Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 185

causing beetle-attracting odors to concentrate. Beetles
are then attracted to such forests from a larger area, and
with high beetle densities, even the more vigorous trees
are susceptible to invasion. 43 Under such conditions,
beetle populations increase rapidly in one year. if the
forest is opened up by whatever means, the odor plume
is dispersed and becomes less effective. Beetle parasites
and beetle predation by birds may also be factors that
reduce bark beetle populations.
once an extensive outbreak is under way, the thou-
sands of adult beetles emerging from infested trees can
overcome the defenses of healthy trees—even some rel-
atively small ones—as well as those of unhealthy trees. 44
the healthy hosts provide a superior food resource for
the beetle larvae; hence, even more new adults emerge
the following year. When conditions are especially
favorable for the beetles, the outbreak may be so exten-
sive that it becomes a regional or even continental phe-
nomenon, as occurred in western north America during
the late 1990s and early 2000s. 45 A n outbreak usually
ends only after a cold winter kills many larvae or when
susceptible trees are no longer available—either because
the conditions causing host-tree stress have been ameli-
orated in the area or because the beetles have simply
killed so many trees of suitable size. the last mountain
pine beetle outbreak in northern colorado and south-
ern Wyoming apparently subsided because the beetles
could no longer find enough suitable trees; tempera-
tures cold enough to terminate the outbreak were not
recorded.
An important feature of the disturbances caused
by most bark beetle species is that nearly all mortality
occurs in the larger trees, namely, those greater than
about 6-8 inches in diameter. Most smaller trees and
saplings usually survive, even where the beetles kill
most of the large trees in the overstory. For example,
severe mountain pine beetle outbreaks in lodgepole
pine forests of southeastern Wyoming and northern
colorado during the 1990s and 2000s reduced the can-
opy by 10 to more than 70 percent in many stands. 46
nevertheless, even in the most severely affected stands,
the smaller trees that were not attacked were generally
numerous enough to meet or exceed U.S. Forest Service
requirements for stand regeneration. Also, less competi-
tion from older trees enables the establishment of new
tree seedlings. 47

With less competition for light, water, and nutrients
following the death of overstory trees, the understory
trees often grow much faster than they did before the
outbreak. For example, height growth rate of many sur-
viving lodgepole pine and subalpine fir trees doubled
after the mountain pine beetle outbreak in a northern
colorado study area, and radial growth rate increased
two- to threefold after a mountain pine beetle out-
break in the Greater Yellowstone ecosystem . 48 Modeling
studies suggest that this rapid growth of the surviving
understory can restore pre-outbreak levels of basal area
within about 80 years . 49 Recovery of basal area may be
even faster under some conditions, as observed after a
mountain pine beetle outbreak in colorado in the 1980s.
ninety percent of the basal area that had been present
prior to the outbreak had regrown after only 30 years in
some lodgepole pine stands. 50 in forests where lodgepole
pine dominated the pre-outbreak overstory but subal-
pine fir and engelmann spruce were more abundant in
the understory, the composition of the post-beetle forest
shifts toward more fir and spruce; however, where lodge-
pole pine dominated both the overstory and the under-
story, lodgepole continues as the dominant specie s. 51

Major Forest Types

Rocky Mountain forests are easily classified into sev-
eral major types (see table 11.1), each of which can be
further subdivided. the most thorough classification
was done using Rexford Daubenmire's habitat-type
approach, where forests are classified according to two
factors: (1) the tree species that would dominate a stand
after a very long time without disturbance, when the
vegetation has presumably equilibrated with prevail-
ing climate and soils, and (2) one or more understory
plant species that are indicators of other environmental
conditions. this classification assists in evaluating sites
for wood production, wildlife habitat, and other forest
values. 52

Ponderosa Pine Forest

Ponderosa pine is the most widespread conifer in the
western United States and is a major tree species at lower
elevations throughout its range. in Wyoming it grows
primarily in the eastern portions of the state, where