Readying For Winter
November’s chilly temperatures, brisk winds and frosty mantle have recently premiered in our area, and harsher, colder days are on their way. How will the maples, oaks, cherries, hemlocks, dogwoods and sumacs of our watersheds manage to survive Western New York’s long, icy months ahead? Actually, our native woody plants are very responsive to the changing environmental conditions of the seasons and have been preparing themselves for winter’s worst since the summer solstice! It was then, in midsummer, when a fascinating sequence of physiological changes began to occur, eventually allowing the woody ones 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 brutal and temperatures bitterly cold, but critical preparations must begin long beforehand. Although we see no obvious outward changes, 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, which in turn triggers chemical messengers and hormonal changes that physically prepare the plants for the challenging winter season ahead.
By late summer, the plants’ preparations become more apparent to human eyes. Little buds have already formed along the twigs, containing next spring’s miniature flower blossoms, cones and leaves wrapped up in protective, waxy waterproofed bud scales. Deciduous trees, which shed their leaves in autumn in a complex process known as abscission, have begun mobilizing sugars and other valuable materials out of the foliage and into the trunk and roots for winter storage. The leaves initially remain their familiar green color, due to the abundance of photosynthetic pigments called chlorophyll. Other colored molecules, such as orange carotenes, yellow xanthophylls, and red anthocyanins are present as well but are greatly outnumbered and masked by the greens. As nights lengthen and temperatures begin to fall, chlorophylls are the first to break down, and the chemistry of the leaf cells is altered, revealing and intensifying autumn’s spectacular blaze of beauty. The golden yellows of aspen, birch, larch and cottonwood, the bronzy oranges of ash, hickory, oak and beech, and the rich crimsons, scarlets and burgundies of maple, dogwood, cherry and sumac bless human viewers with a breath-taking symphony of color, hue and texture — but for the trees, it’s all about timing and preparation for the challenges to come. Colors fade, and 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 brown, dry and devoid of nutrients and water, will be shed at the next gentle nudge of wind, rain or snowfall, and will nourish the soils in which they rest.
It has been estimated that approximately ten million leaves are dropped 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, 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 when 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 local not-for-profit organization that is dedicated to preserving and enhancing the water quality, scenic beauty and ecological health of the lakes, streams, wetlands and watersheds of the Chautauqua region. For more information, call 664-2166 or visit www.chau-tauquawatershed.org or www.facebook.com/chautauquawatershed.