Leaves have started to change here in the Southern Appalachians and I've had lots of questions about them, so here is an excerpt from Mountain Nature:
Leaves are green because they contain chlorophyll, which reflects green light while absorbing red and blue. We see the color of light that is reflected, not absorbed, by the leaf. By reflecting green light, however, the plant misses the valuable energy in that particular wavelength. They therefore trap as much as possible with additional, accessory pigments in the leaves, which capture some of the energy that chlorophyll misses and then transfer that energy to chlorophyll.
During spring and summer, the green of chlorophyll masks the colors of the accessory pigments, which include carotenoids and anthocyanin. The carotenoids are a group of closely related, common pigments that are present in most leaves (and carrots). They absorb green and blue light, reflecting orange and yellow. Anthocyanin also absorbs green and blue, but it reflects red. Thus, the carotenoids appear yellow or orange and anthocyanin red or purple.
On a fall day, the hills are alive with these vibrant colors. In the fall, so many leaves become various shades of yellow and red because the chlorophyll in those leaves breaks down, but the accessory pigments remain or are synthesized. As the days shorten, deciduous plants decrease the production of chlorophyll and it quickly disappears from their leaves. Since the accessory pigments are more stable, they persist and delight our eyes. In some cases, plants even produce more accessory pigments, especially anthocyanin. Once the green of chlorophyll is gone, these other colors shine through.
Beautiful color appears every year at about the same time because the shortening days of fall signal the trees to begin the process of chlorophyll loss, but some years are more colorful than others. Good growing conditions during the summer (warm and wet) ensure that every leaf is packed with pigments and every tree is loaded with leaves, setting the stage for a spectacular show. During the peak period of leaf change, which is usually mid-October for the Southern Appalachians, several converging conditions produce a short, but extremely colorful season. A cold snap intensifies the color because cold temperatures break down chlorophyll quickly, thereby revealing the accessory pigments. Dry weather also intensifies the color because anthocyanin, present in sap, becomes more concentrated as the water in sap evaporates away. Excess water dilutes the sugary sap, as anyone knows who has tasted a watery, wet-weather cantaloupe. Finally, since sunlight also destroys chlorophyll, sunny weather intensifies the color. The best color show, therefore, occurs during a fall season that is dry, sunny, and cool at night. When these conditions are met, a short but spectacular show results, and it is the timing of the cold snap that determines when the show begins.
It is time to get outside and enjoy these brisk and colorful fall days. Enjoy!
Red maple leaves reabsorbing chlorophyll. Image may not be used without permission.