INVASIVE PLANT SPECIES AND THE JOINT FIRE ...Invasive plants may be responsi-ble for serious, long-term ecological impacts; they can cause the decline of native plant species, dis-rupt

Invasive Plant Species and the Joint Fire Science Program

Heather E. Erickson and Rachel White

United States Department of Agriculture

Forest ServicePacific NorthwestResearch Station

General Technical ReportPNW-GTR-707

November 2007

AUTHORSHeather Erickson is a research ecologist, and Rachel White is a science writer, Forestry Sciences Laboratory, 620 SW Main St., Suite 400, Portland, OR 97205.

Cover photoKerry Metlen, University of Montana.

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Invasive PlantSpecies and

the Joint Fire Science Program

Heather E. Erickson Rachel White

U.S. Department of AgricultureForest Service

Pacific Northwest Research StationPortland, Oregon

November 2007

have the potential tospread relatively quicklyover large areas. Invasiveplants may be responsi-ble for serious, long-termecological impacts; theycan cause the decline ofnative plant species, dis-rupt nutrient cycling andhydrology, and alter fireregimes.

Disturbances, such aswildfire and prescribedfire, may promote plantinvasions via a number of mechanisms. On theother hand, fire can be used to control some inva-sives. Determining how to successfully manageinvasives is crucial, but to do so, much moreinformation is needed on the ecology of speciesinvasions, the interactions between fire and inva-sives, and the responses of invasives to differentmanagement practices.

Several government-funded research pro-grams and initiatives are addressing ways to better understand the biology, ecology, and man-agement of invasive species. Since 1999, the JointFire Science Program (JFSP) has funded over 25projects related to invasive plant species and fire.In this paper we summarize completed and on-going JFSP research related to fire and invasives.

INTERACTIONS BETWEEN FIREAND INVASIVES

Fire, like many disturbances, may promotethe invasion of nonnative plant species by

providing canopy openings, reducing cover ofcompeting vegetation, and creating favorable soil

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ABSTRACTErickson, Heather E.; White, R. 2007. Invasive

plant species and the Joint Fire ScienceProgram. Gen. Tech. Rep. PNW-GTR-707.Portland, OR: U.S. Department of Agriculture,Forest Service, Pacific Northwest ResearchStation. 18 p.

Invasive nonnative plants may be responsible forserious, long-term ecological impacts, includingaltering fire behavior and fire regimes. Therefore,knowing how to successfully manage invasiveplants and their impacts on natural resources iscrucial. We present a summary of research oninvasive plants and fire that has been generatedthrough the Joint Fire Science Program—focusingspecifically on ecology of species invasions, theinteractions between fire and invasives, and theresponses of invasives to different managementpractices. Selected findings include (1) prescribed fire may increase invasive species in some ecosys-tems; (2) fuel treatments that leave some over-story canopy, minimize exposure of bare ground,and target sites that already host species capableof resprouting may be less likely to promote invasives; and (3) postfire seeding should beapproached with caution, as it can increase invasives.

Keywords: Invasive plants, fire management, cheatgrass, fuel treatments, postfire seeding, fireregimes, exotic species.

INTRODUCTION

Invasive nonnative plants are plants introducedinto areas outside of their natural ranges, usu-

ally as a result of human activities. Plants that aresuccessful invaders often have high growth ratesand typically reproduce prolifically. Thus, they

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Orange hawkweed(Hieracium aurantiacum),particularly problematicin the NorthwesternUnited States, is anexample of an aggressiveinvader that thrives indisturbed areas.

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conditions such as newly exposed soil surfacesand increased nutrient availability. Prescribed fire,now used in many ecosystems to remove vegeta-tion that has accumulated as a result of decadesof fire suppression, may have the unintendedeffect of spreading invasive species. Similarly, firemanagement activities such as fuel reductions canoften promote plant invasions, potentially replac-ing one type of fire hazard represented by exist-ing fuels with another type of hazard from newtypes of nonnative fuels. Moreover, fire exclusionmay promote the invasion of woody species intograsslands.

Invasive plants also may affect fire behaviorand fire regimes, often by increasing fuel bedflammability, which increases fire frequency. Aclassic example of a widespread invader that hasgreatly altered fire in the Western United Statesand Canada is cheatgrass (Bromus tectorum L.).Cheatgrass is a winter annual, growing rapidlyduring late winter and early spring and dying inearly summer when most other plants are still

green. Dead cheatgrass provides a continuous bedof highly flammable fuel that can readily carry afast-moving fire. When the native flora is poorlyadapted to a more frequent fire regime, the cheat-grass is able to gain a competitive advantage,resulting in a grass/fire cycle (D’Antonio andVitousek 1992). However, not all invasive plantsenhance an aspect of a fire regime; a few mayactually make sites less flammable (D’Antonio2000). Documenting and explaining the causesbehind the sometimes complex interactionsbetween fire and invasive species is a major goal of JFSP research.

