Impacts of Livestock Exclusion
From Wyoming Big Sagebrush
Communities
Jennifer M. Muscha
Ann L. Hild
Larry C. Munn
Peter D. Stahl
Abstract: Fenced exclosures are used by range scientists to determine the effects of livestock grazing on vegetation and soils. In the
1950s and 1960s numerous rangeland exclosures were established
on previously grazed areas in Wyoming big sagebrush steppe.
Current understanding suggests shrublands may not return to the
pristine condition with livestock exclusion. In the summers of 2001
and 2002 we examined nine 30- to 45-year-old exclosures in Natrona,
Fremont, Sweetwater, Bighorn, and Washakie Counties in Wyoming
to compare vegetative cover and presence of biological soil crusts
inside and outside exclosures. Bare soil was greater outside exclosures at three sites, and litter cover was greater inside the
exclosure at three of the nine sites. Biological soil crust cover was
greater inside only one of nine exclosures. Although shrub cover
increased from original levels (data collected at time of exclosure
establishment) both inside and outside exclosures at all sites, shrub
cover was greater inside than outside at only one exclosure site. Our
results are mixed with respect to impacts of livestock exclusion from
Wyoming big sagebrush steppe, partially because original sagebrush levels were low in most exclosures, and because grazing use
outside exclosures differed among the nine sites.
Introduction ____________________
Native rangeland protection is becoming increasingly
important, as managers attempt to enhance and conserve
biodiversity. Because a majority of our rangelands are under
public ownership, management choices become controversial. Grazing of domestic livestock has been suggested as
beneficial to semiarid rangelands in the Western United
States (Vavra and others 1994), while others propose permanent removal of grazing by domestic livestock (Donahue
1999; Fleischner 1994; and others). The effects of grazing
removal from already grazed systems in the West are not
well documented.
In: Hild, Ann L.; Shaw, Nancy L.; Meyer, Susan E.; Booth, D. Terrance;
McArthur, E. Durant, comps. 2004. Seed and soil dynamics in shrubland
ecosystems: proceedings; 2002 August 12–16; Laramie, WY. Proceedings
RMRS-P-31. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
Jennifer M. Muscha is a Graduate Student at the University of Wyoming.
Ann L. Hild, Larry C. Munn, and Peter D. Stahl are Associate Professor,
Professor, and Assistant Professor, Department of Renewable Resources,
University of Wyoming, Laramie, WY 82071, U.S.A., Fax: (307) 766-6403, e-mail:
annhild@uwyo.edu
176
Previous research on exclosures conducted in arid and
semiarid regions document different vegetative responses to
the exclusion of grazing. The most evident difference between grazed and ungrazed areas is greater density of
aboveground vegetation inside the exclosure (Costello and
Turner 1941). Although range managers once assumed that
removal of grazing pressure from rangelands improved
rangeland condition (Dyksterhuis 1949), several exclosure
studies in arid environments demonstrate that recovery of
degraded rangelands can be slow to nonexistent (Holechek
and Stephenson 1983; Robertson 1971; West and others
1984). Other studies in arid and semiarid rangelands examining removal of livestock from an area report no change in
vegetation (Johnson-Barnard 1995; West and others 1984),
an increase in herbaceous cover but not species richness
(Anderson and Holte 1981), or an increase in shrub cover
after removal of grazing (Brady and others 1989; Lang
1973), resulting in a more degraded state. In sagebrush
systems, noticeable response of vegetation after grazing
removal may not occur for many decades, and removal of
grazing can increase density and cover of big sagebrush
(Anderson and Holte 1981; Bock and others 1984; Lang
1973). If the grazing-free period has been short (less than 20
years; Brady and others 1989), shrub canopy may remain
unchanged or increase greatly (sagebrush ground cover
increased 293 percent inside and 30 percent outside an
exclosure in northeastern Wyoming; Lang 1973).
