The Science of Leaf Change
Why Do Leaves Change Colors?
Fall leaf
color change starts very subtly late in September and early October in
temperate North America. 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.
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.
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. 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.