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Livestock grazing effects on fuel loads for wildland fire in sagebrush dominated ecosystem

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Volume 1, 2014
pp. 35-57
ISSN: 2331-5512
35
Livestock Grazing Effects on Fuel Loads for
Wildland Fire in Sagebrush Dominated
Ecosystems
Eva K. Strand
1
, Karen L. Launchbaugh
2
, Ryan Limb
3
, L. Allen Torell
4
Keywords: annual grasses, wildlife fire, prescribed grazing, sagebrush, fuel treatments
AGROVOC Terms: fire ecology, herbivory, invasive species, grazing, grazing
management, sagebrush
Abstract
Herbivory and fire are natural interacting forces contributing to the maintenance of
rangeland ecosystems. Wildfires in the sagebrush dominated ecosystems of the Great
Basin are becoming larger and more frequent, and may dramatically alter plant
communities and habitat. This synthesis describes what is currently known about the
cumulative impacts of historic livestock grazing patterns and short-term effects of
livestock grazing on fuels and fire in sagebrush ecosystems. Over years and decades
grazing can alter fuel characteristics of ecosystems. On a yearly basis, grazing can reduce
the amount and alter the continuity of fine fuels, potentially changing wildlife fire
spread and intensity. However, how grazing-induced fuel alterations affect wildland fire
depends on weather conditions and plant community characteristics. As weather
conditions become extreme, the influence of grazing on fire behavior is limited,
especially in communities dominated by woody plants.
1
Eva Strand (evas@uidaho.edu) - Assistant Professor of Landscape Disturbance Ecology,
Department of Forest, Rangeland, and Fire Sciences, University of Idaho, 875 Perimeter
Drive- MS 1135, Moscow, ID 83844-1135.
2
Karen Launchbaugh - Professor of Rangeland Ecology, Department of Forest,
Rangeland, and Fire Sciences, University of Idaho, 875 Perimeter Drive- MS 1135,
Moscow, ID 83844-1135.
3
Ryan Limb - Assistant Professor of Rangeland Fire Ecology, School of Natural Resource
Sciences, Dept. 7680, North Dakota State University, PO Box 6050, Fargo, ND 58108.
4
Allen Torell - Professor of Agricultural Economics, Department of Agricultural
Economics, New Mexico State University, Box 30003, MSC 3169, Las Cruces, NM 88003.
Strand et al. Journal of Rangeland Applications
v.1, 2014: pp.35-57
36
Contents
Introduction ................................................................................................................................................ 37
Historic Livestock Grazing Patterns ............................................................................................................ 38
Introduction of Exotic Annuals Grasses ...................................................................................................... 38
How Grazing Alters Plant Community Composition in Sagebrush Ecosystems .......................................... 39
Livestock grazing effects on shrub cover/densities ............................................................................. 39
Livestock grazing effects on perennial grass cover .............................................................................. 40
Livestock grazing effects on annual grass abundance ......................................................................... 41
How Livestock Grazing Can Modify Fuel Loads .......................................................................................... 43
Shrub fuel loads .................................................................................................................................... 44
Perennial grass fuel loads ..................................................................................................................... 44
Annual grass fuel loads ......................................................................................................................... 44
Continuity of fuels ................................................................................................................................ 45
Grazing to Manage Fuels Depends On Weather, Topography, and Vegetation Composition ................... 46
Economics of Fuel Treatments ................................................................................................................... 48
Summary and Remaining Knowledge Gaps ................................................................................................ 50
Acknowledgements .................................................................................................................................... 51
Literature Cited ........................................................................................................................................... 52
Key Points
Cover and biomass of perennial herbaceous
plants in sagebrush communities can be
reduced by heavy (or severe) grazing
repeatedly in the spring before the perennial
grasses initiate bolting.
High severity grazing (i.e. >50% utilization),
especially in the spring during initiation of
bolting of perennial grasses, can suppress
competition from native herbaceous plants
and cause soil disturbance that can favor
annual invasive grasses including cheatgrass.
Livestock grazing at low/moderate severity
(i.e., < 50% utilization) generally has little
influence on the cover of perennial grasses
and forbs.
Areas grazed by livestock can have more, less,
or the same density and cover of sagebrush
compared to non-grazed areas. Determining
factors include the season and intensity of
grazing, species of livestock, ecological site,
and site conditions at the time of grazing.
A window of opportunity may exist for
targeted grazing to reduce annual grasses
before perennial grasses initiate bolting or
during dormancy of perennial grasses.
Targeted grazing with sheep or goats can
reduce the fuel load of shrublands in the short
term by reducing woody fuels.
Livestock grazing can reduce the standing
crop of perennial and annual grasses to levels
that can reduce fuel loads, fire ignition
potential, and spread.
Strand et al. Journal of Rangeland Applications
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37
Grazing after perennial grasses produce seed
and enter a dormant state can reduce the
residual biomass left on the site, thereby
decreasing the fire hazard the following
spring and summer.
Grazing can reduce the continuity of fuels,
including the amount of herbaceous biomass
between shrubs, in sagebrush ecosystems.
Economic analyses reveal that fuel
treatments in sagebrush ecosystems have the
highest benefit/cost ratio when the perennial
grasses comprise the dominant vegetation,
i.e. prior to annual grass invasion and shrub
dominance.
Extreme fire weather conditions,
characterized by low fuel moisture and
relative humidity, and high temperature and
wind speed, affect wildland fires more than
do fuel characteristics, and the potential role
of grazing to alter fire behavior is limited.
Introduction
Sagebrush steppe and semi-desert
ecosystems cover vast areas in
western North America and
dominate landscapes of the Great
Basin and Colorado Plateau (Miller et
al., 1994). In this review we focus on
the sagebrush steppe and semi-
desert ecosystems within the Great
Basin. Despite their immense extent,
several forces threaten the
persistence and distribution of these
ecosystems. Climatic conditions,
grazing, exotic plant invasion, habitat
fragmentation, and fire all can alter
the extent and composition of
sagebrush-dominated landscapes
(Miller et al., 1994; Miller & Eddleman, 2000;
Davies et al., 2011). A significant
concern in recent years is
increasingly large and severe
wildfires occurring across the arid
regions in the west; these fires
remove sagebrush and favor more
tolerant annual and perennial grasses (Knick &
Rotenberry, 1997). Invasive early-curing annual grasses
such as cheatgrass, red brome, and medusahead are
filling the interspaces between shrubs on many arid
sites, or are becoming the ecologically dominant
species after a fire. Both situations create a
continuous fuel bed, allowing fire to spread more
readily across the landscape (Stewart & Hull, 1949;
D’Antonio & Vitousek, 1992). A warming climate with
earlier snowmelts contributes to a prolonged fire
season with larger and more severe fires (Chambers and
Pellant, 2008).
Weather, fuel characteristics, and landscape features
all affect fire spread, severity, and intensity (Figure 1).
Efforts to reduce the risk of extensive fires in
sagebrush-dominated ecosystems have focused
considerable attention on how livestock grazing
affects fuels, fire behavior and fire effects. Livestock
grazing influences factors related to fuel
characteristics, including the proportions of
herbaceous and woody fuel, amount of herbaceous
biomass, live/dead fuel mix, and continuity of fuel at
a patch and landscape scale (Figure 1). Fire behavior
and effects are also influenced by weather and
Figure 1. Factors that affect rate of spread, intensity, and severity of wildland fires can
be separated into factors related to weather, fuel characteristics, and landscape
features and context. Grazing can potentially influence factors related to fuel
characteristics.
Strand et al. Journal of Rangeland Applications
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38
landscape features that are independent from grazing
(Figure 1). This paper provides a comprehensive
overview and scientific synthesis of published
research on: 1) how livestock grazing can modify
plant community composition and alter fuel
characteristics of sagebrush-dominated plant
communities; 2) how yearly grazing patterns affect
fuel loads and wildland fire behavior; and, 3) the
comparative economics of grazing as a fuel reduction
treatment.
Historic Livestock Grazing Patterns
The introduction of domestic livestock to the Great
Basin in the 1860s initiated an era of broad-scale
ranching and significant changes in rangeland
ecosystems (Miller et al., 1994). Early grazing practices
during settlement and homesteading were by all
accounts ill-informed and poorly managed (Belsky &
Blumenthal, 1997; Miller & Eddleman, 2000). After several
decades of heavy stocking and season-long use, the
perennial grass and forb understory was considerably
depleted across much of the sagebrush steppe and
semi-desert (Vale, 1974). One of the greatest effects of
this excessive grazing pressure was a reduction of the
fine fuels that had previously carried wildfires (Miller et
al., 1994; Miller & Eddleman, 2000). Concomitant with
excessive grazing pressures was the reduction and
relocation of Native American populations which
reduced the presence of rangeland fire in sagebrush
systems (McAdoo et al. 2013).
With a reduced frequency of wildfires, the woody
plant cover increased, and shrublands and woodlands
expanded (Miller et al., 1994; Miller & Eddleman, 2000).
Livestock grazing also promoted woody plant growth
by suppressing competition from herbaceous plants
through preferential grazing of grasses and forbs
(Miller et al., 1994; Wilcox et al., 2012).
Grazing management programs designed to improve
native perennial grass communities were first
implemented in the 1940s. Managed grazing,
including periods of rest, seasonal deferment, and
reduced stocking rates was widely implemented in
the latter half of the 20th century (Krueger et al., 2002).
As grazing management practices were implemented,
herbaceous fuel loads generally increased, and
wildfires became more common. This reduced the
abundance of non-sprouting shrubs, including most
sagebrush species, across vast areas (Young & Blank,
1995; Davies et al., 2009).
Introduction of Exotic Annuals Grasses
Cheatgrass, an invasive annual grass introduced to
North America in the 18th century (Mack, 1981), has
vastly changed the fire regimes across the Great
Basin and western North America (Brooks et al., 2004).
