Researchers at SUNY College of Environmental Science and Forestry in Syracuse, N.Y., have it all figured out. Basically two things cause the chemical process that make the leaves change colors: temperature and daylight.
The leaf is the food machine for the tree in the spring and summer months as it absorbs energy from sunlight that changes carbon dioxide and water to carbohydrates. The food-making takes place in the leaf's numerous cells that contain chlorophyll, the substance that makes leaves green.
But hidden inside those leaves underneath all that vibrant green are pigments ranging from pale yellow to deep orange. When fall comes and the length of daylight shifts along with changes in temperature, the chlorophyll breaks down. The leaf's green color gives way to the fall splendor of yellow and orange. Other chemical changes may occur at the same time, causing the leaves of some trees to turn red or even purple, such as dogwoods and sumacs.
While the leaf is turning color, other changes are invisibly taking place. A special layer of cells develops where the leaf stem is attached to the tree and gradually severs the tissues that hold the leaf onto the branch. Once the seal is cut, a gentle breeze is enough to make the leaf fall off.
Autumn leaves' color intensity and duration is affected by the weather, including temperature, light and water. For the most brilliant color the best weather is low temperatures that above freezing with lots of rainy and/or overcast days.
To find out more, check out this online pamphlet titled "Why Leaves Change Colors."
--From the Editors at Netscape
