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Community Stability in a Salt-Desert
Shrubland Grazed by Sheep: the Desert
Experimental Range Story
Stanley G. Kitchen
Derek B. Hall
The two-dimensional ball and trough analogy is a useful
method of presenting the multiple stable-state model
(Laycock 1991; Tausch and others 1993). In figure 1, the ball
symbolizes the composition of a community at a given point
in time. It rests on a line curved to a contour of valleys and
hills representing stable states (A and B) and intervening
thresholds (C). The shape of the line is a function of the
physical and biological environment. Trough depth depicts
stability (Laycock 1991).
For as long as the ball (community composition) stays
within a single trough (stable state) it more or less follows
the successional model, meaning, the ball will always return
to rest at the bottom of the trough (climax vegetation)
whenever disturbance is removed. When a disturbance
occurs with adequate force to overcome threshold height, the
ball crosses into a new trough. At this point the Clementian
model is no longer adequate to describe the possible compositional trajectories of a community. The reversibility of a
move across a threshold depends upon threshold height.
Constant disturbance, such as moderate grazing, may lack
the full energy needed to move the ball over a threshold but
will hold it on the slope of the trough. Such a ball (community), though stationary, is less stable than one in the trough
bottom because of its more proximal position to the threshold crest. The breadth of the trough bottom is indicative of
community flexibility, or the degree to which it can vary in
composition without a change in stability.
Ahstract-The effects of 59 years of winter and spring grazing on
a Great Basin salt-desert shrub land were analyzed using frequency
and cover data. Spring grazing altered species composition more
than winter grazing when compared to the nongrazed exclosures.
Grazing in both seasons resulted in significant decreases in shrub
importance. Introduced annuals increased in importance with spring
grazing. Spring-grazed pastures show patterns of destabilization
that are missing from winter-grazed pastures and nongrazed
exclosures.
Traditionally applied concepts used in managing grazing
on Western United States rangelands are founded in what
has been called the "climax" (Friedel 1991) or the "successional" (Westoby and others 1989) model. The framework of
this model was developed by Clements (1916) and applied to
the management of rangelands by Sampson (1919). It is
based on the assumption that: (1) a single climax state exists
for each site; (2) each of a series of seral states predictably
gives way in succession to subsequent states until the climax
state is reached, implying a single pathway for succession to
follow; (3) disturbance, such as that caused by grazing, has
the opposite effect as succession; and (4) all changes in
successional position are reversible. Consequently, all one
must due to restore a community degraded by grazing is
reduce or eliminate grazing, and successional forces will, in
time, complete the restoration. The closed nature of the
model renders irrelevant the concept of stability.
Vegetative communities on many arid and semiarid rangelands worldwide do not respond as predicted by the successional model (Friedel 1991; Laycock 1991). Alternative models have recently gained acceptance as more accurate tools
in describing and explaining changes on rangelands (Laycock
1991). These models incorporate concepts of multiple stable
states and successional pathways and generally hold that
movement between states requires crossing thresholds.
Reversibility, at least on a practical time-scale (several to
many decades), is often lost.
c
A
In: Barrow, Jerry R.; McArthur, E. Durant; Sosebee, Ronald E.; Tausch,
Robin J., comps. 1996. Proceedings: shrubland ecosystem dynamics in a
changing environment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. Rep.
INT-GTR-338. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
Stanley G. Kitchen is a Botanist at the U.S. Department of Agriculture,
Forest Service, Intermountain Research Station, Shrub Sciences Laboratory,
Provo, UT 84606. Derek B. Hall is a Reclamation Scientist with Bechtel
Nevada, Las Vegas, NV 89130.
•
B
Figure 1-The two-dimensional ball and trough
diagram can be used to illustrate relationships
among stable states and intervening thresholds.
102
Abiotic and biotic changes in the environment, such as
climate change and species introductions, alter relationships among thresholds and stable states and are expressed
in the diagram as a change in line curvature (Tausch and
others 1993). Such changes can alter the stability (depth) of
a state (trough) as well as create new stable states. As a
result, communities that have demonstrated considerable
resilience to disturbance beforehand might become susceptible to lower levels of disturbance following changes in
environmental parameters.
and paired with each of the 32 exclosures (two per pasture).
Study area pairs were grouped by grazing season: 20 in
pastures grazed in winter only, and 12 in pastures with
spring grazing. Justification for ignoring grazing intensity
is provided by Harper and others (1990) and Whisenant and
Wagstaff (1991).
