WHAT'S HAPPENING IN THE TREES?
The world is changing. Summer is clearly in the rearview, and you can see it leaving in the trees.
It happens to some earlier than others. The poplars, namely, start to turn and shed, covering the ground with their golden slips sometimes as early as late-August—creating an aesthetic that early season bushytail hunters know well, while confusing the eyes of the chanterelle hunters of summer. The sycamores of the creekbottoms and river valleys turn brown and crispy in September, and a strong cold front’s wind rips them from their branches to cover flats and fill eddies, fouling hooks and boat language, and complimenting the bowhunters’ crispy morning walks in with their crunch underfoot. Others hang on to their branches while they catch fire with the picturesque reds, oranges, and purples of fall.
Despite the bittersweet farewell to summer that it brings, this peak display of colorful foliage is one of the many perks of living in the Old Dominion. But what exactly is going on in the trees around us? And why are some years more colorful than others?
Trees have to eat, too. Or, rather, they must secure energy for growth and reproduction. All spring and summer, trees are engaged in a food production process called photosynthesis. This process is driven by a pigment called chlorophyll, which absorbs energy from sunlight and combines it with carbon dioxide and water to produce energy for the tree. Chlorophyll is also the pigment that’s responsible for the green color of leaves during this time, and it’s what you’re seeing in the lush, green forests of summer in Appalachia.
Alongside chlorophyll, leaves also harbor carotenes and xanthophyll pigments (orange and yellow-brown pigments, respectively), but they are, during peak photosynthesis, mostly masked by green pigments.
As the days grow shorter in the late-summer and early fall, a thin layer of cells known as the abscission layer forms across the base of the stem of a leaf. This layer is comprised of small tubes designed to carry water from the roots of the tree to the leaves, and energy produced in the leaves back into the tree.
As the season progresses, the cells in the abscission layer swell, constricting the tubes and eventually totally cutting off the resource exchange between tree and leaf. Without water provided by the tree’s roots, the leaves can no longer produce chlorophyll, and the green pigment begins to fade, giving way to the previously masked oranges and yellows (carotenes and xanthophylls). Reds and purples make their debut as products of anthocyanin pigments, which are manufactured by the sugars trapped in the leaf after abscission.
Different trees and vines show different autumn colors, and each is due to the specific mixture of the different pigments in the leaf. Maples famously exhibit bright oranges and reds, while birches characteristically turn a gleaming yellow. Sumac and Virginia creeper turn a deep red or purple.
At the same time that the abscission layer is forming at the base of the leaf, the tree is sealing the separation—closing itself off to the elements as it prepares to go dormant for the winter. This seal is what is referred to as a leaf scar.
The leaves, then, will eventually fall from the tree, pulled to the ground by their own weight, or they may be blown off on a gusty autumn day. Some trees, like some oaks, locally, hold onto their dead-brown leaves, which only fall when pushed off by new growth in the spring.
Several factors combine to produce varying levels of autumn foliage intensity from year to year, including temperature, available light, and soil moisture.
Cool temperatures, specifically at night, encourages the production of anthocyanins (red and purples), and sunny skies help to use up chlorophyll quickly, resulting in an earlier, more vibrant display. However, freezing temperatures hinder the leaves’ ability to produce red and purple pigments, resulting in a more drab display.
A dry late-summer and early fall can cause the abscission layer to form earlier, and the leaves may fall before really changing color. Also, heavy winds, as we can sometimes see as a result of a hurricane or tropical storm passing through in October, can rip the leaves from the trees before they completely change, particularly if preceded by dry weather.
The perfect conditions for vibrant fall foliage is a real Goldilocks situation. A wet summer followed by a relatively dry fall with ample sunlight, cool (but not freezing) nights, and stable weather produces the best viewing.
Every year, folks examine the weather and the trees and make judgements as to the quality of the year’s foliage. My advice? Don’t sweat it. The shoulder seasons in Virginia are beautiful not only for their visuals, but also for their remarkable natural processes. Look deeper, and enjoy them.
*Originally published in The Rural Virginian