Abiotic and biotic influences on Bromus tectorum invasionand Artemisia tridentata recovery after fire

Lea CondonA, Peter J. WeisbergA and Jeanne C. ChambersB

ADepartment of Natural Resources and Environmental Science, University of Nevada – Reno,

1000 Valley Road, Reno, NV 89512, USA.BUS Forest Service, Rocky Mountain Research Station, 920 Valley Road, Reno, NV 89512, USA.CCorresponding author. Email: leacondon@yahoo.com

Abstract. Native sagebrush ecosystems in the Great Basin (western USA) are often invaded following fire by exoticBromus tectorum (cheatgrass), a highly flammable annual grass. Once B. tectorum is established, higher fire frequenciescan 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 toexamine the distribution and cover of B. tectorum and A. tridentata 6 years after a large wildland fire. We used structuralequationmodels 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 ahypothesised negative effect of pre-fire tree canopy cover on post-fire cover of A. tridentata. Site- and landscape-levelabiotic 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 anegative effect of perennial herbaceous species cover. Restoration efforts to reduce tree canopy cover should be limited toproductive 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 localenvironmental variation and the relative abundances and com-

petitive abilities of both native and exotic species (Keeley et al.2005; D’Antonio et al. 2009). Higher availability of resourcesafter fire (Badia and Marti 2003; Certini 2005) can increase

the probability of establishment and spread of exotic species asdescribed 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 seedbanks, surviving meristems capable of resprouting, or viableseed sources are present in sufficient abundance (D’Antonioet al. 2009).

In semiarid regions of the western United States, expansionof pinyon and juniper trees into Artemisia tridentata

(sagebrush)-dominated ecosystems has cascading effects.

Expansion of the highly competitive trees results in progressivedecreases in understorey species including A. tridentata andnative perennial herbaceous species (Miller et al. 2011). At the

same time, infilling and growth of the trees result in increasedfuel loads and causes larger and more severe fires (Miller et al.2008, 2011). High-severity fires in pinyon and juniper wood-

lands result in almost complete mortality of trees and Artemisiaspecies (Baker and Shinneman 2004; Bauer and Weisberg2009) and increase the potential for mortality of the residualperennial herbaceous component. The net effect is increased

susceptibility of these ecosystems to invasion and spread ofBromus tectorum L. (cheatgrass), a Eurasian annual grass, afterfire. In the worst-case scenario, native A. tridentata ecosystems

are converted to near monocultures of B. tectorum with signifi-cantly decreased resource values and ecosystem services.

Bromus tectorum is a highly flammable, fire-adapted species

that increases the continuity of fine fuels and causes morefrequent and often larger wildfires (D’Antonio and Vitousek1992; Link et al. 2006). Increased fire frequencies favour

B. tectorum over fire-intolerant native shrubs such as Artemisiatridentata, which are killed by fire and require much longer fire-free intervals for establishment and reproduction (Young andEvans 1978). Sufficient seeds of B. tectorum typically survive

after fire to permit reestablishment and high resource avail-ability can result in rapid population growth (West and Young2000). Seeds of A. tridentata vaseyana can survive after fire

(Mueggler 1956), but are short-lived (i.e. no persistent seedbank) 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 establishmentin the first few years after fire (Ziegenhagen 2003). Thuspropagule limitation is a significant factor reducing seedling

establishment of A. tridentata in competition with B. tectorum

(Mazzola et al. 2010). Recruiting seedlings of native perennialspecies are, in general, poor competitors against B. tectorumseedlings because this annual grass can germinate and exhibit

CSIRO PUBLISHING

www.publish.csiro.au/journals/ijwf International Journal of Wildland Fire 2011, 20, 597–604

� IAWF 2011 10.1071/WF09082 1049-8001/11/040597

greater root elongation earlier in the fall and under colder wintertemperatures (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) thaneither native shrubs or grasses.

