Have you ever wondered how maples, oaks, lilacs, dogwoods, and other trees and shrubs survive our long, cold, snowy winters?
Actually, these woody plants are very sensitive to the changing environmental conditions of the seasons and have been preparing themselves for winter's worst since last June's summer solstice. It was then, in midsummer, when a fascinating sequence of physiological changes began to occur, which would eventually allow the trees to enter into the protective state of cold-hardiness and slowed metabolism known as "winter dormancy."
Dormancy ensures survival during the tough times of winter, when soils are frozen, winds are harsh and temperatures may be brutally cold, but preparations must begin long beforehand.
Deciduous trees become dormant to enable them to survive the rigors of winter, while, at the same time, adding stark beauty to the Chautauqua Watershed landscape.
Although we see no obvious outward changes in the trees, dormancy of woody plants is initiated when light-sensitive, blue-colored leaf pigments, called "phytochrome," detect changes in photoperiod beginning in late June. Even though most humans tend not to notice, it is then that nights begin to lengthen and days shorten, ever so subtly. The biochemical clock within plants is especially sensitive to changing night length, and in turn triggers chemical messengers and hormonal changes, which physically prepare the plants for the challenging times to come.
By late summer, the plants' preparations become more apparent to human eyes. Tiny buds have already formed along the twigs, containing next spring's miniature flower blossoms and leaves wrapped up in protective, waxy waterproofed bud scales.
Deciduous trees, which will soon be shedding their leaves in autumn in a complex process known as "abscission," have begun mobilizing nutrients and other valuable materials out of the leaves and into the trunk and roots for winter storage. The leaves initially remain their familiar green color, due to the great abundance of photosynthetic pigments called "chlorophyll." Other colored molecules, such as orange carotenes, yellow xanthophylls and red anthocyanins are present as well, but they are greatly outnumbered and masked by the greens. As nights lengthen and temperatures begin to fall, however, chlorophylls are the first to break down, and the chemistry of the leaf cells is altered, revealing autumn's spectacular blaze of golden yellows, bronzy oranges, rich crimsons and scarlet-reds.
Leaf drop soon follows. The trees have prepared for this loss by gradually sealing off the connections between each leaf and the twig to which it had been attached. The leaves - now faded, dry, and devoid of nutrients and water - will be shed at the next gentle nudge of wind, rain or snowfall.
It has been estimated that approximately 10 million leaves are shed from an acre of trees each year, or about 50,000 to 70,000 leaves per tree. But why is such tremendous loss necessary? Although it may at first appear to be an extravagance of nature, leaf drop is actually another dormancy preparation and winter survival strategy. To understand this, recall that the most important function of leaves is to provide lots of surface area for capturing light, which in turn drives the food-making machinery of all green plants. The thin, broad leaves of woody deciduous plants are beautifully designed for efficiently absorbing a tremendous amount of sunlight, but at the same time, they are the site of great losses of water by evaporation. In the summer, an acre of forest may lose up to 2,500 gallons of water a day.
This is fine as long as the plants' roots can replace the lost water by absorbing more from the soil. Winter, however, poses new problems, not so much because of the cold, but because water is no longer available for uptake it becomes locked in the soil in frozen form. If the leaves remained on the trees all winter, devastating water losses would occur. Rather than suffer tissue damage or death by dehydration, deciduous trees drop their leaves, minimize their water losses, and in their dormant state, patiently await the coming of spring.
Becky Nystrom is a professor of biology at Jamestown Community College, a long-time CWC supporter and volunteer and a founding trustee of the CWC. The Chautauqua Watershed Conservancy is a private nonprofit organization with a mission to preserve and enhance the water quality, scenic beauty and ecological health of the lakes, streams, wetlands and watersheds of the Chautauqua region. To sign up for e-news updates, find out more information on watershed care or support CWC's conservation activities, visit www.chautauquawatershed.org or call 664-2166.