MAJOR QUESTIONS AND FINDINGSJoint Fire Science Program research projects are addressing many important questions regardinginvasives and fire. We grouped projects accordingto these four broad questions:

1. What Factors Influence Community Susceptibility to Invasion by NonnativeSpecies After Fire?

2. How Do Fuel Reduction Treatments Affect Invasives?

3. How Do Plant Invasions Affect Fire Regimes?

4. What Are Effective Mitigation Strategies Against Invasive Plants?

Following each question are the related research project titles (in italics), brief descriptionsof the projects, and some of the findings to date.


Prescribed fire may have the unintended effect of increasinginvasive species in some ecosystems.

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Question 1:What Factors Influence CommunitySusceptibility to Invasion byNonnative Species After Fire?Community structure and disturbances have long been studied as factors contributing to how easily a community (community susceptibility, com-munity invasibility) may be invaded by non-native plants. Recently, invasibility after fire hasbeen examined more closely.

“Spatial Interactions Among Fuels, Wildfire, and Nonnative Plants.”

In this project, Philip Omi, Molly Hunter, and their team determined factors that contribute topostfire invasion by nonnative plants at threemixed-conifer forests in the Rocky Mountains.Using a survey approach, they selected plotsacross several strata defined by spatial layers ofelevation, aspect, vegetation, fuel treatments, andfire severity and progression. Their main findingwas that fire severity was a consistent predictor ofnonnative species cover and was a more impor-tant predictor of nonnative species establishmentthan other abiotic variables. These results suggestthat high-severity wildfires may be one of themore important mechanisms for continued spreadof nonnative species in the Western United States(Final report to the JFSP,1 Hunter et al. 2006).

“Weed Invasions Following Fire in Southwestern Colorado: Long-Term Effectiveness of MitigationTreatments and Future Predictions.”

Lisa Floyd-Hanna and colleagues also used a survey approach in the Mesa Verde National Parkon the Colorado Plateau to identify plant com-munities susceptible to postfire weed invasion.Extensive fires occurred in the park in the 1990sresulting in major plant invasions. The teamfound that invasive species were prevalent in theseed banks in pinyon-juniper woodlands, thecommunity type most vulnerable to invasionsafter fire. Also, the pinyon-juniper woodlands hada relatively sparse cover of native species capable


JFSP research has shown several factors that can contributeto whether an ecosystem is vulnerable to the spread of invasive plants after a fire.

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1 The final reports and more information about each project areavailable on the Joint Fire Science Program’s Web site, www.firescience.gov. Search under project title or name of scientist.

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of rapid postfire sprouting, potentially giving theinvasives a competitive advantage. The least vul-nerable type of vegetation, mountain shrubland,had a native vegetation that resprouted andquickly grew back, outcompeting the invasives.Soil properties differed substantially in burnedversus unburned areas, supporting the idea thatnutrient conditions are more favorable for plantgrowth (especially weeds) in burned areas. In particular, nonnative plant density after fire wasweakly correlated with soil nitrate and percentageof silt, suggesting these variables may be impor-tant indicators of site susceptibility to invasion(Floyd et al. 2006, JFSP 2006).

“Sagebrush Steppe and Pinyon Juniper Ecosystems: Effects of Changing Fire Regimes,Increased Fuel Loads, and Invasive Species.”

Using observational and experimental approachesto examine specific mechanisms behind invasionof cheatgrass in sagebrush ecosystems, JeanneChambers and colleagues surveyed and thenburned and removed vegetation on sites at differ-ent elevations in Utah and Nevada and comparedresponses of vegetation and soil to untreatedplots. Naturally, cheatgrass had different patternsof establishment and growth across elevations.Cheatgrass had low establishment, biomass, andseed production on high-elevation sites, probablyowing to ecophysiological limitations resulting


Postfire treatments that encourage high vegetative cover, without introducing new nonnativespecies in contaminated seed mixes, can help prevent further spread of nonnative species.

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from cold temperatures. Cheatgrass had variableestablishment, biomass, and seed production atlow elevations, probably due to soil characteris-tics and spatial and temporal variations in soilwater. Annual variation in soil water was greaterat lower than at upper elevation sites. Soil waterand nitrate availability increased with increasingelevation (Wyoming sagebrush to Vasey sage-brush to mountain brush communities), however,unlike soil moisture, nitrate availability was notmore variable at lower elevations. In the experi-ment, the researchers found that removal of herba-ceous perennials increased soil water and nitrateavailability. Cheatgrass biomass and seed produc-tion increased 2 to 3 times following vegetationremoval, 2 to 6 times after burning, and 10 to 30times following removal and burning. Thisresearch showed that invasibility by cheatgrassvaries across elevation gradients and appearsclosely related to temperature at higher elevationsand soil water availability at lower elevations.High variability in soil water and lower averageperennial herbaceous cover may increase invasionpotential in lower elevation Wyoming sagebrushsites. Soil water and nitrate availability increasefollowing either fire or removal of vegetation, buton intact sites native perennials typically increasefollowing fire—limiting cheatgrass growth andreproduction. Thus, following these types ofresource fluctuations, invasibility is lowest onsites with relatively high cover of perennialherbaceous species, i.e., sites in high ecologicalcondition (Chambers et al. 2007, JFSP 2006).

