Press-Republican

October 7, 2012

How and why leaves change color

Richard Gast, Cornell Ag Connection
Press-Republican

---- — I enjoy this time of year for a number of reasons. One is just being able to observe the almost miraculous transformation of the green leaves of summer into a 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 even come close to those of the Adirondacks 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 the other day when the granddaughter of one of them asked, “Why do the leaves change colors?” She added that she really liked the red maple leaves.

It brought me back to a time when my daughters were quite young and one asked me that 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 creation, a great bear roamed the earth entering villages, frightening people, scaring away game and eating the food the tribes had gathered for winter. Legend says many warriors set out to kill the bear, but none returned.

After many years, the bravest and strongest warriors from several tribes joined to form a hunting party and set out in search of their enemy. They hunted for months, chasing the great bear across the earth. One day the greatest hunter was able to sneak up on the bear and get close enough to put one arrow into his side. The arrow did not kill the bear, but it drew blood. Wounded, the great bear reared up and leaped into the heavens.

Some say the bear was met by spirit hunters who continue to hunt him to this day, and the bear can still be seen in the night sky as the constellation Ursa Major. 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 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. Mother Earth, in honoring and remembering the great bear, turns the leaves those same colors to this day.

In European folklore, children were told of an elf named Jack Frost 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 photosynthesis, which means putting together with light. In spring and summer, essential nutrients and water 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 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 flows out of the leaves 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 contain pigments, such as carotene, which gives carrots their orange color. Without the presence of chlorophyll, the yellow and orange pigmentation 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.

Richard L. Gast, Extension program educator II, Horticulture, Natural Resources, Energy, agriculture programs assistant, Cornell Cooperative Extension of Franklin County, 355 West Main St., Suite 150, Malone, 12953. Phone 483-7403, fax 483-6214 or email rlg24@cornell.edu.