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How and why do leaves change color?

October 10, 2012
By Richard Gast , Franklin County Cornell Cooperative Extension

I really enjoy this time of year for a number of reasons. One is just being able to casually observe the almost miraculous transformation of the green leaves of summer into an absolutely stunning panorama of fall color; a transition that never ceases to amaze me.

There are very few places in the world where the colors and textures of the season even come close to the awe-inspiring pageantry of fall in the Adirondack Mountains and the farm country of northern New York. I don't think I'll ever lose my appreciation for it.

I was saying just that to a few friends in conversation the other day when the granddaughter of one of them, who must have been listening to us, asked, "Why do the leaves change colors?" She added that she really liked the red maple leaves. And it brought me back to a time when my own daughters were quite young and one asked me that very same question. We pressed the prettiest leaves between the pages of books to preserve them, but other than to say that the colors appeared as the trees were getting ready for winter, I didn't have an answer.

Native Americans used to tell their children how, soon after the time of the creation, a great bear roamed the Earth entering the villages, frightening the people, scaring away the game, eating the food that the tribes had gathered for winter, and causing immeasurable damage. Legend says that many warriors set out to kill the bear, but that none returned. After many years, the bravest and strongest warriors from several tribes joined together to form a hunting party and set out in search of their common enemy. They hunted for months, chasing the great bear across all the earth until one fateful day when the greatest hunter of them all was able to sneak up on the bear and get close enough to put just one arrow into his side. The arrow did not kill the bear, but it drew blood. Wounded, the great bear reared up and leapt into the heavens.

Some say that the great bear was met there by spirit hunters, who continue to hunt him to this day; that the spirit hunters chase him in circles around and under the earth; that the great bear can still be seen in the night sky as the constellation, Ursa Major; and that as the Great Bear constellation rises above the horizon in the fall, his blood drips down onto the trees below, turning the leaves scarlet red.

Others say that the bear was killed by the spirit hunters and that, as the Great Bear died, its blood fell upon the trees, turning the leaves red. As the spirit hunters cooked the meat, fat dripping from the fire fell upon other trees, turning their leaves orange and gold. The land, Mother Earth, in honoring and remembering the great bear, turns the leaves those wonderful colors to this day.

In European fairy tales and folklore, children were told of an elf named Jack Frost (one of many names) who freezes the ground with his chilly feet, paints lacy white pictures on the winter window panes, and turns the leaves colors with the icy touch of his fingers.

The real answer lies in first understanding how trees manufacture food by way of a process called photosynthesis, which means putting together with light. In spring and summer, essential nutrients and water, taken from the ground, travel from the trees' roots to their leaves. At the same time, carbon dioxide is taken from the air. Chlorophyll, a pigment found in green plants, absorbs energy from the sun and uses it to transform carbon dioxide and water into sugars, which the tree either uses immediately or stores as carbohydrates, until it is needed. Oxygen is released into the atmosphere as a byproduct. So I guess you could say, in essence, that chlorophyll is the source of life. It also gives the leaves their green color.

With the coming of fall, and shorter, cooler days, the trees must ready themselves for winter, when they will not have enough light or water for photosynthesis to occur. Subtle chemical changes take place. Water actually flows out of the leaves and into the roots, where the nutrients can be stored. As the leaves stop producing food, the chlorophyll breaks down, and the green pigment in the leaves slowly disappears.

Besides chlorophyll, leaves also contain pigments, such as carotene, which gives carrots their orange color. Without the presence of chlorophyll, the yellow and orange pigmentation, which was hidden all summer, becomes visible. At the same time, sugars that become trapped in the leaves of certain trees, such as sugar maples, react with sunlight and other acidic leaf chemicals, producing radiant reds and purples.

Temperature, light and available water all affect the intensity and the length of the color season. For example, prolonged temperatures just above freezing will result in sugar maples turning bright red, but an early frost may cause colors to appear faded or dull. Timely rains may cause colors to become more intense.

Trees eventually form a layer of cell tissue at every point of leaf stem attachment, effectively severing the leaves from the limbs. When a leaf falls from a tree, a small scar remains.

Do evergreens like pine, spruce and balsam fir lose their leaves, as well? The answer is yes. These conifers do shed their needles in the fall; not all of their needles, just the oldest, less healthy ones. The younger needles remain on the trees all winter.



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