Removal of livestock grazing from rangelands can be
evaluated by using fenced grazing exclosures to compare
vegetation inside with an adjacent grazed area. Long-term
exclosures can reveal slow vegetative trends; however, several limitations of exclosures have been noted. Vegetative
conditions between the grazed and protected area may have
differed at the time of exclosure establishment (Costello and
Turner 1941). The exclosure itself may act as a barrier to wind
movement and affect soil and moisture deposition, altering
temperature and humidity (Daubenmire 1940). Because
exclosures were often constructed where vegetative change
due to grazing had already occurred, these sites may not
reflect the response of pristine vegetation. However, these
sites do reflect the response of grazed vegetation to cessation
of grazing by domestic livestock. Because range scientists
are finding the Dyksterhuis (1949) view of range succession
is not accurate where woody species dominate, Archer (1994)
has distinguished that shrub-driven succession may deviate
from grass-driven succession. Consequently, long-term
USDA Forest Service Proceedings RMRS-P-31. 2004
Impacts of Livestock Exclusion From Wyoming Sagebrush Communities
Muscha, Hild, Munn, and Stahl
exclosures in shrubland ecosystems can clearly demonstrate
the impacts of grazing removal in much of the semiarid west.
Finally, because biotic crusts are highly susceptible to soil
surface disturbance such as trampling or offroad vehicles
(Belnap and Gardner 1993), and their biomass and biotic
activity are concentrated within 3 mm of the soil surface
(Garcia-Pichel and Belnap 1996), they are usually influenced by domestic livestock use. Although biotic crusts are
often more developed inside Great Basin exclosures
(Kaltenecker and others 1999), little is known of their
presence and richness in Wyoming shrublands. We selected
nine 30- to 45-year-old rangeland exclosures within Wyoming sagebrush steppe and examined shrub and biological
soil crust presence to better understand impacts of grazing
cessation.
Rankin Basin
(1965)
Worland Cattle (1966)
Big Trails (1962)
Lander Ant
(1957)
Between 1959 and 1965 approximately 100 rangeland
exclosures were constructed in Wyoming for a cooperative
study by the University of Wyoming, the Wyoming Agricultural Experiment Station Cooperative program, and the
Bureau of Land Management (Fisser 1967, 1968). Studies of
vegetational change by use of 0.6-m (2-ft) by 6.1-m (20-ft)
and 1.2-m (4-ft) by 1.2-m (4-ft) plots were initiated in 1959
and 1960 (Fisser 1968). The study of vegetation change by
use of permanent 30.3-m (100-ft) transects of 20 plots, each
0.3-m (1-ft) by 0.3-m (1-ft) square was initiated in 1965
(Fisser 1967). Some of these exclosures were sampled
annually until the early 1970s, and all sampling using the
original methods ceased after the early 1980s. A small
number of these exclosures have been extensively studied
since establishment (Garland 1972; Gdara 1977; JohnsonBarnard 1995; Uhlich 1982).
Site Selections and Description
Nine exclosure study sites were selected within Wyoming
big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis
Beetle and Young) in Natrona, Fremont, Sweetwater, Bighorn, and Washakie Counties in Wyoming (fig. 1). Sites were
selected to have comparable slope and elevation with minimal disturbance (no breaks in fences). Ages of exclosures
ranged from 32 to 46 years (table 1) and ranged in size from
2.5 to 12.5 ha (1 to 5 acres). Dominant grass species found at
exclosure sites were western wheatgrass (Agropyron smithii
Rydb.), Sandberg bluegrass (Poa secunda Presl), bluebunch
wheatgrass (Agropyron spicatum (Pursh) Scribn. & Smith),
junegrass (Koeleria macrantha (Ledeb.) Schult.), needle and
thread (Stipa comata Trin. & Rupr.), blue grama (Bouteloua
gracilis (H.B.K.) Lag. ex Steud.), and Indian ricegrass
(Oryzopsis hymenoides (R. & S.) Ricker) (table 1). Donlin was
the only exclosure dominated by threadleaf sedge (Carex
filifolia Nutt.). Cheatgrass (Bromus tectorum L.) was found
at five of the nine sites: Poison Spider, Big Trails, Powder
Rim D, Worland Cattle, and Sand Creek.