Within each study area, four 50 m parallel transects were
spaced at 10 m intervals. On each transect, 10 nested
sampling configurations were randomly located alternating
on either side of the transect line. Each configuration had an
area of 4.0 m 2 (200 x 200 cm) with smaller, nested plots of
1.0,0.25, and 0.06 m 2 • Transects were read in June and the
first week of July 1994.
Study area summed frequency values (SFV's) were determined for each vascular plant species using a modification of
methods described by Smith and others (1987). Maximum
potential SFV for any species was 160. Relative differences
in SFV's among paired study areas were determined for the
12 most abundant species using the formula: relative difference = [SFV(grazed) - SFV(nongrazed)]/SFV(nongrazed)
x 100. Resulting values indicate the effect of grazing season
on species frequency when compared to no grazing both in
direction and magnitude.
We estimated canopy cover from 400 point intercept observations per study area (10 points per nested configuration).
Mean cover percentages were calculated for total vegetation,
shrubs, perennial grasses, and introduced annuals.
Summed frequency data were used to calculate species
diversity values weighted for species abundance for each
study area using MacArthur's Diversity Index (MacArthur
1972). An index of similarity between each pair of study
areas was calculated for all species combined and shrubs
and perennial grasses only (Ruzicka 1958) (table 1).
Plant nomenclature in this paper follows that of Goodrich
(1986). Data were subjected to analysis of variance using the
General Linear Model (Minitab). Significant differences
(p < 0.10) among means were determined using Fisher's least
significant difference (LSD).
Study Site _ _ _ _ _ _ _ __
The Desert Experimental Range was established in Pine
Valley, Millard County, Utah, in 1933 as a site where longterm effects offall, winter, and spring sheep grazing could be
studied (Clary and Holmgren 1982). The experimental area
of this site is representative of about 180,000 km 2 of mixed
salt-desert shrubland found in the Great Basin and neighboring areas of the Western United States (Holmgren 1975).
Winters are cold and summers are warm. Mean January
and July temperatures are -3.5 and 23.3 °c, respectively
(Holmgren 1975). Approximately half of the 157 mm mean
annual precipitation occurs during a 7-month period of soil
moisture accumulation (October-April), mostly as light snowfalls of 5 cm or less.
Prior to establishment of the Desert Experimental Range,
plant communities in Pine Valley had been significantly
altered due to a half century of unrestricted access to public
lands by livestock operators (Holmgren 1975). In 1934, 20
pastures of either 97 or 130 ha were established to study the
long-term effects of sheep grazing on the desert community.
In each of 16 pastures, two exclosures of 0.4 ha each were
established in 1935. Treatment combinations of fall (early
winter), midwinter, and spring (late winter) grazing at light,
moderate, and heavy levels (average of 25, 35, and 42 sheep
days per hectare, respectively) were assigned to each pasture (Hutchings 1966). Grazing treatments have been applied annually from 1935 to present with actual sheep use
days adjusted according to available forage. Earlier investigations (Holmgren and Hutchings 1972) revealed no differences between the effects of fall and winter grazing treatments and led to a modification in which both dormant-season
grazing treatments were applied during a single winter
(dormant) period.
The long-term study conditions at the Desert Experimental Range are well suited for investigating the effects of
sheep grazing on the stability of salt-desert plant communities. Here we examine differences in species composition and
cover between paired grazed and nongrazed study areas and
seek to characterize possible stable states. We consider the
stability of communities under the different grazing treatments and explore the possible ramifications of species
introductions on community stability. Finally, we make
management recommendations based upon our conclusions.
Results
------------------------------------
Fifty-eight species were encountered across all transects
including: shrubs (10), perennial grasses (10), cacti (4),
perennial forbs (17), annual forbs (16), and annual grass (1).
All species except for the annual forbs halogeton (Halogeton
glomeratus) and Russian thistle (Salsola iberica) and the
annual grass cheatgrass (Bromus tectorum) are native to the
site.
Table 1-Similarity indices (Ruzicka 1958) based upon SFV's for
paired grazed and nongrazed study areas for all
species, shrubs only, and perennial grasses only.
Asterisks indicate Significant differences (p < 0.10).
Pastures
Methods
Winter
-----------------------------------
All species
Shrubs
Perennial grasses
The 16 pastures at the Desert Experimental Range with
exclosures were used for this study. Grazed study areas (0.4
ha each), with similar soils and aspect, were located near to
103
62.6
65.2
70.9
Spring
54.8
46.5
68.7
Table 2-Mean SFV's for 12 species across all grazed and exclosed
study areas. Asterisks indicate significant differences
between treatment means for individual species (p < 0.1 O).
