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Why Trees Change Colors
As New England's foliage season begins, we are
commonly asked why trees change color.
Historically, it was believed that autumn colors
were simply a byproduct of dying leaves. However;
recent research suggests that fall colors may have an
expanded role in the life of trees.
During spring and summer,
leaves get their green cast from
chlorophyll, the green colored pigment that captures the energy in
sunlight for photosynthesis.
Leaves also contain other pigments, but whose colors are
masked by the super-abundant chlorophyll. During
autumn,
chlorophyll begins to break down.
Formerly, it was widely held that
autumn colors were simply the
expression of the remaining pigments, now unmasked by the
absence of chlorophyll. In other
words, fall colors were a tree's
gray hair.
Recently, this belief has come
into question. Interestingly, research suggests brilliant
autumn colors may actually contain a visual message
warning insects.
For example, in the fall certain insects choose trees on
which to lay their eggs. In the spring, the eggs hatch
and the juvenile insects feast on their host tree. It has
been theorized that the production of brilliant
colors essentially announces the tree's lethality to
insects. Over time insects have evolved to avoid these
bright leaves and through natural selection,
trees have evolved to produce even brighter foliage.
This 'leaf-signal' hypothesis proved so provocative
among researchers that multiple worldwide studies
were conducted to test its legitimacy. Preliminary
results found that trees with strong fall colors indeed
tend to suffer less insect damage. However compelling,
there are a number of details about insects and trees that conflict the leaf-signaling hypothesis. Namely, when bright autumn colors appear, insects that feed on
foliage may not be present to warn off.
Other research argues an alternative explanation suggesting that changing colors
serve as a sort of leaf sunscreen. The interior of an autumn leaf is a frenzy
of activity. Much of the chlorophyll and other-molecules necessary for photosynthesis
are carefully dismantled. Therefore, the leaves are inherently less efficient.
Autumn leaves cannot capture all the sunlight, and
energy can build up in the leaf,causing damage. Red and orange colors, produced
by pigments called anthocyanins, appear to protect the leaves by blocking excess
sunlight.
Interestingly, recent evidence in favor of the 'leaf-signal' hypothesis actually
supports the 'sunscreen' hypothesis. The link found autumn between bright fall
colors and a lack of damage in the spring may be a result of the trees doing
a better job of protecting their leaves in preparation for winter dormancy. Further
research has helped explain why some trees have bright colors and
others do not. Some trees, like birch for instance, produce no anthocyanins.
Their yellow leaves are produced by a class of pigments called carotenoids which
help chlorophyll absorb sunlight, but do not shield the trees leaves in the fall.
This new debate and research has continued to raise questions rather than provide definitive answers. The discussion is an example of the complexity of tree physiology and lends deeper reverence for the beauty displayed in fall's foliage.
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