October for much of North America is a time of great change. It is a time of casting off the oppressive heat of summer and preparing for the inevitable return of old man winter. October is also a time when the dramatic color of fall begins to sweep across the landscape, inching its way down from the far North. Fingers of scarlet and gold stretch down the spine of the Appalachian Mountains slowly embracing the East in a kaleidoscope of vivid colors, as if nature in all its beauty and grandeur demands to go out with a bang, before succumbing to the dreary bleakness that befalls the eastern deciduous forests in the winter.
But what drives this explosion of color? Spring and summer offer a monotony of green, but then with Autumn, our world takes on the appearance befitting of a Dr. Sues book. In order to understand what we are seeing in the fall we must first understand what color is in the first place. When we look at an object, the color that we see is determined by the physical properties of that object and specifically which color wavelengths of light that objects absorbs and reflects / scatters. For instance, an object that absorbs all wavelengths of light takes on the appearance of being black. Conversely, an object that scatters all wavelengths in turn looks white. Ultimately the color we see on an object is in fact the color of the wavelength of light that the object does not absorb and therefore reflects or scatters off of its surface. This means that when we look at the leaves of a red maple in the fall, the brilliant scarlet red that we see is the one color that the leaf itself does not absorb and therefore reflects back for us to observe.
During the growing seasons of spring and summer, leaves are full of chlorophyll which is where photosynthesis takes place. As the days grow shorter and the nights become cooler, the deciduous trees that characterize the eastern hardwood forests undergo a dramatic transformation as they prepare for the coming winter. Much like a squirrel stockpiling food for the winter, or bears consuming as much as 30,000 calories a day to prepare for hibernation, trees begin the process of storing energy as well.
At first the tree simply begins to develop a specialized layer of cork cells at the base of the leaf. As this is formed water and mineral uptake into the leaf is decreased and finally stopped all together. As the water and mineral uptake slows, the production of chlorophyll begins to cease as well while yellow xanthophylls, orange B-carotene, and red anthocyanin become unmasked by the dwindling chlorophyll. As the structural protein that produces the chlorophyll begins to degrade, the resulting amino acids are taken back up into the tree and stored in the trunk and roots.
All of this begs the question though as to whether or not the color change itself could in fact actually be beneficial for the trees in ways other than just the re-absorption of amino acids? As it tuns out, there has been quite a bit of research done on anthocyanin – the compound that turns leaves red. Researchers have found that not only does anthocyanin help with the absorption of nutrients – especially nitrogen – but when the leaves fall to the ground, this substance also stunts the growth of nearby saplings thus helping to reduce competition in an act known as allelopathy. More research needs to be done on the potential benefits of xanthophylls and carotene but if red leaves tend to aid trees, than we may very well one day find that other colors offer their own specific benefits.
