The Leaves of Fall

Fall is once again upon us here in the Smoky Mountains. October is historically one of our busier months in terms of tourism as the “leaf peepers” arrive to take in the breath-taking vistas of color that bless us each fall. Well, most of the time anyway.

We have had a few Autumns that were disappointing in terms of the vibrancy and longevity of the foliage colors. This year appears to be destined to be one of those years.

Have you ever wondered (or had your kids ask you) why do leaves change colors anyway? Here’s why:

The leaves of trees and other plants contain three main pigments: carotene, anthocyanin, and chlorophyll the photosynthetic pigment which captures the sun’s energy to make food for plants. As the most abundant pigment, chlorophyll is what gives leaves their green hue in spring and summer.

Another chemical in leaves, auxin, controls a special band of cells at the base of each leaf stem, called the abscission layer. During the growing season, auxin prevents this layer from fully developing and blocking the tiny, internal tubes that connect each leaf to the rest of the tree’s circulatory system.

In the fall cooler and shorter days trigger an end to auxin production, allowing the abscission layer to grow and cut off the circulation of water, nutrients and sugar to the leaves. When this happens, chlorophyll breaks down rapidly. When the chlorophyll is out of view carotene creates the yellow in maple, aspen and birch leaves and anthocyanin provides the oranges and reds of maples, sumacs and oaks. When there’s less sun, anthocyanin isn’t as chemically active and leaves are more orange or yellow than red.

Other factors such as the amount of rain and temperature variation can also affect the intensity of color in fall leaves. This year we endured a near record breaking drought and unusually high temperatures. The leaves are therefore not turning the bright colors we look forward to, but are just withering up and falling off the trees. Mostly. There are exceptions.

We have one young maple tree in our yard that must have some extra deep roots, for it is blazing a bright red while the others maples around it are barely orange and their leaves are littering the ground already.

But, even without the usual vibrance of color, fall is still a beautiful time of year and we enjoy sitting on our porch in the evenings and enjoying the views. We also enjoy the crisp evenings in front of a craclking fire in our fireplace. Let winter come; we're ready!

Doug-Bob.

Note: Material for this article was provided by University of Wisconsin-Madison.