In semiarid Artemisia tridentata shrublands, abundance of

native perennial herbaceous species is a major determinantof invasibility by annual grasses. Long-term observational data-sets from sagebrush-steppe recovering from livestock grazing

(Anderson and Inouye 2001), and from sagebrush semi-desertresponding to wildfire and livestock grazing (West and York2002), show an inverse relationship between Bromus tectorum

and total perennial cover. Experimental research shows that the

effects of fire and removal of perennial herbaceous vegetationon invasion ofB. 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 removaland fire (Chambers et al. 2007). The negative effects of native

perennial herbaceous species on growth and reproduction ofB. tectorum may translate to a positive effect on A. tridentata

establishment.

Conversion of diverse Artemisia tridentata ecosystems toBromus tectorum dominance results in habitat loss and frag-mentation, 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 amanagement tool to maintain and restore A. tridentata commu-

nities threatened by pinyon and juniper expansion (Forbis et al.2006; Rau et al. 2008). Effective use of prescribed fire requiresknowledge of the likelihood of recovery of A. tridentata and

the potential for invasion by B. tectorum. We conducted alandscape-scale observational study of the distribution of thesetwo species 6 years following a single large wildfire in centralNevada, USA. We hypothesised that B. tectorum cover follow-

ing fire is positively associated with pre-fire tree canopy coverand negatively associated with cover of herbaceous perennialspecies, and that the converse is true for post-fire recovery of

A. tridentata. We also hypothesised that there would be a directnegative effect of B. tectorum cover on A. tridentata because ofthe ability of B. tectorum to out-compete seedlings of native

species. We used structural equation models (Grace andPugesek 1997) to quantify the interacting influences of pre-firetree canopy cover, perennial herbaceous cover, distance from

potential seed source, and site environment on post-fire cover ofB. tectorum andA. tridentata. The implications of our results forrestoring and maintaining A. tridentata in ecosystems suscepti-ble to B. tectorum invasion are discussed.

Materials and methods

Study design and field measurements

The 2800-ha Wall Canyon study area lies within the ToiyabeRange of central Nevada, USA, ranges in elevation from 2145 to

2455m, and encompasses an area burned by a wildland fire inJuly 2000. The study area is generally xeric, with steep slopesand high levels of solar radiation (Table 1). Pre-fire vegetationwas dominated by singleleaf pinyon (Pinus monophylla Torr.

Table1.

Environmentalvariablesconsidered

forinclusionin

structuralequationmodels

Variable

Description

Units

Mean(s.d.)

Range

Abioticvariables

Solarradiation

Incident,cloud-freesolarradiationestimated

for15May

kJm

�2day

�1

26418.4

(1996.6)

17895–28253

Slope

Field-m

easuredslopesteepness

838.4

(16.3)

0–77

Maxim

um

soildepth

Maxim

um

soildepth

cm53.3

(19.5)

23–85

%coarse

fragment

Percentageofcoarse

fragmentin

surfacesoils(to10-cm

depth)

%42.5

(10.6)

20.2–71.3

TCI

TopographicConvergence

Index:(flowaccumulation�area)/slope

Wetnessindex

5.5

(2.8)

2.2–14.92

Bioticvariables

Pre-firetree

canopycover

Areacovered

bypre-firetree

canopyin

a0.1-haplot

m2

229.9

(71.5)

37.7–423.2

Edgedensity

Edgedensity

ofunburned

patch

edgewithin

a300-m

radiusofeach

plot

m17.9

(22.6)

0–89

Bromustectorumcover

Ocularestimateofcover

toclosest1%

andaveraged

over

each

plot

%8.8

(6.3)

0–22.3

Artem

isia

tridentata

ssp.vaseyanacover

Measuredin

belttransectsbycalculatingthearea

ofeach

shrubusingan

equationforan

ellipse

%0.2

(0.3)

0–1.2

Perennialherbaceouscover

Ocularestimateofcover

toclosest1%

andaveraged

over

each

plot

%0.1

(0.1)

0–0.4

598 Int. J. Wildland Fire L. Condon et al.

and Frem.) 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 blue-grass (Poa nervosa (Hook.) Vasey) at the lower elevations, and

curlleaf mountain mahogany (Cercocarpus ledifolius Nutt.) onnorth-east-facing slopes). In this semiarid ecosystem, annualprecipitation is highly variable but the long-term mean is

