Sagebrush - Mountains and Plains: The Ecology of Wyoming Landscapes - page 115

other sagebrush islands are associated with mima
mounds (see chapter 9), or with pockets of deeper soil
in areas dominated by threetip sagebrush. As is usu-
ally the case, a vegetation pattern can have several
explanations.

Sagebrush Adaptations

Plant distribution is determined more by the toler-
ances of delicate seedlings than by the characteristics
of adult plants. Seedlings of big sagebrush are com-
monly observed in some years, almost always when
the early summer has been relatively cool and wet. 12
Lower temperatures minimize the rate of water loss
from the drought-sensitive seedlings, providing more
time for roots to grow down to a dependable water
supply. though the roots may not grow more than
an inch or two in the first year, the presence of deep
soil water contributes to a relatively moist surface soil
environment in three ways: (1) capillary movement
from below, (2) condensation in the surface soil of
water vapor that emanates from the deeper soil water,
and (3) hydraulic redistribution from deep to shallow
soil via the root system of other plants (see below for
explanation). once established, the adult plants toler-
ate the occasional years when little precipitation falls
during the winter. Surface roots use surface moisture
early in the growing season, but other roots grow
down to 6 feet or more and make use of deep soil mois-
ture after surface soils are dry. the root system extends
laterally to a distance of about 5 feet, well beyond the
shrub's canopy.
Significant soil water recharge at depth usually
occurs only once a year, during the snowmelt period,
but, as noted, surface soil dampening during a hot, dry
summer can happen even without rainfall through
the novel process of hydraulic redistribution. 13 First
described by ecologists James Richards and Martyn
caldwell in 1987, this process is now known to occur
in numerous plants that have both deep and shallow
roots. if the deep roots are in moist soil and the shal-
low ones in dry soil, as often happens in the summer
where big sagebrush grows, then water is transported
by the deep roots up to the shallow roots, where it is
released into the soil. initially the process was referred
to as hydraulic lift (from deep to shallow soil), but sub-

Fig. 7.9. Wyoming big sagebrush sometimes grows in patches,
such as in this area east of Rock Springs. the patches can be
small or large, oval or irregular, and often are associated with
coppice dunes. elevation 7,000 feet.

the shrubs. But how did the shrubs become established
in the first place? one explanation is that the patch
began with a single shrub that survived by chance,
perhaps adjacent to a burrow. its presence caused snow
drifting on the lee side of the shrub, creating condi-
tions favorable for the seedlings of additional shrubs—
perhaps from seed produced by the original plant. in
addition to snow, the taller shrubs could also lead to
the accumulation of blowing organic matter and soil
particles. this accumulation, combined with nutrients
brought to the surface by the animals and litterfall from
the shrubs, creates islands of fertility. 11 Such islands are
found where water and other resources are inadequate
to support a uniform cover of sagebrush. neil West, an
ecologist at Utah State University, suggested in 1988
that, for sagebrush-dominated vegetation in the Great
Basin of Utah, any activity that destroys sagebrush
islands reduces the productivity of the ecosystem as a
whole. the same probably is true for Wyoming.