.
During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the sugars manufactured by chlorophyll.
In late summer the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells form at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the cholorophyll begins to decrease.
Often the veins will still be green after the tissues between them have almost completely changed color.
But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the stem of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brillant the resulting color display that we see. When the days of autumn are bright and cool, and the nights chilly but not freezing, the brightest colorations usually develop.
Anthocyanins temporarily color the edges of some of the very young leaves as they unfold from the buds in early spring. They also give the familiar color to such common fruits as cranberries, red apples, blueberries, cherries, strawberries, and plums.
In our autumn forest (Blue Ridge Mountains) they show up vividly in the maples, oaks, sourwood, sweetgum, dogwood, tupelo, black gum, and persimmon. These same pigments often combine with the carotenoids' colors to give us the deeper orange, fiery reds, and bronzes typical of many hardwood species.
The carotenoids occur, along with the chlorphyll pigments, in tiny structures - called plastids - within the cells of leaves. Sometimes they are in such abundance in the leaf that they give a plant a yellow-green color, even during the summer. But usually we become aware of their presence for the first time in autumn, when the leaves begin to lose their chlorophyll.
Carotenoids are common in many living things, giving characteristic color to carrots, corn, canaries, and daffodils, as well as egg yolks, rutabagas, buttercups, and bananas.
Their brilliant yellows and oranges tint the leaves of such hardwood species as hickories, ash, maple, yellow-poplar, apen, birch, black cherry, sycamore, cotton-wood, sassafras, and alder.
What happens to all those fallen leaves?
Needles and leaves that fall are not wasted. They decompose and restock the soil with nutrients and make up part of the spongy humus layer of the forest floor that absorbs and holds rainfall. Fallen leaves also become food for numerous soil organisms vital to the forest ecosystem.
It is easy to see the benefit to the tree of its annual leaf fall, but the advantage to the entire forest is more subtle. It could well be that the forest could no more survive without its annual replenishment from leaves than the individual tree could survive without shedding these leaves.
Where can I see autumn colors in America?
Look for autumn color in parks, forest, in the cities, woodlands, countryside, and the mountains - everywhere you find deciduous broadleaved trees that drop their leaves in the autumn. The trees include oaks, maples, hickories, beeches, yellow-poplars, dogwoods, sweetgums, and many others.
New England is very famous for the spectacular autumn colors of its mountains and countryside, but the Adirondack, Appalachian, Smoky, and Rocky Mountains are also full with displays of color. in the West are the bright yellows of aspen stands and larches contrasting with the dark greens of the evergreen conifers; in the East are the reds, oranges, golds, and bronzes of the mixed deciduous woodlands.
When is the best time to see autumn colors?
Unfortunately, autumn color is not very predictable, especially in the long term. Half the fun is trying to outguess Nature! But it generally starts in late September in New England and moves southward, reaching the Smoky Mountains by early November. It also appears about this time in the high-elevation mountains of the West. Remember that cooler high elevations will color up before the valleys. The USDA Forest Service's Fall Color Hotline (1-800-354-4595) or there link over to your left can provide you with details as the autumn color display progresses.