Differences for winterfat and purple three-awn approached
significance with p-values of 0.11 and 0.12, respectively.
Grazing season effects on relative differences in sum
frequency are illustrated in figure 2. A significant negative
effect of spring compared to winter grazing was observed for
budsage, Indian ricegrass, and squirreltail. This was in spite
of the significant positive response of Indian ricegrass to
grazing when season is ignored. Spring grazing significantly
favored galleta (Hilariajamesii) and halogeton, both warm
season species, and had a near significant, positive effect for
cheatgrass (p = 0.11).
Vegetative cover expressed both in absolute percentages
and relative to total vegetative cover is shown in table 3.
Shrub cover is significantly lower for both grazing seasons
when compared to nongrazed exclosures. Perennial grass
cover was highest in winter grazed pastures, although
differences were not statistically significant. Spring grazing
resulted in significantly higher cover percentages for introduced annuals compared to both winter grazing and
nongrazed exclosures.
We detected no significant differences in species richness
due to grazing treatment with a mean of 19.7 species detected per sample area. MacArthur's Diversity Index yielded
8.7 and 7.9 species of equal abundance (frequency) in
Summed frequency per study area
Exclosed
Grazed
Species
Shrubs
Shadscale
Winterfat
Budsage
Low rabbitbrush
Perennial Grasses
Indian ricegrass
Sand dropseed
Galleta grass
Purple three-awn
Squirreltail
Perennial Forbs
Gooseberryleaf globemallow
Introduced Annuals
Cheatgrass
Halogeton
36.0
60.7
68.7
20.6
49.2
46.4
30.8
19.8
80.5
71.3
45.5
34.5
26.3
98.6
81.9
48.6
24.1
16.9
15.8
13.3
43.3
14.0
51.5
28.1
Table 3-Mean cover values for exclosures, winter-grazed, and
spring-grazed study areas. Asterisks indicate significant
differences (p < .10) among grazed and exclosed, or
winter-grazed and spring-grazed study areas. Numbers
in parentheses indicate relative cover percentages
(proportional to total vegetative cover).
Mean SFV's for the 12 most common species are found in
table 2. Grazing-related differences for Greenes low rabbitbrush (Chrysothamnus greenei), sand dropseed (Sporobolus
cryptandrus), and gooseberryleaf globemallow (Sphaeralcea
grossulariifolia) were not significant. Values for the shrubs
buds age (Artemisia spinescens) and winterfat (Ceratoides
lanata), and for the perennial grasses squirreltail (Sitanion
hystrix) and purple three-awn (Aristida purpurea) are significantly lower for grazed areas (grazing season ignored)
than for nongrazed areas. Significantly higher grazingrelated values were found for the shrub shad scale (Atriplex
confertifolia), the perennial grass Indian ricegrass (Oryzopsis
hymenoides), and halogeton.
o=
o
60
Cover percentage
Grazing treatment
Exclosures
Winter
Spring
---------------------- Percent ---------------------
Total vegetation
Shrubs
Perennial grasses
Introduced annuals
1 (5)
Winter Grazing
-20
-40
-60
Q)
-80
Qi
P::
1 (5)
20
4 (20)
12 (60)
4 (20)
[SJ Spring Grazing
20
al
21
5 (24)
15 (70)
No Grazing (59 years)
40
~
23
9 (39)
12 (54)
-100
ATCO
CELA
ARSP
CHGR
ORHY
SPCR
I ----- SHRUBS - - - - - - I -------I ------------------- PERENNIAL
HIJA
ARPU
SIHY
SPGR
BRTE
HAGL
I FORB I GRASS I FORB I
- - - - - - - - - - - - - - - - - - I - ANNUAL - I
GRASSES - - - - - - -
104
Figure 2-Relative differences in summed
frequency for winter-grazed and springgrazed pastures when compared to
exclosures. Graph depicts both the direction and magnitude of that difference. Asterisks indicate significant differences
(p < 0.10) between the effects of the two
grazing seasons. ATCO = shadscale,
CELA = winterfat, ARSP = budsage,
CHGR = Greenes low rabbitbrush,
ORHY = Indian ricegrass, SPGR = sand
dropseed, HIJA =galleta, ARPU =purple
three-awn, SIHY = squirreltail, SPGR =
gooseberryleaf globe mallow, BRTE
cheatgrass, and HAGL = halogeton.
nongrazed and grazed study areas, respectively. The difference in values approaches the statistical threshold for significance (p = 0.13). Differences in the indices associated
with grazing season were not significant.
