Why do leaves change color in the fall?

A little Anatomy and Physiology

Tree leaves are complex structures with a single purpose: to manufacture sugars in the form of carbohydrates to feed and sustain the plant. This process is called Photosynthesis and it takes water, carbon dioxide and sunlight energy to produce sugar for the plant to use and expels oxygen into the atmosphere. This process takes place in the Chloroplast, an organelle in the leaf, and the light energy is absorbed by Chlorophyll, which gives leaves their green color. This process continues throughout the growing season as the tree uses the sugars for growth and reproduction and also stores these sugars in the roots, stem and trunk for use in the spring.

How do leaves change color? 

As the days begin to shorten and the nights get longer, the tree begins to experience some changes. There is a layer of cells at the end of the leaf Petiole (the stem of a leaf) called the Abscission layer. These cells begin to break down, creating a corky like layer between the stem and Petiole and cutting off water flow to the leaf. This process is controlled by hormones produced by the plant. When this layer forms it produces a protective barrier for the tree and can be seen as a bud scar on the limb.  As the flow of water is reduced, Chlorophyll begins to break down and the leaf eventually falls off the tree. As the amount of Chlorophyll in the leave lessens the underlying pigments are uncovered and the true colors of the leaf are revealed. The predominant pigments found in leaves are: Chlorophyll which is green, Carotenoids which produce the yellows, oranges and browns such as in carrots, corn and yellow flowers and Anthocyanins which produce the reds such as in berries, apples, grapes. These pigments are in the leaf at certain levels at all times  but are covered over by the dominant green of Chlorophyll. They are also at different levels in the leaf depending on the species of the tree; therefore you will see leaf colors ranging from browns to yellow and orange to reds and sometime purple depending on the levels and concentration of these pigments in the leaf. 

How does Weather affect autumn leaf color?

Weather can have an affect on the brilliance of the color of the leaves. The most important time is when the abscission layer is beginning to shut down and the Chlorophyll levels are dropping, if the weather at this time is dry and hot the color will be less, if, however, the weather is warm and moist the colors will be more brilliant. Warm sunny days, good moisture and cool crisp days without freezing will produce the best color. As the fall approaches and the growing conditions are good the leaves will continue to work, producing more sugars. If these sugars are trapped in the leaf as it begins to shut down the color show will be improved. All these factors affect the overall display. 

What about evergreens and why do deciduous trees lose their leaves?

Deciduous trees lose their leaves because they would not survive the winter cold. These thin leaves do not have a waxy coating to protect them from the cold and therefore must be dropped before they are killed off and damage the plant. Evergreen plants on the other hand have a heavier leaf cuticle with a waxy covering which protects them from the cold and desiccation or drying seen in the winter. Evergreen trees do, however, lose their leaves or needles at some time during the year. White Pines for instance will lose their older interior needles every fall-this leads to many concerned calls about the health of their trees. Some evergreens such as Rhododendrons will lose leaves in the spring. 

So, forget all this scientific mumbo-jumbo and just go out and enjoy the beauty of the season. Look on in awe as these perennial giants put on their annual display for our enjoyment and pleasure. Slow down, kick some leaves, laugh, sing and be a child again.