“Fire and Invasive Grasses in Western Ecosystems.”

In this project Matt Brooks, Jayne Belnap, Jon Keeley, and Robert Sanford are investigating thecomplex relationships among fire, soil nutrientavailability, and plant invasions. Annual grassinvasions are commonly attributed to fire, yetthere are areas of minimal disturbance wherealien annual grasses occur and areas of high disturbance that remain invasion-free. Using acombination of extensive field surveys, and fieldand laboratory experiments, this group is examin-ing some of the mechanisms behind the wide-ranging patterns. The field surveys (across 432sites) showed that soil characteristics differedbetween invaded and uninvaded patches anddepended on climate. Invaded patches in regionswith low winter precipitation had higher avail-able phosphorus than uninvaded patches, but inregions with higher winter precipitation, othernutrients (e.g., manganese and potassium)became important as well. Intensive field experi-ments are ongoing in three ecosystems in theWestern United States: sagebrush steppe, pon-derosa pine, and ponderosa-sagebrush ecotone.At the ponderosa pine (Pinus ponderosa Dougl. exLaws.) site in Kings Canyon National Park, a plot-scale experiment was initiated to examine theeffects of burn season and nine other treatmentssimulating fire and fire-free conditions on soilsand invasives. Early findings show that soil nitro-gen was positively related to cheatgrass biomass,and that this occurred independently of fire sea-son. The scientists conclude that where cheatgrassis established in the forest, management usingshort-return-interval fires is likely to maintain


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cheatgrass indefinitely. Laboratory studiesshowed that cheatgrass grew better in soils thatwere burned compared to unheated soils, andthat soil manganese may also play a role in cheat-grass performance, supporting the field surveys.More information on the project is available athttp://www. werc.usgs. gov/fire/lv/ fireandinvasives (JFSP 2006).

“Fire Effects on Seed Banks and Vegetation in the Eastern Mojave Desert: Implications forPostfire Management.”Matt Brooks and colleagues recently began a project to evaluate the immediate effects of fire onseed banks and vegetation in the Mojave Desert.This project is in response to a specific needexpressed by land managers to understand howfire affects seed banks in hot desert regions, espe-cially contrasting native versus nonnative species.This information is needed to help determine ifpostfire seeding treatments are warranted, espe-cially if they are focused on reducing dominanceof invasive nonnative plants (Matt Brooks 2006,pers. comm.).

“Rapid Response to the 2003 Fires in Southern California: Impact of Fuel Age on Fire Behaviorand Recovery.”Jon Keeley, Tess Brennan, and Anne Pfaff found that fire severity in chaparral shrublands is not amajor determinant of alien plant invasion. Butpast fire history, in particular fire frequency, is acritical factor because it reduces the capacity forthe native shrublands to recover, a necessity forinhibiting alien plant invasion (Keeley et al. 2005).

“Predicting the Invasion and Survival of the Exotic Species Paulownia tomentosa FollowingBurning in Pine and Oak-Pine Forests.”Dane Kuppinger, Peter White, and Michael Jenkins are examining landscape, watershed, andstand-scale conditions that influence the establish-ment of Paulownia after fire. Paulownia has recent-ly invaded extensive areas following fire in theGreat Smoky Mountain National Park. Initialanalyses show that Paulownia occurs in landscape positions associated with high fire intensity andthat a useful predictor for Paulownia at the standscale is the amount of bare soil. Additional stud-ies are examining the survivorship of Paulowniawith time since burning (JFSP 2006).


Paulownia (Paulownia tomentosa (Thunb.) Sieb. & Zucc. ex Steud.) a new invader after fire in theGreat Smoky Mountains, is like many invaders inthat it is strongly associated with bare exposedsoil.

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Question 2: How Do Fuel Reduction Treatments Affect Invasives?Fuels can be reduced by manual or mechanical methods or by prescribed fire, each of which hasconsequences for invasives.

“Pre-Fire Fuel Manipulation Impacts on Alien Plant Invasion of Wildlands.”In this project, Jon Keeley and colleagues exam-ined 24 fuel breaks across California in a range ofvegetation types including chapparal, oak wood-land, coniferous forest, and coastal scrub. Theyfound that fuel breaks provide establishment sites


Fuel break construction methods that leave some overstory canopy and minimize the amount of exposed mineral soil may be less likely to promote invasion by alien plants than more disruptive methods such asthose using bulldozers.

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for nonnative plants, especially when combinedwith livestock grazing and in areas with repeatedburning. They also found that fuel break con-struction methods that leave some overstorycanopy and minimize the amount of exposedmineral soil may be less likely to promote inva-sion by alien plants than more disruptive meth-ods such as those using bulldozers (Merriam et al.2006).

