Geoscience Reference
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which more snow drifts during the next storm. in this
way, the shrubs facilitate the percolation of meltwater
deeper into the snowpack or possibly into the soil (if it's
not frozen), reducing the amount that is lost to subli-
mation while creating the potential for additional snow
accumulation as the snow melts around the shrub tops.
the depressions around the shrubs can be created and
filled several times during a winter, augmenting mois-
ture input above that to be expected if the shrubs were
not there. this phenomenon could be especially impor-
tant where the vegetation is more or less uniformly
covered with snow except for the tops of shrubs—a
common occurrence.
As noted, big sagebrush has roots that are deeper
than those of most grasses and forbs. 29 As a result, more
water is used by plants when the shrub is present. if the
shrubs are removed by fire or other means, total water
consumption by the plants (transpiration) is less, per-
haps by 15 percent, 30 but total plant growth probably is
lower as well—as would be expected based on the cor-
relation of plant growth with evapotranspiration in arid
climates (see fig. 6.5). closer to the mountains, where
snowfall is higher, less transpiration may allow for some
runoff as streamflow.
if a fire occurs. Big sagebrush-dominated ecosystems
burn from time to time, and because the sagebrush
itself cannot sprout, nutrient and water resources are
made available to grasses and forbs for 10 years or
more. As herbaceous plants become more abundant,
the carrying capacity for bison and cattle increases. Per-
haps the optimal habitat for some animals is a mosaic
of old-growth sagebrush intermingled with recently
burned sagebrush. Before the arrival of euroAmericans,
the steppes dominated by sagebrush might have been
more varied where periodic fires were possible, creat-
ing patches of grassland and young sagebrush inter-
mingled with old-growth sagebrush. today, sagebrush
cover might be more uniform and older in some places
than previously, the result of fire suppression and more
extensive livestock grazing. 32 However, elsewhere, sage-
brush landscapes have been heavily fragmented, as dis-
cussed later. Because of threatened species that depend
on sagebrush, the practice of prescribed burning in this
ecosystem may not be advisable in some areas. 33
total annual plant growth is enhanced by the pres-
ence of big sagebrush, in part because of its deeper root
system but also because its green leaves persist through-
out the year, which lengthens the growing season. 34
Sagebrush growth tends to be greatest in the fall and
early spring, whereas the growth of associated grasses
and forbs is more rapid in late spring and early sum-
mer. Fall-through-spring precipitation and temperature
are good predictors of total annual growth of sagebrush
and associated plants. 35
the flow of carbon through ecosystems parallels
the flow of energy, because so much energy is stored
as carbon-rich compounds—that is, organic materials,
such as cellulose and other carbohydrates. they are
important sources of food for large and small herbivores,
aboveground and below. Moreover, animal carcasses
along with plant remains that do not burn become part
of the mulch and soil organic matter—both of which
increase water infiltration and nutrient availability. Big
sagebrush tends to concentrate such materials around
its base more than do herbaceous plants, creating a
small-scale island of fertility that can persist even after
sagebrush is burned or removed in some other way. 36
this patchiness in the surface soil helps promote sage-
brush ecosystem recovery after disturbances. in general,
mineland reclamation specialists have found that con-
Energy, Carbon, and Soil Organic Matter
the food web of sagebrush-dominated shrublands is
similar to that of grasslands, with probably more than
half the herbivory and carnivory occurring in the soil.
However, less than half the annual precipitation comes
during the summer, which limits the success of warm-
season c 4 plants, such as blue grama. Much of the plant
growth is concentrated in the less-palatable big sage-
brush, which competes for moisture that could be used
by other forage plants, namely, cool-season grasses.
consequently, the forage available for large ungulates
might be less than on the grasslands, especially those
on the Great Plains where the plants have evolved with
more bison herbivory (see chapter 6). 31 Sagebrush is
known to be an important food for pronghorn, mule
deer, and elk, but primarily during the winter, when
herbaceous forage is scarce or buried by snow. Bison rely
on grasses and forbs more than shrubs, as do cattle.
Just as in other ecosystems, the energy stored in
stems and leaves aboveground can be released as heat
 
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