A mean similarity index of 60.1 was calculated across all
species and study area pairs. Similarity values were higher
for winter than for spring pastures for all species combined
and for shrubs separately. Across all pairs, values for perennial grasses (mean 70.2) were significantly higher than
those for shrubs (mean 59.4).
(winterfat, shadscale, Indian ricegrass). Conversely, the
rapid growth rate of cheatgrass more than compensated for
the relatively high degree of overlap between its growing
season and time of maximal use by grazers.
Plant species sometimes inhibit herbivory by allocating
carbohydrate resources to structural and chemical characteristics that discourage foraging animals. The spines of
shadscale and secondary metabolites oflow rabbitbrush are
examples at the Desert Experimental Range. These strategies are deployed at a cost to growth rate. This is generally
a beneficial trade-off for perennials where the risk of herbivory is high.
Although single plant responses to herbivory are often
negative and frequently lead to an increase in mortality rate,
whole populations may compensate with increases in seedling survival. Consequently, herbivory has the effect of
lowering mean plant age (Tilman 1988). Thus, species that
reach reproductive maturity more quickly have an advantage over slower maturing species in communities with
significant herbivory-related losses. Shadscale is a shorter
lived, faster maturing species than winterfat or budsage
(Blaisdell and Holmgren 1984). This relatively abbreviated
life history contributes to the success of this species in
grazing impacted communities.
In summary, species that succeed under grazing pressure
do so through strategies of avoidance or tolerance. At the
Desert Experimental Range, avoiding species are cool-season dormant, or employ protective structures or biochemistry to discourage herbivores. Tolerating species have fast
growth rates, extended growing seasons, and/or sh0r.ter
lifespans. Species most impacted by cool-season graZIng
possess traits that may be highly adaptive in the absence of
herbivory but make the species susceptible to grazing damage. These traits include conservative growth rate, obligate
summer dormancy, exposed meristems, long life-span, and
a lack of protective strategies.
Grazing practices in northern Pine Valley prior to the
establishment of the Desert Experimental Range were, in
the absence of introduced annuals, probably not severe
enough to cause irreversible changes in species frequencies.
Therefore, the composition of communities in exclosures,
where sheep have been excluded for 59 years, might approximate that of presettlement times.
Structurally, all protected communities were similarly
dominated by shrubs and/or perennial grasses, often in
roughly equal proportions (table 1). Budsage, winterfat,
Indian ricegrass, and sand drop seed were the most frequently encountered species (MSF > 50). Shadscale, galleta,
purple three-awn, squirreltail, and cheatgrass were also
common (50) MSF > 25). Though minor contributors to total
biomass (Hutchings and Stewart 1953), several species of
native forbs were encountered in most exclosed study areas.
A total of 31 native forb species, representing a wide range
in life-history strategies, were sampled.
The most abundant introduced annuals at the Desert
Experimental Range are cheatgrass, halogeton, and Russian thistle (Harper and others, this proceedings). Their
occurrence in the 32 exclosures was minor except on permanent rodent mounds where they have become the dominant
vegetation.
Discussion __________
Herbivory alters stability by disrupting competitive balance among individuals of co-occurring species, thus providing an opportunity for invasion and/or expansion of species
affiliated with one or more alternate stable states (Harper
1969). The possible effects of pastoral activities on community stability are certainly not limited to those caused by
herbivory. However, the effects of other grazing-related
processes such as trampling, soil compaction, and disruption of cryptobiotic crusts are probably secondary in importance. Therefore, an examination of the mechanisms by
which herbivory modifies competitive relationships should
provide a satisfactory explanation for grazing-related differences in community composition.
Herbivory is the removal of living plant tissue from a plant
for food. When vegetative parts are removed, photosynthetic
capacity is at least temporarily reduced. At some point, this
results in a reduction in growth rate and may reduce reproductive output and/or the ability of the plant to survive
stress. When reproductive parts are removed, the capacity to
replace dying individuals is impaired. Thus, herbivory may
reduce the ability of plants to compete for limited resources
and to leave progeny.
