CSIRO PUBLISHING
www.publish.csiro.au/journals/ijwf
International Journal of Wildland Fire 2011, 20, 597–604
Abiotic and biotic influences on Bromus tectorum invasion
and Artemisia tridentata recovery after fire
Lea CondonA, Peter J. WeisbergA and Jeanne C. ChambersB
A
Department of Natural Resources and Environmental Science, University of Nevada – Reno,
1000 Valley Road, Reno, NV 89512, USA.
B
US Forest Service, Rocky Mountain Research Station, 920 Valley Road, Reno, NV 89512, USA.
C
Corresponding author. Email: leacondon@yahoo.com
Abstract. Native sagebrush ecosystems in the Great Basin (western USA) are often invaded following fire by exotic
Bromus tectorum (cheatgrass), a highly flammable annual grass. Once B. tectorum is established, higher fire frequencies
can lead to local extirpation of Artemisia tridentata ssp. vaseyana (mountain big sagebrush) and have cascading effects
on sagebrush ecosystems and the species that depend on them. We conducted a landscape-scale observational study to
examine the distribution and cover of B. tectorum and A. tridentata 6 years after a large wildland fire. We used structural
equation models to quantify the interacting influences of pre-fire tree canopy cover, perennial species cover, distance from
potential seed source, and site environment on post-fire cover of B. tectorum and A. tridentata. Results confirmed a
hypothesised negative effect of pre-fire tree canopy cover on post-fire cover of A. tridentata. Site- and landscape-level
abiotic factors influenced pre-fire tree canopy cover, which, in turn, influenced the probability of rapid recovery to
A. tridentata. However, B. tectorum cover was primarily influenced by a positive effect of incident solar radiation and a
negative effect of perennial herbaceous species cover. Restoration efforts to reduce tree canopy cover should be limited to
productive sites with sufficient cover of perennial herbaceous species to facilitate site recovery.
Additional keywords: fire effects, Great Basin, landscape-scale, structural equation modelling, succession.
Introduction
Ecological resilience following wildfire is influenced by local
environmental variation and the relative abundances and competitive abilities of both native and exotic species (Keeley et al.
2005; D’Antonio et al. 2009). Higher availability of resources
after fire (Badia and Marti 2003; Certini 2005) can increase
the probability of establishment and spread of exotic species as
described by the fluctuating resource hypothesis (Davis et al.
2000). However, many native species are highly competitive
with exotics and can dominate post-fire sites if residual seed
banks, surviving meristems capable of resprouting, or viable
seed sources are present in sufficient abundance (D’Antonio
et al. 2009).
In semiarid regions of the western United States, expansion
of pinyon and juniper trees into Artemisia tridentata
(sagebrush)-dominated ecosystems has cascading effects.
Expansion of the highly competitive trees results in progressive
decreases in understorey species including A. tridentata and
native perennial herbaceous species (Miller et al. 2011). At the
same time, infilling and growth of the trees result in increased
fuel loads and causes larger and more severe fires (Miller et al.
2008, 2011). High-severity fires in pinyon and juniper woodlands result in almost complete mortality of trees and Artemisia
species (Baker and Shinneman 2004; Bauer and Weisberg
2009) and increase the potential for mortality of the residual
perennial herbaceous component. The net effect is increased
Ó IAWF 2011
susceptibility of these ecosystems to invasion and spread of
Bromus tectorum L. (cheatgrass), a Eurasian annual grass, after
fire. In the worst-case scenario, native A. tridentata ecosystems
are converted to near monocultures of B. tectorum with significantly decreased resource values and ecosystem services.
