Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 189

the absence of disturbance. in Wyoming, Douglas-fir is
most common in the western half of the state, where it
occurs just below the lodgepole pine forest zone. it is also
abundant on the east slopes of the colorado Front Range,
and in adjacent idaho and Montana, such as to the west
and north of Yellowstone national Park (fig. 11.10).
As with ponderosa pine forests, tree density and
understory cover in Douglas-fir forests vary with local
environmental conditions and disturbance frequency
and severity, ranging from sparse, savanna-like stands to
closed-canopy stands. Historical fire regimes were simi-
lar to those in ponderosa pine forests, with fire intervals
perhaps somewhat longer and high-severity fire perhaps
more common in Douglas-fir, especially on wetter sites. 63
Large stand-replacing fires occurred in years when cli-

matic conditions were very dry. For example, widespread
fires in 1856 burned Douglas-fir forests in portions of
Jackson Hole, northern Yellowstone national Park, and
the centennial Valley just west of Yellowstone Park . 64
insects can also cause variation in forests of Doug-
las-fir, especially the Douglas-fir beetle and the western
spruce budworm (figs. 11.11 and 11.12). the bark beetles
kill the larger trees, whereas the budworm kills mainly
smaller individuals. Rarely do the insects kill all trees
in a stand . 65 Mixed-age Douglas-fir stands develop on
dry sites or where fires or insects have caused only par-
tial mortality of the canopy; even-aged stands (that is,
those made up of trees that are roughly the same age)
also develop, especially after high-severity fires or other
disturbances that kill all the trees . 66

Fig. 11.11. Douglas-fir woodland in northern Yellowstone
national Park, 15 years after a severe outbreak of Douglas-fir
bark beetle. the beetles killed most of the larger trees in the
stand, which stimulated the growth of small trees, shrubs,
and herbaceous plants. notably, in contrast to lodgepole pine,
Douglas-fir snags tend to break off rather than uproot. Photo
by Dan Donato.

Fig. 11.12. Larvae of western spruce budworm, a moth, com-
monly attack the buds and young needles of Douglas-fir, as seen
here in northern Yellowstone national Park in 2012. this bud-
worm also attacks white fir in colorado, Utah, and idaho, and,
to a lesser extent, engelmann spruce. it is said to be the most
widespread forest defoliator in western north America (Leather-
man et al. 2009). tree growth is slowed, but unlike bark beetles,
the western spruce budworm usually does not kill its host.