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Michael A. Stecker
mike@mstecker.com
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New England's Autumn
Leaves
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During the first half of October the New England
states of Connecticut, Massachusetts, New Hampshire, Vermont and Maine put
on a color foliage show that attract thousands of visitors.
But why do the leaves change color? The answer is a change in the pigment
mixture within the leaves. During the spring and summer growing seasons
most tree leaves are green because they are full of chlorophyll bound up
in chloroplasts. Plants use chlorophyll to produce sugars from water and
carbon dioxide through a sunlight mediated process called photosynthesis.
The amount of chlorophyll is so high during the summer that its green
color masks all other pigments present in the leaf. However, chlorophyll
is not a very stable compound and bright sunlight causes it to decompose.
The synthesis of chlorophyll in plants requires sunlight and warm
temperatures. Therefore, during summer chlorophyll is continuously broken
down and regenerated so the leaves maintain their green color. But, during
autumn the days shorten (less sunlight) and the temperature cools
triggering changes in the tree. One of these changes is the growth of a
corky membrane between the branch and the leaf stem. This membrane
interferes with the flow of nutrients into the leaf. Because the nutrient
flow is interrupted, the production of chlorophyll in the leaf declines,
and the green color of the leaf fades. Other pigments then become dominant
and the leaves change color before they drop. The three most prominent
pigments are:
Chlorophyll -- gives leaves their green color.
Carotenoids -- provide the yellow, orange, and brown colors
Anthocyanins -- give the red and purple colors.
Unlike chlorophyll and carotenoids (carotene), anthocyanins are not
attached to cell structures, but are dissolved in the cytoplasm. The color
produced by these pigments is sensitive to the pH of the cell. If the
tissue is quite acidic, the pigments impart a bright red color; if less
acidic, its color is more purple.
The range and intensity of autumn colors is greatly influenced by the
weather. Low temperatures destroy chlorophyll, and if they stay above
freezing, promote the formation of anthocyanins. Bright sunshine also
destroys chlorophyll and enhances anthocyanin production. Dry weather, by
increasing sugar concentration in sap, also increases the amount of
anthocyanin. So the brightest autumn colors are produced when dry, sunny
days are followed by cool dry nights.
Autumn leaf color is specific to the species of tree:
Oaks turn red, brown, or russet
Hickories turn golden bronze
Dogwood turns purplish red
Beech turns light tan
Red maple turns brilliant scarlet
Sugar maple turns orange-red
Black maple turns glowing yellow
Sourwood and black tupelo turn crimson
Aspen, birch, and yellow-poplar turn golden yellow
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