Bromus tectorum is a highly flammable, fire-adapted species
that increases the continuity of fine fuels and causes more
frequent and often larger wildfires (D’Antonio and Vitousek
1992; Link et al. 2006). Increased fire frequencies favour
B. tectorum over fire-intolerant native shrubs such as Artemisia
tridentata, which are killed by fire and require much longer firefree intervals for establishment and reproduction (Young and
Evans 1978). Sufficient seeds of B. tectorum typically survive
after fire to permit reestablishment and high resource availability can result in rapid population growth (West and Young
2000). Seeds of A. tridentata vaseyana can survive after fire
(Mueggler 1956), but are short-lived (i.e. no persistent seed
bank) and seedling establishment is typically low after fire
(Young and Evans 1989). Seed sources of A. tridentata from
outside the burn perimeter are often important for establishment
in the first few years after fire (Ziegenhagen 2003). Thus
propagule limitation is a significant factor reducing seedling
establishment of A. tridentata in competition with B. tectorum
(Mazzola et al. 2010). Recruiting seedlings of native perennial
species are, in general, poor competitors against B. tectorum
seedlings because this annual grass can germinate and exhibit
10.1071/WF09082
1049-8001/11/040597
37.7–423.2
0–89
0–22.3
0–1.2
0–0.4
m2
m
%
%
%
Area covered by pre-fire tree canopy in a 0.1-ha plot
Edge density of unburned patch edge within a 300-m radius of each plot
Ocular estimate of cover to closest 1% and averaged over each plot
Measured in belt transects by calculating the area of each shrub using an equation for an ellipse
Ocular estimate of cover to closest 1% and averaged over each plot
229.9 (71.5)
17.9 (22.6)
8.8 (6.3)
0.2 (0.3)
0.1 (0.1)
17 895–28 253
0–77
23–85
20.2–71.3
2.2–14.92
26 418.4 (1996.6)
38.4 (16.3)
53.3 (19.5)
42.5 (10.6)
5.5 (2.8)
kJ m2 day1
8
cm
%
Wetness index
Mean (s.d.)
Units
Description
Incident, cloud-free solar radiation estimated for 15 May
Field-measured slope steepness
Maximum soil depth
Percentage of coarse fragment in surface soils (to 10-cm depth)
Topographic Convergence Index: (flow accumulation area)/slope
Abiotic variables
Solar radiation
Slope
Maximum soil depth
% coarse fragment
TCI
Biotic variables
Pre-fire tree canopy cover
Edge density
Bromus tectorum cover
Artemisia tridentata ssp. vaseyana cover
Perennial herbaceous cover
Materials and methods
Study design and field measurements
The 2800-ha Wall Canyon study area lies within the Toiyabe
Range of central Nevada, USA, ranges in elevation from 2145 to
2455 m, and encompasses an area burned by a wildland fire in
July 2000. The study area is generally xeric, with steep slopes
and high levels of solar radiation (Table 1). Pre-fire vegetation
was dominated by singleleaf pinyon (Pinus monophylla Torr.
Variable
greater root elongation earlier in the fall and under colder winter
temperatures (Harris 1967; Aguirre and Johnson 1991). Also,
B. tectorum typically has higher nutrient uptake rates (Monaco
et al. 2003) and higher growth rates (Arredondo et al. 1998) than
either native shrubs or grasses.
In semiarid Artemisia tridentata shrublands, abundance of
native perennial herbaceous species is a major determinant
of invasibility by annual grasses. Long-term observational datasets from sagebrush-steppe recovering from livestock grazing
(Anderson and Inouye 2001), and from sagebrush semi-desert
responding to wildfire and livestock grazing (West and York
2002), show an inverse relationship between Bromus tectorum
and total perennial cover. Experimental research shows that the
effects of fire and removal of perennial herbaceous vegetation
on invasion of B. tectorum in sagebrush ecosystems are additive,
with B. tectorum biomass and seed production increasing two to
three-fold following removal of perennial herbaceous species,
three to six-fold after fire, and 10–30-fold after both removal
and fire (Chambers et al. 2007). The negative effects of native
perennial herbaceous species on growth and reproduction of
B. tectorum may translate to a positive effect on A. tridentata
establishment.
Conversion of diverse Artemisia tridentata ecosystems to
Bromus tectorum dominance results in habitat loss and fragmentation, has placed several species, including sage-grouse
(Centrocercus spp.), at risk for federal listing. This has resulted
in a degree of urgency in developing management solutions
(Knick et al. 2003). Prescribed fire is increasingly used as a
management tool to maintain and restore A. tridentata communities threatened by pinyon and juniper expansion (Forbis et al.
