Will Lentz, a reader from Fairbanks, asks a question that flares
every fall: why do some aspens turn red?
few scientists from Fort Collins, Colorado, pondered that subject
in the late 1970s. Curious about red aspen trees people had noticed
for half a century, they studied why these existed amid those with
the more common leaf color, yellow.
Before getting to the scientists results, a quick refresher
on why tree leaves change color. Deciduous trees (the broad-leafed
ones that drop their leaves in the fall, in contrast to our needle-leaved
spruce) are gamblers now folding their hands after a few months
Sensing shorter periods of daylight, trees have quit refreshing
their leaves with chlorophyll, the green pigment that helps capture
the suns energy and allows the tree to convert it to the sugars
that make it taller and bushier.
Right about now, when a tree reaches its daylight threshold,
cork-like cells develop where leaf meets stem. This abscission layer,
which later reveals itself as a handsome scar, is an ever-clogging
pipeline that restricts the flow of sugars from leaf to tree.
As the tree informs its solar panels their services will no
longer be needed, chlorophyll production stops. In mid-summer, chlorophyll
was the loudest kid in the family. The parent tree replenished its
leaves chlorophyll as the sun faded them like colored paper
left on the windowsill. The trees autumn refusal to ante any
more chlorophyll allows the quiet pigments in the leaf to express
These include yellow (xanthophylls) and orange pigments (carotenoids).
Reds and purples come from anthocyanins.
Kuo-Gin Chang, Gilbert Fechner and Herbert Schroeder, then at
Colorado State University, a few decades ago dropped aspen leaves
into a blender and pushed the button. From the solution of liquid
leaves, they determined that a sugary red pigment was indeed present
in red and orange aspens, but not in yellow aspens.
The scientists hinted that the red occurs on only some trees,
meaning it is probably a genetic trait a red aspen is sort
of like a person with red hair. The researchers also wrote that
yellow trees remained yellow from year to year but one tree they
selected for its redness at the start of their five-year observation
was red only for the first year and yellow each following year.
And most of those lovely reds and oranges did not endure to carpet
the forest floor, fading to bland yellow within a week of falling.
Another intriguing question: in this dont-waste-a-molecule
world, why do trees invest in creating pigments other than energy-gathering
green? Nobody seems to have answered that, but scientists have speculated
that red acts as a sunscreen to keep played-out leaves from getting
overexcited by photons. Another idea is that color might also either
throw off green-munching bugs or be a trees way of showing
insects its vigor compared to its drab, easy-to-attack neighbor.
Since the late 1970s, the director of the Geophysical Institute
at the University of Alaska Fairbanks has supported the writing
and free distribution of this column to news media outlets. 2014
is Ned Rozells 20th year as a science writer for the Geophysical