Medusahead is another annual grass, introduced
from the Mediterranean region in the late 1800s and
spread rapidly across the Great Basin (DiTomaso et al.,
2008). These and other invasive annual grasses,
including red brome, have changed fuel
characteristics and fire regimes of the ecosystems
they invaded (D’Antonio & Vitousek, 1992; Brooks et al.,
2004). These fine-textured, flammable, and early
maturing grasses have lengthened the annual fire
season and shortened the return interval of wildfires
across the Great Basin (Hull & Pechanec, 1947; Stewart &
Hull, 1949; Davison, 1996; Bradford & Lauenroth, 2006; Balch et
al., 2013). Their rapid spread was exacerbated by
excessive stocking rates and inappropriate grazing
practices (Knapp, 1996; Young & Sparks, 2002; Chambers et al.,
2007). Thus a discussion of the effects grazing has on
fire patterns in sagebrush-dominated ecosystems
necessitates a discussion of how grazing affects
annual grass abundance.
Fire is a widespread disturbance type in sagebrush
ecosystems, but when cheatgrass and other annual
grasses become established, they change fuel
characteristics and shorten the fire return interval in
these ecosystems (Stewart & Hull, 1949; Brooks et al., 2004;
Balch et al., 2013). Fires can occur more frequently
because it only takes a few years post-fire (i.e., three
to six) to develop a sufficient fuel continuity to
facilitate another fire (Peters & Bunting, 1994). The
abundance of cheatgrass also increases the likelihood
of fire ignition and spread (Bunting et al., 1987; Link et al.,
2006; Balch et al., 2013). For example, the estimated fire
ignition risk more than doubled (i.e., from 46% to
100%) in bunchgrass communities in southwestern
Washington when the cover of cheatgrass increased
from 12% to 45% (Link et al., 2006). The continuity and
flammability of cheatgrass contribute to a highly
Strand et al. Journal of Rangeland Applications
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39
connected fuel bed which facilitates rapid spread
across the landscape (Figure 2). The fire return
interval can be halved and the fire size greatly
increased on rangelands dominated by cheatgrass as
compared to fires in vegetation communities without
cheatgrass (Balch et al., 2013). As wildfires become more
frequent, perennial grasses and native shrubs are
generally lost from the plant community (Peters &
Bunting 1994). With repeated fires, the seedbank of
perennial herbaceous species eventually becomes
depleted, permanently altering vegetation
composition in sagebrush communities (Knapp, 1996;
Humphrey & Schupp, 2001).
How Grazing Alters Plant Community
Composition in Sagebrush Ecosystems
How grazing affects the plant composition of
sagebrush ecosystems depends on several factors:
precipitation is key, followed by soil characteristics,
season and intensity of grazing, and species of grazing
herbivore. Plant community composition also has
important implications for fire regimes and potential
fire behavior. Different types of plants exhibit very
different fuel characteristics that affect fire ignition,
fire behavior, and fire effects (Figure 2). Fine
herbaceous fuels cure over the summer, rapidly
equilibrate with the ambient relative humidity, and
facilitate easy ignition in the summer and early fall.
Fire spread through these fuels is usually low
intensity because of the lower amount of biomass per
unit area (Scott & Burgan, 2005).
Sagebrush-dominated ecosystems support an
overstory of shrubs composed of fine woody fuels
(i.e., less than 7.6 cm [3.0 inches] diameter). Fine
woody fuels are more difficult to ignite but typically
burn longer and hotter than the herbaceous grass
and forb fuels in the understory. Fine woody
vegetation increases flame length and fire intensity.
Increasingly greater shrub biomass and fuel loads
lead to more severe fire effects, e.g. plant mortality,
smoke emissions, soil heating, and biomass
consumption (Sikkink et al., 2009). Woody plants such as
sagebrush can also contain volatile oils that can
create highly flammable fuel loads and increase both
flame lengths and fire spread (Buttkus & Bose, 1977).
Livestock grazing effects on shrub
cover/densities
An examination of a variety of grazing studies and
comparisons reveals no clear and consistent effect of
grazing on cover, density, or biomass production of
shrubs. For example, researchers in eastern Oregon
recorded increased density of juvenile sagebrush
plants under high stocking rates (1.2 AUM/ha or .48
AUM/ac) compared to no grazing or a
low stocking rate (0.6 AUM/ha or
0.24 AUM/ac) in Wyoming big
sagebrush with a crested wheatgrass
understory (Angell, 1997). Likewise,
sagebrush density increased in
response to early season grazing,
before perennial grasses flower and
set seed, in a threetip sagebrush
community (Laycock, 1967; Bork et al.,
1998).
The variable effect of grazing on
shrubs can also be assessed by
comparing the plant community in
areas where grazing has been
excluded to adjacent similar areas
where grazing has continued. We
examined eleven exclosure studies in
sagebrush ecosystems where grazing
Figure 2. The fuels of sagebrush-dominated ecosystems can be categorized and
described as herbaceous (i.e., grasses and forbs) and fine woody fuels (i.e., < 7.6 cm
[3.0 inches] diameter woody stems). The fuels vary in how they contribute to fire
behavior and effects.
Strand et al. Journal of Rangeland Applications
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40
had been excluded for ten years or more. In these
comparison studies, sagebrush and other shrubs’
response to the removal of grazing varied depending
on the species of shrub, soil type, community
condition at the time of exclosure, species of
herbivore, and season and intensity of grazing. In
seven of the eleven studies; there was no consistent
or discernible difference in shrub cover or density
between grazed and ungrazed sites (Rice & Westoby,
1978; Daddy at al., 1988; Courtois et al., 2004; Yeo, 2005; Davies
et al., 2010). For example, Davies et al. (2010) found no
difference in Wyoming big sagebrush cover in areas
grazed at moderate intensity (30-50% utilization and
a deferred rotation grazing system) over the past 70+
years compared to areas that had been excluded
from grazing in Wyoming big sagebrush steppe. A
similar comparison of rangeland vegetation in
fourteen grazing exclosures with mountain and
Wyoming big sagebrush in southeastern Idaho
revealed no difference in shrub cover inside and
outside the exclosures in areas available for grazing
by wildlife and livestock (primarily cattle) under a
variety of grazing systems. (Yeo, 2005).
Several exclosure studies revealed that grazing
affected shrub cover or density, but the effect was
not consistent. Laycock (1967) described greater
production of threetip sagebrush in areas grazed (at
levels describe as “heavy”) in the spring by sheep
compared to those areas excluded from grazing (for
25 years) or areas grazed in the fall. Holechek and
Stephenson (1983) similarly showed greater cover of
basin big sagebrush on grazed lowland sites in an
exclosure study in northern New Mexico. However,
on upland sites Holechek and Stephenson (1983)
reported greater cover of sagebrush in the exclosure
compared to the adjacent grazed area with 30 to 50%
utilization levels. Manier and Hobbs (2006) showed
greater cover of mountain big sagebrush in
exclosures than on adjacent grazed areas at 17
exclosure sites in western Colorado. Similarly,
Whisenant and Wagstaff (1991) reported greater
relative cover of bud sage in exclosures without
grazing for 53 years compared to adjacent grazed
areas. Exclosure studies may be valuable in discerning
the effects of the recent grazing regimes on specific
areas. However, exclosure studies, collectively, do
not reveal global trends due to grazing. The results of
these studies suggest that the effects of grazing on
shrub cover and production are site-specific, and
depend on the site conditions; the historic grazing
regimes; plant community composition at the time
the exclosures were constructed; and, the specific
grazing regime after the exclosures were established.
Several researchers also attributed plant community
change to the removal or reduction of grazing by
comparing observations before and after changes in a
grazing regime. For example, Yorks et al. (1992) found
an increase in basin big sagebrush cover (0.5% to 13%
from 1933 to 1989) along a 37-km (30-mile) transect
in sagebrush semi-desert in Utah. During this 56-year
period, grazing pressure was reduced as a result of
the Taylor Grazing Act of 1934; however, a general
increase in annual average precipitation may have
had a greater influence on shrub cover than did the
reduction in grazing. Similarly, Wyoming big
sagebrush cover on a site in south-central Idaho
increased from 18% in 1950 to 25% in 1975 after the
removal of grazing (Anderson & Holte, 1981). In this study,
the increase can be attributed to succession and
adequate precipitation. In a subsequent study on the
same site, sagebrush cover declined from 25% in
1975 to 13% in 1995 because of wide-spread die off
of sagebrush likely related to drought, insect and
rodent damage, and/or fungal pathogens (Anderson &
Inouye, 2001). Increased shrub cover was also observed
ten to fifteen years after removal of grazing by free-
roaming horses in sagebrush ecosystems across the
Great Basin (Beever et al., 2008). On the other hand, big
sagebrush cover decreased on a site in northern Utah
eleven years after livestock grazing was removed
(Austin & Urness, 1998). This decrease was largely
attributed to increased grazing pressure by mule deer
and more competition with sagebrush from perennial
grasses that were not grazed after the removal of
cattle.
Livestock grazing effects on perennial grass
cover
The effects of grazing on perennial grass cover in
sagebrush communities depends on factors similar to
those affecting sagebrush cover, including
precipitation, soil characteristics, season of grazing,
grazing intensity, and type of herbivore. Severe
grazing that occurs repeatedly in the spring, before
Strand et al. Journal of Rangeland Applications
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41
plants produce seeds, has been shown to reduce the
cover of perennial grasses and forbs (Vale, 1974; Bork et
al., 1998); the effect of light to moderate intensity
livestock grazing on vegetation is more obscure.