Herbivory is expressed selectively in all natural communities. Selectivity varies with species of herbivore , season of
use, and density of target species (Harper 1969). Ofparticular interest at the Desert Experimental Range are changes
in utilization due to seasonal differences in desirability of
target species to sheep. For example, sheep consume smaller
amounts of Indian ricegrass during winter when it is dormant than in the spring when succulent green shoots are
available. Other species that are more highly selected in
spring than in winter include budsage and squirreltail. As
expected, these three species had significantly lower SFV's
in spring-grazed versus winter-grazed pastures (fig. 2). We
observed no seasonal differences for shrub species that
provide forage of comparable palatability in both winter and
spring (winterfat and shadscale). Higher SFV's for warm
season herbaceous species (galleta, sand dropseed, and
halogeton) are attributable to dormancy during grazing
season and competitive release.
The magnitude of the impact of herbivory is inversely
proportional to growth rate and to length of growing season.
At the Desert Experimental Range, we observed that palatable species that grow slowly and only during the spring
(budsage and squirreltail) were more negatively impacted
by grazing than those that grow at moderate rates and are
able, at least in some years, to "recover" after spring grazing
105
Winter-grazed areas had fewer and smaller shrubs and
more perennial grass plants than nongrazed areas. Among
shrubs, shadscale increased in importance while winterfat
and budsage decreased. Indian ricegrass and sand dropseed
increased while the less abundant squirreltail decreased.
Native forb abundance and diversity did not appear to be
affected by grazing. However, low density size made the
detection of significant differences difficult. The distribution
ofintroduced annuals in winter-grazed pastures was similar
to that of exclosures.
Shrubs in spring-grazed pastures were small, often oflow
vigor, and widely scattered. Budsage has almost been eliminated. The effects on perennial grasses were mixed. Coolseason grasses had frequencies equal to or lower than those
in exclosures. Warm-season grasses generally did better
than in exclosures. One exception was purple three-awn.
This grass may suffer more from trampling or other secondary grazing effects than from herbivory. There were no
measurable effects of spring grazing on native forbs. Introduced annuals were significantly more abundant in springgrazed than in winter-grazed pastures and nongrazed study
areas. This effect was amplified in 1995, a year of near record
precipitation, creating clear contrasts among neighboring
pastures of different-season grazing treatments (Harper
and others, this proceedings).
Results suggest that winter-grazed study areas and their
nongrazed pairs are not separated by any significant threshold and are probably equally stable, indicating plasticity in
community stability. Conversely, reversion of spring-grazed
study areas to shrub-dominated landscapes may require
more than reductions or even elimination ofli vestock. Harper
and others (1990) demonstrated that downward trends for
budsage and winterfat in spring-grazed pastures have continued since establishment, while trends for the same species on winter and nongrazed areas have continued to rise
during the same time period. Assuming the rates of change
in community composition were somewhat constant in the
spring-grazed pastures, it would take at least 120 years after
elimination of grazing to fully restore these species to levels
found in exclosures. This process could be slowed further by
the virtual loss of seed sources and increased dominance of
introduced annuals. Thus, we argue that, for all practical
purposes, spring-grazed pastures have passed into an alternative compositional state of unknown stability.
Locations at the Desert Experimental Range with shrub/
perennial grass communities have been almost completely
displaced by introduced annuals (Harper and others, this
proceedings). The rodent mounds mentioned previously,
which make up approximately 10 percent of the area in
experimental pastures, are a case in point. Alien annuals
now dominate large areas on the fine-textured soils of valley
bottoms that were once dominated by winterfat. Similar
areas of annualization are found in many of the valleys of the
Great Basin. There are probably many causes for the loss of
perennial cover (McArthur and others 1990), including but
not limited to, mismanagement of domestic livestock grazing. Apparently, intact perennial communities on some soil
types are at risk of annualization regardless of how grazing
is managed (Harper and others, this proceedings; Tausch
and others 1994). At the Desert Experimental Range, this
change in stability is due primarily to the advent of exotic
weeds. In the ball and trough model, this community is
represented by a change in line curvature, creating a new
"valley," or stable state, dominated by annuals (Tausch and
others 1993).
Management Recommendations _
Continued winter (dormant season) grazing with sheep at
moderate levels appears to pose little threat to the stability
of these communities. Spring grazing increases the risks of
shrub loss and conversion to annuals. Common sense suggests that the effects of spring grazing might be minimized
under a conservative deferred grazing system. Using current technology, attempted restoration of annualized lands
may not be prudent due to costs and high probability of
failure. Even when restored, such communities may be
highly unstable due to the presence of introduced annuals.
More exhaustive studies are clearly needed to evaluate
management options for annualized ranges in the Intermountain West.
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