Sagebrush - Mountains and Plains: The Ecology of Wyoming Landscapes - page 116

sequent research found that water can be moved up or
down (always from wet to dry soil). 14 this process pro-
vides more water in the summer for plants that have
only shallow roots, thereby extending the growing
season and increasing the overall productivity of the
sagebrush ecosystem. Hydraulic redistribution also can
prolong microbial activity in the surface soil, which
facilitates decomposition and increases the supply of
nutrients for the plants.
Several mechanisms allow the efficient use of water
by big sagebrush. the stomata close rapidly as water
stress develops during the day, limiting transpiration
until the next morning, when they are again open. 15
the plant re-equilibrates at night, when water uptake
exceeds losses. Stomatal closure limits photosynthesis
as well, but prolonging the growing season by con-
serving water seems to be adaptive. in addition, the
plant is evergreen, retaining most of its leaves for one
year. Moreover, acclimation to changing temperatures
occurs readily, even down to freezing. Big sagebrush is
capable of photosynthesis in the early spring and even
during relatively warm winter days. 16 in general, ever-
green plants fix carbon whenever water and tempera-
ture conditions are within tolerance ranges. With no
time lost for the production of new leaves, the growing
season is longer.
Water is most readily available in the spring, when
big sagebrush has the largest leaf area for photosyn-
thesis. two leaf types are produced: ephemeral leaves
in early spring, which fall as water stress develops
during the summer, and overwintering leaves that
remain on the plant until the following spring (see fig.
7. 6 ). 17 essentially, the plant is evergreen, but the loss
of ephemeral leaves by early summer allows for some
reduction in total leaf area as water becomes more
limiting, providing another mechanism for conserv-
ing water and extending the length of the growing
season. Shadscale, another shrub in the basins, also
produces both ephemeral and persistent leaves. the
leaves of many species of sagebrush and other xero-
phytic plants often have a pale, silvery-green color,
attributable to dense, minute silvery hairs on the sur-
face. this color reflects light more readily than dark
green does, which reduces water loss because the
leaves remain cooler.

the carbohydrates produced by photosynthesis are
distributed to all plant parts where energy is needed. in
big sagebrush, a large proportion of these energy-rich
compounds is stored in the twigs. thus, both the twigs
and the leaves are a good source of food for herbivores.
Furthermore, the shrubs are tall enough to extend above
the snow during most winters. Little wonder, then, that
historically the shrubs have been subjected to consid-
erable browsing pressure and that they have evolved
a mechanism to minimize this herbivory, namely, the
terpenes that give sagebrush its characteristic aroma.
the terpenes apparently cause sufficient indigestion to
minimize the amount eaten by some animals. Without
this adaptation, the shrub may not have survived the
intense browsing that might have resulted from large
herbivore populations, especially those limited most
directly by winter food supply. 18
considering the genetic variation that exists in popu-
lations of widespread plants, it should not be surprising
that different species of sagebrush, and even different
individuals of the same species, vary in the amount of
terpenes they produce. Herbivores have been observed
to preferentially browse on those plants with the low-
est terpene concentrations. Some animals tolerate the
terpenes. A good example is the pronghorn, which
eats sagebrush all year long (the shrub provides 70-95
percent of its food in the winter). other herbivores eat
sagebrush as well, such as sage-grouse—especially when
other foods are not available. 19
Big sagebrush, unlike most of its neighboring plants,
lacks the capacity to sprout from roots or the root crown.
consequently, longevity and seed production are espe-
cially important for its persistence. Some plants live for
100 years or more, though 40-50 years is more com-
mon. 20 the shrub produces annual rings in the wood
similar to temperate zone trees. With regard to sexual
reproduction, big sagebrush produces thousands of tiny
seeds each year, some of which remain viable for up to
4 years. 21 Like many other plants, germination occurs
in late winter or early spring, when moisture is avail-
able and the temperature is warm enough for growth.
no cold period is required to break seed dormancy,
although some investigators have found that this treat-
ment causes higher germination rates in mountain big
sagebrush. even very low salt concentrations can inhibit