2006; Rau et al. 2008). Effective use of prescribed fire requires
knowledge of the likelihood of recovery of A. tridentata and
the potential for invasion by B. tectorum. We conducted a
landscape-scale observational study of the distribution of these
two species 6 years following a single large wildfire in central
Nevada, USA. We hypothesised that B. tectorum cover following fire is positively associated with pre-fire tree canopy cover
and negatively associated with cover of herbaceous perennial
species, and that the converse is true for post-fire recovery of
A. tridentata. We also hypothesised that there would be a direct
negative effect of B. tectorum cover on A. tridentata because of
the ability of B. tectorum to out-compete seedlings of native
species. We used structural equation models (Grace and
Pugesek 1997) to quantify the interacting influences of pre-fire
tree canopy cover, perennial herbaceous cover, distance from
potential seed source, and site environment on post-fire cover of
B. tectorum and A. tridentata. The implications of our results for
restoring and maintaining A. tridentata in ecosystems susceptible to B. tectorum invasion are discussed.
L. Condon et al.
Range
Int. J. Wildland Fire
Table 1. Environmental variables considered for inclusion in structural equation models
598
Effects of the pre-fire plant community on post-fire recovery
Int. J. Wildland Fire
599
Legend
Wall Canyon plot locations
Oregon
Idaho
Burn perimeter
Wyoming
dNBR
Nevada
Value
Utah
High: 185
California
Low: ⫺199
Arizona
0
250
500
1000
1500
N
2000
Metres
Fig. 1. The location of the 2800-ha Wall Canyon fire in central Nevada, USA, and the sampling design within and outside the burn
area. Low values of differenced normalised burn ratio (dNBR) (darker areas) represent portions of the landscape that did not burn.
Horizontal lines delineate equal-area sections used for stratified random sampling of survey plots along gradsects.
and Frém.) and Utah juniper (Juniperus osteosperma (Torr.)
Little), with small pockets of sagebrush shrubland (mountain big
sagebrush (Artemisia tridentata ssp. vaseyana (Rydb.) Beetle),
Sandberg bluegrass (Poa secunda J. Presl) and Wheeler bluegrass (Poa nervosa (Hook.) Vasey) at the lower elevations, and
curlleaf mountain mahogany (Cercocarpus ledifolius Nutt.) on
north-east-facing slopes). In this semiarid ecosystem, annual
precipitation is highly variable but the long-term mean is
170 mm and most precipitation arrives during the winter as
snow. Mean precipitation after the fire in 2000 was below
average for 2000–05. The mean for this period was 119 mm. In
2005, precipitation totalled only 83.3 mm and immediately
before sampling (October 2005 to May 2006) was 79 mm
(Western Regional Climate Center; see http://www.wrcc.dri.
edu/summary/Climsmnv.html, accessed 29 June 2008). Soils
are lithic, deep Arborolls–Haplargids–Torriorthents and the
terrain is rugged (USDA NRCS 2006). The study area is managed by the US Forest Service and has been subject to a range
of anthropogenic disturbances including cattle grazing, salvage
logging, recreational off-highway vehicle use and mining.
Although these disturbances occur throughout the study area,
they are more frequent near the canyon mouth. Survey plots,
constrained to be at least 100 m apart, were selected randomly
along east–west transects that spanned the canyon from ridgeline to ridgeline. Transects (i.e. gradsects as described in
Gillison and Brewer 1985) provided representative sampling
across gradients of elevation and distance from burn perimeter,
given that the burn perimeter closely followed the major
ridgelines. The east–west transects were randomly located
within three equal-area sections to adequately sample the
geographic extent of the Wall Canyon burn. A total of one
hundred and two 20 50-m (0.1-ha) plots were surveyed,
including 71 within the burn, 16 outside the burn and 15 in
unburned patches within the burn area (Fig. 1).
