Desert Shrublands and Playas - Mountains and Plains: The Ecology of Wyoming Landscapes - page 140

sensitive to drought, with dramatic annual variations
in plant cover associated with spring precipitation and
the abundance of a scale insect that feeds on roots . 33
Drought is not the only explanation offered for des-
ert shrub mortality. Between 1977 and 1986 the shrubs
over large areas in the Great Basin of Utah died, appar-
ently stemming from four factors triggered by several
years of unusually high precipitation: (1) frequent
periods when the soil was anaerobic, (2) increased soil
salinity, (3) increased susceptibility of shrubs to soil-
borne diseases, and (4) the loss of shrub tolerance to
the droughts that occurred after the wet period ended . 34
thus far, a similar dieback of desert shrubs has not been
reported for Wyoming.
excessive livestock grazing has been a common
disturbance in most desert shrublands . 35 cattle drives
first started on the Great Plains, where the rangelands
were adapted to the grazing of large mammals because
of large numbers of bison there. Moving westward, the
cattle foraged in big sagebrush steppe, which also had
considerable grass cover and a long history of grazing by
bison. Sometimes the concentrations of cows and sheep
were much higher than livestock managers would allow
today, and without careful herding, the same numbers
of animals easily wandered into the adjacent desert
shrublands, causing dramatic changes. 36 Some of the
palatable plants declined with this added grazing pres-
sure, while others increased . 37 Recovery after reducing
the numbers of livestock was very slow because of the
arid environment.
one of the first adverse effects of livestock graz-
ing was probably the degradation of delicate biologi-
cal soil crusts, where they occurred, by trampling (as
discussed in chapter 7) . 38 When this happens, soil ero-
sion increases, nitrogen fixation may be reduced in
some years, and favorable sites for seed germination
and seedling establishment can be lost. considerable
research has been done on the importance of the crusts
for desert shrubland ecosystems in the Great Basin, but
their importance in Wyoming desert shrublands is still
uncertain. today they are most easily found in the Big-
horn Basin, in places where grazing has been minimal.
Were the crusts less common in Wyoming to begin
with, because of a higher cover of perennial grasses
between the shrubs and heavier grazing by bison? An
answer is not possible at this time. 39

in addition to the loss of soil crusts, introduced
annual plants have greatly affected some desert shrub-
lands, just as they have some shrublands dominated by
sagebrush (see fig. 7.13). three common weeds are cheat-
grass, halogeton, and Russian thistle . 40 of the three,
cheatgrass is by far the most invasive and has become
a major problem in western intermountain basins (see
chapter 7). Some varieties of the species are adapted to
tolerate saline soils and can invade even with little or no
livestock grazin g. 41 As in sagebrush, cheatgrass typically
grows so densely in favorable years that fires burn in
places where previously they could not because of in-
sufficient fuel. And the fires destroy the patches of soil
crust that might still persist. Arguably, cheatgrass is the
most severe problem facing desert shrubland managers
in Utah, nevada, and southern idaho, and with climate
change and ongoing evolution, it could become a severe
problem in Wyoming as well. Douglas Shinneman and
William Baker compared many sites in the Great Basin
states, 42 finding more rapid cheatgrass invasion where
soil crust cover had declined—another indication that
managers must consider microbial organisms along
with large animals, fire, and the dominant plants as
they strive to achieve sustainable land management.
though not yet reported for Wyoming, an unusual
effect of cheatgrass invasion was to convert a shrubland
in eastern Washington dominated by big sagebrush to
one dominated by greasewood. the following mecha-
nism was proposed by William H. Rickard: (1) a high
cover of cheatgrass led to the interception and use
of more of the annual precipitation, resulting in in-
adequate percolation of water to the depths required by
big sagebrush; (2) fire frequency was increased by the
flammable cheatgrass, but greasewood (already present
in the community) persisted because it could sprout,
unlike big sagebrush; and (3) greasewood caused an
increase in surface soil salinity, which big sagebrush
seedlings could not tolerate . 43 Similar changes could
occur in Wyoming if cheatgrass were to become abun-
dant where big sagebrush and greasewood coexist.

Scenario for the Future

extrapolating from what is known about desert eco-
systems, a warmer climate would increase potential
evapotranspiration and rates of soil drying. there could