Escarpments and the Foothill Transition - Mountains and Plains: The Ecology of Wyoming Landscapes - page 157

Fig. 10.3. Aspen groves on the east slopes of centennial
Ridge, in the Medicine Bow Mountains, occur in most ravines
and along the contour where the impermeable granite meets
the permeable soils that have developed on sedimentary
rocks. Sagebrush steppe, dominated by threetip sagebrush,

occurs on windswept ridges between the ravines. Douglas-fir
and limber pine occur just above the aspen. Forests at higher
elevations are dominated by lodgepole pine, engelmann
spruce, and subalpine fir.

Plant Adaptations and Vegetation Dynamics quently browsed by deer, especially in the winter. nota-

bly, the shrubs respond by growing lateral branches.
the more heavily the shrubs are browsed, the more
spiny the branches become, making further browsing
more difficult. in this way the plants develop their own
defense mechanism.
Some ecologists have observed expansion of curlleaf
mountain-mahogany into adjacent plant communities
during long periods without fire, suggesting that fre-
quent fires historically restricted mountain-mahogany
to rocky sites . 6 c urlleaf mountain-mahogany can live
150 years. the shrubs spread by layering or seedling
establishment and can sprout after ire s. 7 Large seed
crops are common, though often not in consecutive
years.

Mountain-Mahogany Shrublands

one of the more conspicuous foothill shrublands is
dominated by mountain-mahogany ( Cercocarpus spp . ).
this shrub forms dense thickets on rocky or shallow
soils from low elevations on the western Great Plains
(~4,500 feet) up to elevations of about 8,000 feet in
the mountains (fig. 10.4; see table 10.1). two species
of Cercoc arpus occur in Wyoming: the deciduous true
mountain-mahogany in the southern Black Hills and
across the southern half of the state, and the ever-
green curlleaf mountain-mahogany in the foothills
of the Bighorn Mountains and also to the west and
south (fig. 10.5). there is little overlap in the distri-
bution of the two species. Both grow on a variety of
bedrock types, including sandstones, limestones, and
shales . 4 the abundance of both species is undoubtedly
facilitated by their ability to fix nitrogen ; 5 and perhaps
because of their more nitrogen-rich foliage, they are fre-

Juniper Woodlands

A characteristic feature of some foothills and escarp-
ments are picturesque woodlands dominated by long-
lived Utah juniper or Rocky Mountain juniper (figs. 10.6