Desert Shrublands and Playas - Mountains and Plains: The Ecology of Wyoming Landscapes - page 132

High

are less than in grasslands and sagebrush steppe, primar-
ily because of reduced water availability , 3 and perennial
grasses and forbs are less abundant. Fires are infrequent.
Variation in plant distribution can be caused by slight
changes in topography. Playas occur in low depressions,
where water sometimes accumulates before evaporating,
leaving a dry soil surface that is white with precipitated
salt s. 4 Plant life is sparse in such areas.
Salt accumulation in desert soils is a formidable prob-
lem for the survival of many plants, especially seedlings.
Most notable is that many plants cannot extract water
from soil if the salt (solute) concentration of the soil water
is higher than that of the root cells. Plants known as halo-
phytes have evolved adaptations that solve this problem
by maintaining cellular concentrations at higher levels
than those found in the soil solution. in the leaves of
some halophytes, the salt concentrations promote water
uptake that causes leaf succulence. Greasewood and red
swampfire are examples. 5 o ther halophytes have evolved
special adaptations for excreting salts (fig. 8.3 ). 6
Halophytes essentially function as pumps, collecting
salts from throughout the rooting zone and depositing
them in a smaller area as leaves fall. Some ecologists
have hypothesized that salt deposition under shrubs
might prevent the establishment of other plants nearby,
reducing competition for limited water and nutrients—
a form of allelopathy.
As in grasslands and sagebrush steppe, competition
for resources determines which desert plants and animals
manage to survive. this competition is often minimized
because of seasonal differences in activity, or because

Big sagebrush
Winterfat

Gardner's
saltbush

Shadscale

Greasewood
Pickleweed

Low

Lo w H igh

SALINITY

Fig. 8.2. Distribution of six desert shrubs in relation to gradi-
ents of salinity and aridity. All are found in Wyoming except
pickleweed, the most salt-tolerant species, which is found in
Utah and nevada. Adapted from West (1988).

except for soil salinity, desert shrublands have many
features in common with sagebrush steppe: dominance
of drought-tolerant shrubs with small leaves; exten-
sive, sometimes deep root systems for obtaining water
and nutrients; sensitivity to various kinds of herbivores
and invasive plants; and processes that enhance the
efficiency of water and nutrient use, such as hydraulic
redistribution in the soil and nutrient reabsorption from
leaves prior to leaf fall . 2 Plant cover and annual growth

Fig. 8.3. Scanning electron micrograph
of salt-secreting glands on the leaves of
a salt-tolerant plant in the genus Cheno-
podium. the bulbous glands, which are
modified leaf hairs, enlarge until they
burst, after which rainwater washes
the salts onto the soil. Magnification
about 33 times. Microphotograph by
Howard c. Stutz.