It is difficult to discern grazing effects from other
biotic effects and abiotic environmental conditions
(Miller et al., 1994, Holechek et al., 2006). When grazing was
removed from a Wyoming big sagebrush site,
increases in perennial grass cover occurred
sometimes (Robertson, 1971), but not always (Rice &
Westoby, 1978; West et al., 1984). Yorks et al. (1992)
observed a ten-fold increase in perennial grass cover
from 1933 to 1989 in Utah semi-desert where grazing
pressure by livestock was reduced. On the other
hand, Davies et al. (2010) found no difference in
current year’s herbaceous production when
comparing long-term (i.e., 70 years) moderately
grazed rangeland (30-50% utilization) with areas
excluded from grazing in Wyoming sagebrush steppe
communities in eastern Oregon.
Livestock grazing effects on annual grass
abundance
Livestock grazing and annual grasses are interacting
factors that affect fuel characteristics and wildland
fire occurrence and behavior throughout sagebrush
ecosystems. Intense (high stocking rate), severe (high
utilization levels), and repeated (multiple defoliation
events in the same season) grazing can suppress
competition from native plants and cause soil
disturbance that can favor annual invasive grasses
including cheatgrass (Klemmedson & Smith, 1964; Mack,
1981; D’Antonio & Vitousek, 1992; Knapp, 1996; Bradford &
Lauenroth, 2006; Chambers et al., 2007; Loeser et al. 2007).
Perennial grasses are strong competitors with
cheatgrass (Booth et al., 2003; Chambers et al., 2007; Blank &
Morgan, 2012), so grazing that adversely affects
perennial grasses can actually increase annual
grasses.
Exclusion of livestock does not necessarily slow
invasion or reduce abundance of annual grasses
(Cottam & Evans, 1945; West et al., 1984; Young & Allen, 1997;
Anderson & Inouye, 2001; Courtois et al., 2004; Young & Sparks,
2002). A comparison of grazed and ungrazed canyon
vegetation in Utah showed that cheatgrass was 1.5
times more frequent in an ungrazed than a grazed
canyon (Cottam & Evans, 1945). Substantial invasion by
cheatgrass and other exotic annual grasses can also
occur on sites that have never been grazed by
livestock but where there is a seed source (Daubenmire,
1940; Tisdale et al., 1965; Svejcar & Tausch, 1991; Goodwin et al.,
1999). However, caution should be applied to site
comparisons aimed at ascertaining the effects of
grazing because the spread of cheatgrass across an
area depends on the level of site degradation when
the annual grasses were introduced (Young & Sparks,
2002), frequency of wildfire (Cottam and Evans, 1945) and
on the relative resistance of different ecological sites
to cheatgrass invasion (Chambers et al., 2007).
Though severe and poorly timed grazing can promote
annual grasses, in some situations livestock grazing
can suppress annual grasses, including cheatgrass
(Daubenmire, 1940; Mosley, 1994; Vallentine & Stevens, 1994;
Mosley & Roselle, 2006; Loeser et al., 2007) and medusahead
(DiTomaso et al., 2008). The intensity of grazing can
influence whether annual grasses are suppressed or
promoted. For example, in northern Arizona high
elevation semi-arid grasslands, sites with moderate
grazing intensity (about 50% utilization) in the
summer grazing season had lower cheatgrass
abundance than either intensely grazed (stocked to
accomplish high utilization >70% in a 12-hour grazing
period) or ungrazed treatments (Loeser et al., 2007). In
southeastern Washington bluebunch wheatgrass
communities, high-intensity sheep grazing pressure
during winter dormancy and the spring grazing
season eliminated cheatgrass from a site within a few
years. However, a reduction in perennial grasses also
occurred and the rapid reinvasion by annual grasses
was observed after cessation of grazing (Daubenmire,
1940).
The impact of grazing on invasive annual grasses is
highly variable and site specific, which gives rise to
opposing research and field observations that either
implicate grazing in the spread and abundance of
annual grasses, or describe the suppression of annual
grasses by livestock grazing. Important factors
contributing to these conflicting results include
resistance to cheatgrass as determined by soil
temperature, the timing and amount of available soil
moisture, the relative abundance of perennial
herbaceous species, and the season and intensity of
grazing (Chambers et al., 2013).
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42
The timing and amount of precipitation and winter or
spring temperatures strongly affect the germination,
survival and growth of annual grasses such as
cheatgrass (Mack & Pyke, 1983; Chambers et al., 2007).
Cheatgrass also is favored after fire or other
disturbances when the community of perennial
herbaceous plants has been depleted (Chambers et al.,
2007; Hoover & Germino, 2012). Precipitation timing and
amount are immensely important factors
determining the response of cheatgrass to grazing
(Young et al., 1987). Because cheatgrass responds quickly
to early season rains, grazed cheatgrass plants may
exceed the growth of ungrazed plants if moisture is
available following spring grazing (Vallentine & Stevens,
1994).
However, grazing can also increase annual grass
abundance in dry years. A study in a high-elevation,
Great Basin grassland in Arizona revealed
similar levels of cheatgrass in high-intensity
cattle grazing with high stocking rates
compared to ungrazed pastures until a
drought year occurred (Loeser et al., 2007). In
the two years after the drought year, the
high-intensity grazing treatment resulted in
an 80% increase of cheatgrass cover and a
frequency of occurrence of nearly 100%
compared to about 40% on ungrazed sites
(Loeser et al., 2007).
The effects of grazing on annual grass
abundance also varies by season in which
grazing occurs. There appears to be a
window of opportunity for grazing to
reduce annual grasses if grazing occurs
when annuals begin to produce seeds but
before native perennial grasses initiate
bolting (Figure 3; Mosley, 1994; Vallentine &
Stevens, 1994; Mosley & Roselle, 2006; Smith et al.,
2012). Cheatgrass is very palatable to
livestock and has high nutritional value in
the vegetative stage and is preferentially
selected over many perennial grasses in
early spring throughout the Great Basin
(Young & Clements, 2007). Early and late spring
clipping that simulated grazing reduced the
biomass of cheatgrass compared to an
unclipped control, though density of
cheatgrass was unaffected (Tausch et al.,
1994). A similar clipping study found no effect of
clipping on cheatgrass seed density except when
plants were clipped in the boot stage and then
clipped again two weeks later, resulting in reduced
seed density (Hempy-Mayer & Pyke, 2008).
The timing of grazing is critical because annual
grasses may flourish if perennial plants are grazed
preferentially at times when the perennial grasses are
sensitive to damage by grazing (Pyke, 1986; Ganskopp,
1988). If bunchgrasses are routinely heavily grazed
(exceeding 50% utilization) in the period from bolting
through seed-set, and particularly if multiple
defoliation events in the same season occur, the
competitive advantage can be shifted toward
cheatgrass (Daubenmire, 1940; Young et al., 1987). Late
season grazing, after perennial grasses have
produced seed and begin to senesce, has minimal
Figure 3. Conceptual depiction of how livestock grazing can influence cheatgrass
abundance in sagebrush-dominated ecosystems with a significant component of
perennial grasses. Grazing can suppress or promote cheatgrass depending primarily
on the season of grazing. Grazing suppresses cheatgrass when applied in early spring
when annuals begin to produce seeds and before native perennial grasses initiate
bolting; and when applied during the dormant season.
Strand et al. Journal of Rangeland Applications
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43
impact on these grasses (Ganskopp, 1988; Hempy-Mayer &
Pyke, 2008). Several years of fall grazing by cattle on
semi-desert (27 cm [10.6 inches] annual
precipitation) sites in Nevada dominated by Wyoming
big sagebrush and salt desert shrub plant
communities has been shown to reduce cheatgrass
density and cover and increase cover of perennial
grasses compared to sites without fall or winter
grazing (Schmelzer et al., in press).
Well-timed and closely managed spring grazing can
be an effective tool to suppress annual grasses
including cheatgrass and medusahead (Mosley, 1994;
Vallentine & Stevens, 1994; Mosley & Roselle, 2006; DiTomaso et
al., 2008; Smith et al., 2012). One of the best opportunities
to reduce the abundance and cover of cheatgrass is
before most perennial grasses begin active growth
(Vallentine & Stevens, 1994; Young & Allen, 1997; Mosley &
Roselle, 2006; Smith et al., 2012). The challenge is to
remove livestock before perennial plants begin active
growth in order to avoid reduced vigor in the
perennial grasses (Laycock, 1967; Miller et al., 1994; Loeser et
al., 2007). Regardless, perennial
grasses with similar phenologies
as annual grasses, like
bottlebrush squirreltail, may be
reduced (Booth et al., 2003). On
cheatgrass-dominated sites,
high grazing intensity and
annual use must be maintained
or annual grasses will quickly
re-invade and dominate an area
(Daubenmire, 1940; Klemmedson &
Smith, 1964; Pyke, 1986).
How Livestock Grazing
Can Modify Fuel Loads
Management practices can
greatly affect a landscape’s fuel
amount and distribution. Fuel
load, or biomass, is one of the
most influential and easily
manipulated fuel variables
affecting fire intensity (Figure
4). Fuel load is the portion of
the biomass that will actually
burn in a wildfire or prescribed
fire, and is closely related to vegetation biomass. Fuel
loads are the primary drivers of heat, and all
measures of heat increase with increasing fuel loads
(Vermeire & Roth, 2011). The likelihood of fire-induced
bunchgrass mortality depends upon the amount of
heat received and the type of plant tissue exposed to
lethal heat (Miller, 2000; Wright, 1971). Livestock grazing
is one management technique that has been shown
to decrease fine fuel loading and subsequent wildfire
severity (Archibald et al., 2005; Davies et al., 2010).
Fuel management objectives aimed at reducing flame
lengths and fire spread in grassland and shrubland
ecosystems could be accomplished by altering the
fuel bed depth, fine fuel loading, cover, and
continuity such that the flame length never reaches
1.2 meters (3.9 feet; Nader et al., 2007). Livestock (i.e.,
cattle horses, sheep and goats) grazing primarily
impacts small diameter fuels (< 0.51 cm [0.2 inch]
diameter), including grass and small woody stems
that equilibrate with the ambient humidity and
temperature within 1 hour (i.e., the 1-hour time lag
Figure 4. The plant composition in sagebrush-dominated ecosystems is variable across the
landscape; this has important implications for fire behavior because different types of plants
exhibit different fuel characteristics affecting fire ignition, behavior, and effects. Fire spreads
quickly through cured grass usually at low intensity because of the low amount of biomass per
unit area. Fine woody vegetation increases flame length and fire intensity. Higher shrub loads
lead to more severe fire effects when the area burns. The continuity and flammability of
cheatgrass contributes to fire connectivity and spread.