170mm and most precipitation arrives during the winter assnow. Mean precipitation after the fire in 2000 was belowaverage for 2000–05. The mean for this period was 119mm. In2005, precipitation totalled only 83.3mm and immediately

before sampling (October 2005 to May 2006) was 79mm(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 theterrain is rugged (USDA NRCS 2006). The study area is man-aged by the US Forest Service and has been subject to a range

of anthropogenic disturbances including cattle grazing, salvagelogging, 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 100m apart, were selected randomlyalong east–west transects that spanned the canyon from ridge-line to ridgeline. Transects (i.e. gradsects as described in

Gillison and Brewer 1985) provided representative samplingacross gradients of elevation and distance from burn perimeter,given that the burn perimeter closely followed the major

ridgelines. The east–west transects were randomly locatedwithin three equal-area sections to adequately sample thegeographic extent of the Wall Canyon burn. A total of onehundred and two 20� 50-m (0.1-ha) plots were surveyed,

including 71 within the burn, 16 outside the burn and 15 inunburned patches within the burn area (Fig. 1).

Aerial cover, topographic and soils data were collected at

each plot. Aerial cover of herbaceous species including Bromustectorum was ocularly estimated to the closest 1% withintwenty-five 0.5-m2 quadrats. Quadrats were positioned along

transects beginning at 1, 10 and 17m along the 20-m side ofthe 0.1-ha rectangular plot. Aerial cover and frequency of all

Legend

dNBR

Value

High: 185

Low: �199N

2000Metres

150010005002500

Wall Canyon plot locations

Burn perimeterOregon

Nevada

Wyoming

Utah

Arizona

California

Idaho

Fig. 1. The location of the 2800-haWall Canyon fire in central Nevada, USA, and the sampling designwithin 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.

Effects of the pre-fire plant community on post-fire recovery Int. J. Wildland Fire 599

Artemisia tridentata individuals were surveyed along three2� 50-m belt transects. Shrub canopy dimensions along thelongest axis and the axis perpendicular to the longest were

measured for each shrub to estimate aerial cover using theequation for the area of an ellipse.

Abiotic data were collected from each plot, including topo-

graphic position, aspect and slope. Slope and aspect values wereused 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 byrock (Harner and Harper 1976). Soil depth measurements wererecorded 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 wereanalysed for pH and% coarse fragments. Soil pH was measuredwith a Corning 320 pH meter (Columbus, OH, USA) using

,10 g of soil, 19mL of deionised water and 1mL of CaCl2.Coarse fragments were sieved and weighed to determine theirpercentage 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 solarradiation, pre-fire tree canopy cover, topographic convergence

index, unburned patches and burn perimeter. The intensity ofincident solar radiation assuming clear-sky conditions (Kumaret al. 1997) was modelled using a 30-m-resolution digital ele-

vation model (DEM) for 15 May, to correspond with the springseed germination period. Tree canopy cover was delineatedfrom 1996 panchromatic digital orthophotography (1-m reso-lution). Panchromatic digital orthophotoquads (DOQs) were

corrected for topographic shadowing with IDRISI Kilimanjaro

version 14.02 software (Clark Labs, Worcester, MA, USA) andused to create a polygon layer of tree canopies that existed

before the fire (Greenwood andWeisberg 2009). An automated,object-oriented classification method, implemented in eCogni-

tion Professional version 4.0 software (Trimble, Westminster,

CO, USA), used brightness, patch shape, patch area, distance,textural homogeneity and local neighbourhood relationshipsto 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 ln a=tan bð Þ ð1Þ

where a is the upslope contributing area of water drainage tothe centre of the plot and b the local slope angle. High valuesindicate sites that collect and retain water in runoff events (e.g.

depressions, low in the watershed), and low values indicate siteswith 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 beforethe 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 burnseverity using a ratio of short-wave infrared bands, Band 4

(0.76–0.90mm) and Band 7 (2.08–2.35mm) in the equation(Cocke et al. 2005):

NBR ¼ Band 4� Band 7ð Þ= Band 4þ Band 7ð Þ ð2Þ

Unburned patches throughout Wall Canyon were identifiedusing differenced NBR (dNBR) values, subtracting the post-

fire NBR from the pre-fire value for each pixel, such thatpositive dNBR values indicate vegetation damage or burnedareas. To develop an unburned patch layer, dNBR values were

generalised to two classes, burned and unburned, using aniterative, 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 vege-tation sampling locations within the burn perimeter. Twentyof twenty-three (87%) unburned patches and 71 of 71 (100%)

burned patcheswere correctly classified. Edge density (mkm�2)of unburned patches was then calculated for a 300-m neighbour-hood surrounding each surveyed plot.