Soil surface texture (Blake and Hartge 1986) was analyzed on soil samples retrieved from inside and outside
exclosures. Grazing history at each exclosure site was
USDA Forest Service Proceedings RMRS-P-31. 2004
Donlin (1958)
Poison Spider
(1965)
Red Wash #2
(1958)
Powder Rim D (1970)
Materials and Methods ___________
Exclosures in Wyoming
Sand Creek (1970)
Figure 1—Locations of nine range exclosures
sampled in Wyoming in 2001 and 2002 (year of
establishment is indicated in parentheses).
maintained at the Bureau of Land Management Regional
offices. Current grazing use and annual precipitation is
reported in table 1. Seven exclosures in our study had sandy
loam soil textures (table 1). The Poison Spider exclosure had
15 percent more sand content inside the exclosure than
outside. Poison Spider and Big Trails have the greatest
annual precipitation of the exclosures examined. Donlin and
Powder Rim D have comparable annual precipitation and
soil textures. Sand Creek and Worland Cattle also have
comparable annual precipitation and soil texture, but are
drier than Donlin and Powder Rim D sites. Rankin Basin
and Red Wash #2 are the driest sites in our study because of
low precipitation, and their soils have greater clay content
than the other seven exclosures (table 1). Most exclosure
sites are grazed in spring or in all seasons on a rotational
basis. Rankin Basin site is different from other exclosures in
its light grazing use only in winter.
Methods
In 2001 and 2002 we placed vegetative line transects 20 m
in length at each exclosure. The number of transects (three
to four inside and outside each exclosure) at each site was
determined by the size of the exclosure. Basal and canopy
cover was recorded on each transect. Transects were evenly
spaced inside the exclosure and permanently marked on
both ends with a metal stake. Outside the exclosure, transects
were placed the same distance from the fence line as the
inside transects. Basal and cover data at each site was
analyzed using a two-group t-test (alpha 0.05) to compare
grazed to grazing removal treatments. We compared our
vegetative data to original records collected by Fisser at the
time of exclosure construction. Because Fisser’s initial
samples included only a single transect in each exclosure
and position (no replicates), we did not conduct analysis of
variance on initial data sets. Some differences in vegetative
cover between initial samples and our data may be partially
177
Muscha, Hild, Munn, and Stahl
Impacts of Livestock Exclusion From Wyoming Sagebrush Communities
Table 1—Site characteristics of nine Wyoming shrubland exclosures sampled in 2001 and 2002.
Exclosure
name
Established
Elevation
Annual
precipitation
Big Trails
year
1962
m
1,515
mm
348
Poison Spider
1965
1,758
Lander Ant
1957
Donlin
Dominant grass/
grasslike species
(in order of abundance)
Soil
texture
Domestic
animal
Months
grazed
Stocking
rate
Acres
per
AUM
3.0
Sandberg bluegrass,
western wheatgrass,
bluebunch wheatgrass,
cheatgrass
Sandy
loam
Cattle,
horses
Nov. to Jan.,
first year
April to June,
second year
Rested
third year
303
Cheatgrass, Sandberg
bluegrass, western
wheatgrass, junegrass
Sandy
loam
Cattle,
sheep
March to June
5.5
1,621
221
Needle and thread,
blue grama, Sandberg
bluegrass, western
wheatgrass
Sandy
loam
Cattle
March to April
Nov. to Feb.
10.0
1958
1,879
285
Threadleaf sedge,
needle and thread
Sandy
loam
Cattle
April-June
Sept. to Oct.
Powder Rim D
1970
1,939
277
Needle and thread,
Sandberg bluegrass
Sandy
loam
Cattle,
sheep
Deferred
rotation, all
Worland Cattle
1970
1,300
194
Needle and thread,
cheatgrass, western
wheatgrass
Sandy
loam
Cattle,
sheep
March to May,
Oct. to Dec.a
9.0
Sand Creek
1966
1,333
194
Needle and thread,
Sandberg bluegrass
Sandy
loam
Cattle,
sheep
March to May,
Oct. to Dec.a
11.9
Rankin Basin
1958
1,932
172
Blue grama,
needle and thread
Sandy
clay loam
Cattle
Oct. to May
18.0
Red Wash #2
1965
1,576
258
Needle and thread,
Indian ricegrass,
bluebunch wheatgrass
Sandy
clay loam
Cattle
Dec. to April
10.0
a
6.1
10.0
Grazing use highly varied in this exclosure since established.
attributed to differences in sampling methods. However,
because our data include only a single observer (Muscha)
this effect is uniform across the nine exclosure sites.
