Sagebrush - Mountains and Plains: The Ecology of Wyoming Landscapes - page 120

Recovery after Disturbances

western Wyoming and neighboring states. 53 in contrast,
shrub dieback during the mid-1980s in the Great Basin
was attributed to unusually wet conditions that persisted
for 5 years. 54 Shrubs that were affected included antelope
bitterbrush, big sagebrush, fourwing saltbush, rabbit-
brush, shadscale, and winterfat.
extreme, ecologically significant weather events are
likely to become more frequent with climate change.
Higher mean annual temperatures will lead to less snow
accumulation and earlier snowmelt, and also a higher
potential for evapotranspiration throughout the year.
Because of added warmth, drier conditions for longer
periods during the summer could result even if annual
precipitation stays the same. if plants begin grow-
ing earlier in the spring, late-spring frosts could kill
some of them. Predicting precipitation changes with
global warming is currently difficult, but if droughts
are more severe, some shrublands now dominated by
sagebrush could become dominated by more drought-
tolerant shrubs or possibly grasslands. 55 Silver sagebrush
might be affected most severely, but big sagebrush also
depends on higher levels of precipitation than do most
grassland species. Sagebrush reinvasion after distur-
bances probably would be slowed, if it occurs at all,
because the extended cool, moist conditions required
for seedling establishment may become less frequent.
Drought-tolerant invasive species, such as cheatgrass,
may have a better chance of becoming established.

Big sagebrush is easily killed because it cannot sprout,
but historically it re-established after disturbances.
the rate of recovery varies considerably in relation to
the amount of precipitation, the amount of litter on
the soil, the degree of competition from herbaceous
plants, the intensity of grazing, the number of sage-
brush seeds in the soil, the number of live shrubs that
remain, and the kind of big sagebrush on the site. 46
Mountain big sagebrush has been observed to re-
establish after about 40 years, whereas Wyoming big
sagebrush takes much longer. 47 Without big sagebrush,
the ecosystem appears and functions like a grassland,
with junegrass, needle-and-thread grass, western
wheatgrass, Sandberg bluegrass, and numerous other
species. in relatively moist environments—and in the
absence of cheatgrass—sagebrush cover becomes quite
high as the shrubs age, persisting until the next fire or
drought. 48
even without planting, managers have come to
expect that big sagebrush and other native plants will
eventually invade areas where the plants have grown
before. that happened, for example, on abandoned
farmland in the Powder River Basin. 49 interestingly, the
establishment of big sagebrush during mineland recla-
mation is often difficult. Some research suggests that
survival is higher when mycorrhizal fungi are available
in the topsoil and when the recommended seeding rate
of perennial grasses is not too high. 50

Grasshoppers

Drought, Spring Frost, and Extended Wet Periods outbreaks of grasshoppers killed approximately 50 per-

cent of the big sagebrush in a portion of the Powder
River Basin during the drought years of the 1930s.
Allred wrote, “the grasshoppers swarmed in such hordes
that they devoured all of the edible vegetation, ate the
leaves and bark from the twigs of the sagebrush, and
completely girdled the more tender stems.” 56 other
insects known to kill big sagebrush include gall midges,
the aroga moth, some beetles, and Mormon crickets.
Small mammals (such as voles) may girdle the stems of
big sagebrush, causing a significant disturbance when
their populations are high. 57
Grasshopper outbreaks tend to occur during drought
years that are relatively warm. thus, continued warm-

Unusual weather can kill shrubs, though the degree of
disturbance depends on the species. in a study in eastern
Montana, the shoots of silver sagebrush were more sus-
ceptible to drought than were those of big sagebrush, but
the silver sagebrush recovered more rapidly because of its
ability to sprout from the root crown. 51 Winter mortality
of sagebrush can occur as well, probably from water stress
created by frozen soils, low soil water availability, and
below-average snowfall. 52 Another cause may be the pre-
mature breaking of dormancy during periods of above-
average air temperature, making the plants susceptible to
subsequent frosts. extensive areas of mountain big sage-
brush have been killed by spring frost damage in south-