What Makes Tree Leaves Change Color in the Fall?

Simple sugars feed a tree in the process known as photosynthesis.“Chlorophyll” is the agent for food making in green plants.

Yellow and orange "carotenoids" are also present in the leaves during warm weather, but they are "masked" by the greater amounts of the green pigments. Cool temperatures stop the production of green pigments and cause chlorophyll to degrade. The yellow and orange pigments are then "unmasked" as the green pigments disappear.

Red and purple autumn colors come from another group of cell pigments called "anthocyanins," (Persimmons, dogwoods, maples, sumacs, sweetgums and ashes) stimulated by lower temperatures and high light levels. Mild drought conditions stimulate production of the red pigments. Acidic sap also contributes.Alkaline sap causes purple coloration.Carotenoids and anthocyanins often combine in leaves to give the deep oranges, fiery reds, and bronzes typical of many hardwood species.

Brown autumn leaf color of oaks and beech is due to the presence of the brownish tannin compounds in combination with the carotenoids.

Several environmental factors can diminish the fall foliage colors:

1. Very warm weather conditions.
2. An early frost.
3. Long periods of wet, cloudy weather in fall.

In a nutshell, the conditions that create the brightest fall colors are:

1. Cool but not freezing temperatures
2. Mild late-season drought
3. Sunny days