Escarpments and the Foothill Transition - Mountains and Plains: The Ecology of Wyoming Landscapes - page 162

rangeland ecologist W. J. Waugh found that juniper
in the Bighorn Basin had been restricted to limestone
outcrops until about 10 years after the introduction of
large herds of cattle—a time that also marks the initia-
tion of fire exclusion. Juniper has since expanded onto
adjacent sites. Waugh noted that more than 90 percent
of the juniper seedlings were becoming established
under sagebrush, which apparently was functioning as
a nurse plant, and he hypothesized that an increase in
sagebrush cover following livestock grazing had created
a more favorable environment for juniper invasion. 13
Another important indirect effect of livestock graz-
ing is to reduce the fine fuels available for fire. Fires
at 30- to 60-year intervals formerly prevented juniper
expansion in parts of the Great Basin of Utah, and
observers have noted that burning can reduce the abun-
dance of sagebrush and juniper. 14 n either juniper nor
big sagebrush reproduce by sprouting, and woodlands
with old junipers are often restricted to rocky ridges,
where fires are less frequent. Heavy livestock grazing
could also reduce the rate of evapotranspiration from
the rangeland ecosystem, which would reduce competi-
tion for water from other plants. Moreover, in combi-
nation with an unusually wet period, livestock grazing
could enable the establishment of a large number of
juniper seedlings. However, this scenario may not apply
everywhere, as other observers have observed that
drought, in combination with grazing and fire exclu-
sion, favors juniper invasion . 15
Although fire exclusion and grazing are important
in some places, some young juniper stands represent
natural recovery by seedling establishment following
past fires or other disturbances of mature woodlands . 16
near the edges of the current ranges of both species,
pinyon pine and the two junipers may still be migrating
into suitable habitats from their Pleistocene refugia. For
example, some juniper stands in Wyoming may repre-
sent the first generation to colonize the sites since the
last glacial advance. 17 infill is also occurring in many
stands that contain no evidence of past fire, that is, no
basal fire scars on living trees and no charred wood. 18
Some of these woodlands have also been grazed only
lightly or not at all. in such places, fire exclusion and
grazing cannot be the drivers of infill. climate variation
probably is more important. Several warm, moist peri-
ods during the 1900s were especially favorable for tree

establishment in many parts of the West. 19 increasing
atmospheric carbon dioxide may also be facilitating tree
growth by increasing water-use eficienc y. 20 For what-
ever reason, woodland expansion and infill are occur-
ring in many parts of the West, though the causes may
not be the same everywhere.
Regardless of the mechanism, herbaceous plant
growth is greatly reduced with the establishment of
juniper in a grassland or shrubland, owing to more
competition for water, light, and nutrients. For a time,
fires may become less frequent if the shrubs are widely
spaced, thereby perpetuating conditions favorable for
juniper infilling. However, eventually fuel continuity
increases and fires become more probable. Prescribed
fires are often used to reduce juniper density, but man-
agers know that an undesired consequence could be
the invasion of introduced plants, such as cheatgrass
(see chapter 7). Another concern is that burning could
reduce the amount of food and thermal cover avail-
able for deer and other wildlife. Animals stay warmer at
night under the juniper canopy because of less exposure
to the cold sky. Daytime shade during the summer also
is important.
in northwestern colorado, on the colorado Plateau,
extensive woodlands are dominated by pinyon pine
and Utah juniper. 21 the two species coexist, but pin-
yon pine commonly is found at slightly higher eleva-
tions than Utah juniper—suggesting that the juniper
is more drought tolerant than the pine. Pinyon pine is
found in only one location in Wyoming, near Flaming
Gorge Reservoir in the foothills of the Uinta Mountains.
Rocky Mountain juniper is common, often occurring
with limber pine and ponderosa pine in the foothills. 22

Ponderosa Pine-Limber Pine-Douglas-fir Woodlands

Ponderosa pine, limber pine, and Douglas-fir occur com-
monly in the foothills and on escarpments (fig. 10.8).
Growing in association with grasslands, juniper, and
various kinds of shrubland, some of the tree-dominated
woodlands have been increasing in density and expand-
ing their range—possibly as a result of fire exclusion,
although climate conditions in the twentieth century
have been favorable for these tree species just as for juni-
per and pinyon . 23 i increasing densities of ponderosa pine
can pose problems for ranchers, primarily because the