Results ________________________
Exclosure sites represent environments with varieties of
plant available moisture because of precipitation and soil
disparities between sites (table 1). Consequently our tables
and figures present the sites roughly ordered from most
mesic (Big Trails and Poison Spider) to most xeric (Red Wash
#2) based on precipitation and soil texture differences.
Basal cover differed between inside and outside positions
at four of the nine exclosure study sites (P < 0.05). At Donlin,
Big Trails, and Poison Spider, bare soil was greater, and
litter cover was less outside than inside exclosures (fig. 2). In
two of these sites (Poison Spider and Big Trails) litter was
the dominant cover inside exclosures (76 percent and 64
percent, respectively), while at all other sites bare soil was
the dominant cover inside exclosures. Litter in the Poison
Spider exclosure was primarily due to invasion of cheatgrass
178
at this site (fig. 3). Cheatgrass was also present at the Big
Trails site, but absent from Lander Ant. Cheatgrass cover
was greater inside Poison Spider and Big Trails exclosures.
Powder Rim D exclosure was unique because it was the only
site where cover of biological soil crusts differed between the
two positions and was greater inside the exclosure. At all
sites, biological soil crust cover was less than 10 percent
regardless of position.
Vegetative canopy cover differed between positions (inside and outside) at Big Trails, Poison Spider, Powder Rim D,
and Red Wash #2 sites (fig. 3). Native grass cover was
greater inside the exclosure at Powder Rim D (P < 0.05); at
Red Wash #2 and Poison Spider, native grass cover was
greater outside the exclosures.
Only Poison Spider and Rankin Basin exclosures had
initial shrub canopy cover more than 10 percent at the time
of exclosure construction (figs. 4 and 5). In general, shrub
cover increased from time of construction to present at all
exclosure sites, regardless of position, and overall increases
were greater inside exclosures than outside. In the most
recent samples, shrub cover differed between positions (t-test,
<0.05) only at Poison Spider (fig. 5).
USDA Forest Service Proceedings RMRS-P-31. 2004
Impacts of Livestock Exclusion From Wyoming Sagebrush Communities
Muscha, Hild, Munn, and Stahl
Percent ground cover
100
80
*
60
40
20
0
In
Out
Big Trails
In
Out
Poison Spider
In
Out
Lander Ant
In
Out
Donlin
In
Out
In
Out
In
Out
Powder Rim D Worland Cattle Sand Creek
Soil crust
Litter
In
Out
Rankin Basin
In
Out
Red Wash #2
Bare soil
Figure 2—Soil cover (crusts, litter, and bare ground) in two positions (inside and outside grazing exclosures) at nine
sagebrush study sites in Wyoming (within a site and cover type asterisks delineate significance at P < 0.05, t-test).
Percent canopy cover
100
90
80
70
60
50
40
30
20
10
0
In
Out
Big Trails
In
Out
Poison Spider
In
Out
Lander Ant
Shrub
In
Out
Donlin
Native grass
In
Out
In
Out
In
Out
Powder Rim D Worland Cattle Sand Creek
Cheatgrass
In
Out
In
Out
Rankin Basin Red Wash #2
Forbs and succulents
Figure 3—Canopy cover (shrub, native grass, cheatgrass, and forbs and succulents) in two positions (inside and
outside grazing exclosures) at nine sagebrush study sites in Wyoming (within a site and cover type asterisks
delineate significance at P < 0.05, t-test).
Discussion _____________________
Effects of Grazing Removal
The range of plant available moisture represented by the
nine exclosure sites offered a variety of responses to removal
of domestic stock from sagebrush steppe systems. Overall, at
more xeric sites there were few differences inside and outside exclosures. Shrub cover increased both inside and outside exclosures as they aged, regardless of precipitation and
soil differences. The Red Wash #2 site is unique in having
more native grass cover outside the exclosure. This difference
is likely due to a low stocking rate and use in winter months,
allowing for favorable grass regrowth prior to our sampling in
June. Other xeric sites were used in the spring, prior to
sampling. In general, the most xeric sites have shown limited
response to grazing removal so far. Moderately moist sites
USDA Forest Service Proceedings RMRS-P-31. 2004
such as Powder Rim D seem to reflect more response to
grazing removal, and the greatest response is seen in the
most mesic sites (Big Trails and Poison Spider). The Lander
Ant site may be unique in lack of response; however, this site
differs from Big Trails and Poison Spider in its use only by
cattle, lower stocking rate, and slightly lower precipitation.
