Present-Day Environments and Climate Change - Mountains and Plains: The Ecology of Wyoming Landscapes - page 36

Fig. 3.8. Surface soil texture has a strong influence on plant
distribution, as shown here for a grass—blue grama—and
two species of shrubs, Gardner saltbush and big sagebrush.
Adapted from nichols (1964).

100

Blue grama
Gardner saltbush
Big sagebrush

90 10

80 20

70 30

clay

60 40

50 50

silty
clay

sandy
clay

40 60

silty clay
loam

clay loam

30 70

sandy clay
loam

20 80

loam

silt loam

10 90

loamy
sand

sandy loam

silt

sand

100

100 90 80 70 60 50 40 30 20 10

% sand

a common shrub around the edges of alkali flats, is well
adapted to saturated soils with low oxygen, whereas big
sagebrush cannot tolerate standing water within 10 cm
of the surface. 16 Well-drained, well-aerated soils are nec-
essary for the survival of big sagebrush and the growth
of crops (fig. 3.8).
Salinity, and other chemical traits such as alkalinity
and sodium concentration, depend on the composition
of the soil's parent material, the amount of water that
has evaporated from the soil over the centuries, and
little or no potential for the leaching of salts from the
soil (that is, where the precipitation-evaporation ratio is
less than one). 17 Plants that tolerate saline, alkaline, and
high-sodium soils are known as halophytes, for example,
greasewood, saltgrass, and the various species of salt-
bush. Less salt-tolerant species, such as big sagebrush,
are sometimes found growing with halophytes but usu-
ally in areas where salt concentrations are lower (along
drainages where snow accumulates and some of the
salts are dissolved and washed below the rooting zone;
see chapter 7).

the weathering of some rock types leads directly to
the formation of soils with high salt concentrations,
especially cretaceous marine shales, such as those in
the Steele, niobrara, cody, and Pierre formations. Saline
soils often develop in basins where considerable water
accumulates on the surface during wet periods and
where subsequent evaporation leads to the accumula-
tion of salts on the surface, often forming white crusts.
Soils derived from bentonite tend to have high con-
centrations of sodium (because bentonite is composed
largely of montmorillonitic clay, which develops in
high-sodium sedimentary environments).

Periodic Disturbances

the environment is usually described in terms of tem-
perature, wind, water availability, and soil characteris-
tics, but just as important are the various disturbances
that have affected ecosystems for millennia. examples
include fires; windstorms; floods; burrowing; epidemics
of certain insects; and, in the alpine tundra, frost heav-