Yellowstone's Northern Range - Greater Yellowstone Science ...

GRASSLANDS
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Mammalian grazers, such as elk and other
ungulates, process plant matter through their
digestive systems with significantly different
results for the ecosystem than if those plants were
allowed to simply die and accumulate on the
surface of the soil as litter. Grazers accelerate and
enhance the cycling of nutrients through the soil
system by consuming and digesting plants and then
producing feces and urine that are cycled back into
the system; they also do it by returning their dead
carcasses to the system. A major difference
between a wild native grazing system and a
commercial livestock operation is that virtually all
of the organic matter in the bodies of the wild
grazers makes its way back into the system (often
through the digestive tracts of predators and
scavengers, who add another layer to the processing),
while virtually all of the organic matter in the
bodies of a commercial livestock herd is taken
from the system to be consumed and processed
elsewhere, usually through human digestive tracts.
An interesting side note to this recycling
theme is the relational dimension of how nutrients
move from the summer to the winter range. Since
ungulates grow so rapidly and gain so much weight
and fat on the summer range, and then lose weight
and mostly die on the winter range, they probably
move nutrients down the elevational gradient (F.
Singer, U.S. Geol. Surv., pers. commun.).
GRASS SIZES AND SHAPES
In a study of the effects of elk herbivory on
northern range grasslands, heights of seed stalks
and vegetative leaves were taller on grazed winter
range sites than on ungrazed sites in three of four
years (Singer and Harter 1996). Vegetative leaves
were shorter on grazed sites than on ungrazed sites
in one year, 1986, apparently when growth conditions
were sub-optimal, possibly due to a very early
melting of the winter's snowpack (Singer 1996a,
Singer and Harter 1996). No increases in bunchgrass
mortality. no differences in species diversity,
and no differences in soil moisture due to winter
elk herbivory were documented (Singer and Harter
1996). Morphological parameters of grazed plants
were reduced on summer range where plants were
grazed through the active growing season (Wallace
1996). This might be evidence grazing effects on
plant morphology does not necessarily damage the
plants or their production.
LITTER
Accumulated organic litter was about 3.5
times greater in ungrazed sites within exc10sures
(Frank 1990, Singer 1996a). As a result of the
increase in litter and soil crust lichens over a period
of 24-28 years of protection from grazing, exposed
surface (bare ground and rock combined) was 11
percent higher on grazed surfaces.
GRASSROOTS
Coughenour (1991,1996) stndied root
biomass and nitrogen respo~ses to grazing of
upland steppe on the northern range, and determined
that, "grazing had no effect on root biomass,
. .. an important measure of the fitness of long-lived
perennial grass genets." Coughenour emphasized
the importance of climatic conditions in affecting
grazing responses of plants. Root biomass on
grassland sites was not affected by grazing (Men-ill
et al. 1994a). There were more root-feeding
nematodes (roundworms) in the soil of grazed
areas, and they probably increased nitrogen
mineralization rates (Merrill et al. 1994a, 1994b, 1996).
FORBS
The forb biomass technique was used to
gather data on forbs because it is most closely tied
to the productivity of the site. The results of forb
biomass analysis showed some variability between
years and study sites. At the Blacktail Plateau
exc1osure, there was less forb biomass on grazed
sites in two of three years of sampling. However. a
study of six exc10sures on the northern range found
that there was no statistically significant difference
in forb biomass in 1986 or 1987. Coughenour
(1991), in an independent sampling of the same six
excIosures in 1987, found no difference in forb
biomass on grazed and ungrazed plots, and in 1988
found more forbs on grazed plots. Singer (1995)