Aerial cover, topographic and soils data were collected at
each plot. Aerial cover of herbaceous species including Bromus
tectorum was ocularly estimated to the closest 1% within
twenty-five 0.5-m2 quadrats. Quadrats were positioned along
transects beginning at 1, 10 and 17 m along the 20-m side of
the 0.1-ha rectangular plot. Aerial cover and frequency of all
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Int. J. Wildland Fire
L. Condon et al.
Artemisia tridentata individuals were surveyed along three
2 50-m belt transects. Shrub canopy dimensions along the
longest axis and the axis perpendicular to the longest were
measured for each shrub to estimate aerial cover using the
equation for the area of an ellipse.
Abiotic data were collected from each plot, including topographic position, aspect and slope. Slope and aspect values were
used to validate their respective geographic information systems
(GIS) layers. Soil depth was measured by pounding a 0.5-cm
metal rod into the ground until further pounding was resisted by
rock (Harner and Harper 1976). Soil depth measurements were
recorded as an average of three readings from each of ten 0.5-m2
quadrats. A 1-L composite soil sample was collected from the
same ten 0.5-m2 quadrats to a depth of 10 cm. Soils were
analysed for pH and % coarse fragments. Soil pH was measured
with a Corning 320 pH meter (Columbus, OH, USA) using
,10 g of soil, 19 mL of deionised water and 1 mL of CaCl2.
Coarse fragments were sieved and weighed to determine their
percentage in each soil sample. Soil texture was assessed using a
ribbon test according to the classification of Thien (1979).
Geographic information systems (GIS)-derived data
GIS was used to construct several map layers including solar
radiation, pre-fire tree canopy cover, topographic convergence
index, unburned patches and burn perimeter. The intensity of
incident solar radiation assuming clear-sky conditions (Kumar
et al. 1997) was modelled using a 30-m-resolution digital elevation model (DEM) for 15 May, to correspond with the spring
seed germination period. Tree canopy cover was delineated
from 1996 panchromatic digital orthophotography (1-m resolution). Panchromatic digital orthophotoquads (DOQs) were
corrected for topographic shadowing with IDRISI Kilimanjaro
version 14.02 software (Clark Labs, Worcester, MA, USA) and
used to create a polygon layer of tree canopies that existed
before the fire (Greenwood and Weisberg 2009). An automated,
object-oriented classification method, implemented in eCognition Professional version 4.0 software (Trimble, Westminster,
CO, USA), used brightness, patch shape, patch area, distance,
textural homogeneity and local neighbourhood relationships
to segment images into homogeneous patches and delineate
polygons dominated by tree crowns (Weisberg et al. 2007).
Topographic convergence index (TCI) was calculated as:
TCI ¼ lnða=tan bÞ
ð1Þ
where a is the upslope contributing area of water drainage to
the centre of the plot and b the local slope angle. High values
indicate sites that collect and retain water in runoff events (e.g.
depressions, low in the watershed), and low values indicate sites
with steep slopes that are high in the watershed.
GIS layers for unburned patches and burn perimeter were
developed with a classification of Landsat Thematic Mapper
(TM) imagery from 2 June 2000 and 20 July 2000, both before
the 22 July 2000 fire, and from 8 October 2000, after the fire.
All image processing procedures were implemented in IDRISI
Kilimanjaro version 14.02 software. Normalised burn ratio
(NBR) was calculated to highlight areas of differing burn
severity using a ratio of short-wave infrared bands, Band 4
(0.76–0.90 mm) and Band 7 (2.08–2.35 mm) in the equation
(Cocke et al. 2005):
NBR ¼ ðBand 4 Band 7Þ=ðBand 4 þ Band 7Þ
ð2Þ
Unburned patches throughout Wall Canyon were identified
using differenced NBR (dNBR) values, subtracting the postfire NBR from the pre-fire value for each pixel, such that
positive dNBR values indicate vegetation damage or burned
areas. To develop an unburned patch layer, dNBR values were
generalised to two classes, burned and unburned, using an
iterative, unsupervised classification (ISOCLUST algorithm,
using IDRISI Kilimanjaro version 14.02) that is a variation of
the commonly used ISODATA technique (Ball and Hall 1965).