“Fuels Management and Nonnative Plant Species:An Evaluation of Fire and Fire SurrogateTreatments in Chaparral Plant Community.”Similarly, Jennifer Gibson and colleagues found that fuel treatments in shrub (primarily whiteleaf

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manzanita [Arctostaphylos viscida Parry]) com-munities that retain greater levels of overstoryshading and litter or surface cover greatly miti-gate risk of increasing exotic plant cover. Theytook advantage of manipulative treatments imple-mented in the Whiskeytown National RecreationArea in northern California where native brushwas masticated to various degrees and examinedthe responses of invasive and native plant cover.Invasive plants increased most in masticated plotsthat were also burned, suggesting that this treat-ment should be used discriminantly (JFSP 2006).

“Fuel Reduction in Oak Woodlands, Shrublands, and Grasslands of SW Oregon: Consequences for Native Plants and Invasion by NonnativeSpecies.”Using a retrospective analysis in thinned chap-paral and oak communities in southwesternOregon, Keith Perchemlides and Patricia Muirfound that although the relative dominance ofexotic and native species did not change betweenareas thinned 4 to 7 years previously and areasnever thinned, among the exotic species, annualgrass cover increased strongly in response to thin-ning (nearly doubling to almost 20 percent cover),whereas exotic annual forb cover decreased.


Canada thistle (Cirsium arvense), an invasive forb widely distributed across North America, increased afterburning and thinning at the Lubrecht Experimental Forest. Canada thistle is widespread in part because it survives fire and colonizes exposed bare soil in recently burned areas.

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These changes occurred across treat-ments (hand-piled and burned andmechanical mastication), suggesting thatfuel reduction in these systems may havethe unintended effect of changing fueldynamics and increasing exotic annualgrasses (Pat Muir 2006, pers. comm.).

“Fire Effects and Fuels Management in Blackbrush (Coleogyne ramosissima[Torr.]) Shrublands of the MojaveDesert.”Matt Brooks and colleagues examinedthe effect of using fire to control black-brush. Blackbrush is considered a haz-ardous fuel and is often burned, espec-ially at urban-wildland interfaces. Using a field sampling approach, they exam-ined vegetation at three paired (burned andunburned) sites and found that as fire removedthe blackbrush, the cover of nonnatives increased.They caution that because blackbrush is slow torecover after fire, invasive annual grasses mayreplace blackbrush and change the fire cycle. Pastland use also may play a role in invasive estab-lishment in that if lands were previously grazed,native herbaceous perennial grasses and forbsmay be depleted and invasive grasses mayalready be present. Rangelands were historicallyburned across much of the West to replace poorforage species with annual grasses that often provided superior forage (Brooks and Matchett2003, JFSP 2006).

“Fire and Fire Surrogate Program.”As part of the national Fire and FireSurrogate Program (http://www.fs.fed.us/ffs),Andrew Youngblood and colleagues at theHungry Bob Site in the Blue Mountains of

Oregon and Kerry Metlen and Carl Fiedler at the Lubrecht Site in western Montana examinedhow understory vegetation, including nonnatives,responded to thinning, burning, and thinningwith burning in mixed ponderosa pine/Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests.In Oregon, three invasive grasses were found inareas that were thinned and burned, althoughtwo of these were present before the treatmentswere implemented and all had very low levels ofabundances (< 4 percent). Continued monitoringwill determine whether these invasives willincrease in the future. In Montana, exotic forbsincreased 2 and 3 years after treatment with thegreatest response in the thin-and-burn treatment.According to the authors, this increase may be anunavoidable short-term consequence of makingforests more resilient to exotic invasion shouldwildfire occur (Metlen and Fiedler 2006,Youngblood et al. 2006).


JFSP scientists have found that light or moderate-severity burns thatkeep exposure of bare soil to a minimum are least likely to promoteplant invasions.

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“Fire and Fire Surrogate Program.”In another analysis from the Lubrecht Site in Montana, Erich Dodson and Carl Fieldler exam-ined how invasive plants, in particular thosethought to alter ecosystem properties, (i.e., “trans-formers”) responded to the thin, burn, and thin-and-burn restoration treatments. The thin-and-burn treatment yielded the greatest number andabundance (percentage of cover) of exotic plantsand transformer species, whereas the controlyielded the least. Multiple regression analysisshowed that the transformer species were relatedto the reduction of overstory tree cover andscorch height, suggesting that treatment intensitywas important in facilitating invasion. A compan-ion study (Gundale et al. 2006) showed that soilinorganic nitrogen increased most in the thin-and-burn treatment, suggesting increasedresource availability may also have enhancedinvasion. Specific strategies, such as seeding ofnative species, limiting grazing, and harvesting in a manner to minimize soil disturbances, maybe necessary to limit invasions in these systems(Dodson and Fiedler 2006).

“Effects of Fuel Management Treatments in Pinyon-Juniper Vegetation at a Site on theColorado Plateau, Evaluating the Effects ofPinyon-Juniper Thinning Treatments at aWildland/Urban Interface.”In two ongoing projects on the Colorado Plateauand the southern Great Basin, Matt Brooks and colleagues are evaluating the effects of varioustypes of mechanical and chemical thinning treatments for pinyon and juniper woodlands.Response variables are focused on fuel bed andpotential fire behavior in addition to general plant

community characteristics, especially the subse-quent dominance of cheatgrass and other invasivenonnative plants. Preliminary results indicate thatsome of the treatments are clearly most effectiveat shifting the woodland landscape towards thedesired native shrub-steppe condition. Moreinformation is available at http://www.werc.usgs.gov/fire/lv/pj/lakemead (2004 and 2005annual reports to the JFSP, unpublished).

