In the 1800’s the allure of gold brought people to the Rocky Mountains.
Today, that same allure still brings people to the Rockies. Instead
of shiny flakes of metal, the golden attraction today is the sight
of hillsides painted with the golden leaves of aspen.
Around Labor Day each year
our thoughts turn to fall: harvests, hunting, football, and, in
the Rocky Mountains…fall colors. As the day length begins to shorten,
the deep green color of summer begins to pale, and we see the first
hints of yellow in the trees. In most years, we expect to see the
height of the fall color display in the Rockies around the third
and fourth weeks of September.
Fall colors follow a typical
progression, beginning in the north and spreading to the south in
response to shorter day lengths. The color display also begins
earliest in the higher elevations and works its way down the slopes.
While the annual transition from summer to fall is determined
primarily by the shortening day length, many factors contribute
to both the precise timing of the change and the intensity of the
display. The ideal conditions for the formation of brilliant fall
colors are warm, sunny days, and cool, clear evenings. Drought
conditions can place the trees under stress. One reaction to that
stress is a slowing or cessation of active growth. Stress can even
cause trees to begin the process of dropping their leaves for the
year. Consequently, fall colors often develop a little earlier
than normal in drought years. Extended periods of cool, wet weather
will decrease the intensity of the color display. Strong winds
or heavy snows could put an early end to the fall color season.
The early season color change allows fall color viewers to observe
a unique characteristic of aspen growth. Aspen propagate primarily
by sprouting from an expanding root system, creating groups of trees,
or clones, ranging in size from several trees to many acres. These
clones are genetically identical. One factor in how and when a
tree changes color is the balance of various chemicals in the plant.
This balance is, to some degree, genetically determined and varies
between clones. Because of these differences, it is possible to
see hillsides with one small group of trees that have already changed
to vibrant gold colors standing among otherwise green aspen. This
color change allows viewers to readily see individual aspen clones.
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