 Sagebrush
Steppe.
Two sagebrush steppe classes are distinguished on
the vegetation map:
Sagebrush Steppe on Lava and Sagebrush Steppe off
Lava.
The distinction is largely a consequence of
differences in soil reflectance rather than vegetation.
Soils overlying the basaltic uplands of the
southern two-thirds of the INEEL tend to be darker than
those of sagebrush-dominated communities off lava because
of the presence of dark-colored pebbles or sand on the
surface or because small patches of bare lava are exposed.
On the broken, irregular basalt topography, the
soil surface often is sandy and friable and less likely to
form the platy vesicular crusts that characterize many
areas off the basalt flows.
This may influence species composition.
For example, cheatgrass
has invaded many of the communities on the basalt flows,
but is rare or absent from some of the sagebrush dominated
areas off the flows.
Soils in areas where cheatgrass is absent tend to
be slightly saline and to form thick surface crusts.
Distinguishing between these two classes of
sagebrush steppe also makes it easy to identify the extent
of upland areas where shallow soils overlay relatively
recent basalt flows.
One must keep in mind, however, that the
classification is not completely accurate;
some sagebrush communities on the basalt flows may
be classified as “off lava” and vice versa.
Sagebrush steppe may be dominated by
either Wyoming big sagebrush or basin big sagebrush, or by both.
The differential distributions of the two subspecies are
related to gradients of soil texture (Shumar and Anderson 1986).
Basin big sagebrush occurs on sandy soils that are usually deep
and well drained, whereas Wyoming big sagebrush tends to occur on
fine-textured, shallow soils having limited depths of water
infiltration. Wyoming
big sagebrush is the most abundant sagebrush at the INEEL, but
extensive patches of basin big sagebrush occur on the eastern
basalt uplands (Shumar 1983).
Pockets of basin big sagebrush within areas generally
dominated by Wyoming big sagebrush occur on the lee sides of lava
ridges where sand accumulates, within former channels of the Big
Lost River, and on the linear sand dunes near the Mud Lake basin.
The influence of these linear dunes is readily apparent on
the vegetation map.
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Some areas of
sagebrush steppe are dominated by one of the species of “low
sage,” black sagebrush (Artemisia
nova) or Artemisia
arbuscula. Black
sagebrush typically occurs on shallow, stony, calcareous or
limestone-derived soils (Shultz 1983, Hironaka 1979), although it
also occurs on dry, windswept ridges and on shallow soils
overlying basalt. A. arbuscula also occurs on shallow soils, but it is more of an
upland species, often found on foothill slopes.
Aside from the
dominance of sagebrush, the sagebrush steppe communities do not
have a unique species composition.
However, most sagebrush steppe has an abundance of
perennial grasses, and the combined cover of perennial grasses can
approach that of the sagebrush.
These are the same grasses that occur in the grasslands
described earlier. In
fact, the only apparent difference between many areas classified
as sagebrush steppe and others classified as grasslands is the
density of sagebrush. In
some areas, this reflects fire history.
Sagebrush is killed by fire and must recolonize burned
sites from seed. Most
perennial grasses and forbs resprout following fire from roots or
other organs that are protected below ground.
Therefore, these herbaceous species can become the
dominants following fire and it may be decades before sagebrush
again reaches a high density.
Such areas likely will be classified as grasslands.
Shrubs other
than sagebrush occur in most of these steppes. Green
rabbitbrush is essentially ubiquitous;
other common shrubs include winterfat,
prickly phlox, and spiny
hopsage.
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