CHARLESTON, W.Va. -- The evening chill hangs in the air, the days grow shorter and the vegetation all around plans its last hurrah before the winter slumber. Autumn flowers light up the roadsides, vegetable plants (if they survived) are feverishly producing a glut of last-minute produce, and leaves begin to change color. But why do leaves change color?
Each year, as the day-length shortens and the nights cool, trees get the message that winter is soon approaching. Deciduous trees, those that lose their leaves during winter, then set to work doing several things to prepare. The tender leaves of these trees are not meant to withstand cold weather and would not survive the winter. So the tree begins processes to rid itself of the leaves before winter, a process called senescence (Latin: to grow old).
Deciduous trees and shrubs use hormonal signals to induce what is called programmed senescence, which is different from senescence in response to poor growing conditions (say a dying plant from lack of water). The plant begins to break down any molecules in the leaf that it can, and then pull the products of that process back into the tree/shrub for winter storage. Just like a squirrel will store acorns for winter, the tree wants to preserve as much as it can for the cold months ahead. The tree invests a lot of energy in forming those compounds, so it stands to reason that preserving as much of that investment is in the tree's best interest.
One of the things that breaks down for re-absorption into the tree is chlorophyll, which is the green pigment responsible for giving leaves their color. Chlorophyll is the major molecule that drives photosynthesis, which is the process plants use to harness solar energy to produce food in the form of sugar. It is broken down into smaller molecules, which can then be moved from the leaves back into the plant. Other pigment molecules that aid in photosynthesis and other plant processes are present throughout the growing season, but are not broken down by the plant. Most of the time, we do not see these colors until the chlorophyll is removed from the leaves. The result of their debut are the vibrant fall colors of the West Virginia hills (granted they have fall colors elsewhere, but none so lovely as ours).
Now here's where we are going to get technical. There are four main classes of plant pigments; porphyrins, carotenoids, anthocyanins, and betalains. These pigments provide colors to plant leaves, stems, fruits, flowers, and roots. The porphyrin group includes chlorophyll, while the carotenoids are yellow and orange, anthocyanins are blue and red, and the betalains are red and yellow (but are found in relatively few species). The pigments are usually found in specific combinations that give leaves of species specific colors. For example, sugar maple leaves contain carotenoids that make them orange, whereas yellow birch leaves contain carotenoids that make them, well, yellow.
When the tree has absorbed as much as it can from the leaves, or when the temperatures get too cold to sustain the leaves, the tree disconnects the leaf in a process called abscission (more Latin: to cut away). Basically, the plant builds a wall between itself and the leaf and allows the leaf to fall. There are a handful of native trees that, despite the fact that the leaves have been abscised, retain the dead leaves throughout the winter. This persistent foliage occurs in oak, beech, sycamore, and hornbeam trees. And just to throw another Latin term at you, this phenomenon is called marescence (withering).
Some years have great fall colors, while others do not. It all comes down to weather. A dry summer or drought will cause abscission to occur early, resulting in brown leaves that drop early. This has happened in several recent years. Early freezes and hard frosts can damage pigments and leaves, cutting short any chance of a colorful fall.
The best colors are usually the result of a summer growing season with plenty of moisture (check!) followed by a fall with cool evening temperatures and lots of sunshine (check!). It looks like we are in store for some stunning fall foliage. Be sure to get out and enjoy it in your landscape and in the beautiful hills of West Virginia. Just remember that in no time you'll be raking those leaves up. In the next few weeks, we'll talk about how you can use those leaves to work for your garden.