“Reducing Wildfire Risk by Integration of Prescribed Burning and Biological Control ofInvasive Saltcedar (Tamarix spp.).”Another ongoing fuel reduction project seeks toreduce hazardous fuels created by the invasive


Cheatgrass (Bromus tectorum), a noxious invasive annualgrass, is found across North America and has exceptionalecosystem altering potential. JFSP research has shown thatcheatgrass dominance often increases with fuel reductiontreatments. Here, cheatgrass is in both the foreground andacross the slope in the distance in the Great Basin desert,north of Reno, Nevada.

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nonnative tree, saltcedar (Tamarix spp.), in ripari-an zones in the southern Great Basin. The specifictreatment combinations are biocontrol to defoliateand weaken saltcedar trees followed by pre-scribed fire. There are no results to report yet on this project (Matt Brooks 2006, pers. comm.).

“Fire and Invasive Grasses in Western Ecosystems.”Jon Keeley and Thomas McGinnis further explored how fire affected the persistence ofcheatgrass in low-elevation ponderosa pineforests at Kings Canyon National Park. Theirfindings suggest that increasing time betweenprescribed fires may inhibit cheatgrass establish-ment. This is somewhat counter to the goal ofrestoring historical fire regimes, but may be anappropriate compromise between reducing seri-ous fire hazards and exacerbating alien plantinvasions (Keeley and McGinnis 2007).

Question 3: How Do Plant Invasions Affect FireRegimes?

“Fire and Invasive Plant Ecology and Management: The Need for Integration toEffectively Restore Ecosystems.”Matt Brooks and colleagues organized a special session at an international scientific meeting topresent summaries describing the interrelation-ships between invasive plants and fire. This sym-posium reviewed the history of fire and invasiveplant research and management, summarized theinterrelationships between fire and invasivespecies, highlighted the many complex interrela-tionships that remain unstudied, provided exam-ples of promising new lines of research, and

identified gaps in existing research programs. Asignificant product from this symposium was adescription of the various ways that plant inva-sions can alter fuel bed characteristics, fire behav-ior, and fire regimes—which the authors calledthe invasive plant/fire regime cycle (Brooks et al.2004).

“Fire Management Options to Control Woody Invasive Plants in the Northeastern and the Mid-Atlantic U.S.”As mentioned previously, not all invasive plants increase fire frequency or intensity. Occasionally,invasive shrubs and trees can reduce fire frequen-cies below their natural range. Alison Dibble andher team have shown that black locust (Robiniapseudoacacia L.), a prominent invasive in easternpitch pine (Pinus rigida P. Mill.) forests, is not asflammable as native vegetation and may actuallylengthen fire-return intervals (Dibble and Rees2005).

Question 4: What Are Effective MitigationStrategies Against Invasive Plants?Invasives can be removed by several means including fire, biological control, herbicide use,and mechanical cutting. These and followup seed-ing practices hold promise for mitigation, but fewstudies have documented their effectiveness.

“The Use of Fire as a Tool for Controlling Invasive Plants.”Fire is often used as part of integrated pest man-agement programs to control invasive plants.Matt Brooks, additional collaborators, and theCalifornia Invasive Plant Council convened a


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panel of experts to develop state-of-the-sciencesummaries and recommendations for using fire to manage invasive plants. This panel concludedthat annual species that produce seeds well afterthe fire season begins, that have flowering struc-tures embedded within the fuel bed, and thathave short-lived seed banks are most amenable to control using fire. In contrast, perennial specieswith perennating tissue that is either below-ground or well above the fuel bed (and thus pro-tected from heating), and that resprout readilybecause they are adapted to fire or some otherform of recurrent disturbance, are not generally

amenable to control by fire. Invasive plants thatalter the fuel bed structure making it less flamma-ble may also be difficult to control with firebecause they produce fuel beds that are relativelyinflammable. In all cases, followup monitoringand plans for retreatment are required, and inmost cases, fire should be integrated with othercontrol methods. The net effects of any treatmentplan on the entire plant community, higher tropiclevels, and ecosystem properties, need to be con-sidered before treatments are implemented(DiTomaso and Johnson 2006, DiTomaso et al.2006, JFSP 2006).


Researchers have found that certain mitigation treatments can be effective at shifting the woodland landscape towards thedesired native shrub-steppe condition.

