Escarpments and the Foothill Transition - Mountains and Plains: The Ecology of Wyoming Landscapes - page 156

usually snow-free, providing easier access to forage. in
such places, shelter and forage are available nearby—
unlike the mountain forests, where there is abundant
shelter but little winter forage (owing to continuous
snow cover), or the surrounding lowlands, where both
shelter and forage may be hard to find.
the vegetation mosaic of escarpments and foothills
consists of shrublands, grasslands, woodlands, and
windswept ridges (fig. 10.2). Shrublands dominated
by mountain-mahogany are abundant on many rock
outcrops with little or no soil. on deeper soils where
snow accumulates, the common plants are antelope
bitterbrush, mountain big sagebrush, serviceberry,
skunkbush sumac, and snowberry (table 10.1). Black
sagebrush or threetip sagebrush commonly occur on
windswept ridges or plateaus with shallow soils, and
tufted hairgrass and other species sometimes are
found in leeward arroyos where unusually deep snow-
drifts develop, sometimes called nivation hollows (see
fig. 7.8). Foothill grasslands support a sparse cover of
bluebunch wheatgrass along with other grasses and
forbs. the woodlands have varying densities of juni-
per, limber pine, ponderosa pine, aspen, Douglas-fir,

and—in one or two localities in extreme south-central
Wyoming—pinyon pine or Gambel oak. Windswept
ridges with shallow soils and little snow accumulation
have bluebunch wheatgrass and a variety of cushion
plants (see fig. 8.9).
in general, the foothill-escarpment mosaic is patchy,
a reflection of abrupt changes in rock type, soil depth,
snow accumulation, microclimate, slope, and the in-
verse texture effect. Vegetation banding is sometimes
observed, such as in the centennial Valley, west of Lara-
mie, where different kinds of vegetation are associated
with bands of sandstone, limestone, shale, siltstone,
and claystone. 2 Similarly, elongated aspen groves can
be found at the contact between impervious granite in
the mountains and the overlying sedimentary strata of
the foothills, where runoff from the mountain encoun-
ters deeper soils (fig. 10.3). trees also grow on ridges,
sometimes because the groundcover there is usually in-
sufficient to fuel a fire. thus, trees have a greater chance
of becoming established than in the adjacent grassland
or sagebrush steppe, where the competition for water
from grasses and forbs is more intense and where the
time between fires is shorter . 3

2800

9000

Lodgepole pine,
Douglas-fir, or
Ponderosa pine
forest

2600

Limber
pine
woodland
Juniper
woodland

8000

2400

2200

7000

Mixed-
foothill
shrubland
or
Ponderosa
pine
savanna

Basin or
Foothill
grassland

Sagebrush
steppe

2000

6000

1800

1600

Saltbush
desert
shrubland

Sagebrush
desert
shrubland

Mixed-
grass
prairie

5000

Greasewood
shrubland

Woody
draw

1400

Riparian
woodland

4000

1200

Low Moderate High

WATER AVAILABILITY

Fig. 10.2. Distribution of various kinds of foothill grasslands,
shrublands, and woodlands in relation to gradients of water
availability and elevation in the Bighorn Basin, near the

Bighorn Mountains (Bighorn canyon national Recreation
Area). Vegetation types that occur above and below the foot-
hills are shown for comparison.