Geoscience Reference
In-Depth Information
Fig. 10.4. (above) curlleaf mountain-mahogany and a few
ponderosa pine grow on shallow, rocky soils in this area on
the east side of the Bighorn Mountains. Mixed-grass prairie
occurs on deeper soils. true mountain-mahogany is found in
similar habitats in the southern and eastern parts of Wyo-
ming (see chapter 17).
Fig. 10.5. (right) true mountain-mahogany is easily distin-
guished from curlleaf mountain-mahogany. Drawing by Judy
Knight.
and 10.7). Depending on soil characteristics, associated
plants can include black sagebrush, big sagebrush, ante-
lope bitterbrush, mountain-mahogany, limber pine,
and ponderosa pine (see table 10.1). Rocky Mountain
juniper is common in eastern Wyoming and the north-
ern Great Plains, where summer precipitation is higher
than farther west. Usually it occurs in ravines or on
steep slopes, along with yucca, skunkbush sumac, and
ponderosa pine. Utah juniper grows on escarpments in
the more arid basins of western Wyoming, for example,
in the Wind River canyon and Bighorn Basin.
the cause of juniper presence or absence is a puz-
zle, because both species are absent from many habi-
tats that seem suitable. on the upper or northerly edge
of its range, such as in Wyoming, the junipers seem
to survive only in warmer thermal belts, where frosts
are less frequent at night. 8 Like mountain-mahogany,
both species are found on a variety of geologic forma-
tions. However, unlike mountain-mahogany, juniper is
more capable of expanding into adjacent rangelands on
deeper soils. Differences in the distribution of juniper
and mountain-mahogany may be the result of differ-
ences in microclimate, relative ability to spread into
adjacent habitats, or soil-bedrock features that have not
yet been studied in sufficient detail . 9 Juniper distribu-
tion in the Bighorn Basin ranges from about 3,600 to
6,000 feet, overlapping with but generally lower than
mountain-mahogany shrublands. the lower limits in
specific areas seem to be where shallow rocky soils meet
deeper alluvial or colluvial soils. the upper limit is prob-
ably controlled by a still-undetermined climatic factor. 10
Most shrublands at low elevations in the Bighorn Basin
 
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