Autumn’s spectacular blaze blesses human viewers with a breathtaking symphony of color, but for the trees, it’s all about timing and preparation for the challenges to come. Photo by Becky Nystrom

November’s chilly temperatures, brisk winds, and damp dreariness have arrived, and darker, colder days draw near. As the natural world brings autumnal reminders of seasons swiftly passing, our native trees and shrubs gently reveal the ancient wisdom of synchrony, adaptation, and preparation for the wintry challenges to come.

How do the maples, oaks, cherries, dogwoods, and sumacs manage to survive the long, icy months ahead? These native woody plants are highly attuned to the advancing seasons and have been preparing for winter’s worst since the summer solstice! It was then, in midsummer, when a series of key changes began, gradually leading to the protective state of cold-hardiness, slowed metabolism, and quiescence 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 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 in preparation for the challenging 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 in protective, waxy, waterproofed bud scales. Deciduous trees, which shed their leaves in autumn, 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 predominance of the photosynthetic pigment chlorophyll, while other colored molecules such as orange carotenes, yellow xanthophylls, and red and purple anthocyanins are greatly outnumbered and masked by the greens. As nights lengthen and temperatures fall, however, the chlorophylls 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, bronzy oranges of ash, hickory, oak, and beech, and rich crimsons, scarlets, and burgundies of maple, dogwood, cherry, and sumac bless human viewers with a breathtaking 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 follows. The leaves, now brown, dry, and devoid of nutrients and water, are shed at the next gentle nudge of wind, rain, or snowfall, and will nourish the soils in which they rest.

It’s 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 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 2500 gallons of water a day, which 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 desiccation 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 warmth and light of spring.

The Chautauqua Watershed Conservancy is a not-for-profit organization with the mission to preserve and enhance the water quality, scenic beauty and ecological health of the lakes, streams, wetlands and watersheds of the Chautauqua region. For more information, visit chautauquawatershed.org and follow us on Facebook and Instagram.