Values of environmental variables were extracted by over-laying 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 3unit (Sunnyvale, CA, USA) at sub-metre precision and differ-entially corrected. Reconstructed plots were used to sample pre-

fire tree canopy cover. Plot centroids were sampled in GIS forelevation, slope, aspect, estimated solar radiation and topo-graphic convergence index.

Data analysis

Structural equation modelling (SEM) allows the testing ofcomplex dependency relationships and partitions direct andindirect effects of explanatory variables such as pre-fire treecanopy cover (Grace and Pugesek 1997). One strength of SEM is

that it accounts for correlations between variables that may bemasking a relationship of interest (Grace 2006). SEM requiresformulation of explicit conceptual models (i.e. path diagrams)

representing causal and correlational relationships amongmeasured variables. The resulting relationships of SEM areequivalent to standardised partial regression coefficients (Grace

2006). In the present study, SEM was used to describe thehypothesised 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 environ-ment 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 coverfollowing fire will be higher on mesic sites, as indicated bygradual, shaded slopes, higher position in the watershed (i.e.

lower TCI), and deeper soils with a lower proportion of coarsefragments. Pre-fire tree canopy cover was predicted to havesimilar associations with site environmental variables. Green-

wood and Weisberg (2009) observed that sites with deeper,more clayey soils in central Nevada generally supported greatertree cover, associated with high levels of tree establishment inrecent decades. Greater pre-fire tree canopy cover was expected

600 Int. J. Wildland Fire L. Condon et al.

to negatively influence the post-fire abundance of A. tridentata

and perennial herbaceous vegetation owing to the well-knowninverse relationship between overstorey and understorey coverin pinyon–juniper ecosystems (Miller et al. 2005, 2011). Post-fire establishment of A. tridentatawas predicted to be positively

correlated with increased proximity of unburned patches thatserve as propagule sources, because seed dispersal of thisspecies occurs over short distances (�30m; Meyer 1994).

Proximity to unburned patches was hypothesised to bepositively influenced by slope as extremely steep slopes tendto be quite rocky with limited fuel loading and continuity.

Bromus tectorum cover was predicted to negatively influenceA. tridentata cover through direct competition, given that shrubseedlings are thought to use water from the same soil depth asB. tectorum (Booth et al. 2003).

We hypothesised that Bromus tectorum cover would begreatest, and perennial herbaceous cover least, on xeric sitesas indicated by high levels of solar radiation (Fig. 2a); cover of

native plant species in the Great Basin following wildfire isoften greater in more mesic sites (Reilly et al. 2006). The coverof perennial herbaceous species was predicted to exert a nega-

tive influence on B. tectorum cover through direct competitioneffects, consistent with our hypothesis that the presence ofperennial herbaceous species increases site resistance to inva-

sion byB. tectorum (Anderson and Inouye 2001; Chambers et al.2007). Perennial herbaceous cover was predicted to positivelyinfluence Artemisia tridentata cover through facilitation effectsor as an indicator of improved site conditions not captured by the

modelled environmental variables.We further hypothesised thatincreased cover of perennial herbaceous species would have anindirect, 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 indicatesthe magnitude and direction of influence of the predictorvariable on the response, accounting for other causal and

correlational relationships in the model (Grace and Pugesek

1997). SEM analyses were implemented in AMOS (Analysis ofMoment Structures) version 7.0 software (SPSS, IBM, Armonk,NY). Models were assessed using fit indices, referring to thecorrespondence between the hypothesised model and the

observed covariance matrix. Chi-square statistics, their associ-ated P values and the root mean square error of approximation(RMSEA) provide complementary measures of model fit

(Kaplan 2000). Individual pathways were evaluated usingcritical ratios, defined as the covariance estimate divided bythe standard error. Critical ratios were evaluated for statistical

significance assuming a standard normal distribution (Arbuckle2006). The most parsimonious model (Fig. 2b), representing asubset of the full, hypothesised path model (Fig. 2a), wasselected for interpretation.