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“Fire Management Options to Control Woody Invasive Plants in the Northeastern and the Mid-Atlantic U.S.”Although burning is often used to control inva-sive shrubs in the Northeast and Mid-AtlanticStates, it may be counterproductive if burns areconducted during dormancy. Using an experi-mental approach, Alison Dibble and colleaguesexamined the effect of dormant-season burn,growing-season cut with same-season burn, andgrowing-season cut followed by a second cutacross three sites in the Northeast. For the sevenspecies that were studied, a single treatment,whether applied during the dormant or earlygrowing season, decreased root reserves (essentialfor resprouting) for less than one growing season.With 2 years of treatment, resprouting stilloccurred, but growth tended to be less vigorousthan for those treated only once. Across species,the authors concluded that carefully applied cut-ting and prescribed fire treatments, in tandem oralone, can reduce the fire hazard in invaded fuelbeds (JFSP 2006).

“Evaluate Treatments to Reduce Hazardous Fine Fuels Created by Nonnative Plants in Zion Canyon.”A project led by Denise Louie and colleagues is evaluating different combinations of three phases of control treatments for invasive annual grassesin riparian zones at Zion National Park (ZNP): (1) biomass reduction (mowing vs. fire); (2) her-bicide to target the grasses (different types, rates,

and timings); and (3) followup seeding of early-successional native species to compete with inva-sive grasses that escaped the first two treatments.Preliminary results indicate strong differencesamong treatments, suggesting that definitive rec-ommendations for future management of invasiveannual grasses at ZNP will emerge from this proj-ect. These early results were used by the NationalPark Service to design Burned Area EmergencyResponse treatments to manage invasive annualgrasses after the Kolob Fire of 2006, the largestfire in the history of ZNP (Matt Brooks 2006, pers.comm.).

“Management of Fuel Loading in the Shrub-Steppe.”In field trials, Steven Link and his team tested two herbicides for use against invasives (mostlycheatgrass) in a shrub-steppe community in east-ern Washington. Imazapic2 reduced cheatgrasscover by about 40 percent and increased nativevegetation cover by about 10 percent 2 years afterapplication, whereas glyphosphate reduced thecover of both cheatgrass and native species.Imazapic is recommended to reduce cheatgrassbut should be used with caution where rarenative mustards are present (the mustards werereduced with treatment). The team found thatprescribed fire can be used in the fall to preparesurfaces for preemergent herbicide applicationwhere cheatgrass cover is less than 50 percent.Link and colleagues also conducted a restorationstudy and found that fire risk of two communities8 years after establishment of large bunchgrasses


2 The use of trade or firm names in this publication is for readerinformation and does not imply endorsement by the U.S.Department of Agriculture of any product or service.

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was approximately 77 percent, and decreased further to 66 percent 18 years after restoration.The savings associated with reducing fire riskfrom 100 percent to 66 percent may be enough to cover the costs of restoring shrub-steppeecosystems. More information can be found onthe project Web site at http://www.tricity.wsu.edu/shrub_ steppe/fire_publications (JFSP 2006).

“Effectiveness of Postfire Seeding to Reduce Cheatgrass (Bromus tectorum) Growth andReproduction in Recently Burned SagebrushSteppe.”Postfire seeding is often used to suppress the growth and reproduction of cheatgrass in low-elevation sagebrush steppe. Nonnative perennialgrasses are traditionally seeded to compete withinvasive annuals because of their presumedgreater growth rates compared with native peren-nials, yet few controlled studies have tested thisassumption. With a field experiment, Matt Brooksand associates examined the response of cheat-grass and other invasive grasses to postfire seed-ing of native vs. nonnative perennial grassspecies. Because of low rainfall, the growth ofseeded plants was low the first 2 years. Also,across seeded plots, an invasive perennial grass(bulbous bluegrass [Poa bulbosa L.]) increased indominance, presumably because of soil distur-bance that occurred with the seeding technique (a mechanized drill). Researchers will continue toevaluate the responses after seeded plants reachmaturity. A Web site with further information isavailable at http://www.werc.usgs.gov/fire/lv/postfireseeding/greatbasin (JFSP 2006).

“Evaluating Postfire Seeding Treatments Designed to Suppress Cheatgrass (Bromus tectorum) in Ponderosa Pine Forests on theColorado Plateau.”A similar project evaluated the effects of seeding native perennial grasses after fire with and with-out seedbed preparation on cheatgrass dominancein a basin big sagebrush community on theColorado Plateau. Preliminary results 3 yearsposttreatment indicate that seedbed preparationimproved establishment of the seeded nativespecies, but also led to increased dominance of anonnative forb. Responses of cheatgrass amongtreatments have been nonsignificant so far. Thisdelayed response is likely due to the timing of thefire prior to seeding, which occurred when manycheatgrass plants had emerged as seedlings andwere likely killed, thus reducing the onsite popu-lation levels (Matt Brooks 2006, pers. comm.).

