Present-Day Environments and Climate Change - Mountains and Plains: The Ecology of Wyoming Landscapes - page 35

to the formation of a coarse-textured soil with a high
infiltration capacity for water. Sometimes the resulting
sand is sorted by wind into extensive dune fields, such
as northeast of Rock Springs and between Shoshoni and
casper (see figs. 1.5 and 9.1).
Fine-textured and deeper soils usually develop from
more easily weathered shales, mudstones, and siltstones,
or from material transported by wind and water (such as
on much of the western Great Plains). infiltration rates
are slower in fine-textured soils, but the water-holding
capacity is higher. Such soils can be quite fertile,
though some are saline or high in toxic elements (such
as selenium). Saline playas develop where fine-textured
sediments accumulate in closed basins with no or little
outflow. Some shale-derived soils (for example, from the
Mowry, Steele, Belle Fourche, Pierre, and thermopolis
formations) are high in bentonite—a clay that swells
and shrinks during wetting and drying cycles, which
typically prevents the establishment of young plants.
Similarly, soils high in soluble salts, gypsum (hydrated
calcium sulfate), and sodium may form crusts on the
surface that slow the rate of plant establishment (see
chapter 8).
Predictably, soil characteristics change with eleva-
tion. organic matter content increases from arid sites in
the basins up to montane meadows, but then decreases
farther up into the alpine zone. Plant production is
highest at mid-elevations, where more moisture is avail-
able but the climate is still not as cold as higher on the
mountain. Severe climates, such as in the alpine tun-
dra, tend to restrict soil development as well as plant
growth.
Salts and nutrients accumulate in the soil where
the precipitation-evaporation ratio is less than one, as
would be expected during most years in intermountain
basins and on the Great Plains. A layer of lime (calcium
carbonate) develops near the average depth of water
infiltration, forming a hardpan or caliche layer that
may restrict root development. in contrast, a hardpan
does not develop in the mountains where the precipita-
tion-evaporation ratio is greater than one and the soils
tend to be slightly to strongly acidic. With more water
moving downward through the soil profile, the poten-
tial for nutrient leaching is higher.
Five soil features seem especially critical in deter-
mining vegetation patterns in Wyoming and adjacent
states: infiltration rate, depth, water-holding capacity,

aeration, and salinity. infiltration is especially impor-
tant in semi-arid regions, where the potential for evapo-
ration is high due to relatively warm, dry air. Water that
percolates rapidly to a depth of 2 inches or more evapo-
rates more slowly, remaining in the soil long enough
to be used by plants. coarse-textured soils in semi-arid
regions tend to have higher infiltration rates and con-
sequently higher rates of plant growth—a principle
referred to as the inverse texture effect. the name stems
from the fact that, in eastern north America—where
the precipitation is generally higher than in the low-
lands of Rocky Mountain states—coarse-textured soils
sustain less plant growth than those with fine texture
(more silt and clay). 15
the principle of the inverse texture effect can be
extended to rock outcrops, where the soil is thin or even
nonexistent but where water is funneled into cracks.
Plant roots capable of growing through these fissures,
such as the roots of mountain-mahogany and limber and
ponderosa pine, probably have more water available to
them than would be the case for adjacent fine-textured,
deeper soils. in fact, shrubs and trees throughout the
region, which generally require more water than do
herbaceous plants, are often found on rock outcrops
or coarse soil, not on fine-textured soils, where water
often evaporates before plants can use it (see fig. 3.3).
Mountain-mahogany and other shrubs that grow in such
places are an important source of food for wildlife.
Water-holding capacity is determined by soil tex-
ture, soil depth, and the amount of organic matter.
Fine-textured soils—with a relatively high percentage of
silt, clay, and organic matter—hold more water against
the flow of gravity than do coarse-textured soils. they
also enable more plant growth if the soil is saturated
at least once during the year, such as often occurs near
snowdrifts. Abrupt transitions between grasslands and
some shrublands are often associated with changes in
soil depth or soil texture (see figs. 17.9, 17.11, and 17.17),
with the more drought-tolerant grasses abundant on
shallower soils with a low water-holding capacity (see
chapter 7).
Soil aeration is determined by texture and the
amount of water in the soil; it is important to plants
because air spaces in the soil have the oxygen required
for root respiration. oxygen is less readily available if
the spaces are filled with water, which occurs more
often in fine-textured soils or in wetlands. Greasewood,