Into every Autumn, some leaves must fall

For all of us who do not have to rake them, there are few fall events to equal the splendor of autumn leaves. They are everywhere, and they offer a wide variety of color. It is a scene replayed every year, the green fades to reds and gold and crimson. Sometimes it looks as if someone has thrown a palette of paints onto the leaves. Then those colors too, begin to fade and we hear the soft rustling as the leaves fall to the ground. Did you ever ask yourself, what makes the leaves change color or why they change just when they do.

Leaves are green because of the process of photosynthesis. Sunlight is absorbed by chlorophyll in the leaves and serves as the energy for protein production. Chlorophyll strongly absorbs the red and blue portions of sunlight best. Since chlorophyll absorbs green light poorly, green light is reflected and that’s the color we end up seeing as leaf color. Leaves are the means by which the trees gather carbon dioxide, and give off oxygen. It is the roots that gather water and nutrients, but it is the leaves that process it. So we know what makes the green in leaves, but what makes the other colors? Have they been there all along?

For a long time, Jack Frost got fingered for the change of color in leaves. It is commonly believed that when the temperatures get low enough for frost to form, it kills the leaves. When the leaves die, they can no longer produce chlorophyll, and thus exhibit their death colors. But frost, and cold temperatures have nothing to do with leaves changing color.

The real cause of the leaves’ demise, is the trees themselves. As the days get shorter in the fall, there is not enough sunlight to make photosynthesis worthwhile. The leaves become a liability. Water moisture is lost through the leaves, and in the winter, cannot be replenished easily.

In the summer, our days are much longer than our nights. At the autumnal equinox, we have approximately equal amounts of day and night. After the equinox in September, the days grow shorter and shorter until they reach the shortest day in December, the Winter solstice.

How does the tree know that this is happening? The trees are able to monitor the amount of sunlight they are receiving. Once the daily amount of sunlight is reduced enough, the trees shut down the process of photosynthesis. They remove the sugars and proteins stored in the leaves and cease production of chlorophyll. When the chlorophyll stops absorbing sunlight, the blue and red portions of light are free to show themselves, and they quickly dominate the green, thus the reds and oranges of fall color.

The Coup de Gra for the leaves comes when the trees send a signal for hormones in the leaf stems to die and form a corky like seal between the leaf and the branch. Once this is completed, it takes only a bit of wind to separate the leaf from the tree, right down to the waiting rakes!