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

THE NORTHERN RANGE
showed that grazing selected against some species
and for others. For example, there was less rosy
pussy toes (85 percent less) and fringed sage (33
percent less) on some grazed sites, but there was a
great deaI of site-by-site variation. On the other
hand, another native forb, Arabis holboelii, was
found on grazed plots but on no ungrazed plots.
No consistent effects of grazing on forbs was
revealed, as measured either by plant frequency or
by plant basal cover. When plant frequency was
measured, forbs were more often encountered on
grazed sites in 8 of 14 comparisons, and less often
encountered on grazed sites in 6 of 14 comparisons
(Coughenour et al. 1995). Plant basal cover was
less for forbs on grazed sites at the Blacktail and
Lamar exclosures, but greater on grazed sites at the
Gardiner exclosures (Reardon 1996).
From the evidence collected to date, there
was no consistent response of forbs to ungulate
grazing on the northern range. Green forb biomass
was reduced by grazing at some locations, years,
and sites but the reverse of this was also commonly
measured. Some of the annual variations observed
may have been related to variations in precipitation.
Coughenour et al. (1995) felt that "forbs
responded positively to high spring rainfall and, in
those years, forbs became increasingly competitive
with grasses. While some reductions in forbs were
observed, they were usually non- or marginallystatistically
significant, not consistent between
years, and no species of forb was being eliminated
or reduced to a low level."
SOILS
No consistent trends were found in soil
nitrates, soil organic matter, or soil nutrients
between grazed and ungrazed sites (Lane 1990,
Lane and Montagne 1996). Soil surface bulk
densities were consistently higher on grazed sites
(Lane 1990, Lane and Montagne 1996). High
surface bulk densities may restrict movement of air
and water through the soil; higher densities are'
usuaIly the result of soil compaction. Some soil
compaction is a logical consequence of the hoof
action of ungulates on the soil surface. But several
studies verified that soil moisture levels were not
40
affected by grazing (Lane 1990; Coughenour 1991;
Merrill et aI. 1994a, 1996; Lane and Montagne
1996; Singer and Harter 1996).
For more on soils, see Chapter 5 .
RESEARCH
RECOMMENDATIONS:
GRASSLANDS
The area of greatest emphasis during the
most recent research initiative concerned grazing
effects on grasslands. While we now understand
the grasslands better than the other components of
the winter range, continuation of some aspects of
grassland research is strongly recommended. First,
a more intensive and consistent monitoring
program for grassland production, grassland
nutrient content, and climatic changes is needed.
Second, the effects of the mechanisms of grazing
on grasslands need to be better understood. For
example, what factors contribute to the apparent
stimulation of aboveground and belowground
production and nutrient cycling by grazing? Third,
fitness of some grasses may be enhanced by
grazing, and this question should be pursued.
Other important topics requiring further investigation
include system functioning, the role of
grasshoppers, nematodes and other invertebrate
herbivores, and the role of small mammals such as
pocket gophers in soil disturbance and plant
succession (GruellI973, Houston 1982). Research
is also needed to develop methods of restoring
native grasses to areas now dominated by such
non-native species as smooth brame, crested
wheatgrass, timothy, and cheatgrass.
CONCLUSIONS
Houston (I982) wrote that, "the effects of
herbivores upon the vegetation of the park require
another look, because preoccupation with ungulates
gives a distorted view of their herbivory in
ecosystem dynamics." An important aspect of the
studies conducted since Houston published that
statement is their attention not only to ungulates
but also to many other environmental factors.
Rather than focus on the immediate effects of