Every autumn we revel in the beauty of the fall colors. The mixture
of red, purple, orange and yellow is the result of chemical processes
that take place in the tree as the seasons change from summer to winter.
During the spring and summer the leaves
have served as factories where most of the foods necessary for the
tree's growth are manufactured. This food-making process takes place
in the leaf in numerous cells containing chlorophyll, which gives
the leaf its green color. This extraordinary chemical absorbs from
sunlight the energy that is used in transforming carbon dioxide and
water to carbohydrates, such as sugars and starch.
Along with the green pigment are yellow to orange pigments, carotenes
and xanthophyll pigments which, for example, give the orange color
to a carrot. Most of the year these colors are masked by great amounts
of green coloring.
Chlorophyll Breaks Down
But in the fall, because of changes in
the length of daylight and changes in temperature, the leaves stop
their food-making process. The chlorophyll breaks down, the green
color disappears, and the yellow to orange colors become visible and
give the leaves part of their fall splendor.
At the same time other chemical changes
may occur, which form additional colors through the development of
red anthocyanin pigments. Some mixtures give rise to the reddish and
purplish fall colors of trees such as dogwoods and sumacs, while others
give the sugar maple its brilliant orange.
The autumn foliage of some trees show
only yellow colors. Others, like many oaks, display mostly browns.
All these colors are due to the mixing of varying amounts of the chlorophyll
residue and other pigments in the leaf during the fall season.
Other Changes
Take Place
As the fall colors appear, other changes
are taking place. At the point where the stem of the leaf is attached
to the tree, a special layer of cells develops and gradually severs
the tissues that support the leaf. At the same time, the tree seals
the cut, so that when the leaf is finally blown off by the wind or
falls from its own weight, it leaves behind a leaf scar.
Most of the broad-leaved trees in
the North shed their leaves in the fall. However, the dead brown leaves
of the oaks and a few other species may stay on the tree until growth
starts again in the spring. In the South, where the winters are mild,
some of the broad-leaved trees are evergreen; that is, the leaves
stay on the trees during winter and keep their green color.
Only Some Trees
Lose Leaves
Most of the conifers - pines, spruces,
firs, hemlocks, cedars, etc. - are evergreen in both the North and
South. The needle- or scale-like leaves remain green or greenish the
year round, and individual leaves may stay on for two to four or more
years.
Weather Affects
Color Intensity
Temperature, light, and water supply
have an influence on the degree and the duration of fall color. Low
temperatures above freezing willfavor anthocyanin formation producing
bright reds in maples. However, early frost will weaken the brilliant
red color. Rainy and/or overcast days tend to increase the intensity
of fall colors. The best time to enjoy the autumn color would be on
a clear, dry, and cool (not freezing) day.
Enjoy the color, it only occurs for a
brief period each fall.
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