Why Do Leaves Change Color?
Posted: October 8, 2012
Chlorophyll pigments, which help the plant manufacture food, are plentiful in leaf cells during the growing season. Their green color masks the colors of other pigments also present in the leaf. The presence of chlorophyll depends on the presence of sunlight, so as day length shortens, chlorophyll production declines.
The pigments responsible for yellows, browns, oranges, and many other intermediate colors are called carotenoids (KAR-eh-ten-oids). You can see these pigments in abundance in the autumn leaves of aspen, birch, ginkgo, hickory, and honeylocust trees. Carotenoids also give characteristic color to carrots, corn, daffodils, and bananas.
The reds and purples that color the autumn leaves of red maple, white ash, and red oak come from another groups of pigments called anthocyanins (An-tho-SI-ah-nins). Unlike carotenoids, anthocyanins are not present in the leaf during the growing season. Instead, they develop in late summer, in the sap of leaf cells. The red and purple pigments are the result of a series of chemical reactions involving sugar buildup. And the chemical reactions are a result of falling temperatures, as autumn nights grow cold.
Not all trees can produce anthocyanin pigments, and even those that can depend on the breakdown of sugars in the leaf in the presence of bright light. It is when autumn days are bright and cool, and nights are chilly but not freezing, that the most brilliant colors usually develop. Trees that grow in dense shade usually do not produce the vibrant colors that trees of the same species produce in full sunlight.
There is no simple way to predict fall color for a given area. The intensity, type, and duration of color, and the dates of peak color are determined by a variety of environmental factors and the genetic characteristics of the plants, themselves.
One thing we do know, though, is that nearly everybody loves the beautiful fall colors of Pennsylvania forests. Take time to enjoy the view!
There are lots of ways to get more information about Pennsylvania forests and trees. Visit www.patrees.org or call your local Penn State Extension office 570-963-6842 or Bureau of Forestry office.