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44
[htl] fuels). Livestock can also impact larger fuels
(0.51-2.54 cm [0.2 -1 inch] diameter or 10-htl fuels)
through browsing and trampling as suggested in a
review by Davison (1996). Hence, grazing could be a
useful management tool for reduction of grass and
shrub biomass (1-htl and 10-htl fuels).
Shrub fuel loads
Targeted grazing can be applied to reduce the fuel
load of shrublands through “brush-clearing”
strategies (Nader et al., 2007). Strategies to reduce shrub
abundance generally rely on goats or sheep because
these species generally consume greater quantities of
shrubs than cattle (Taylor, 2006; Papanastasis, 2009).
Grazing by cattle would not be expected to affect
sagebrush cover through direct consumption of
sagebrush.
Perennial grass fuel loads
The effect of grazing on fire behavior and extent is
predictably less pronounced on sites dominated by
woody plants compared to those with more
herbaceous biomass. However, reduced fire
frequency and spread in grazed shrublands and
forests have been observed because the herbivores
remove the fine herbaceous fuels that are most likely
to ignite and initiate fire spread (Zimmerman &
Neuenschwander, 1984; Hobbs, 1996).
Grazing with the goal of reducing herbaceous fuel
loads generally is more effective if it occurs right
before the season of greatest fire risk, which
generally coincides with peak biomass and the
initiation of dormancy (Taylor, 2006). If grazing occurs
early in the growing season, grasses can regrow and
biomass can be reestablished to levels similar to
ungrazed areas (Anderson & Frank, 2003). Grazing or
mowing after plants have initiated seed formation
and reached peak biomass can reduce biomass levels
below those of ungrazed plants and paddocks (Miller et
al., 1990; Anderson & Frank, 2003). Grazing late in the
growing season (after seed set) and in the dormant
season can thereby reduce the residual biomass
carried over to the following spring and summer
(Launchbaugh et al., 2008). Furthermore, grazing after
seed production has lower impact on plant vigor and
survival than grazing before floral initiation (Adler et al.,
2001).
Ungulate grazing reduces the standing herbaceous
plant material available for burning; this in turn can
potentially reduce the frequency, extent and intensity
of fires in grass, shrub, and forest understory fuel
types (Vale, 1974; Zimmerman & Neuenschwander, 1984;
Tausch et al., 1994; Hobbs, 1996; Belsky & Blumenthal, 1997;
Blackmore & Vitousek, 2000). In relatively moist Wyoming
big sagebrush steppe (30 cm [11.8 inches] annual
precipitation), Davies et al. (2010) found that grazing
reduced the amount of herbaceous fuel. The fine
flammable grassy fuel load, including dead standing
crop, was two-fold greater in plots that had not been
grazed for 70 years compared to adjacent areas that
had been grazed long-term at moderate grazing
intensity (30-50% utilization). In grasslands without
shrubs, fire intensity is inversely related to standing
crop biomass (Stronach & McNaughton, 1989; Hobbs, 1996),
and grazed patches burn less completely and
intensely than ungrazed patches (Hobbs et al., 1991);
however, these relationships have not been well
researched in shrubland systems of the Great Basin.
Beyond the amount of residual fuel remaining after
grazing, the proportion of live versus dead
herbaceous biomass may be an important factor
affecting a fire’s ability to spread in grasslands.
Grazing can in some instances increase the
propensity for fire to spread because herbivores
selectively remove green biomass and thereby
increase the proportion of dead to live biomass
(Leonard et al., 2010). Though alteration of the live-dead
ratio of herbaceous biomass is possible through
grazing, it is unlikely to be important in late season
wildfires in the Great Basin when most vegetation is
dormant.
Annual grass fuel loads
The effects of livestock grazing on fuel characteristics
of communities with significant amounts of annual
grasses can be viewed in two ways. First, as noted,
grazing can promote or suppress annual grasses over
years or decades. Second, livestock grazing can
reduce the standing biomass of annual grasses within
a year to reduce fuel loads and alter fuel continuity.
Only a few studies have addressed the potential
effect of livestock grazing on fuel loads in annual
grasslands. Diamond et al. (2009) examined the effect
of grazing by cattle on fire behavior on a cheatgrass-
dominated site in Nevada. Targeted grazing when
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45
cheatgrass was in the boot stage was applied to
reduce 80 to 90% of the herbaceous biomass. This
treatment resulted in reduced flame length and rate
of fire spread in a prescribed burn conducted in
October: fuel characteristics were so greatly reduced
that the fire would not spread across the grazed plots
(Diamond et al., 2009). Note, however, this study was
conducted in a confined area that may not be easily
replicated on landscape scales.
Cheatgrass is most palatable and nutritious before
the seeds mature and plants turn purple (Hull &
Pechanec, 1947; Young & Allen, 1997). However, livestock
will eat cheatgrass throughout the season and it has
been considered by some as important winter forage
in the Great Basin (Emmerich et al., 1993; Vallentine &
Stevens, 1994). This may create an opportunity to graze
cheatgrass almost year-round to manage fuel loads.
Research in Nevada examined the potential value of
winter grazing by cattle to reduce cheatgrass fuel
loads (Schmelzer et al., in press). In this study, cheatgrass
fuel loads were reduced 70 to 80% by winter cattle
grazing. The cattle favored cheatgrass over perennial
grasses, and with a protein supplement were able to
maintain their weight. This study suggests that winter
livestock grazing could be accomplished on landscape
scales as a part of regular grazing practices to manage
fuel loads of cheatgrass. Winter (dormant season)
grazing reduces fuel carryover to the next summer
(Figure 5), can reduce the thick litter layer known to
facilitate the germination of medusahead and
cheatgrass seed, may decrease the size of the annual
grass seedbank, and has few if any adverse effects on
the dormant, desired perennial grasses.
A significant challenge to managing fuel loads of
annual grasses with livestock grazing is the highly
variable biomass production related to rainfall
patterns (Young et al., 1987). One study in southern
Idaho showed that cheatgrass biomass varied ten-
fold depending on annual precipitation; from 404 to
3879 kg/hectare [452 to 4344 lbs/acre] in a dry
compared to a wet year (Hull & Pechanec, 1947). Thus, a
program using livestock grazing to manage fuel loads
created by annual grasses will need to be flexible, and
responsive to annual moisture regimes that will alter
plant growth and biomass. Winter grazing of
cheatgrass has one distinct advantage for livestock
production: the amount of potential forage is known
months in advance, so the livestock numbers needed
to achieve desired utilization levels can be easily
determined. For spring grazing of cheatgrass, biomass
production can fluctuate dramatically in a short
period due to sudden and unpredictable changes in
precipitation and temperature. Matching livestock
numbers with that forage base is much more difficult.
Continuity of fuels
Fuel continuity describes the spatial arrangement or
distribution of fuel and is a major factor affecting the
spread of fire across a landscape (Cheney & Sullivan,
2008). Greater fuel continuity leads to faster rates of
spread, and spread with lower fireline intensity
(NWCG, 1994). Horizontal continuity is the relationship
of the horizontal distance between
fuel particles and is related to
percent cover of vegetation.
Management actions that alter
vegetation species composition
and abundance can strongly affect
the fuel continuity (Brooks et al.,
2004). Fuel continuity generally
increases as fuel load increases.
However, if major shifts in
vegetation composition occur, then
fuel load can decrease while fuel
continuity increases. Invasion of
the sagebrush ecosystem by
annual grasses is a classic example
of this phenomenon: bunchgrasses
Figure 5. Sagebrush steppe where cheatgrass dominates the herbaceous vegetation.
Winter grazing has been applied in the image to the left while the image to the right
has been excluded from grazing for the past 20 years. Both photos were taken in late
May in neighboring pastures.
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v.1, 2014: pp.35-57
46
and shrubs, with abundant open space between
plants, are being replaced with smaller-statured
grasses with less space between individual plants. A
major factor of larger wildfires in recent years is the
increased fuel continuity across the landscape
(Davison, 1996).
Livestock grazing can alter the spatial pattern of
vegetation which in turn can have important
consequences for fire occurrence and spread (Adler et
al., 2001). Grazing can increase or decrease the spatial
heterogeneity of vegetation depending on the
existing plant community’s grazing animal
distribution, and the scale of observation (Adler et al.,
2001). Typically, grazing increases patchiness when the
grazing pattern is stronger than the vegetation
pattern and when grazing increases the contrast
among vegetation types. In grassland systems, grazed
patches may be more likely to be re-grazed in
subsequent years because they typically contain a
greater proportion of new growth (Hobbs et al., 1991).
In grasslands, landscape mosaics created by variable
grazing intensity can provide “firebreaks” and
prevent fires from becoming larger (McNaughton, 1992).
However, these relationships may not apply to
shrublands because fire can be carried by the shrubs.
Davies et al. (2010) observed larger fuel gaps in
moderately grazed areas compared to ungrazed areas
in a Wyoming big sagebrush community, and the
continuous perennial grass patches were larger in
ungrazed areas. Furthermore, herbaceous fuel
between shrubs in sagebrush ecosystems may be
effectively reduced by livestock grazing. France and
colleagues (2008) documented grazing by cattle in
Wyoming big sagebrush ecosystems was focused on
bunchgrasses in interspaces between shrubs with
only negligible removal of grasses under shrub
canopies at moderate (i.e., 40%) utilization levels. A
case study in sagebrush steppe in northern Nevada
demonstrated success keeping landscape scale fire at
a minimum using livestock to reduce fuels and
implementing range improvement projects, such as
flanking existing roads with green-strip seedings,
managing brush, seeding projects, and improving
riparian areas to function as green-strips (Freese et al.,
2013).