The resulting classification was ground-truthed at the 94 vegetation sampling locations within the burn perimeter. Twenty
of twenty-three (87%) unburned patches and 71 of 71 (100%)
burned patches were correctly classified. Edge density (m km2)
of unburned patches was then calculated for a 300-m neighbourhood surrounding each surveyed plot.
Values of environmental variables were extracted by overlaying boundaries of 0.1-ha (20 50-m) plots on GIS layers.
Plot boundaries were reconstructed from global positioning
system (GPS) points recorded with a Trimble GeoExplorer 3
unit (Sunnyvale, CA, USA) at sub-metre precision and differentially corrected. Reconstructed plots were used to sample prefire tree canopy cover. Plot centroids were sampled in GIS for
elevation, slope, aspect, estimated solar radiation and topographic convergence index.
Data analysis
Structural equation modelling (SEM) allows the testing of
complex dependency relationships and partitions direct and
indirect effects of explanatory variables such as pre-fire tree
canopy cover (Grace and Pugesek 1997). One strength of SEM is
that it accounts for correlations between variables that may be
masking a relationship of interest (Grace 2006). SEM requires
formulation of explicit conceptual models (i.e. path diagrams)
representing causal and correlational relationships among
measured variables. The resulting relationships of SEM are
equivalent to standardised partial regression coefficients (Grace
2006). In the present study, SEM was used to describe the
hypothesised interacting effects of pre-fire tree canopy cover,
distance from potential seed sources (i.e. proximity to unburned
patches), perennial herbaceous species cover and site environment on post-fire cover of Bromus tectorum and Artemisia
tridentata (Table 1).
The hypothesised network of causal relationships tested
using SEM (Fig. 2a) predicts that Artemisia tridentata cover
following fire will be higher on mesic sites, as indicated by
gradual, shaded slopes, higher position in the watershed (i.e.
lower TCI), and deeper soils with a lower proportion of coarse
fragments. Pre-fire tree canopy cover was predicted to have
similar associations with site environmental variables. Greenwood and Weisberg (2009) observed that sites with deeper,
more clayey soils in central Nevada generally supported greater
tree cover, associated with high levels of tree establishment in
recent decades. Greater pre-fire tree canopy cover was expected
Effects of the pre-fire plant community on post-fire recovery
(a)
Int. J. Wildland Fire
601
(b)
Slope
Slope
Topographic
convergence
index
% coarse
fragment
Proximity to
unburned
patches
Perennial
herbaceous
cover
Perennial
herbaceous
cover
⫺0.320
Topographic
convergence
index
⫺0.284
0.250
⫺0.223
A. tridentata
ssp. vaseyana
cover
A. tridentata
ssp. vaseyana
cover
% coarse
fragment
⫺0.234
⫺0.225
Maximum
soil
depth
Pre-fire
canopy
cover
Bromus
tectorum
cover
Solar
radiation
Maximum
soil
depth
⫺0.362
Pre-fire
canopy cover
⫺0.400
⫺0.485
⫺0.316
Solar
radiation
Bromus
tectorum
cover
0.381
Fig. 2. Path diagrams of the (a) hypothetical and (b) most parsimonious structural equation modelling (SEM) model explaining field-measured cover
values of Artemisia tridentata ssp. vaseyana, Bromus tectorum and perennial herbaceous species following fire. Correlations among abiotic variables
are shown with dotted double-headed arrows. Negative path coefficients are shown with dashed lines and positive path coefficients are shown with solid
lines. Standardised path coefficients show the strength and the direction of the relationship between variables after accounting for the influence of
variables that correlate with those variables.
to negatively influence the post-fire abundance of A. tridentata
and perennial herbaceous vegetation owing to the well-known
inverse relationship between overstorey and understorey cover
in pinyon–juniper ecosystems (Miller et al. 2005, 2011). Postfire establishment of A. tridentata was predicted to be positively
correlated with increased proximity of unburned patches that
serve as propagule sources, because seed dispersal of this
species occurs over short distances (30 m; Meyer 1994).