Results

Several of the abiotic predictor variables were significantlycorrelated with one another within the SEM framework(Fig. 2b). Positive correlations existed between % coarse frag-

ment and slope. Negative correlations occurred between slopeand both maximum soil depth and solar radiation, between solarradiation and TCI, and between % coarse fragment and TCI.

These relationships indicate that sites lower in the watershedwere more likely to be shaded and to have finer soil textures andmore gradual slopes owing to their topographic position. Sig-nificant correlations among abiotic variables were accounted for

when modelling their influences on plant cover within the SEMframework.

The most parsimonious model (RMSEA, 0.001, x2¼18.477, P¼ 0.779) for post-fire recovery of Artemisia triden-

tata, perennial herbaceous species and invasion and spread ofBromus 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 networkof causal relationships (compare Fig. 2a,b). Site environment

Slope

Proximity tounburnedpatches

Pre-firecanopycover

Pre-firecanopy cover

A. tridentatassp. vaseyana

cover

A. tridentatassp. vaseyana

cover

Perennialherbaceous

cover

Perennialherbaceous

cover

Bromustectorum

cover

Bromustectorum

cover

Topographicconvergence

index

% coarsefragment

Maximumsoil

depth

Solarradiation

Slope

Topographicconvergence

index �0.284

�0.362

�0.485

�0.400

�0.225

�0.223

�0.320

�0.234

�0.316

0.250

0.381

% coarsefragment

Maximumsoil

depth

Solarradiation

(a) (b)

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 shownwith dotted double-headed arrows. Negative path coefficients are shownwith dashed lines and positive path coefficients are shownwith 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.

Effects of the pre-fire plant community on post-fire recovery Int. J. Wildland Fire 601

factors did not influence A. tridentata directly and had no effecton perennial herbaceous cover. However, there were indirecteffects of site environment on A. tridentata mediated through

negative influences of TCI and % coarse fragment on theinhibitory variable, pre-fire tree canopy cover. Overall, treecover was greater on higher positions in the watershed and

sparser on rockier sites (Fig. 2b). Solar radiation was the onlysite environment factor to influence B. tectorum cover, exhibit-ing 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 didnot have significant effects on A. tridentata cover and are notincluded in the final model.

The biotic variables exhibitedmany of the predicted relation-ships (Fig. 2b). Pre-fire tree canopy cover values were spatiallyvariable, but intermediate overall (mean¼ 24.91%, 95% CI¼�1.80%). Aerial perennial herbaceous cover values were gen-erally low (mean¼ 0.09%, 95% CI¼�0.02%), and Bromus

tectorum cover valueswere comparatively high (mean¼ 8.81%,

95% CI¼�1.50%) (Table 1). These values indicate that thestudy site as a whole was likely in an intermediate to late stage oftree expansion before the fire. Pre-fire canopy cover of trees was

negatively correlated with Artemisia tridentata cover, but wasnot correlated with B. tectorum cover. Also, perennial herba-ceous 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 measuredabiotic variables had no apparent effect on the post-fire coverof 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 apparentpreference of B. tectorum for post-burn xeric conditions islargely attributable to the ecological amplitude of the species

(Chambers et al. 2007). In the absence of perennial herbaceousspecies, B. tectorum establishment, growth and reproduction ishighest 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 estab-lishment, growth and reproduction on higher-elevation sites

with cold soil temperatures (Evans and Young 1972; Mack andPyke 1983). Local environmental conditions and the composi-tion 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, evenafter accounting for the effects of relevant environmental vari-

ables (Fig. 2b). However, pre-fire tree cover did not exhibit theexpected positive effect on Bromus tectorum cover or negativeeffect on perennial herbaceous cover. Low cover values of

perennial herbaceous species 6 years after fire likely indicatethat the perennial herbaceous species had been depleted beforethe fire by inappropriate livestock grazing, as shown formultiple

post-fire sites in the Great Basin (Koniak 1985). In our study,grazing was reinitiated 2 years following the fire. Thus, lack ofan effect of pre-fire tree cover on perennial herbaceous specieslikely resulted from low initial perennial herbaceous cover that

was maintained by grazing after the fire. Establishment andpersistence 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-significantrelationship 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 thewatershed, and patterns of B. tectorum spread following the firethat were reinforced by grazing and anthropogenic disturbance.