Although high-intensity grazing can reduce biomass
and fine fuel loads, light grazing can produce patchy
burn patterns in continuous sagebrush steppe fuels
(Bunting et al., 1987). Low- to moderate-intensity grazing
can remove sufficient fuel and break up fuel
continuity to significantly reduce fire spread (Bunting et
al., 1987). Patchy burn patterns are particularly
important in sagebrush regions where maintenance
of sagebrush cover (e.g., for wildlife habitat) is a
management objective. Patchy burns leave islands of
unburned sagebrush, thereby creating a seed source
for reestablishment of sagebrush plants across the
affected area (Colket, 2003).
Much of the evidence on fire behavior in herbaceous
fuels is extrapolated from grassland ecosystems in
both North America and Africa. However, because
fire itself is a physical process driven by fuel, it is
largely unaffected by the specific plant species, but
rather the amount and structure of the fuel source.
For example, one classification for wildland fuels is
the time required for dead fuel to equilibrate to
changes in relative humidity (largely a function of fuel
diameter). Additionally, fuel loading and fuel
continuity are used in fire spread models, whereas
vegetation species is generally not included (Scott and
Burgan, 2005). This allows for comparisons of fire
behavior among ecosystems with similar fuel classes,
but completely different species composition.
Grazing to Manage Fuels Depends On
Weather, Topography, and Vegetation
Composition
Carefully targeted grazing can be used as a tool to
reduce fine fuel loads, the rate of spread and final
extent of fires, and ultimately fire frequency, in
sagebrush-dominated ecosystems. However, the
level to which grazing affects fire behavior depends
on a number of physical and environmental
conditions, such as ambient temperature, wind
speed, humidity, fuel composition, fine-scale
continuity (tuft-scale), spatial distribution, and
topography (Figure 1). Fuel loading and fuel moisture
directly affect the fire behavior and consumption
rates in sagebrush ecosystems under most
environmental conditions. In the absence of
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47
sagebrush cover, if fine fuel loading is less than 560 to
650 kg/hectare (627 to 728 lbs/acre), fires will sustain
only under environmental conditions characterized
by less than 15% relative humidity, temperatures
exceeding 29°C (84.2 °F), dead fuel moisture less than
12%, and wind speeds greater than 16 km/hour (9.9
miles/hour (Britton et al., 1981; Bunting et al., 1987;
Launchbaugh et al., 2008). However, when fine fuel
loading is above 1700 kg/hectare (1904 lbs/acre), fire
will spread under a wide array of environmental
conditions (Bunting et al., 1987). Given these estimates,
based on models, livestock grazing could remove
sufficient fine fuel to reduce the risk for fire ignition
and spread throughout most of the year.
Consequently, areas that are selected for a
prescribed fire should not be grazed the season
before the planned fire to allow fine fuel
accumulation (Bunting et al., 1987).
In sagebrush steppe and semi-desert, the shrub
component adds vertical structure to an understory
of herbaceous forbs and grasses. Brown (1982)
suggested that at 20% sagebrush canopy cover, a
cured herbaceous fuel load of at least 340 kg/hectare
(381 lbs/acre) would be required to sustain a fire with
a 16 km/hour (9.9 mile/hour) wind. Areas with
greater sagebrush cover may burn at lower
herbaceous fuel loads. Lower fuel moistures, typical
in the fall, increase the rate of spread, flame lengths
and fire intensity when compared to spring burns
(Sapsis & Kauffman, 1991). Consumption rate of 10-htl
and 100-htl woody fuel also increases with lower fuel
moisture content (Sapsis & Kauffman, 1991). In addition
to fuel moisture and weather, topography also affects
fire behavior. At 30% slope the fire rate of spread is
two to three times greater than a flat area, while at
50% slope the rate of spread increases four to seven
times (Brown, 1982). Thus, fuel reduction by grazing will
have the most pronounced effects and potential to
benefit suppression activities on more level parts of
the landscape.
Reducing levels of fine fuels, as could be
accomplished with livestock grazing, reduced the
modeled surface rate of spread and fire intensity in
simulated shrub and grassland communities
(Launchbaugh et al., 2008). Model assumptions using
Behave Plus software include uniform fuel continuity,
weather, and slope (Andrews, 2008). In addition, the
model does not include potential spotting to advance
the fire ahead of the containment line. The effects of
reduced fuel load on fire behavior were more
pronounced at low wind speeds and high fuel
moisture. When burning conditions became extreme,
changes in the amount of herbaceous fuels (1-htl fuel
classes) had little effect on fire behavior variables.
Under less extreme fire weather conditions, livestock
grazing to reduce herbaceous fuel loads could
influence fire behavior, making fire in these
sagebrush communities easier to contain.
A similar study with similar fire model assumptions
and results was conducted at study sites near Las
Cruces, New Mexico and Tucson, Arizona (Varelas,
2012). This study confirmed that with moderate fuel
moisture and light winds the reduction of fine fuels
by grazing could reduce flame lengths below a 1.2-
meter (4-feet) level, permitting direct attack by hand
crews. However, the grazing treatments were not
effective under more extreme burning conditions and
the cattle grazing treatment had limited potential to
alter fire behavior when a significant shrub
component was present.
Important factors driving the behavior and effects of
fire in sagebrush steppe and semi-desert systems are
fuel characteristics and fire weather (Figure 6).
Livestock grazing has the highest potential to reduce
fire spread and intensity in areas dominated by
herbaceous fuels with low sagebrush cover under
moderate or better weather conditions, (i.e.,
conditions represented in the upper left region of
Figure 6). Grazing by cattle is generally focused on
grasses and other herbaceous forage, therefore cattle
grazing would have limited potential to alter fire
behavior that is driven primarily by sagebrush cover
(i.e., conditions represented in the lower left region
of Figure 6). However, under moist and cool
conditions, grazing can influence fires that move
through sagebrush communities by slowing the
movement of fire along the herbaceous understory
between shrubs. Under extreme burning conditions,
characterized by low fuel moisture and relative
humidity, and high temperature and wind speed,
wildland fires are driven more by weather conditions
than by fuel characteristics. Therefore, as fire
weather conditions become extreme, the potential
role of grazing on fire behavior decreases and may
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48
become meaningless (e.g., conditions represented on
the right side of Figure 6).
Economics of Fuel Treatments
Fuel treatments are designed to alter fuel conditions
so that wildfire is easier to control and less
destructive (Reinhardt et al., 2008). As noted above,
cattle grazing primarily alters fuel conditions by
reducing the amount of herbaceous fine fuels,
whereas goat and sheep grazing can potentially also
reduce the shrub component. Other fuel treatments
that can be used to accomplish these same objectives
include, herbicides, mechanical treatments such as
mowing, prescribed/controlled fires, or a
combination of these treatments (Nadar et al., 2007;
Diamond et al., 2009).
The costs of fuel treatments vary widely, yet the
relative costs and success of alternative treatments is
an obvious concern and must be considered when
evaluating fuel management options. Several studies
review and describe the many factors affecting fuel
treatment costs on forested areas where
management of the woody
overstory is of key concern (Cleaves
et al., 2000; Hesseln, 2000; Kline, 2004)
similar cost estimates on
rangelands are limited. Least cost
fuel treatments will vary with
conditions and objectives, but
grazing alternatives appear to be
cost-competitive especially if the
objective is reduced fine fuel loads
where mowing or a prescribed
burn are potential alternatives.
As described by Mercer et al. (2007)
and Kline (2004), expanding beyond
costs to consider net economic
benefits of fuel treatments is a
complex analysis. The most
important unanswered economic
question is whether the resource
expended to reduce wildfire risk
and damages result in net
economic gains. Tradeoffs also
exist between increased
expenditures on fire suppression
versus fuels management (Mercer et al., 2007). The
benefit/cost (B/C) assessment requires definition of a
wildfire production function that defines the
relationship between size and intensity of wildfires as
it relates to alternative fuel management treatments,
climate variables, and site-specific characteristics.
Potential benefits of fuel treatments such as reduced
wildfire risk, reduced fire suppression costs, and
reduced structural losses will be site-specific. Thus,
the site-specific analysis must account for the
cumulative cost of fuel treatments, the likelihood of
wildfire events with and without treatments, the
effects and costs of fire suppression and post-fire
restoration, and the effect of management actions
and wildfires on resource conditions, structural
damages, and saleable products over time (Kline, 2004).
Given these complexities, only a few studies have
estimated net economic benefits of fuel treatments
in forested areas (Mercer et al., 2007; Prestemon et al.,
2012), and only one recent study considered net
economic benefits of fuel treatments on rangelands
(Taylor et al. 2013).
Figure 6. The potential for grazing to influence fire behavior occurs along continuums of
fuel and weather conditions. In this conceptual model, fuel composition is displayed on
the y-axis and fire weather condition is displayed on the x-axis. Low fire weather severity
is characterized by high fuel moistures, high relative humidity, low temperature, and low
wind speeds, while extreme fire weather is characterized by the opposite conditions.
The potential for grazing to be effective in reducing the risk of fire initiation and spread
is greatest when the sagebrush cover is low and the fire weather severity is low to
moderate.
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49
The net economic benefits of selected fuel
treatments in the sagebrush ecosystems of the Great
Basin were estimated in a study by Taylor et al. (2013).
State-and-transition models were used to define
vegetative characteristic changes expected to occur
based on natural succession and disturbance
interactions for sites dominated by Wyoming big
sagebrush and mountain big sagebrush. The analysis
was a probabilistic benefit/cost assessment where
the benefit of the treatment was considered to be
fire suppression costs averted over the next two
hundred years because alternative fuel management
treatments were undertaken. The success of fuel
treatments (movement to a state with less shrubs
and invasive annual grasses) was considered to be
uncertain with re-treatment required when the
simulation projected a treatment failure.