Proximity to unburned patches was hypothesised to be
positively influenced by slope as extremely steep slopes tend
to be quite rocky with limited fuel loading and continuity.
Bromus tectorum cover was predicted to negatively influence
A. tridentata cover through direct competition, given that shrub
seedlings are thought to use water from the same soil depth as
B. tectorum (Booth et al. 2003).
We hypothesised that Bromus tectorum cover would be
greatest, and perennial herbaceous cover least, on xeric sites
as indicated by high levels of solar radiation (Fig. 2a); cover of
native plant species in the Great Basin following wildfire is
often greater in more mesic sites (Reilly et al. 2006). The cover
of perennial herbaceous species was predicted to exert a negative influence on B. tectorum cover through direct competition
effects, consistent with our hypothesis that the presence of
perennial herbaceous species increases site resistance to invasion by B. tectorum (Anderson and Inouye 2001; Chambers et al.
2007). Perennial herbaceous cover was predicted to positively
influence Artemisia tridentata cover through facilitation effects
or as an indicator of improved site conditions not captured by the
modelled environmental variables. We further hypothesised that
increased cover of perennial herbaceous species would have an
indirect, positive effect on the establishment of A. tridentata
through competitive effects on B. tectorum (Fig. 2a).
To make variable effect sizes comparable despite disparate
units, path coefficients were standardised by dividing each
variable by its standard deviation. The path coefficient indicates
the magnitude and direction of influence of the predictor
variable on the response, accounting for other causal and
correlational relationships in the model (Grace and Pugesek
1997). SEM analyses were implemented in AMOS (Analysis of
Moment Structures) version 7.0 software (SPSS, IBM, Armonk,
NY). Models were assessed using fit indices, referring to the
correspondence between the hypothesised model and the
observed covariance matrix. Chi-square statistics, their associated P values and the root mean square error of approximation
(RMSEA) provide complementary measures of model fit
(Kaplan 2000). Individual pathways were evaluated using
critical ratios, defined as the covariance estimate divided by
the standard error. Critical ratios were evaluated for statistical
significance assuming a standard normal distribution (Arbuckle
2006). The most parsimonious model (Fig. 2b), representing a
subset of the full, hypothesised path model (Fig. 2a), was
selected for interpretation.
Results
Several of the abiotic predictor variables were significantly
correlated with one another within the SEM framework
(Fig. 2b). Positive correlations existed between % coarse fragment and slope. Negative correlations occurred between slope
and both maximum soil depth and solar radiation, between solar
radiation and TCI, and between % coarse fragment and TCI.
These relationships indicate that sites lower in the watershed
were more likely to be shaded and to have finer soil textures and
more gradual slopes owing to their topographic position. Significant correlations among abiotic variables were accounted for
when modelling their influences on plant cover within the SEM
framework.
The most parsimonious model (RMSEA , 0.001, x2 ¼
18.477, P ¼ 0.779) for post-fire recovery of Artemisia tridentata, perennial herbaceous species and invasion and spread of
Bromus tectorum, given the influences of site environment,
proximity to unburned patches and pre-fire tree canopy cover,
represents only a subset of the originally hypothesised network
of causal relationships (compare Fig. 2a,b). Site environment
602
Int. J. Wildland Fire
factors did not influence A. tridentata directly and had no effect
on perennial herbaceous cover. However, there were indirect
effects of site environment on A. tridentata mediated through
negative influences of TCI and % coarse fragment on the
inhibitory variable, pre-fire tree canopy cover. Overall, tree
cover was greater on higher positions in the watershed and
sparser on rockier sites (Fig. 2b). Solar radiation was the only
site environment factor to influence B. tectorum cover, exhibiting a strongly positive path coefficient. Solar radiation did not
directly influence A. tridentata or perennial herbaceous cover.
The proximity to unburned patches and cover of B. tectorum did
not have significant effects on A. tridentata cover and are not
included in the final model.
