|Red Oak in Tennessee, photo courtesy Ashley Bradford|
Leaves contain many pigments. During most of the season the green chlorophyll is the predominant pigment and it masks the presence of the other pigments. Chlorophyll is the pigment that allows leaves to photosynthesize, creating the sugars that provide energy for the plant. Chlorophyll is not particularly stable so it is continuously synthesized and broken down when the weather is warm.
In the autumn, when the temperatures cool and day length shortens, chlorophyll synthesis shuts down. As it breaks down, the yellow and orange carotenes are unmasked. This gives many plants their characteristic yellow fall color.
Anthocyanins, which are red to purplish-blue, are the third type of plant pigment. Anthocyanin production can be triggered by many things including high sugar concentration in the leaf which happens when sugars from photosynthesis are trapped in the leaf as it prepares to separate from the plant. Anthocyanins can also be triggered by environmental stresses to a plant, such as too much or not enough water, too much or not enough nutrients, fungal disease, damage to the bark by animals or string trimmers, and many other things. If you see a tree, or part of tree, change color significantly earlier than others of the same species, it is likely due to stress!
Weather can also affect fall colors. The best colors are seen when there are sunny, warm days and cool, crisp (but not freezing) nights, as well as adequate soil moisture. So hope for good weather, enjoy nature’s spectacle this fall, and report your observations to Project BudBurst!
(Editor's note: Kay Havens is on travel and asked me to submit this for her.)