Because healthier ecological states with relatively
high perennial grass cover and without an overgrown
sagebrush canopy (sagebrush present but not
ecologically dominant) were considered to be
resilient and responsive to treatment, and with a
marked reduction in fire frequency following
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50
relatively low-cost fuel treatments, these healthy
areas were found to be more economical to treat
than were mature sagebrush areas with a depleted
perennial herbaceous understory or areas invaded
with annual grasses. The estimated B/C ratio ranged
from a high of 13.3 for productive Wyoming
sagebrush steppe sites (a relatively low-cost
controlled burn treatment) to less than one (i.e., not
economically efficient) for high shrub densities and
levels of annual grass invasion (requiring expensive
and often unsuccessful treatments). Similarly, the
estimated B/C ratio for mountain big sagebrush sites
decreased with a rise in brush canopy and annual
grass invasion. The implication is that the desired
time for fuel treatments is before a decadent shrub
canopy (one with considerable dead standing
biomass) occupies the area and annual grass invasion
occurs.
No known studies have quantified the net economic
benefits of grazing treatments for fuels management.
The analysis would be quite different from that of
Taylor et al. (2013) because the effects of grazing
treatments would generally only last one or two
years: the herbaceous understory regrows and the
treatment must be reapplied. However, several broad
conclusions might be drawn. First, grazing treatments
would potentially be an economical alternative to the
prescribed burn treatment suggested by Taylor et al.
(2013) specifically for areas with relatively low shrub
cover where perennial grasses dominate. Areas
where sagebrush fuel loads are low and herbaceous
fuel loads are high are the conditions most favorable
for grazing treatments (Figure 6). Second, given the
relatively short benefit period for the grazing
treatment, unless the brush canopy is significantly
altered, the cost of the treatment must remain
relatively low. The harvested forage would contribute
an additional grazing benefit to livestock production
if previously unused forage were harvested.
Negative potential ecological impacts from grazing
treatments are of concern, as are treatment costs
(Table 1). Varelas (2012) estimated the cost of targeted
cattle grazing treatments increased by about
$18/hectare ($7.30/acre) for each 89 kg/hectare (100
lbs/acre) of herbaceous material removed by grazing
animals. Targeted cattle grazing treatments using
herding and low moisture blocks to hold cattle on
targeted areas were found to be effective and cost
competitive ($123/hectare [$50/acre]) when the
standing herbaceous materials were reduced by
about 45% (from 1,161 kg/hectare [1,300 lbs/acre] to
about 536 kg/hectare [600 lbs/acre]).
Summary and Remaining Knowledge
Gaps
The legacy of early post-settlement livestock grazing
has played an important role in shaping vegetation
dynamics in sagebrush ecosystems. High intensity
and severe grazing in the late 1800s contributed to a
dramatic reduction in both fine herbaceous fuels and
fire frequency, and provided a competitive advantage
for, and consequent increase in, woody plants. The
introduction of exotic annual grasses in the late
1800s to early 1900s radically altered the fuel
characteristics of many sites in the Great Basin. Over
the last several decades, reduced grazing pressure,
increased cover of flammable exotic annuals,
increased human activity, and more recently, a longer
climate-induced fire season (Chambers & Pellant, 2008),
have all led to the current situation in the Great Basin
where fires are larger and more frequent than 25+
years ago. Wildfires burn frequently enough to
prevent establishment of sagebrush and cause a
change in vegetation types across this vast region.
There are several ways contemporary livestock
grazing practices can affect the extent and behavior
of fires in sagebrush-dominated ecosystems. These
include cumulative effects that occur on decadal time
scales and that alter plant community composition
(i.e., woody versus herbaceous) and those influenced
annually through changes in fuel loads. Over decades,
livestock grazing can change the relative proportions
of shrubs, perennial grasses, and annual grasses,
altering the fuel composition. On an annual basis,
grazing can reduce the amount of herbaceous fine
fuels, including cheatgrass, forbs and small twigs of
woody plants. Grazing can reduce fire spread and
intensity by removing understory vegetation,
reducing the amount of fuel, and accelerating the
decay of litter through trampling. This altering of
fuels continuity can create patchy burns that result in
unburned islands of vegetation providing seed
sources for re-establishment of plants after the burn.
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51
The effects of grazing could result in fires that burn at
lower intensity, increased patchiness, decreased rate
of spread, and increased subsequent survival of
plants after fire. The specific outcome will depend on
the fire weather conditions and the structural
composition of the plant community when a fire
occurs. As fire weather conditions become extreme,
the potential role of grazing on fire behavior is
limited.
Fuels management programs that incorporate grazing
treatments must consider the long-term effects of
such treatments on both desired and undesired plant
species, with desirability defined by site-specific
management goals and objectives. Grazing practices
can alter plant communities such that shrub density
increases, perennial grasses decrease, and exotic
annual grasses and other invasive species gain a
foothold, an outcome that would decrease resistance
to and resilience from fire. Sound grazing practices
and targeted grazing efforts aimed at wildland fuel
reduction, however, have a strong potential to
decrease undesirable fire behavior. Reductions of fine
fuels and the desirable alterations to wildfire
behavior are often overlooked benefits from
including sound grazing practices on the landscape.
We identified four main research gaps in
understanding how grazing influences fire behavior in
sagebrush ecosystems and the related fuel treatment
economics. First, research and observations clearly
support the statement that grazing can influence
wildland fuels and thereby fire behavior. However,
residual herbaceous biomass level thresholds
required to stop or carry the spread of fire under
various weather conditions are largely unknown.
Second, it is not known how shrub properties (cover,
height, structure, etc.) influence the probability that
an area will burn under different weather conditions.
Third, further research is needed to discern the
effects of landscape scale grazing patterns on fire
behavior. Fence-line contrasts suggest that uneven
utilization or spatial variation in grazing systems at
the pasture scale can contribute to stopping or
carrying fires, thereby reducing the area burned.
However, this hypothesis has not been tested at
meaningful scales. Fourth, an important economic
question is whether the resources expended to
reduce wildfire risk result in net economic gains.
From an ecological point of view, many questions
remain unanswered. Sagebrush ecosystems evolved
with fire. However, invasive annual grasses have
altered the nature and impact of fire in these
systems. Fire will always play an important role in
sagebrush steppe and semi-desert, with effects
ranging from rejuvenation to destruction. Grazing is
one of the tools rangeland managers can apply to
moderate these effects.
Acknowledgements
The authors would like to thank Dr. Stephen Bunting,
the Bureau of Land Management, Owyhee Initiative
(http://www.owyheeinitiative.org/), and Great Basin
Fire Science Delivery Project
(http://www.gbfiresci.org/) for support to gather this
information and for providing valuable review in the
development of this synthesis. The research was also
supported by Agricultural Experiment Stations at
Idaho, Oregon, and New Mexico.
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52
Common and Scientific Names of Plants Listed in Text According to the USDA PLANTS Database
(http://www.plants.usda.gov/).
Common Name Scientific Name
Basin big sagebrush Artemisia tridentate Nutt. ssp. tridentata
Bluebunch wheatgrass Pseudoroegneria spicata (Pursh) Á. Löve
Bottlebrush squirreltail Elymus elymoides (Raf.) Swezey
Cheatgrass Bromus tectorum L.
Crested wheatgrass Agropyron cristatum (L.) Gaertn.
Medusahead Taeniatherum caput-medusae (L.) Nevski
Mountain big sagebrush Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle
Red brome Bromus rubens L.
Sagebrush Artemisia spp.
Threetip sagebrush Artemisia tripartita Rydb.
Wyoming big sagebrush Artemisia tridentate Nutt. ssp. wyomingensis Beetle & Young
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... In many parts of the continental US and Europe, grazers are paid by local governments for targeted grazing, or managing livestock for deliberate fire risk reduction in rangelands, as part of coordinated, regional fire risk reduction plans [134][135][136][137][138][139]. The most obvious effects of grazing are reductions in quantity and connectivity (i.e., continuity) of fine, grassy fuels through consumption and trampling [125, 136,140]. Grazing also reduces the amount of fuel load in the form of standing dead grass relative to live grass, thereby increasing the "greenness" of grasslands which also reduced fire risk [130,140,141]. ...
... The most obvious effects of grazing are reductions in quantity and connectivity (i.e., continuity) of fine, grassy fuels through consumption and trampling [125, 136,140]. Grazing also reduces the amount of fuel load in the form of standing dead grass relative to live grass, thereby increasing the "greenness" of grasslands which also reduced fire risk [130,140,141]. ...
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Well-managed rangelands provide important economic, environmental, and cultural benefits. Yet, many rangelands worldwide are experiencing pressures of land-use change, overgrazing, fire, and drought, causing rapid degradation. These pressures are especially acute in the Hawaiian Islands, which we explore as a microcosm with some broadly relevant lessons. Absent stewardship, land in Hawai‘i is typically subject to degradation through the spread and impacts of noxious invasive plant species; feral pigs, goats, deer, sheep, and cattle; and heightened fire risk. We first provide a framework, and then review the science demonstrating the benefits of well-managed rangelands, for production of food; livelihoods; watershed services; climate security; soil health; fire risk reduction; biodiversity; and a wide array of cultural values. Findings suggest that rangelands, as part of a landscape mosaic, contribute to social and ecological health and well-being in Hawai‘i. We conclude by identifying important knowledge gaps around rangeland ecosystem services and highlight the need to recognize rangelands and their stewards as critical partners in achieving key sustainability goals, and in bridging the long-standing production-conservation divide.
