Unusually mild weather and an abundance of
rain have slowed the color change in aspen in the lowest elevations
of Rocky Mountain National Park this fall. A marked green
color can still be seen some aspen leaves, though the height
of autumn color should be reached this week in areas easily
accessible to visitors.
What causes this shift of colors as the weather
cools and days shorten? Chlorophyll is the green pigment responsible
for the green color in plants and for most of the photosynthesis
or food production in plants. As temperatures cool at night,
plants start to break down chlorophyll and pull its building
blocks back into the trunks and roots, conserving these vital
resources for the future. However, other "accessory"
pigments in the leaves are not broken down and stored. Accessory
pigments are masked by chlorophyll in spring and summer, but
remain to color the leaves in the fall. In aspen, the main
accessory pigments are yellow and orange carotenes- the family
of pigments that give carrots their distinctive orange color.
In other plants such as wild geraniums, anthocyanins- red
pigments-function as accessory pigments and provide the distinctive
red fall colors. Accessory pigments capture light from other
parts of the spectrum than that captured by chlorophyll, and
thus make plants more efficient at making their food.
You may wonder why the aspen trees you see
in suburban yards seem to turn colors independent of one another,
while whole clumps of aspens in the park seem to turn color
at once, while other clumps turn earlier or later. In suburban
yards, each aspen tree is usually an independent plant put
in place by the home owner. Aspens in the wild reproduce by
sending out underground shoots more often than by seed. These
shoots eventually pop up some distance away from the original
aspen tree that produced them. This forms something we think
of as a separate tree, but in fact, they are just stems of
one plant. All these connected aspen stems, known more accurately
as a clone, turn color and lose leaves in the fall in unison
because they are one plant.
The brief and brilliant season of fall is
caused by processes as varied as its vibrant colors.
Please click on the photo captions for larger
pictures