The biotic variables exhibited many of the predicted relationships (Fig. 2b). Pre-fire tree canopy cover values were spatially
variable, but intermediate overall (mean ¼ 24.91%, 95% CI ¼
1.80%). Aerial perennial herbaceous cover values were generally low (mean ¼ 0.09%, 95% CI ¼ 0.02%), and Bromus
tectorum cover values were comparatively high (mean ¼ 8.81%,
95% CI ¼ 1.50%) (Table 1). These values indicate that the
study site as a whole was likely in an intermediate to late stage of
tree expansion before the fire. Pre-fire canopy cover of trees was
negatively correlated with Artemisia tridentata cover, but was
not correlated with B. tectorum cover. Also, perennial herbaceous cover was positively correlated with A. tridentata cover,
but was negatively correlated with B. tectorum cover.
Discussion
The effects of both the abiotic and biotic variables on Artemisia
tridentata and Bromus tectorum were complex. The measured
abiotic variables had no apparent effect on the post-fire cover
of A. tridentata, likely because of its widespread distribution
within the watershed and low cover values. As hypothesised,
B. tectorum cover was greatest on xeric sites. The apparent
preference of B. tectorum for post-burn xeric conditions is
largely attributable to the ecological amplitude of the species
(Chambers et al. 2007). In the absence of perennial herbaceous
species, B. tectorum establishment, growth and reproduction is
highest on warmer and more xeric sites and lowest on cold and
mesic sites in these upland watersheds (Chambers et al. 2007).
Ecophysiological constraints severely limit B. tectorum establishment, growth and reproduction on higher-elevation sites
with cold soil temperatures (Evans and Young 1972; Mack and
Pyke 1983). Local environmental conditions and the composition and abundance of perennial herbaceous species determine
the relative abundance and persistence of B. tectorum over time.
As predicted, pre-fire tree canopy cover had a significant
negative effect on post-fire cover of Artemisia tridentata, even
after accounting for the effects of relevant environmental variables (Fig. 2b). However, pre-fire tree cover did not exhibit the
expected positive effect on Bromus tectorum cover or negative
effect on perennial herbaceous cover. Low cover values of
perennial herbaceous species 6 years after fire likely indicate
that the perennial herbaceous species had been depleted before
the fire by inappropriate livestock grazing, as shown for multiple
post-fire sites in the Great Basin (Koniak 1985). In our study,
grazing was reinitiated 2 years following the fire. Thus, lack of
an effect of pre-fire tree cover on perennial herbaceous species
likely resulted from low initial perennial herbaceous cover that
L. Condon et al.
was maintained by grazing after the fire. Establishment and
persistence of grass species like Festuca idahoensis and Poa
secunda under pinyon and juniper on more mesic sites (Miller
et al. 2005) also may have contributed to the non-significant
relationship between pre-fire tree cover and perennial cover.
The lack of a direct effect of pre-fire tree cover on B. tectorum
may be due to relatively high abundance of B. tectorum in the
watershed, and patterns of B. tectorum spread following the fire
that were reinforced by grazing and anthropogenic disturbance.
Perennial herbaceous cover exhibited the predicted positive
relationship with Artemisia tridentata and negative relationship
with Bromus tectorum. These results reflect a growing body of
evidence from long-term observational studies (Anderson and
Inouye 2001; West and York 2002) and experimental studies
showing an inverse relationship between abundance of
B. tectorum and cover of native perennial herbaceous species
(Chambers et al. 2007). They also show for the first time that
reestablishment of A. tridentata following fire is positively
related to the cover of native perennial herbaceous species.
These results indicate that management aimed at maintaining or
increasing the abundance of perennial herbaceous species has
the potential to increase both resistance to B. tectorum invasion
and recruitment of A. tridentata following fire.