... Prescribed fire and mow plots consistently kept modeled fire intensity reduced enough that direct suppression activities were possible, which ultimately reduces resources needed, total fire size, and suppression costs. Our results confirmed our prediction that modeled fire behavior would increase across all treatments with increased fuel curing, mimicking increased risk of higher intensity fire later in the fire season (Strand et al., 2014). This is a function of decreasing fuel moisture from early to late season. ...
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Increased fire size and frequency coupled with annual grass invasion pose major challenges to sagebrush (Artemisia spp.) ecosystem conservation, which is currently focused on protecting sagebrush community composition and structure. A common strategy for mitigating potential fire is to use fuel treatments that alter the structure and amount of burnable material, thus reducing fire behavior and creating access points for fire suppression resources. While there is some recent information on the impacts of fuel treatments on ecological communities, we have little information on fuel treatment effectiveness at modifying fire behavior in sagebrush ecosystems. We present 10 years of data on fuel accumulation and the resultant modeled fire behavior in prescribed fire, mowed, herbicide (tebuthiuron or imazapic), and untreated control plots in the Sagebrush Treatment Evaluation Project (SageSTEP) network in the Great Basin, USA. Fuel data (i.e., aboveground burnable live and dead biomass) were collected in each treatment plot at Years 0 (pretreatment), 1, 2, 3, 6, and 10 posttreatment. We used the Fuel and Fire Tool fire behavior modeling program to test whether treatments impacted potential fire behavior. Prescribed fire initially removed 49% of the total fuel load and 75% of shrubs, and fuel loads remained reduced through Year 10. Mowing shifted fuels from the shrub canopy to the ground surface but did not change the total fuel amount. Prescribed fire and mowing increased herbaceous fuel by the second posttreatment year and that trend persisted through Year 10. Tebuthiuron treatments were ineffective at altering fuel loads. Imazapic suppressed herbaceous vegetation by 30% in Years 2 and 3 following treatment. The modified fuel beds in fire and mow treatments resulted in modeled flame lengths that were significantly lower than untreated control plots for the duration of the study, with shorter term reductions in reaction intensity and rate of spread. Understanding fuel treatment effectiveness will allow natural resource managers to evaluate trade‐offs between protecting wildlife habitat and reducing the potential for high‐intensity wildfire.
... This study supports other research that has highlighted the benefits of managing cattle grazing as an effective tool to reduce fine herbaceous fuels on arid and semiarid regions in the western United States ( Strand et al. 2014 ;Bruegger et al. 2016 ). Our results add to the research base by establishing that strategically placed supplements along a desired corridor can establish a grazed fuel break to reduce buildup of fine fuels during the dormant season for up to 4 km from a single watering point on a relatively flat, cheatgrass-invaded area during fall and early winter. ...
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Management of areas invaded by cheatgrass (Bromus tectorum) continues to be one of the greatest challenges for US Great Basin ecosystems. Targeted cattle grazing in the fall and winter has shown positive results as a management tool to reduce dormant fine fuel biomass within cheatgrass-invaded areas, but management of targeted grazing within large pastures can be challenging. We evaluated the use of strategically placed liquid protein supplement stations over a 4-wk period in the fall to focus cattle grazing along a linear transect stretching away from water to reduce residual cheatgrass biomass on a production-scale, working ranch from 2014 to 2017. Liquid protein supplement stations were moved approximately 1 km farther from water during each week of the study, eventually reaching 4 km from a single water source. Global Positioning System−tracked cattle visited supplement stations 52% ± 4% of the days during the study period and were within 100 m of the supplement station transect line 17.7% ± 2.6% of the time: more than 3 × greater (P = 0.07) than random locations (5.1% ± 2.6%). Week of the study and the subsequent supplement distance from water did not influence the number of visits cattle made to supplement. The duration that cattle remained at supplement was greater in wk 4, when supplement was placed 4 km from water, compared with wk 1 and 2, when the supplement was 1 and 2 km from water, respectively. At the conclusion of grazing, utilization along the supplement station transect averaged 66.0% ± 5.7% and did not differ between supplement stations at 1 km, 3 km, or 4 km from water. Strategic supplementation provides a valuable tool to target cattle grazing at specific locations within cheatgrass-invaded systems to reduce fine fuel buildup during the dormant season.
... Evidence from the long-term exclosures examined in our study agrees with others in demonstrating that long-term herbivory alters fire dynamics (Kerby et al., 2007;Werner et al., 2021). Although we did not measure patchiness directly, our results are consistent in suggesting that grazed areas burn less completely than areas excluded from grazing and, therefore, experience patchier fires (Hobbs & Norton, 1996;Strand et al., 2014;Zimmerman & Neuenschwander, 1984). Our findings also suggest that within a northern fire-dependent mixedgrass prairie ecosystem, areas with patchy fire spread support greater persistence of a fire-sensitive shrub. ...
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Fire refugia and patchiness are important to the persistence of fire‐sensitive species and may facilitate biodiversity conservation in fire‐dependent landscapes. Playing the role of ecosystem engineers, large herbivores alter vegetation structure and can reduce wildfire risk. However, herbivore effects on the spatial variability of fire and the persistence of fire‐sensitive species are not clear. To examine the hypothesis that large herbivores support the persistence of fire‐sensitive species through the creation of fire refugia in fire‐prone landscapes, we examined the response of a fire‐sensitive plant, Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & Young]) to fire and grazing in the fire‐dependent mixed‐grass prairie of the northern Great Plains. We carried out a controlled burn in 2010 within pre‐established exclosures that allowed differential access to wild and domestic herbivores and no record of fire in the previous 75 years due to fire suppression efforts. The experiment was set‐up with a split‐plot design to also examine potential changes in plots that were not burned. Canopy cover of big sagebrush was recorded before the burn in 2010 and again in 2011 with percent area burned recorded within one‐month post‐fire in the burned plots. Percent area burned was the greatest in ungulate exclosures (92 ± 2%) and the least in open areas (55 ± 21%) suggesting that large herbivores influenced fire behavior (e.g. reducing fire intensity and rate of spread) and likely increased fire patchiness through their alterations to the fuel bed. Regression analysis indicated that the proportion of sagebrush cover lost was significantly correlated with the proportion of area burned (R2 = 0.76, p = 0.05). No differences in the non‐burn plots were observed among grazing treatments or among years. Altogether, this illustrates the potential importance of large herbivores in creating biotic‐driven fire refugia for fire‐sensitive species to survive within the flammable fuel matrix of fire‐dependent grassland ecosystems like the mixed‐grass prairie. Our findings also attest to the resiliency of the northern Great Plains to fire and herbivory and underscore the value of managing grasslands for heterogeneity with spatial and temporal variations in these historic disturbances.
... Climate change, changing land uses, and invasion of exotic annual grasses contribute to larger, hotter, and more frequent fires on US western rangelands ( Abatzoglou and Kolden 2011 ;Balch et al. 2013 ;Coates et al. 2016 ). But due to the extensiveness and mixed-ownership of these lands, disparate fuel-management treatments such as prescribed burning or herbicide spraying are singularly inadequate for influencing fire behavior at the landscape scale ( Davies et al. 2015a but currently underused tool for reducing fuel loads and leveraging existing fire risk management activities ( Diamond et al. 2009 ;Strand et al. 2014 ;Davies et al. 2015b ). In federal land management, a combination of policies, local culture and norms, and beliefs of managers and users can create barriers to widespread use of fuels management tools such as grazing (e.g., Moseley and Charnley 2014 ;Schultz et al. 2019 ). ...
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In the United States, the Bureau of Land Management (BLM) manages rangeland resources under dynamic conditions such as drought, annual grass invasion, and larger and more frequent wildfires. But federal policies governing rangelands are not structured to respond to annual variability or unexpected events. To integrate flexibility into public rangeland administration and potentially leverage fuels management treatments at the landscape scale, the BLM and livestock grazing permittees are exploring outcome-based rangeland management approaches to achieve desired ecological, social and economic conditions. This paper examines administrative policies and barriers to using outcome-based approaches to manage fire risk in Idaho through 70 semistructured interviews with permittees, BLM staff, and other agency and nongovernmental stakeholders in three Idaho BLM field areas. We analyzed how rules and norms in policy implementation contributed to perceptions of barriers within and among different field areas. Factors affecting perceptions of outcome-based rangeland management implementation included BLM staff tenure, permittee-agency relationships, beliefs about the efficacy of grazing to manage fire risk, and leadership and staff experience in navigating National Environmental Policy Act requirements or potential lawsuits. Differences in the informal institutions among field areas led to different interpretations of latitude found within formal institutions (“gray zones”) for implementation. This study highlights the importance of local context and the interactions between administrative policies and agency culture for implementing adaptive approaches to managing wildfire risk on public rangelands.
... However, like many large carnivores, black bears were extirpated throughout Nevada by the early 1900s due to landscapescale habitat loss, targeted removals, and unmanaged hunting Lackey et al., 2013). At the same time, a change from a grass-dominated biome to a sagebrush-steppe ecosystem created by increased livestock grazing facilitated the irruption of mule deer herds (Berger & Wehausen, 1991;Miller et al., 1994;Strand et al., 2014) and the resulting concomitant increase in the cougar population. This expansive growth of both mule deer and cougar populations occurred as bears were being extirpated, allowing cougars to dominate the predatory landscape in western Nevada for nearly a century. ...
