Environment, tree genetics determine leaves’ fall colors
In mid-October, the green of summer has faded, and hues of yellow, orange and red take its place. What brings on this seasonal kaleidoscope that prompts people to take long walks in the woods and along treelined streets? The answer lies in environmental factors, as well as in the genetic makeup of the trees themselves.
Fall colors result when trees begin the process of going dormant, temporarily halting their growth to conserve resources during the winter. Shortening day length and lower temperatures trigger trees to cast off their leaves.
“When trees start getting ready to shed their leaves, they stop the production of chlorophyll, the green pigment that captures the light energy of the sun,” said Ed Hedborn, plant records manager and fall color scout at the Morton Arboretum.
In the absence of green, other colors that have always been present in the leaf are unmasked. Cottonwood, buckeye and ginkgo trees show yellow leaves due to carotenoids, the same pigments that bring the buttery color to springblooming daffodils. In other trees, chemically similar pigments, carotenes, read as orange, as is the case with hickory and birch trees.
To create red and purple leaves, trees need to make pigments called anthocyanins, which are also found in grapes and apples. These pigments are on display in sumacs, which turn a bold scarlet, and in the leaves of white oaks, which transform to a wine-purple color in autumn. Scientists are still working out why a tree would go to the trouble of converting light energy into anthocyanins.
While no one can forecast fall colors with total certainty, a few key elements help determine its intensity and duration.
Sugar maples, a showy fall favorite, can turn a range of colors from year to year due to the combination and amount of yellow and red pigments in their leaves.
“The brightest fall color for an area occurs when autumn days are bright, sunny and warm, when nights are cool, but not below freezing, and when we’ve seen enough rainfall for trees to have the water they need,” Hedborn said.
As a general rule, trees in shaded areas do not produce the brilliant colors that trees of the same species produce in full sunlight. Leaves that see the most sun, including those on the top, begin to change first and are typically more dazzling.
An individual tree’s fall color also depends on how it is faring. A healthy tree will give a better show than one challenged by stress, whether that’s due to inadequate moisture, pests or disease.
“How long leaves remain on the tree depends on how quickly cold weather moves in,” Hedborn said.
Another blow to fall leaves? Wind. With even one blustery storm, leaves can make a quick descent from the tree canopy overhead to crunching underfoot.
While the science behind fall colors takes into account many variables, one aspect is constant— the order in which trees change color. The sequence begins with trees such as sumacs and walnuts before moving to redbuds, beeches and many species of maples. Native oaks are among the last to turn as October comes to a close.
For tree and plant advice, contact the Arboretum’s Plant Clinic (630-719-2424 or plantclinic@mortonarb .org).