AUTUMN LEAF COLORS REMAINS UNPREDICTABLE
November 14, 2010
the color of tree leaves in the fall, and why do colors vary so much from
year to year and place to place? I asked Linda Lee, a botanist at the
Savannah River Ecology Laboratory, if recent scientific research has made
predicting the timing and regional occurrence of fall colors more reliable.
According to her, scientists are a long way from predicting accurately
what fall foliage will be like in a given year. Despite what you may read
about the best time to view fall colors, no one knows for sure.
We do know
that temperate zone forests change color in autumn. And thousands of leaf
peepers make an annual pilgrimage to view trees in the Smoky Mountains
and elsewhere, despite not knowing exactly when or where fall colors will
be the most spectacular or whether colored leaves will even still be on
Why do some
trees have brown leaves, others have brilliant red or yellow ones, while
still others stay green all year? Three basic pigments are responsible
for most annual color patterns in plants. Chlorophyll, which makes leaves
green, is the dominant pigment. As the days get shorter, some leaves display
two additional pigments: carotenoid, which results mainly in yellow and
orange, and anthocyanin, which results in red. Carotenoids absorb energy
and help protect leaves from sun damage. Anthocyanins are less well understood
but may also reduce sun damage and deter fungal pathogens. As far as leaves
falling, Linda Lee notes that "autumn weather per se is not what
kills the leaves; the tree itself does, by pulling nutrients out of leaves
and sealing them off. Once the leaf is an empty shell, it falls. In other
words, the tree sort of beats winter to the punch."
that produce the variety of fall colors in tree leaves are themselves
at the mercy of three primary environmental factors--temperature, day
length and rainfall. All are critical in determining how pigments express
themselves. Cool autumn temperatures cause chlorophyll to degrade in many
deciduous trees. Thus the carotenoid and anthocyanin, normally masked
by the chlorophyll, are accentuated. Colorful displays of reds, yellows
and oranges are created. But a sudden, heavy frost may break down the
accessory pigments as well as chlorophyll, which means colors are muted.
cooling is gradual, colors may be dull because chlorophyll remains in
the leaves, preventing full expression of the brightest yellows, reds
and oranges. To further complicate predictions, pigments respond differently
to temperatures based on the timing and amount of rain during previous
days or weeks. Predicting exactly when fall colors will appear in a particular
year and what they will look like is as complicated as forecasting the
weather--and just as unreliable.
rainfall can have a profound effect on fall foliage, as the plants will
produce more sugars and more pigments. If chlorophyll is broken down rapidly,
the remaining colorful pigments will be at highest density, creating intense
autumn colors and delighting the leaf peepers. A wet summer means more
leaves, more leaf surface, and therefore more color displayed in the forest.
region and the types of trees also have a major influence on leaf color.
Brown autumn leaves characteristic of many trees in warm regions of the
South are often a result of pigments associated with tannin, which mask
the red and yellow colors. Tannins are believed to discourage insects
from feeding on the leaves. Evergreens, such as pines and magnolias, are
another special case. Many evergreens have tough, waxy leaves and less
watery sap that allow them to withstand extreme winter cold. The chlorophyll
remains visible in such trees and other pigments are seldom expressed.
about leaf color remain unanswered, and scientists may never be able to
predict exactly when and where fall colors will be at their most intense.
Nonetheless, each year people will venture out to admire Mother Nature's
fall spectacular, a pastime that will always bring its own rewards.
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