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Apex predators can shape communities via cascading top–down effects, but the degree to which such effects depend on predator life history traits is largely unknown. Within carnivore guilds, complex hierarchies of dominance facilitate coexistence, whereby subordinate species avoid dominant counterparts by partitioning space, time, or both. We investigated whether a major life history trait (hibernation) in an apex carnivore (black bears Ursus americanus) mediated its top–down effects on the spatio-temporal dynamics of three sympatric mesocarnivore species (coyotes Canis latrans, bobcats Lynx rufus, and gray foxes Urocyon cinereoargenteus) across a 15,000 km² landscape in the western USA. We compared top–down, bottom–up, and environmental effects on these mesocarnivores using an integrated modeling approach. Black bears exerted top–down effects that varied as a function of hibernation and were stronger than bottom–up or environmental impacts. High black bear activity in summer and fall appeared to buffer the most subordinate mesocarnivore (gray foxes) from competition with dominant mesocarnivores (coyotes and bobcats), which were in turn released by black bear hibernation in winter and early spring. The mesocarnivore responses occurred in space (i.e., altered occupancy and site visitation intensity) rather than time (i.e., diel activity patterns unaffected). These results suggest that the spatio-temporal dynamics of mesocarnivores in this system were principally shaped by a spatial predator cascade of interference competition mediated by black bear hibernation. Thus, certain life history traits of apex predators might facilitate coexistence among competing species over broad time scales, with complex implications for lower trophic levels.
... B. tectorum is highly palatable, especially in the spring when grazing is most damaging to native perennials (D. D. Austin et al. 1994;Strand et al. 2014), and so native species that persist at low abundances at sites dominated by B. tectorum may be more vulnerable to local extirpation via grazing. Annual grass dominance can also suppress native perennial bunchgrasses (Jeanne C. Chambers et al. 2007;Reisner et al. 2013) via competition for light which reduces their growth and reproduction (Dyer and Rice 1999). ...
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Ecosystem transformations are very difficult to reverse and are likely to become more common with accelerating global change. The novel systems that often result are maintained by stabilizing feedbacks that are composed of forcing mechanisms that can be either external or internal to the system. Increasing our understanding of these feedback components and how they relate to each other is crucial to know when they can be reversed and the original ecosystem restored. Here, we studied the sagebrush biome in the western United states, where annual grass invasion and fire are converting vast areas of shrublands into annual grasslands, and these are thought to be two alternative species assemblages. In four separate studies, we aimed to understand the components of positive feedback mechanisms that maintain these two assemblages, and the impacts that conversion from shrubland to grassland has on ecosystem function. In chapter one we found that the spatial connectivity of fuel influences the burn severity of fire, which then favors the occurrence of fire-tolerant annuals in the seedbank. In chapter two we investigated how invasion and the loss of shrubs and perennial grasses by fire influenced soil nutrient cycling, and found that the annual grass dominance in the post-fire state converted the system from a source to a sink of soil C and N. In chapter three, we constructed a fire history atlas to isolate the effect of time since fire and remove the effect of repeated fires. We found that there was very little evidence of recovery towards the pre-fire state even after 30 years. Rather, we found that livestock grazing and annual grass abundance interact to maintain the post-fire, grass-dominated state. Finally, in chapter four we created an allometric equation to calculate biomass from cover estimates.
... Many factors may affect fire regimes, including average climate and interannual variation in weather [54,118], available fuels [119][120][121], historical fire management practices [52,53], and interactions between climate, fuels, and management [14]. There is a lack of understanding how seeding treatments affect fire regimes across arid landscapes [52,53,122]. ...
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Wildfire size and frequency have increased in the western United States since the 1950s, but it is unclear how seeding treatments have altered fire regimes in arid steppe systems. We analyzed how the number of fires since 1955 and the fire return interval and frequency between 1995 and 2015 responded to seeding treatments, anthropogenic features, and abiotic landscape variables in Wyoming big sagebrush ecosystems. Arid sites had more fires than mesic sites and fire return intervals were shortest on locations first treated between 1975 and 2000. Sites drill seeded before the most recent fire had fewer, less frequent fires with longer fire return intervals (15–20 years) than aerially seeded sites (intervals of 5–8 years). The response of fire regime variables at unseeded sites fell between those of aerial and drill seeding. Increased moisture availability resulted in decreased fire frequency between 1994 and 2014 and the total number of fires since 1955 on sites with unseeded and aerially pre-fire seeding, but fire regimes did not change when drill seeded. Greater annual grass biomass likely contributed to frequent fires in the arid region. In Wyoming big sagebrush steppe, drill seeding treatments reduced wildfire risk relative to aerial seeded or unseeded sites.
Article
On the Ground •land resilience is influenced by a variety of ecosystem properties that fall into two broad categories, 1) abiotic and 2) biotic. •Abiotic properties cannot be directly influenced with management. •In contrast, biotic properties of the ecosystem can be readily influenced by management. The formula for robust biotic resilience to wildfire and resistance to invasive annual grasses in the northern Great Basin sagebrush ecosystem is largely about maintaining and promoting perennial bunchgrasses. Meeting these imperatives in a highly variable, invasive annual grass-prone environment modifies the very nature of the problem from seemingly simple, to one that is highly complex, particularly in areas where bunchgrasses are depauperate. •Success in such an environment requires a process- rather than an event-based approach. It is unlikely that a singular management treatment will be effective, so the problem should be managed accordingly. •The management system itself must also possess properties of resilience if we hope to promote ecosystem resilience in an ever-changing risk and recovery environment. A successful strategy will first require securing the necessary components of a resilient management system, and a shift in paradigm from random acts of opportunistic restoration to a sustained, organized, process-based approach for promoting ecosystem resilience.
Article
On the Ground •More often than not, there is untapped potential for win-wins between livestock production and conservation. On the other hand, it is impossible to achieve every objective everywhere, all the time. Sometimes the tradeoffs are real. •We need to spend less time searching for general rules and more time embracing the complexity and context-dependence within rangeland science. •Rather than writing off findings that do not fit our current worldview, we should challenge ourselves to broaden our views in ways that reconcile multiple findings or multiple truths. It is possible we are all partly or mostly right, and we just need to figure out why, how, and in what contexts. •There is value in doing research in a way that focuses on really listening to and respecting multiple perspectives so that the results we produce not only qualify as facts, but also as truths that many people can buy into and get behind.
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The Nevada Plots exclosure system was constructed in 1937 following passage of the Taylor Grazing Act to assess long-term effects of livestock grazing oil Nevada rangelands. A comparison of vegetation characteristics inside and Outside exclosures was conducted during 2001 and 2002 at 16 sites. Data analysis was performed with a paired t test. Out of 238 cover and density comparisons between inside and outside exclosures at each site, 34 (14% of total) were different (P < 0.05). Generally, where differences occurred, basal and canopy cover were greater inside exclosures and density was greater outside. Shrubs were taller inside exclosures at 3 sites grazed by sheep (Ovis aries). perennial grasses showed no vertical height difference. Aboveground plant biomass production was different at only 1 site. Plant community diversity inside and outside exclosures were equal at 11 of 16 sites. Species richness was similar at all sires and never varied > 4 species at any site. Few changes in species composition, cover, density, and production inside and outside exclosures have occurred in 65 years, indicating that recovery rates since pre-Taylor Grazing Act conditions were similar under moderate grazing and grazing exclusion oil these exclosure sites.
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This state-of-knowledge review about the effects of fire on flora and fuels can assist land managers with ecosystem and fire management planning and in their efforts to inform others about the ecological role of fire. Chapter topics include fire regime classification, autecological effects of fire, fire regime characteristics and postfire plant community developments in ecosystems throughout the United States and Canada, global climate change, ecological principles of fire regimes, and practical considerations for managing fire in an ecosytem context.
Book
Grassfires: Fuel, Weather and Fire Behaviour presents information from CSIRO on the behaviour and spread of fires in grasslands. This second edition follows over 10 years of research aimed at improving the understanding of the fundamental processes involved in the behaviour of grassfires. The book covers all aspects of fire behaviour and spread in the major types of grasses in Australia. It examines the factors that affect fire behaviour in continuous grassy fuels; fire in spinifex fuels; the effect of weather and topography on fire spread; wildfire suppression strategies; and how to reconstruct grassfire spread after the fact. The three meters designed by CSIRO for the prediction of fire danger and rate of spread of grassfires are explained and their use and limitations discussed. This new edition expands the discussion of historical fires including Aboriginal burning practices, the chemistry of combustion, and the structure of turbulent diffusion flames. It also examines fire safety, including the difficulty of predicting wind strength and direction and the impact of threshold wind speed on safe fire suppression. Myths and fallacies about fire behaviour are explained in relation to their impact on personal safety and survival. Grassfires will be a valuable reference for rural fire brigade members, landholders, fire authorities, researchers and those studying landscape and ecological processes.
Article
Livestock grazing potentially has substantial influence on fuel characteristics in rangelands around the globe. However, information quantifying the impacts of grazing on rangeland fuel characteristics is limited, and the effects of grazing on fuels are important because fuel characteristics are one of the primary factors determining risk, severity, continuity, and size of wildfires. We investigated the effects of long-term (70 yr) livestock grazing exclusion (nongrazed) and moderate levels of livestock grazing (grazed) on fuel accumulations, continuity, gaps, and heights in shrub-grassland rangelands. Livestock used the grazed treatment through 2008 and sampling occurred in mid- to late summer in 2009. Nongrazed rangelands had over twofold more herbaceous standing crop than grazed rangelands (P<0.01). Fuel accumulations on perennial bunchgrasses were approximately threefold greater in nongrazed than grazed treatments. Continuity of fuels in nongrazed compared to grazed treatments was also greater (P<0.05). The heights of perennial grass current year's and previous years' growth were 1.3-fold and 2.2-fold taller in nongrazed compared to grazed treatments (P<0.01). The results of this study suggest that moderate livestock grazing decreases the risk of wildfires in sagebrush steppe plant communities and potentially other semi-arid and arid rangelands. These results also suggest wildfires in moderately grazed sagebrush rangelands have decreased severity, continuity, and size of the burn compared to long-term nongrazed sagebrush rangelands. Because of the impacts fuels have on fire characteristics, moderate levels of grazing probably increase the efficiency of fire suppression activities. Because of the large difference between fuel characteristics in grazed and nongrazed sagebrush rangelands, we suggest that additional management impacts on fuels and subsequently fires need to be investigated in nonforested rangelands to protect native plant communities and prioritize management needs.