According to a Cherokee legend, back in the early days, when plants and animals were first made, they were instructed to fast and stay awake for 7 days in order to gain spirit power. Most stayed awake the first night, but on the second day many began to fall asleep. By the 7th night, only a few of the plants were still awake. Among these were the cedar, spruce, pine, holly and laurel. As a reward for this these trees were allowed to keep their green leaves all year while the others must lose their leaves in the fall.
As interesting and fun as this story is to believe, it's probably not why the leaves change colors. When I began to research the real reason leaves change colors I came across some useful (and sometimes confusing) information.
The largest factor in determining fall color is a function called photoperiodism, the length of day and night. When the nights start to get longer in autumn, the trees begin their preparation for the dormant winter period. The production of chlorophyll (which gives leaves their green tint) begins to slow and allows the carotenoids, yellow and orange pigments contained in the leaves, to show. These carotenoids are in the leaves year round, but in the spring and summer months the amount of chlorophyll in the leaves is so great that the green blocks out the orange and yellow pigments.
Some leaves are a little more complex and get a red or purple color. Old stories say the reason for this is Jack Frost moving through the forests during the fall, breathing his icy breath on the leaves and pinching their stems with his icy fingers to make them the shades of red and purple. The scientists have a different explanation for the red and purple colors. Another pigment called anthocyanin, contained in the leaves of some trees is produced by a process known as "senescence" and reacts with certain sugars in the trees to cause the red and purple colors.
Senescence is the term used to describe the aging and death of a tree or a part of it. As the leaf dies, the anthocyanin pigments are produced. Reduction of chlorophyll also plays a small part in the coloring of the red leaves, as does intensity of light, temperature and the water supply that the tree has. Because of these factors, trees or branches that are shaded might not get the bright colors that those in the bright sunlight will get. In some years where there is a high percentage of cloudy days towards the end of August and/or cloudy, rainy days in September, the colors may not be as bright, or may not appear at all.
Because of genetic differences, not all trees change at the same time. Some trees start changing their colors in early August, while others will hold on until late October. Because of these genetic differences and the number of factors that influence the color change, it is almost impossible to make an accurate scientific prediction of how the color change will be in a given year. I guess it would be asking a lot for the scientists to predict the color change every year. Most science is merely speculation on the part of an individual anyway, just like many of the old legends are merely speculation by someone.
Maybe we should believe the old legends then, maybe the trees really do have to go without sleep for a week and maybe Jack Frost really does pinch the stems of maple leaves with his icy fingers. As I walk down a wooded path on an Autumn day, kicking up leaves and watching my breath vaporize and drift away...I can't help but believe in the magic of old stories.