Now is the time to enjoy the weather outside and talk about the science behind leaves changing colors.
Trees respond to such factors as autumn drying conditions, temperature change, altered sun position, and light. It takes approximately two weeks to begin and complete the fall color change so timing and a little luck are essential for the "perfect" view.
Fall color change and flow 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.
Autumn Leaf Color Change, The Anatomy of a Fall Leaf The major factor influencing autumn leaf color change is the lack of water. Not a lack of water 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 leaves from the stem (called 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.
Fall Leaf Color Change Follows a Predictable Process of Chemical Leaf ChangeThis 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. Thus 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.
Carotene (the pigment found in carrots and corn) causes maples, birches, and poplars to turn yellow. The brilliant reds and oranges in this fall landscape are due to anthocyanins. Tannins give the oak a distinctively brown color and is the final persistent color most leaves turn before becoming part of the forest floor. Review Leaf Color by Tree Species.
View a Time-lapse Video of the Fall Leaf Color ChangeThe Virginia Tech Dendrology department has two fascinating time-lapse films, one on a leaf turning color and one on a forest turning into autumn gold.
Time-lapse movie of a leaf changing color
Time-lapse movie of a forest changing color
Dr. Teri Hamlin
North Region Agriculture Education
Georgia Department of Education
204C Four Towers University of Georgia
Athens, Ga 30602
706-552-4461 / 706-540-0032
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