Additionally, cheatgrass is a large part of the vegetative
cover in the Big Trails and Poison Spider sites and is absent
from Lander Ant. The difference between positions at these
two exclosures demonstrates the potential for transitions to
shrub-driven succession as suggested by Archer (1994) and
others. The unique interactions of available moisture, grazing animal, and season of use are impossible to separate in
this study but provide important insight relative to the
mechanisms involved in sagebrush increase. Additionally,
none of the nine exclosures have been burned since establishment, and so generalizations relative to cheatgrass
179
Muscha, Hild, Munn, and Stahl
Impacts of Livestock Exclusion From Wyoming Sagebrush Communities
Shrub cover (percent)
Inside
Outside
Big Trails
Lander Ant
40
40
30
30
20
20
10
10
0
0
1966
1978
2002
1959
Donlin
40
30
30
20
20
10
10
0
0
1980
1967
2001
40
40
30
30
20
20
10
10
0
0
1978
1987
2001
Sand Creek
Worland Cattle
1966
2001
Powder Rim D
40
1965
1980
1971
2002
Rankin Basin
1978
2002
Red Wash #2
40
40
30
30
20
20
10
10
0
0
1965
1978
2002
1961
1981
2001
Figure 4—Shrub cover change at eight Wyoming exclosure sites from year of
establishment to present (2001 or 2002 sampling dates). Order based on total
canopy cover inside exclosures, soils, and precipitation (Poison Spider shown
in fig. 5).
180
USDA Forest Service Proceedings RMRS-P-31. 2004
Impacts of Livestock Exclusion From Wyoming Sagebrush Communities
shrub canopy to increase. In most cases, shrub canopy cover
increased comparably with or without domestic grazing
livestock use. Explanation for these differential responses to
grazing exclusion requires further examination.
Percent shrub cover
Poison Spider
A
40
30
B
20
10
0
1965
1980
2001
Year
Inside
Outside
Figure 5—Shrub cover in Poison Spider
exclosure from time of establishment to 2001
(within a year, columns with the same letter
do not differ P > 0.05, t-test).
dynamics in Wyoming sagebrush steppe are speculative in
the absence of fire.
Biological Soil Crusts
Biological soil crusts have received increased attention in
arid land research, and our research has found that biological soil crusts are a visible presence in some shrubland
ecosystems in Wyoming. Although we have no records of the
cover of biotic crusts at time of exclosure establishment, we
can only assume that they were similar across the sites prior
to the time of fence construction. Although total cover of
biological soil crusts was significantly greater inside only
one exclosure (Powder Rim D; fig. 2), our data indicate a
general trend of higher cover inside all exclosures. Additionally, crusts remain to be detailed for morphological characteristics or richness as suggested by Rosentreter (this proceedings). Several studies have shown biological soil crusts
are negatively impacted by and highly susceptible to livestock trampling (Anderson and others 1982; Beymer and
Klopatek 1992; Jeffries and Klopatek 1987; and others).
Cover of biological soil crusts was less than 10 percent inside
and outside exclosures at all sites even after more than 40
years of domestic livestock removal and do not appear to
reflect stocking rate differences among our sites.
Conclusions ____________________
The small differences between the inside and outside
positions at our nine exclosure sites indicate that some
shrubland systems will not necessarily improve to a more
productive system after 40 years of livestock exclusion. Our
data demonstrate shrub cover will not always increase
where stock are excluded at a greater rate than with livestock present. It is clear that the release of grazing pressure
does not decrease shrub canopy; in some cases it may allow
USDA Forest Service Proceedings RMRS-P-31. 2004
Muscha, Hild, Munn, and Stahl
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