Sagebrush - Mountains and Plains: The Ecology of Wyoming Landscapes - page 119

serving as much soil organic matter as possible, along
with its associated microorganisms, is fundamental for
restoration.
Because of concerns about the effects of carbon di-
oxide on climate change, ecologists have been conduct-
ing experiments to determine whether the sagebrush
ecosystem is a source of atmospheric carbon or a sink:
does an ecosystem with big sagebrush tend to store
more or less carbon than is released from the ecosystem
as carbon dioxide? 37 As with most semi-arid shrublands
around the world, sagebrush steppe tends to be a sink
during wet years but a source in dry years. Generally,
the sagebrush ecosystem accumulates carbon as the
shrubs age, in the form of organic matter above- and
belowground. When most of the shrubs die, the eco-
system could become a source if the unburned woody
sagebrush biomass decays rapidly. However, Wyoming
ecologists Meagan cleary, elise Pendall, and Brent
ewers did not detect an increase in carbon emissions
after fire—possibly because the growth of grasses and
forbs increases greatly soon after fires, and the sage-
brush roots and stems decompose slowly. 38
Still, the benefits of improving habitat or forage avail-
ability for some species by burning must be balanced
against the undesirable effects of additional atmo-
spheric carbon dioxide from the fire, not to mention
the invasion of undesirable plants (such as cheatgrass)
and the loss of critical habitat for threatened species (see
below). Federal and state guidelines often call for man-
aging ecosystems on public lands so that they sequester
carbon, resist the invasion of exotic plants, and offer
habitat for such species as the greater sage-grouse and
pygmy rabbit—all ecosystem services that are now
highly valued.

other plants, such as indian ricegrass. 39 Losses of nitro-
gen may occur through erosion, denitrification, or the
emission of nitrogen gases. 40
Several investigators have studied the rates of litter-
fall and decomposition in shrublands dominated by
big sagebrush, both important processes for nutrient
cycling. 41 Some evidence suggests that nutrients are
reabsorbed by twigs before the leaves fall, thereby con-
serving nutrients in the shrubs. 42 Still, soil nitrogen
is enhanced under shrubs. there also is great varia-
tion in soil organic matter and nitrogen availability—
higher in ravines with mountain big sagebrush to less
on ridgetops with black sagebrush. 43 Burrowing mam-
mals and harvester ants affect nutrient dynamics as
well. 44
the major effect on nutrient cycling of having big
sagebrush or any shrub in the ecosystem is to have more
woody biomass aboveground. nutrient accumulation is
therefore greater aboveground than where grasses and
forbs predominate. When the shrub dies, or as twigs
and branches die, more of the nutrients are added to
the soil surface as wood rather than as herbaceous plant
material. Also, woody fuels accumulate that can create
conditions for a longer, hotter fire.
Another influence of shrubs is to create islands of
fertility, as discussed previously. Shrubs accumulate
windblown debris and associated nutrients, and the
root system of the shrubs extracts soil nutrients from
several feet beyond the canopy while dropping them on
the surface as litter in a smaller area—possibly another
mechanism creating patchiness in the vegetation. the
death of the sagebrush root system must be important
as well, not only in reducing competition for water and
nutrients with neighboring grasses and forbs, but also
in providing a substantial pool of nutrients as the roots
decompose. the decaying root system can also alter soil
structure, possibly leading to greater infiltration rates.
Such effects may be lost at some point after big sage-
brush has been removed, whether by fire or herbicides.
Deeper soil organic matter may gradually be lost as well,
along with the potential for hydraulic lift and the tap-
ping of deep soil nutrients, including phosphorus. 45 of
course, big sagebrush typically becomes re-established,
as discussed in the next section, restoring the benefits
of having this shrub in the ecosystem—unless cheat-
grass changes everything.

Nutrient Availability

nutrient distribution in sagebrush-dominated eco-
systems is similar to that of grasslands, with the largest
amounts in the soil. Water is usually more limiting to
plant growth than are nutrients, though nitrogen may
be limiting during wet years. As in grasslands, the major
source of nitrogen is probably precipitation, but symbi-
otic nitrogen fixation occurs locally in biological crusts
on the soil surface; in the nodules of lupine and other
legumes; and possibly adjacent to the roots of some