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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<http://forestry.about.com/cs/fallcolor/a/fallwave.htm>
* 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<http://forestry.about.com/library/weekly/aa090103.htm>
* 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<http://forestry.about.com/od/fallcolor/a/fall_leaf_color.htm>.


View a Time-lapse Video of the Fall Leaf Color ChangeThe Virginia Tech
Dendrology <http://www.cnr.vt.edu/DENDRO/DENDROLOGY/features.htm> 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<http://www.cnr.vt.edu/DENDRO/DENDROLOGY/leaf.swf>
Time-lapse movie of a forest changing
color<http://www.cnr.vt.edu/DENDRO/DENDROLOGY/stand.swf>

--
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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