“Weed Invasions Following Fire in Southwestern Colorado: Long-Term Effectiveness of Mitigation Treatments and Future Predictions.”Lisa Floyd and colleagues reexamined an aerial native plant seeding experiment 7 years aftertreatments were applied following a fire at MesaVerde National Park. Density of a persistent post-fire invasive (muskthistle [Carduus nutans L.]) wasreduced by 66 percent, and density of a nativegrass was doubled in areas that received seedingtreatments compared to areas that were not seed-ed. Although aerial application of native grassseeds did not completely prevent weed invasion,it greatly reduced postfire weed density and rap-idly increased the native component of the plantcommunity. Mechanical treatment of muskthistle(removing seed heads and digging plants out


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with a shovel) was effective only in areaswhere native grasses were present: musk-thistle was completely absent from theseareas 7 years later. Chemical treatment ofCanada thistle (Cirsium arvense (L.) Scop.)(by using Curtail) for 2 years following fireresulted in almost complete mortality ofCanada thistle, which was nearly absentfrom treated areas 7 years later. Althoughchemical treatment of Canada thistle wasvery effective, it is labor-intensive andprobably feasible only in localized areas(Floyd et al. 2006, JFSP 2006).

“Spatial Interactions Among Fuels, Wildlife, and Nonnative Plants”Seed mixes are often contaminated with non-native species, and their application on landscapescales may introduce a large number of nonnativeweed seeds. Using research from study sites inthe Rocky Mountains of Colorado and NewMexico, Philip Omi and colleagues showed thatwhere seed mixes were applied aerially, there wasa positive association between the cover of non-native species and seeded grasses. However,where seed mixes were applied by hand or a seed drill on a more limited scale, postfireseeding successfully deterred nonnative speciesestablishment, similar to the effect of abundantnative cover. Postfire treatments that encouragehigh vegetative cover, without introducing new nonnative species in contaminated seedmixes, would best prevent further spread of non-native species. These researchers suggest thathand application of native grass seed mixes maybe preferable to broad application of nonnativegrass seed (Hunter et al. 2006, JFSP 2006).

“Pre-fire Fuel Manipulation Impacts on Alien Plant Invasion of Wildland.”In another look at the benefits of postfire seeding on invasives, Jon Keeley and colleaguesrecommend against postfire seeding, particularlyin the shrublands and ponderosa pine forests ofCalifornia and the southwest owing to lack ofstrong scientific support (Keeley et al. 2006).

ADDITIONAL PROJECTS: “Invasive Plant and Fire Interactions: Use of the Fire Effects Information System to Provide Information for Managers.”Kevin Ryan and Jane Kapler Smith headed a teamthat has completed detailed literature reviews on60 nonnative invasive plant species in the FireEffects Information System (FEIS, at www.fs.fed.us/database/feis). These reviews synthesize thescientific literature on each species’ basic biology,


Application of contaminated seed mixes on landscape scales mayintroduce a large number of nonnative weed seeds.

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ecology, and relationship to fire. Reviews empha-size how fire affects each species, how the speciesmay alter fire regimes, and potential managementactions for specific ecosystems. A comprehensivesearch of the scientific literature forms the basisfor each review. An additional 10 to 20 nonnativeinvasives will be covered in FEIS under anotherJFSP task entitled “Bringing the Fire EffectsInformation System Up-to-Date and ImprovingService to Land Managers” (JFSP 2006, JaneKapler Smith 2006, pers. comm.).

“Publication of Literature Synthesis Entitled, ‘Effects of Fire on Nonnative Invasive Plants’as 6th Volume in the General Technical Report,‘Wildland Fire in Ecosystems’ (‘Rainbow’Series).”Jane Kapler Smith and colleagues are nearing completion of an edited volume summarizing andsynthesizing current knowledge about the ecolo-gy and management of invasive plants and fire inthe United States. This document includes threesections: (1) overview chapters highlighting thepotential for nonnative species to increase afterfire, the effect of these species on fire regimes, andthe use of prescribed fire to control them; (2)analysis of the relationship of fire to invasionsand invasible plant communities in seven regionsof the United States: the Northeast, Southeast,Central, Interior West, Pacific Northwest andAlaska, Southwest coast, and Hawaiian Islands;and (3) issues of nationwide concern, includingknowledge gaps in relation to fire and invasives,postfire rehabilitation, prescribed fire monitoring,and effects of fire surrogate treatments on inva-sive species (Jane Kapler Smith 2006, pers.comm.).

SUMMARY OF FINDINGSHave we answered our questions? Partly, yes! We’ve learned (see detailed bullets below) aboutseveral factors that can contribute to whether anecosystem is vulnerable to invasion, we’velearned how prescribed fire and fuel treatmentsmight actually increase invasive species, andwe’ve learned about some successful mitigationstrategies against invasives. Many of the projectsare ongoing or just beginning. More questionswill be answered and tools for managers devel-oped and refined as the word continues to getout. So stay tuned!

• Invasibility is complex: Soil seed bank composition, competing native vegetation,soil disturbance and fertility, fire severity,and climatic regime and weather all affecta community’s susceptibility to invasionafter fire.

• Prescribed fire can be a problem:Prescribed fire may have the unintendedeffect of increasing invasive species insome ecosystems, especially if the ecosys-tems are in poor ecological condition andthe native species capable of establishingor resprouting following fire have beendepleted.

• Fuel treatments matter: Fuel treatments that leave some overstory canopy, mini-mize exposure of bare ground, and targetsites with species capable of establishingor resprouting may be less likely to pro-mote invasion by alien plant species.

