Desert Shrublands and Playas - Mountains and Plains: The Ecology of Wyoming Landscapes - page 139

under water stress. infiltration rates vary considerably
and appear to influence the distribution of different
plants (see fig. 8.7). nutrient leaching is rare, but low
levels of nitrogen can limit plant growth . 27 nitrogen fix-
ation results from species of cyanobacteria, bacteria, or
lichens found in biological soil crusts, but atmospheric
inputs of nitrogen may be more important. 28 nutrient
losses are of little consequence, primarily because water
is usually the primary limiting factor, but also because
most nitrogen requirements for new plant growth prob-
ably are met by mineralization of soil organic matter
and reabsorption from senescing leaves. As in grass-
lands and other shrublands, herbivores and detritivores
increase the rate of nutrient cycling (see chapter 6), and
the presence of shrubs creates islands of fertility (see
chapter 7).
except for the effects of livestock grazing, relatively
little is known about plant-animal interactions in des-
ert shrublands, compared to grasslands and sagebrush-
dominated shrublands. Jackrabbits are thought to prefer
winterfat for food but use nearby patches of big sage-
brush for hiding cover (fig. 8.10). in one study, winter-
fat nearest the sagebrush was subject to heavier grazing
by the hares, which in turn facilitated the expansion
of big sagebrush. in this way the jackrabbits influenced
the vegetation mosaic, at least during years when their
numbers are high. Some investigators believe that jack-
rabbits in large numbers can graze rangelands more
heavily than livestock or big game . 29
in addition to jackrabbits, townsend's ground squir-
rels can be abundant in desert shrublands during some
years, providing prey for raptors. 30 in southern idaho,
the rodent populations drop significantly when desert
shrublands are converted to communities dominated
by annuals, such as cheatgrass.
Little research has been done on the hydrology,
energy flow, and nutrient cycling of Wyoming desert
shrublands, but some interesting opportunities exist.
For example, just as it is helpful to consider the effects
that the presence of big sagebrush has on ecosystem
properties (see chapter 7), it also is interesting to con-
sider the effects of salinity and selenium. Does nitrogen
become more or less available as salinity increases? Are
soils high in sodium more susceptible to erosion? How
do patterns of herbivory and decomposition change, if
at all, because of higher salt concentrations in the halo-

Fig. 8.10. White-tailed jackrabbit in the Red Desert, near
Adobe town. Photo by Ken Driese.

phytic plant biomass? in the case of greasewood shrub-
lands, with finer-textured soils that are wet and possibly
anaerobic during much of the year, is most herbivory
still belowground, as in grasslands and some shrub-
lands? How do high concentrations of selenium affect
microbial activity?

Disturbances and Succession

the major disturbances in desert shrublands are
drought and extraordinarily heavy herbivory, whether
from grasshoppers, Mormon crickets, bison, or live-
stock. Fires are less frequent in more arid environments
because of insufficient fuel, but they surely occur in
greasewood and mixed desert shrublands during years
when fuel accumulates, especially after periods of rela-
tively high rainfall that coincide with light grazing, or
where cheatgrass has invaded (see chapter 7). Harvester
ants are abundant in some saltbush shrublands, creat-
ing patchy disturbances , 31 and sometimes two years or
more of drought will cause portions of a shrub to die,
if not the entire plant. 32 S hadscale is apparently very