Escarpments and the Foothill Transition - Mountains and Plains: The Ecology of Wyoming Landscapes - page 163

Fig. 10.8. escarpments and foothills on the eastern plains
typically have ponderosa pine woodlands, such as along the
Hat creek Breaks, north of Lusk (oligocene and Miocene
sandstones and claystones). this ridge is part of Pine Ridge,

which extends westward to north of casper and eastward into
southern South Dakota. Mixed-grass prairie is also common,
sometimes with silver sagebrush and Wyoming big sagebrush.
elevation 5,000 feet.

additional trees can reduce the amount of forage and
increase evapotranspiration (thereby reducing stream-
flow), and also because the pine needles can be toxic if
their livestock are hungry enough to eat them . 24
in Wyoming, ponderosa pine is found primarily in
comparatively warm areas with higher summer pre-
cipitation. Such environments are found in the eastern
half of the state, primarily in the Black Hills and Bear
Lodge Mountains, at lower elevations on the east and
north slopes of the Bighorn Mountains and the Lara-
mie Mountains, in a few localities around the Medi-
cine Bow and Seminoe mountains, and on lowland
escarpments scattered throughout the Great Plains. 25
Associated plants include skunkbush sumac, Rocky
Mountain juniper, and two grasses—sideoats grama
and little bluestem (see table 10.1). Ponderosa pine is
essentially absent from western Wyoming, perhaps
because of a growing season that is currently too short
and dry. it does occur in the foothills of the Uinta
Mountains of Utah (such as in Dinosaur national
Park), where the elevations are lower and the Arizona
Monsoon provides slightly more summer precipitation
than in Wyoming. 26

Although ponderosa pine is often common on escarp-
ments in eastern Wyoming, where deep water percola-
tion is presumably possible, some geologic substrata
provide better conditions for tree growth than others.
Wyoming ecologist Jim States found that the growth
of ponderosa pine was best on Mowry shale, where the
trees had wider and less variable annual ring widths in
their wood . 27 Low annual variability in ring width sug-
gests that this marine shale, despite the variable climatic
conditions from year to year, provided a consistent envi-
ronment for growth. in contrast, trees on the Pine Ridge
and cloverly sandstones had narrower and more variable
annual rings, even though they grew near the Mowry
shale and surely had the same climatic conditions. 28 t he
association of some foothill species with fracture lines in
the bedrock can be striking (see fig. 3.3).
Limber pine sometimes occurs with ponderosa
pine, but it is more typical where the climate is drier
and colder. Unlike ponderosa, it extends upward to the
alpine treeline. the species tends to be restricted to rocky
soils and ridges, where the seedlings have relatively
little competition from other plants. 29 i it is commonly
found on the lee side of boulders, where the seedlings of