The weather is not the only process affecting the foliage. In fact, the weather is a distant second in importance to the biology of the trees and leaves.
There are many species of trees and shrubs, each with different characteristics. There are two types of trees, evergreen and deciduous. Evergreen trees (e.g., pine, spruce, fir, etc.), sometimes referred to as conifers, keep their leaves (needles) year round. Deciduous trees however, shed their leaves during the winter or during prolonged droughts.
The leaves are responsible for producing food and nutrients for the trees and shrubs. Deciduous trees produced enough nutrients, which are stored by the tree in the trunk and roots, to last throughout the winter. The leaves also allow the trees and shrubs to release excess moisture. The process through which leaves produce food is known as photosynthesis. Photosynthesis breaks down carbon dioxide (CO2) into organic materials such as carbohydrates (e.g., sugars and starches). In order for photosynthesis to take place, energy is required. For plants, this energy is provided by sunlight.
To use the sunlight, the plant must have a mechanism for absorbing the light. Plants use chlorophyll and carotenoids, both chemical pigments, to absorb light. Light has color components collectively known as the spectrum. The spectrum consists of red, orange, yellow, green, blue, indigo, and violet. The green light portion of the spectrum is not effectively absorbed by chlorophyll in plants. The green light is either reflected by or passed through the leaf. This is why leaves generally appear green.
Carotenoids represent a collection of two groups of pigments, carotenes, and xanthophylls. Carotenes are similar to vitamin A and are responsible for the coloration of carrots (the name carrot is derived from this pigment), pumpkins, and yellow and orange leaves. Xanthophylls are yellow pigment responsible for the coloration found in dandelions, sunflowers, corn, egg yolks, and yellow leaves.
In addition to carotenes and xanthophylls, three other chemical pigments play a role in foliage coloration. Anthocyanins produce blue, red, and violet colors. If the plant cell fluid is acidic, the coloration will be red. Conversely, if the fluid is basic, then the color will be blue. Tannins produce brown colors often seen in tea, chestnut bark, and in oak leaves in the fall. The final pigment is a group known as flavones. They are also yellow and are found in sumac, horse chestnut, tea and onions.
Now that you have a basic understanding of trees, leaves, and the chemical pigments found in the leaves, we can now look at how leaves alter their colors in the fall. So often, people talk about the "changing of the leaves". It may be more accurate to say that the leaves are actually losing their color. Leaves contain the pigments mentioned in the earlier section but are overshadowed during the spring and summer by an abundance of chlorophyll, which is green.
As autumn approaches, the amount of available sunlight decreases. This signals the tree that winter is approaching and that it is time to begin the process of shedding its leaves. As a result, the production of chlorophyll ceases and breaks down. Because of this, the green color of the leaf disappears allowing the other pigments to show off their hues.
A maple leaf loses its chlorophyll and other pigments become visible.
Other changes are also taking place during the autumn season. At the point where the leaf stem joins with the branch, a layer of cells develops which blocks the transfer of water to and from the leaves. The veins that carry fluid into and out of the leaf gradually close, trapping sugars and promoting the production of anthocyanins in the leaf. Once the cell buildup is complete, the leaf is ready to fall.
We hope that this brief tutorial has been helpful in explaining the reasons for and the factors which influence the fall foliage. If you have any questions, please e-mail us and we'll do our best to answer them.