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 To understand the process that creates color, we need to know a
little about basic tree growth. A tree has two parts in its
vascular system, the xylem and the phloem. A tree’s xylem cells
can be thought of as thousands of minute soda straws packed end
to end, going from the roots to the leaves. Water and nutrients
are taken up by the roots and transported to the leaves through
the xylem cells in the tree’s sapwood. In the leaves, water and
nutrients are converted into sugar, the energy that feeds the
tree’s growth. This conversion process, known as photosynthesis,
happens in the presence of chlorophyll and sunlight.
The phloem is a thin layer of cells found in the inner bark of
the tree. This is where the sugars move from the leaves to the
roots and other storage sites within the tree. The location of
the phloem shows how a tree can be severely injured or killed if
its bark is damaged. If the phloem is disrupted, food can’t flow
through the phloem and the roots starve to death.
Fall coloration starts with the onset of senescence, a natural
process that disrupts the tree’s vascular system. This is the
orderly process in which the light gathering and carbon
capturing substances in the leaves, including the pigments that
capture sunlight and the proteins that use the captured energy
are disrupted and broken down. The change is started by the
tree’s genetic ability to “sense” day length and temperature
variations. Fall’s shorter days with less light and different
light intensity, along with the cooler and longer nights affect
the production of growth regulators that trigger senescence.
The long and warm days of summer produce high levels of the
auxins and gibberellins that stimulate tree growth and low
levels of growth inhibitors. These stimulate a variety of
changes, including the formation of corklike cells at the base
of the leaf petiole, which produces a brittle zone around the
vascular tissue so that it is easy for the leaf to break off
from the branch. Eventually only the dead xylem cells are left
holding the leaf on the tree. Heavy winds or rains can easily
break this fragile connection, causing leaves to fall to the
ground.
The shorter days and cooler temperatures get the tree ready for
dormancy. Chlorophyll production drops dramatically from the
high levels of the growing season to virtually nothing. The
tree’s priorities then switch to the production of sugars that
will be stored for next season’s growth. This reduction in
chlorophyll production starts the visible fall colors.
Chlorophyll is the predominant pigment and makes the leaves
green during the growing season. Chlorophyll is also very
fragile and must be replaced by plants on a continual basis
until the days grow short and temperatures fall. The fading of
the green color, due to much lower chlorophyll production,
causes the other pigments once masked by the green chlorophyll
to come through. These other pigments include yellow, orange,
and buff colors of the carotenoid, xanthophyll, and tannin
pigments.
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Carotenoids are always present in the leaves, so fall’s
yellow to orange colors are usually fairly consistent from year
to year. Xanthophyll is a yellow to tan colored pigment and
tannins are responsible for the brown earth tones found in oak
leaves. A fourth pigment called anthocyanin does not naturally
occur in the leaves, but is a product of senescence and
concentrated sugar sap in the leaf cells. Anthocyanins appear
red and generate the varying shades of blue, purple, and red
that provide some of the most vibrant color displays. The actual
color depends on the pH of the cell sap, with acidic saps
causing red to orange and neutral to alkaline saps will appear
purple to blue. Not all trees produce anthocyanins with sugar
and red maples, dogwoods, sumac, blackgum, sweetgum, scarlet
oak, sassafras, persimmon, hawthorn, and white oak producing the
most brilliant shades of red, maroon, purple, and blue.
Hopefully this somewhat scientific explanation of fall colors
will cause you to understand a little better what went on within
trees to bring about an abundance of fall color. With the wide
variations we’ve had in temperatures and moisture, it will be
interesting to see what colors we get to enjoy – and for how
long.
[By JOHN FULTON, COUNTY EXTENSION
DIRECTOR SERVING LOGAN, MENARD, AND SANGAMON COUNTIES]
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