Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 197

Fig. 11.20. Aspen groves in
Wyoming tend to be small,
except on the southwestern
slopes of the Sierra Madre,
east of Baggs. Here the aspen
grow over large areas, similar
to the aspen farther south in
colorado and new Mexico.
Patches of Gambel oak also
are found in this area. note
that some of the aspen clones
are still green (see text for
explanation).

because there is a lack of seed but because the environ-
mental requirements for establishment are rarely met . 96
Aspen seedlings are delicate and cannot tolerate even a
brief period of desiccation. they often die before their
roots can penetrate through forest floor litter to the
mineral soil. Also, they may be unable to compete with
other plants for limited water. Whatever the mecha-
nisms that limit seedling establishment in undisturbed
forests, aspen seedlings have been observed in recently
burned forests where mineral soil is exposed and poten-
tially competing plants have been temporarily reduced,
such as in Yellowstone national Park after the 1988 fires
(see chapter 15). Seedlings also have appeared on sedi-
ments exposed in abandoned beaver ponds.
With sprouts as the principal form of regeneration,
nearly all groves develop as a genetically uniform clone
or a mix of several discrete clones. essentially, what
nearly everyone refers to as a tree is more appropri-
ately termed a branch or shoot. Botanists use the term
ramet. Although ramets typically die after about 100
years, the plant itself—the genet —lives for much longer,
most likely for centuries. thus, more genetic variabil-
ity exists between aspen groves than within one grove,
as is apparent when, for example, two adjacent clones
lose their leaves at different times in the fall. of course,

the ramets of some clones (genets) may intermingle . 97
Genetic diversity undoubtedly explains why aspen is
found over such wide environmental gradients in the
Rocky Mountain region and, in fact, why it is the most
widespread tree in north America.
Because of aspen's ability to sprout, it dominates
temporarily after severe disturbances in many conifer-
ous forests, most likely because the sprouts have more
stored energy available for rapid growth than do conifer
seedlings. More than 10,000 sprouts per acre may reach
a height of 3 feet or more within a year following a fire.
conifers, especially subalpine fir, sometimes persist
in the understory and eventually overtop the aspen—
because its seedlings become established on the aspen
forest floor more readily than do those of other trees
(see fig. 11.19). Lodgepole pine and engelmann spruce
also replace aspen under some condition s. 98
Where other trees do not become established, aspen
continues to dominate stands indefinitely, with new
sprouts replacing the senescing older stems. Generally,
most aspen sprouts do not survive in the understory
shade of aspen—an interesting anomaly, as it would
seem that the small trees could tap the energy stored
by the larger trees (which have leaves fully exposed to
sunlight). Plant hormones produced by the dominant