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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.
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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.
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 while
neutral to alkaline saps will appear purple to blue. Not all trees produce
anthocyanins, with sugar and red maples, dogwoods, sumac, black gum, sweet gum,
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 this abundance of fall
color.
[John
Fulton]
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