Perennial herbaceous cover exhibited the predicted positiverelationship with Artemisia tridentata and negative relationshipwith Bromus tectorum. These results reflect a growing body ofevidence from long-term observational studies (Anderson and

Inouye 2001; West and York 2002) and experimental studiesshowing an inverse relationship between abundance ofB. tectorum and cover of native perennial herbaceous species

(Chambers et al. 2007). They also show for the first time thatreestablishment of A. tridentata following fire is positivelyrelated to the cover of native perennial herbaceous species.

These results indicate that management aimed at maintaining orincreasing the abundance of perennial herbaceous species hasthe 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. Duringthe first year after a fire, B. tectorum populations are typically

small (Young and Evans 1978). Two to three years are oftenrequired for B. tectorum densities to increase sufficiently for thespecies to be highly competitive. This lag-time in population

increase can provide a window of opportunity for native speciesestablishment. It is likely that most mature A. tridentata plantsobserved in our study established from seed bank sources in

the growing season immediately following the burn and beforewidespread B. tectorum dominance. This was the wettest yearfollowing 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 duringthe year of the study may have resulted in lower overallestablishment of both A. tridentata and B. tectorum. Successful

establishment and relative cover of annual grass species canvary 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 dispersallimitations (Meyer 1994). In sagebrush ecosystems exhibitingpinyon and juniper expansion, unburned patches typically havegreater densities of A. tridentata seed than burned areas follow-

ing fire (Allen et al. 2008). We did not find the predictedrelationship, possibly because the residual seed bank can maskthe importance of unburned seed sources in post-fire establish-

ment. In an experiment that used covered plots to preventestablishment from wind-borne seed, the residual seed bankcontributed substantially to A. tridentata establishment follow-

ing fire (Mueggler 1956). Also, A. tridentata may establish inphases following fire, and the relative importance of seedsources from outside the burn perimeter or from the seed bankmay diminish with time since fire (Ziegenhagen 2003).

602 Int. J. Wildland Fire L. Condon et al.

As A. tridentata individuals that established immediatelyfollowing fire mature, they contribute their own seed to sub-sequent A. tridentata establishment. This phased establishment

further confounds any influence of distance from unburnedpatches on A. tridentata distribution.

Management implications

Our study was conducted across a 2800-ha landscape andexamined a single fire. Although our data captured a range of

environmental and biotic conditions such as pre-fire canopycover, cover of perennial herbaceous plants, proximity tounburned patches and cover of Artemisia tridentata and Bromustectorum, these data are from a single landscape and therefore

generalisations of our results are limited. Yet, considering therange of conditions examined, it seems likely that our resultswould be confirmed at other sites.

Landscape-scale preventive management for maintainingand restoring resistance to Bromus tectorum invasion and siteresilience following fire is ongoing throughout much of the

Great Basin region.Management treatments include allowing orintroducing disturbance in the form of fire or mechanical treeremoval. Our results suggest that these management activities

will be most effective if they target productive areas with highcovers of perennial herbaceous species. Site selection should bebased on the stage of tree dominance and on the abundance ofshrubs and herbaceous species in the understorey that are

capable of establishing or resprouting following fire (Chambers2005; D’Antonio et al. 2009). High-priority areas for preventivemanagement should be those at the early to intermediate stages

of tree increase where native herbaceous perennials are still asignificant component of the community.

In the absence or near-absence of residual native shrubs and

herbaceous perennials, these ecosystems are at high risk ofinvasion and spread of Bromus tectorum. Active restoration inthe form of revegetation immediately after fire is often neces-sary to prevent B. tectorum dominance. This research indicates

that restoring productive and more mesic sites could increaseoverall resilience and resistance by increasing competitionbetween native perennial shrubs and herbs and B. tectorum.

Introducing disturbance to more xeric sites will likely lead toincreased 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 andSchupp 2004), this exotic species likely will remain a compo-nent 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 Moun-

tain Research Station and the Nevada Agricultural Experimental Station.

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