Sagebrush - Mountains and Plains: The Ecology of Wyoming Landscapes - page 112

Rawlins, black sagebrush is found on windswept ridges
or slopes with shallow soils and very little snow. on
lower slopes with more snow accumulation and deeper
soils (but minimal salt accumulation), Wyoming big
sagebrush occurs. Mountain big sagebrush grows in
moist ravines with deep snow. Desert shrublands (see
next chapter) occur in dry basins, where snow depth is
moderate and salt accumulation is high. 9 Where snow
accumulation is exceptionally deep, only grasses and
forbs can survive (fig. 7.8). clearly, sagebrush distribu-
tion in the landscape depends on soil moisture, salin-
ity, depth, and texture, all of which vary over short
distances because of wind and topography.
Wyoming big sagebrush sometimes occurs in oval-
shaped patches 10-50 feet across. commonly referred
to as sagebrush islands (fig. 7.9), some are simply patches
of tall sagebrush surrounded by shorter sagebrush.
Most are patches of sagebrush in a matrix of grassland
or desert shrubland. one type of island forms where
small silt dunes or sand dunes develop on the lee side
of taller shrubs. these dunes could have been initiated
by a period of wind erosion, possibly after heavy graz-
ing by bison or livestock during a dry period. Where
an occasional shrub managed to survive or a new one
became established, sand and silt accumulated, creat-
ing conditions favorable for the growth and establish-
ment of other shrubs. With time, a larger number of
taller shrubs increased the deposition of windblown
particles, including snow, resulting in the develop-
ment of small dunes that are 6 feet deep or more. Such
dunes are known as coppice dunes and typically have
spiny hopsage, rubber rabbitbrush, green rabbitbrush,
and sometimes basin wildrye in addition to Wyoming
big sagebrush. Spiny hopsage is an indicator of sandy,
relatively moist soils in the semi-arid intermountain
basins (due to the inverse texture effect, explained in
chapter 3). A desert pavement is sometimes found sur-
rounding the islands, often with scattered plants of bud
sagewort. 10
elsewhere, the sagebrush islands are not associated
with windblown material and are less easily explained.
in such places, the islands typically have numerous
small mammal burrows, which can disrupt shallow
hardpans or create soil conditions in other ways that
favor infiltration of water and taller shrub growth. Per-
haps the animals are attracted by the shade provided by

Alpine sagewort

Mtn silver
sagebrush

10000

Mtn big
sagebrush

Threetip
sagebrush

9000

Wyoming big
sagebrush

8000

Black
sagebrush
or
Bud
sagewort
or
Birdfoot
sagewort

7000

Basin
big
sagebrush
or
Silver
sagebrush

6000

5000

Ravine
or
flood-
plain

Ridge
or
dry
basin
MOISTURE GRADIENT

North
or
east
slope

South
or
west
slope

Fig. 7.4. Approximate distribution of different species of
Arte misia (sagebrush and sagewort) in relation to elevation
and topographic position.

Silver sagebrush, black sagebrush, and low sagebrush
are also common. Silver sagebrush has two subspecies,
plains silver sagebrush—found on relatively moist sites
at low elevations, especially east of the continental
Divide—and mountain silver sagebrush, which is com-
mon in moist meadows in the mountains throughout
the region (figs. 7.5 and 7.6). Unlike most woody species
of Artemisia, silver sagebrush is capable of sprouting from
the root crown following fire or other disturbances. 7
Black sagebrush is usually less than 18 inches tall and is
found on relatively dry soils associated with the slopes
of escarpments or where a shallow hardpan of calcium
carbonate (caliche) develops. Similarly, low sagebrush,
which is usually less than 10 inches tall, forms patches
on shallow soils interspersed with patches of big sage-
brush, such as in the lowlands of Jackson Hole and on
the Sweetwater Plateau. in the Bighorn Basin and other
areas of low precipitation, shallow soils commonly have
grasslands (fig. 7.7). 8
Another factor influencing the shrubland mosaic is
snow drifting. For example, in the foothills south of