The Falling Leaves
By Sean Henahan, Access Excellence
 Ithaca,
N.Y (10/4/97)- As Autumn comes, so too do discussions of
the hows and whys of the changing colors of the leaves. One area open to
debate is which factors influence the relative brilliance of the foliage
from one year to the next.
"Science agrees on the mechanism of fall color, but there is debate
as to what precedes it," said Peter J. Davies, Cornell University professor
of plant physiology. "Is it a wet summer or a dry summer that increases
the brilliance? Without a doubt, cool nights and bright days contribute
quite a bit to fall color."
The basics are not in dispute. The fall colors come from two main sources:
pigments, such as yellow and orange carotenoids, and red anthocyanins.
Yellow and orange carotenoids are present in the leaves all the time but
are masked by the green chlorophyll. As the leaves become senescent
(or age) at the end of the season, the green chlorophyll in certain tree
species degrades, allowing the oranges and yellows of the carotenoids to
show through.
During the warm days of fall, the leaves can still make sugars by photosynthesis
-- provided the leaves still possess chlorophyll, explained Davies.
When the night temperatures fall, the transport of the sugars from the
leaves is slowed and these sugars are converted into the red anthocyanins.
This process also is enhanced if the plants are under stress, he said.
The weather at the time of fall color has the most influence. The most
impressive colors will develop under warm sunny days with cool (but not
freezing) nights. Cool, rainy days cause the leaves to fall without
developing much color, as the rain and wind knock the leaves off more rapidly,
he explained:
"If you look at trees at the edge of a woodland area, the trees exposed
to sun are always more colored than those that are more shaded. There
are many opinions on the role of weather during the preceding summer.
I think most are anecdotal and I don't know if anyone has done a long-term
study on the phenomenon," he added. Even fertilizing a tree late
in the season will decrease the fall color of the leaves, he said.
"It is my opinion that the more the tree is under (physiological) stress,
the more color will be developed," Davies said. "Thus a dry summer
(leading to water stress or drought stress) will probably give more color
the following fall than a moist, rainy one."
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