Cover of Bromus tectorum did not exhibit the hypothesised
negative association with cover of Artemisia tridentata. During
the first year after a fire, B. tectorum populations are typically
small (Young and Evans 1978). Two to three years are often
required for B. tectorum densities to increase sufficiently for the
species to be highly competitive. This lag-time in population
increase can provide a window of opportunity for native species
establishment. It is likely that most mature A. tridentata plants
observed in our study established from seed bank sources in
the growing season immediately following the burn and before
widespread B. tectorum dominance. This was the wettest year
following the fire and over the 6 years preceding the study
(http://www.wrcc.dri.edu/summary/Climsmnv.html, accessed
10 May 2011). Below-average precipitation before and during
the year of the study may have resulted in lower overall
establishment of both A. tridentata and B. tectorum. Successful
establishment and relative cover of annual grass species can
vary dramatically among years (Bradford and Lauenroth 2006;
Keeley and McGinnis 2007), and low precipitation may have
influenced both B. tectorum abundance and study results.
We predicted that post-fire cover of Artemisia tridentata
would be positively correlated with closer proximity of
unburned patches that serve as seed sources owing to dispersal
limitations (Meyer 1994). In sagebrush ecosystems exhibiting
pinyon and juniper expansion, unburned patches typically have
greater densities of A. tridentata seed than burned areas following fire (Allen et al. 2008). We did not find the predicted
relationship, possibly because the residual seed bank can mask
the importance of unburned seed sources in post-fire establishment. In an experiment that used covered plots to prevent
establishment from wind-borne seed, the residual seed bank
contributed substantially to A. tridentata establishment following fire (Mueggler 1956). Also, A. tridentata may establish in
phases following fire, and the relative importance of seed
sources from outside the burn perimeter or from the seed bank
may diminish with time since fire (Ziegenhagen 2003).
Effects of the pre-fire plant community on post-fire recovery
As A. tridentata individuals that established immediately
following fire mature, they contribute their own seed to subsequent A. tridentata establishment. This phased establishment
further confounds any influence of distance from unburned
patches on A. tridentata distribution.
Management implications
Our study was conducted across a 2800-ha landscape and
examined a single fire. Although our data captured a range of
environmental and biotic conditions such as pre-fire canopy
cover, cover of perennial herbaceous plants, proximity to
unburned patches and cover of Artemisia tridentata and Bromus
tectorum, these data are from a single landscape and therefore
generalisations of our results are limited. Yet, considering the
range of conditions examined, it seems likely that our results
would be confirmed at other sites.
Landscape-scale preventive management for maintaining
and restoring resistance to Bromus tectorum invasion and site
resilience following fire is ongoing throughout much of the
Great Basin region. Management treatments include allowing or
introducing disturbance in the form of fire or mechanical tree
removal. Our results suggest that these management activities
will be most effective if they target productive areas with high
covers of perennial herbaceous species. Site selection should be
based on the stage of tree dominance and on the abundance of
shrubs and herbaceous species in the understorey that are
capable of establishing or resprouting following fire (Chambers
2005; D’Antonio et al. 2009). High-priority areas for preventive
management should be those at the early to intermediate stages
of tree increase where native herbaceous perennials are still a
significant component of the community.
In the absence or near-absence of residual native shrubs and
herbaceous perennials, these ecosystems are at high risk of
invasion and spread of Bromus tectorum. Active restoration in
the form of revegetation immediately after fire is often necessary to prevent B. tectorum dominance. This research indicates
that restoring productive and more mesic sites could increase
overall resilience and resistance by increasing competition
between native perennial shrubs and herbs and B. tectorum.
Introducing disturbance to more xeric sites will likely lead to
increased cheatgrass dominance and should be avoided.
Because of the strong affinity of B. tectorum for xeric sites,
and the difficulty of restoring these types of sites (Humphrey and
Schupp 2004), this exotic species likely will remain a component of these drier ecosystems.
Acknowledgements
This manuscript benefited from the comments of David Board, Erin
Goergen, Steve Jenkins, Dongwook Ko, Ben Rau and Ashley Sparrow.
Michael Clark, Teresa Olson, Jon Propp and Chelsea Robison assisted with
field work. Bob Blank contributed the use of the USDA Agricultural
Research Service Soils Laboratory. This project was funded by the Joint Fire
Sciences Program (05-JFSP-2-1-94), USDA Forest Service, Rocky Mountain Research Station and the Nevada Agricultural Experimental Station.
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http://www.publish.csiro.au/journals/ijwf