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As the days get cooler and frost is in the air, deciduous trees and shrubs put on an autumn show
in all shades of red, yellow, purple, and brown. The splendor of crisp fall days and vibrant leaves
brings to mind memories for nearly everyone who lives in an area where
deciduous forests are
the dominant vegetation. In many parts of the country, autumn leaves are an important factor in tourism.
Many think that cool weather or frost cause the leaves to change color. While temperature may dictate
the color and its intensity, it is only one of many environmental factors that play a part in painting
deciduous woodlands in glorious fall colors.
To understand the whole process, it is important to understand the growth
cycle of deciduous trees and shrubs. Most have a relatively short period of
annual growth. New stems begin to grow from overwintering buds when the days
become long enough and the weather is warm enough to support growth. For most
trees, growth is usually completed by late June in the Northern Hemisphere.
Next year's leaf buds are set at this time and will not open until they experience
the chill and short days of winter followed by the warmth and longer days of
spring. Once the leaves are fully expanded and the buds are set,
the work of manufacturing and storing
carbohydrates to
support next year’s growth goes full speed ahead. These carbohydrates are
stored in the branches, roots, and buds throughout the growing season to support
next year's growth.
The process that starts the cascade of events that result in fall color is
actually a growth process. In late summer or early autumn, the days begin
to get shorter, and nights are longer. Like most plants, deciduous trees and
shrubs are rather sensitive to length of the dark period each day. When nights
reach a
threshold
value and are long
enough, the cells near the juncture of the leaf and the stem divide rapidly,
but they do not expand. This abscission
layer is a
corky layer of cells that slowly begins to block transport of materials such
as carbohydrates from the leaf to the branch. It also blocks the flow of minerals
from the roots into the leaves. Because
the starting time of the whole process is dependent on night length, fall colors
appear at about the same time each year in a given location, whether temperatures
are cooler or warmer than normal.
During the growing season, chlorophyll
is replaced constantly in the leaves. Chlorophyll breaks down with exposure
to light in the same way that colored paper fades in sunlight. The leaves must
manufacture new chlorophyll to replace chlorophyll that is lost in this way.
In autumn, when the connection between the leaf and the rest of the plant begins
to be blocked off, the production of chlorophyll slows and then stops. In
a relatively short time period, the chlorophyll disappears completely.
This is when autumn colors are revealed. Chlorophyll normally masks the yellow
pigments known as
xanthophylls and
the orange pigments called carotenoids
— both then become visible when the green chlorophyll is gone. These colors
are present in the leaf throughout the growing season. Red and purple pigments
come from
anthocyanins.
In the fall anthocyanins are manufactured from the sugars that are trapped in
the leaf. In most plants anthocyanins are typically not present during the growing
season.
As autumn progresses, the cells in the abscission layer become more dry and
corky. The connections between cells become weakened, and the leaves break
off with time. Many trees and shrubs lose
their leaves when they are still very colorful. Some plants retain a great
deal of their foliage through much of the winter, but the leaves do not retain
their color for long. Like chlorophyll, the other pigments eventually break
down in light or when they are frozen. The only pigments that remain are
tannins,
which are brown.
Temperature, sunlight, and soil moisture greatly influence the quality of the fall foliage display.
Abundant sunlight and low temperatures after the time the abscission layer forms cause the chlorophyll
to be destroyed more rapidly. Cool temperatures, particularly at night, combined with abundant sunlight,
promote the formation of more anthocyanins. Freezing conditions destroy the machinery responsible for
manufacturing anthocyanins, so early frost means an early end to colorful foliage. Drought stress during
the growing season can sometimes trigger the early formation of the abscission layer, and leaves may drop
before they have a chance to develop fall coloration. A growing season with ample moisture that is followed
by a rather dry, cool, sunny autumn that is marked by warm days and cool but frostless nights provides the
best weather conditions for development of the brightest fall colors. Lack of wind and rain in the autumn
prolongs the display; wind or heavy rain may cause the leaves to be lost before they develop their full
color potential.
The character of autumn color is different in different parts of the world. In New England and the
northeast sections of Asia, a few species dominate the deciduous forests. The display there tends to
be short but intense because the change is rapid and rather uniform. In the southern Appalachians, the
change is often gradual and the fall foliage season may last for more than a month because of the greater
diversity of plant species found in the forest there. Mixed forests that have both evergreen conifers
such as spruce and deciduous trees such as aspen or larch are found in the far north or at high elevations.
Here, the dominant color is yellow and the change is rapid, with trees often going from green through brilliant
yellow to bare over a period of two weeks. Tropical forests often have many deciduous trees that lose their
leaves in response to drought; typically the leaves do not change color before they drop. In areas that are
often cloudy for much of the autumn, with rather warm temperatures, fall colors are dull at best. This is
often the case in much of Europe.
While the whole process of fall color is fairly well understood, the reason
for it is less clear. Scientists have long known that xanthophylls and carotenoids
play an important part in photosynthesis by helping to capture light energy,
but the benefit of anthocyanins is not well understood. It might seem
more logical for plants to remove all the carbohydrates they possibly can from
the leaf before making it fall off. If this were the case, we wouldn’t have
the red and purple pigments that we see in sugar maple, black gum, burning
bush, or sweet gum. Carbohydrates are needed to manufacture these pigments.
Some entomologists believe that the evolutionary reason that plants expend
energy to produce fall color is to warn pests. A plant that is healthy is
able to produce lots of carbohydrates, and therefore more anthocyanin. This
may cause certain insect pests laying eggs in the fall to seek another host
plant for their offspring that is weaker and drab by comparison. Some scientists
believe that anthocyanins may act as a sunscreen to inhibit the destruction
of the chlorophyll, help to prevent frost injury to leaf tissues, or limit
water loss during dry spells in autumn. As far as the fall foliage watcher
is concerned, their purpose is simple—they signal a last hurrah for the growing
season and delight the optic nerve.
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Fall Foliage Photos by Scott Aker, Susan Bentz, Erin Clark and Thomas Elias
Last Updated October 6, 2011 6:08 PM
URL = http://www.usna.usda.gov/PhotoGallery/FallFoliageGallery/ScienceFallColor.html
