of hues so varied that it is, as Walt Whitman said, `enough to make the
colorist go delirious.’ Here in the forests of the Blue Ridge, where well
over a hundred kinds of native deciduous trees are to be found, the spectacle
challenges description; the writer feels humbled and gropes for words.”
Arthur Stupka, The Great Smokies and the Blue Ridge (1943)
This past weekend, my wife, Elizabeth, and I were in the Boone-Blowing Rock area in the northwest corner of the state. Up there, the colors were reaching their peak and the highways were jammed with “leaf lookers.” But as I write this on Monday, Oct. 9, the full fall color season hasn’t as yet arrived here in the Smokies region.
I’m conducting a workshop this week near Highlands, where the color is pretty good, but in about a week or less things up here and throughout the region should be spectacular. The reason full color hasn’t arrived is because we haven’t had night-time temperatures that are cool enough as yet. Let me explain.
Spring and summer leaves are green because of chlorophyll, the pigment that enables leaves to manufacture food via photosynthesis. Leaves also contain other pigments at this time, but the dominant green of the more abundant chlorophyll masks them.
Lots of folks have complicated theories involving the cause of fall color, but the basic factors are pretty simple. The first factor is a constant from year to year. With late summer and fall’s shorter days, the leaves stop producing as much chlorophyll. Other colors — especially yellow — emerge as pure pigments. Carotin and other yellow pigments cause pawpaw, spicebush, witch hazel, mulberry, hickory, black locust, and other species to glow softly in the woodlands like candles.
The second factor varies from year to year as to when it kicks in. A cool snap of about 35- to 37-degrees Fahrenheit in late September or early in October (usually before Oct. 10, which is the average true frost date) activates the formation of a corky layer at the point where the leaf stems connect to their twigs. The connection is marked by a swollen nodule. As it hardens, this corky “abscission layer” cuts off the flow of water and minerals to the leaf so that it can no longer produce chlorophyll.
At this point the previously masked colors really start to appear. Compounds called anthocyanins brighten the landscape with the reds and oranges. If the leaf contains significant amounts of tannin — as is the instance with the various oak species — brown and maroon colors are dominant.
Leaves that contain significant amounts of sugar undergo a chemical reaction that results in the brightest red colors. Sugar maple is, of course, an example. But the brightest red of all — lipstick red — is displayed by black gum.
A leaf eventually breaks off at the abscission layer and falls to the ground. Usually this occurs in late fall or early winter. Some trees, however, don’t shed their leaves until late winter or early spring. This is the instance with some oak species. And one of the prettiest sights while on a winter walk is the tan curls of American beech leaves.
Just below our house, a trail passes beside a cut bank where young beech trees still hang onto their leaves long after most other trees have dropped theirs. Each tan leaf curls in upon itself, forming a cylinder that can sound like a tiny chime when the wind passes across it just so.
Have you ever wondered why young beech trees (and some young oaks) don’t lose their leaves in winter? I had, in fact, pondered that very question for many years. And only recently have I come up with an explanation that satisfies me.
In a book titled 1001 Questions Answered About Trees (Dodd, Mead & Co., 1959), the naturalist Rutherford Platt explained it this way: “Trees drop their leaves at the same time that they turn color in the fall because a disk of cells called the abscission layer forms at the base of the leaf stem. These are loose dry cells that cause the leaf to break off of its own weight. A peculiarity of oak and beech is that this abscission layer often does not form until their wood is older.” It seems that, unlike most deciduous trees, which lose significant amounts of nutrients in their discarded leaves, young beech and some oak trees hold onto theirs so as to reabsorb nutrients from them during the winter months.