Why
do autumn leaves change colours?
CHAMPAIGN, IL (September 19, 2003) - Skies luminous like a
paint palette of blended colors, as greens transform to various hues of
red, gold, yellow, brown, and purple in autumn. Each year, leaves change
colors and create beautiful scenery, but why and what exactly gives them
their bright colors?
As
we enjoy autumn, we can also take pleasure in understanding the color
changing process in leaves. ¡§Numerous factors including geography,
growth habits, exposure to light, plant pigments, and weather conditions
all play important roles in the fascinating process,” said Jim Skiera,
Associate Executive Director of the International Society of
Arboriculture.
Where
leaf colors are the most vivid, only a few places in the world have the
combination of tree species and climatic conditions necessary for vivid
autumn foliage. Deciduous forests and trees, with their many broad leaves
that change color almost in unison, display the most noticeable autumn
colors. Evergreen species also display changes in color, just slowly and
gradually.
The
best locations with deciduous trees and favorable conditions for brilliant
autumn colors are:
-
the eastern U.S.;
-
southeastern Canada;
-
scattered mountainous locations in western North America; and
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Seattle, Portland, and San Francisco surrounding areas.
Why
do leaves change color?
A
chain reaction occurs when sunlight strikes the pigment in leaves,
allowing us to see various leaf colors throughout the year. Four broad
categories of pigment play a crucial role in autumn beauty: chlorophylls,
carotenoids, anthocyanins, and tannins.
During
the spring and summer, leaves are the principal site for the
photosynthetic process that transforms carbon dioxide and water into the
carbohydrates that fuel tree growth. This food-making process takes place
in numerous cells containing chlorophyll, the pigment that gives leaves
their green color.
As
days shorten and temperatures cool in autumn, the synthesis of new
chlorophyll drops off, green color disappears, and the rate of
photosynthesis declines. Trees become frugal and more efficient, pulling
nutrients such as nitrogen and phosphorus into twigs and branches for
winter storage, further enhancing the loss of chlorophyll.
Along
with chlorophyll, leaves contain yellow or orange carotenoid, the same
pigment that gives carrots their familiar color. Masked for most of the
year by chlorophyll, the carotenoids reveal themselves in autumn, most
noticeable as yellows and golds in tree species such as Norway maples,
Ohio buckeyes, sycamores, birches, and hickories. For example, the golden
yellows seen in beech leaves result from the presence of tannins and
carotenoid pigments.
The
vivid pink, red, and purple leaves seen on maples, sassafras, sumacs,
white and scarlet oaks, and many other woody plants are formed by
reactions between various sugars and complex compounds called anthocyanins.
A mixture of red anthocyanin pigment and yellow carotene often results in
the bright orange color seen in some species of maples.
Autumn
is a unique, distinctive season. It displays an array of vivid bold colors
that beautifies the surroundings,¡¨ says Skiera.
Fast
Facts: Autumn Colors
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Fertile soil enhances the intensity of reds in leaves.
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The more light a leaf is exposed to, the more likely it is to turn
red.
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Weather conditions most favorable to brilliant color are warm sunny
days followed by cool nights with temperatures below 45 degrees (not to
freezing levels).
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Rainy or cloudy days near ¡§peak coloration time¡¨
will dampen the intensity of autumn color by limiting photosynthesis.
-
Freezing temperatures and heavy frost can kill the brilliance of
autumn color by severely injuring leaves before pigments are fully
developed.
The
International Society of Arboriculture (ISA) is a nonprofit organization
supporting tree care research around the world. Headquartered in
Champaign, IL, ISA is dedicated to the care and preservation of the shade
and ornamental trees. For more information, contact a local ISA certified
arborist or visit www.treesaregood.org.
