Why There is a Fall Leaf Color Change in Autumn

The Autumn leaf color change starts very subtly late in September and early October in temperate North America. Trees respond to such factors as late summer drying conditions, temperature change, altered sun position, and light. It takes approximately two weeks to begin and complete the brilliant fiery eruption and fall color change.

This fall color change and resulting movement to lower latitudes (and lower elevations) takes place as three primary waves in mixed hardwood forests. A simple flow and wave model was designed at the University of Georgia to illustrate what leaf experts call the fall color wave. This Leaf Wave Model is used to explain the movement of autumn leaf color change.

The major factor influencing autumn leaf color change is lack of water and nutrients which effect formation of chlorophyll. Not a lack of water and nutrients to the entire tree, but a purposeful weaning of water from each leaf. Every leaf is affected by colder, drier, and breezy conditions and begins a process which results in its own demise and removal from the tree. The ultimate sacrifice of a leaf-bearing tree is the ultimate in visual pleasure for us.

The broadleaf tree goes through a process of sealing off the leaf from the stem (abscission). This halts the flow of all internal water to the leaf and causes a color change. It also seals the spot of leaf attachment and prevents precious moisture from escaping during winter dormancy. You just might want to view An Autumn Leaf Cross Section for more graphic details.

This lack of water to each leaf causes a very important chemical reaction to stop. Photosynthesis, or the food-producing combination of sunlight, water, and carbon dioxide, is eliminated. Chlorophyll must be renewed (by photosynthesis) or be taken in by the tree along with photosynthetic sugar. So chlorophyll disappears from the leaves. Chlorophyll is the green you see in the leaf.

Once the overwhelming chlorophyll color is removed, true leaf colors will dominate over the receding green pigment. True leaf pigments vary with the species of tree and thus the different characteristic leaf colors. And because true leaf colors are water soluble, that makes the color disappear very quickly after drying out.
Carotenes and xanthophylls (the pigment found in carrots and corn) cause beech, birch, ash, hickory and poplars to turn yellow. The brilliant reds and oranges in this fall landscape are produced by anthocyanins and color maple, sweetgum, dogwood, sumac, sourwood and tupelo . Tannins give some oak species a distinctively brown color and is the final persistent color most leaves turn before becoming part of the forest floor.