Escarpments and the Foothill Transition - Mountains and Plains: The Ecology of Wyoming Landscapes - page 167

Fig. 10.13. Aspen groves occur in some places on the lee
side of the continental Divide south of Rawlins, where deep
snowdrifts develop during winter. the snow provides ade-
quate water for shrubs and trees in an area that elsewhere is
semi-arid. this photo shows a portion of a doughnut-shaped
grove, known as an aspen atoll, where the center is domi-
nated by shrubby cinquefoil, mountain silver sagebrush,

lupine, tufted hairgrass, and various other grasses and forbs.
Deep snow that persists into early summer prevents tree
establishment in the center. the shrubs in the foreground
and on the left are old aspen sprouts that are regularly
broken by the heavy snowpack. Aspen trees develop just
beyond the shrubby aspen, where snow accumulation is less.
elevation 7,600 feet.

oak—all of which are deciduous and occur on mesic
sites with deeper soils. the additional moisture appar-
ently comes from a higher soil water-holding capacity,
more snow accumulation (from drifting), more summer
precipitation, or runoff from the slopes above.
chokecherry woodlands, also known as hardwood
draws or woody draws, are commonly part of a veg-
etation mosaic on the eastern plains that includes
grassland and woodlands dominated by ponderosa
pine (fig. 10.12). in addition to chokecherry, common
shrubs include American plum, skunkbush sumac,
silver sagebrush, and western snowberry. 39 trees com-
monly found with the shrubs are boxelder and green
ash (see table 10.1). except for the lack of cottonwood,
chokecherry woodlands have the characteristics of
some riparian woodlands. Specifically, they have more
plant biomass than is found on adjacent shrublands,
which provides good wildlife cover as well as forage
and shade for livestock.

Aspen-dominated woodlands tend to occur where
the soils are deeper, such as on lower slopes where water
seeps to the surface or on the lee sides of ridges where
snow accumulates. Associated plants include species
that occur in aspen woodlands at higher elevations (see
chapter 11). Sometimes the aspen completely surround
a snowdrift, forming a doughnut-shaped grove around
a meadow that has been referred to as an aspen atoll
(fig. 10.13). Such groves develop when trees first become
established near ridgetops, which causes further accu-
mulation of snow downwind. if the snow is deep
enough, it persists until midsummer and only herba-
ceous plants capable of surviving with a short growing
season can persist. Gradually the aspen grow completely
around the snowdrift and meadow by root sprouting.
the trees also expand inward, toward the edge of the
central meadow. Here the average snow accumulation is
sufficiently shallow to allow the aspen to persist, but the
weight of the snowpack causes breaking and bending,