With wind, rain and biological processes now rapidly stripping the leaves from deciduous trees, time is running out for enjoying what has been spectacular fall color this year. While along Skyline Drive the browns and grays of winter have already taken over, patches of forest at lower elevations and in sheltered places still flash gold and red on a sunny day.
Virginia is renowned for its great show of fall color; it used to cause major traffic jams going up to Skyline Drive when I was a kid in the ’50s. While New England is also noted for its fall color, that color mostly comes from just one tree species — the sugar maple, although this species arguably provides the best show every year, even down here in the South. In the southern Appalachians, with the most diverse forests in North America, we get to enjoy a wider variety of trees, and therefore leaves, throughout the year. Through time-lapse videos, the Virginia Tech dendrology department website shows a single red maple leaf and an entire stand of trees change color.
This color change is at the end of an energy-storage cycle that, for a deciduous tree, starts in the spring, when leaf buds start to unfurl and grow. The green pigment in them, chlorophyll, begins to convert sunlight to energy that the plant can then store in its trunk, stems and buds. The cycle completes in the fall, when fading light and cold make the process of acquiring and storing energy too costly. Trees then reverse the process, drawing nutrients down out of the leaves to store elsewhere for the next growing season.
In 1991 Laurel Hill Press published a great thin, pocket-sized guide to sorting out leaves in fall by their color, “Fall Color Finder: A Pocket Guide to the More Colorful Trees of Eastern North America.” The guide uses color photos, along with a key, and also has a really useful index in the back that shows the basic outline of leaves in the two most common fall colors, yellow or red, as appropriate to the species. Unfortunately, this guide now seems to be out of print, but used copies are available online.
Tree ID can be tricky even at the height of summer, when leaves help immensely. Once winter comes, and leaves are gone, identification relies mostly on bark, growing habit and size and the appearance of the overwintering buds — not nearly as big a help as leaf structure and color. Fortunately, Nature Study Guild Publishers has been publishing a series of identification guides, in its Nature Study Series, that includes the popular “Tree Finder: A Manual for Identification of Trees by their Leaves,” but also “Winter Tree Finder: A Manual for Identifying Deciduous Trees in Winter.” These guides are available from many online sources, including Menasha Ridge Press and the Shenandoah National Park Association, which offers the books online, by phone (540-999-3582) and at the Byrd Visitor Center.
As the U.S. Department of Agriculture explains on its website the light energy captured by chlorophyll is tied up in carbohydrates. Chlorophyll breaks down with exposure to light, so is constantly replaced during the growing season. When nights become long enough, the cells near the juncture of the leaf form a corky layer of cells at the base of the leaf that slowly begins to block transport of materials, including carbohydrates, from the leaf to the branch and blocks the flow of minerals from the roots into the leaves. “In a relatively short time period, the chlorophyll disappears completely,” says USDA. Here in Virginia, this process starts in early fall and generally peaks around the third week in October.
Fall leaf color varies with the species of tree and the amount of light a particular tree receives. Underneath the green of summer and the red, purple and orange of fall are yellow pigments, xanthophylls, which remain throughout the growing season. In the fall, as the tree reabsorbs the chlorophyll, most leaves appear yellow. Some drop to the ground before much color change has occurred, or appear to go almost straight to brown.
With some species, such as maples, added chemicals in the leaf turns it red, if it is exposed to enough light. These “anthocyanins” are manufactured from sugars trapped in the leaf and not usually present during the growing season. Some trees have enough to turn their leaves purple, while others have just enough to combine with the yellow pigments to create orange.
Eventually, as the abscission layer become drier and corkier, the connections between cells become weakened, and the leaves break off, often while still sporting bright color. In the Appalachian forest, this can create gorgeous carpeting on the forest floor, with leaves of varying shapes, sizes and colors comingled. Not all deciduous trees lose all their leaves in winter, but the color in those that remain does not last for long, with pigments breaking down in light or freezing temperatures, not to be replaced. All that is left are the tannins, which are brown.
As the USDA notes, temperature, sunlight, and soil moisture “greatly influence” the quality of the fall foliage display. The best color comes from “a growing season with ample moisture that is followed by a rather dry, cool, sunny autumn that is marked by warm days and cool but frostless nights.” When a lot of sunlight and cool temperatures come after the abscission layer forms, chlorophyll is destroyed through the formation of more anthocyanins. Freezing conditions “destroy the machinery responsible for manufacturing anthocyanins, so early frost means an early end to colorful foliage.” The formation of the abscission layer can also be triggered early by drought, causing leaves to drop before they change color or the color display to be shortened, and wind and heavy downpours can strip the trees of their leaves before they reach their peak color.
Leaf color can vary within a single species, and even one tree, depending on the amount of light individual leaves receive. Sugar maples, for example, can have a gorgeous array of bright-red leaves on the outer edge of their canopy, where they receive more light, while having yellow and even green leaves deeper in the foliage or on other parts of the tree that are shaded.
While the fall color process is “fairly well understood,” USDA notes, the benefit of anthocyanins “is not well understood.” Why do trees put energy into producing red color in leaves late in the energy cycle? According to the USDA, the bright color may warn insect pests that the tree is healthy, and therefore not a suitable host for their eggs; or the anthocyanins may act as a sunscreen to inhibit the destruction of the chlorophyll, help to prevent frost injury to leaf tissues or limit water loss during dry spells in autumn. For us humans, their purpose is simple — “they signal a last hurrah for the growing season and delight the optic nerve.”
This year was “a very good year by all accounts” for leaf turning, says John Seiler, a tree scientist at Virginia Tech. It’s one of the best I can remember, which we can chalk up to the wet, cool summer. It definitely has delighted my optic nerve. Although we may mourn the loss of leaves in the winter, especially after such a great autumn show, as the leaves fall, they reveal the structure of the tree — its bones — which has its own, albeit more subtle, beauty, one we can admire until the leaf buds pop open next spring.
© 2014 Pam Owen