• Fuel reduction treatments: Fuel treat-ments that encourage high vegetative


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cover of native species, without introduc-ing new nonnative species (e.g., in con-taminated seed mixes or by soil distur-bance) may prevent further spread of nonnative species.

• Mitigation strategies may be effective:On sites that exhibit species invasions following wildfire, active interventionwith herbicides or other treatmentsdesigned to control or eliminate the invasive can be highly effective.

• Postfire seeding: Seeding treatments should be used with caution, as studiesare showing these can increase invasives.

ACKNOWLEDGMENTSWe thank Jeanne Chambers, Matt Brooks, and Seth White for technical reviews; the JFSP staff inBoise for helping us access project information;and the scientists for providing feedback on theirfindings.

FOR MORE INFORMATION, PLEASE CONTACT:Tim [email protected](208) 387-5865

FURTHER READINGBrooks, M.L.; D’Antonio, C.M.; Richardson,

D.M.; Grace, J.; Keeley, J.J.; DiTomaso, J.M.;Hobbs, R.; Pellant, M.; Pyke, D. 2004. Effectsof invasive alien plants on fire regimes.BioScience. 54: 677–688.

Brooks, M.L.; Matchett, J.R. 2003. Plant com-munity patterns in unburned and burnedblackbrush (Coleogyne ramosissima Torr.)shrublands in the Mojave Desert. WesternNorth American Naturalist. 63: 283–298.

Brooks, M.L.; Pyke, D. 2001. Invasive plants and fire in the deserts of North America. In:Galley, K.; Wilson, T., eds. The role of fire inthe control and spread of invasive species:Proceedings of invasive species workshop.Fire conference 2000: the first national con-gress on fire ecology, prevention and manage-ment. Misc. Publ. 11. Tallahassee, FL: TallTimbers Research Station: 1–14.

Chambers, J.C.; Roundy, B.A.; Blank, R.R.; Meyer, S.E.; Whittaker, A. 2007. What makesgreat basin sagebrush ecosystems invasible by Bromus tectorum? Ecological Monographs. 77: 117–145

D’Antonio, C.M. 2000. Fire, plant invasions, and global changes. In: Mooney, H.; Hobbs, R.,eds. Invasive species in a changing world.Covelo, CA: Island Press: 65–94.

D’Antonio, C.M.; Vitousek, P.M. 1992. Biological invasions by exotic grasses, the grass/firecycle, and global change. Annual Review ofEcology and Systematics. 23: 63–87.

Dibble, A.C.; Rees, C.A. 2005. Does the lack of reference ecosystems limit our science? A casestudy in nonnative invasion plants as forestfuels. Journal of Forestry. 103: 329–338.


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DiTomaso, J.M.; Brooks, M.L.; Allen, E.B.; Minnich, R.; Rice, P.M.; Kyser, G.B. 2006.Control of invasive weeds with prescribedburning. Weed Technology. 20(2): 535–548.

DiTomaso, J.M.; Johnson, D.W., eds. 2006. The use of fire as a tool for controlling invasiveplants. Cal-IPC Publ. 2006-01. Berkeley, CA:California Invasive Plant Council. 56 p.

Dodson, E.K.; Fiedler, C.E. 2006. Impacts of restoration treatments on alien invasion inPinus ponderosa forests, Montana, USA.Journal of Applied Ecology. 43: 887–897.

Floyd, M.L.; Hanna, D.; Romme, W.H.; Crews, T.E. 2006. Predicting and mitigating weedinvasions to restore natural post-fire succes-sion in Mesa Verde National Park, Colorado,USA. International Journal of Wildland Fire.15: 247–259.

Gundale, M.J.; Metlen, K.L.; Fiedler, C.E.; DeLuca, T.H. 2006. Nitrogen spatial hetero-geneity influences understory diversity fol-lowing restoration treatments in a ponderosapine/Douglas-fir forest, Montana. EcologicalApplications. 16: 479–489.

Hunter, M.E.; Omi, P.N.; Martinson, E.J.; Chong, G.W. 2006. Establishment of nonnative plant species after wildfires: effects of fuel treat-ments, abiotic and biotic factors, and postfiregrass seeding treatments. InternationalJournal of Wildland Fire. 15: 271–281.

Keeley, J.E.; Allen, C.D.; Betancourt, J.; Chong, G.W.; Fotheringham, C.J.; Safford, H.D. 2006.A 21st century perspective on postfire seeding. Journal of Forestry. 104: 103–104.

Keeley, J.E.; Baer-Keeley, M.; Fotheringham, C.J. 2005. Alien plant dynamics following fire in Mediterranean-climate California shrublands.Ecological Applications. 15: 2109–2125.

Keeley, J.E.; McGinnis, T.W. 2007. Impact of prescribed fire and other factors on cheatgrasspersistence in a Sierra Nevada ponderosa pineforest. International Journal of Wildland Fire.16: 96–106

Merriam, K.E.; Keeley, J.E.; Beyers, J.L. 2006.Fuel breaks affect nonnative species abun-dance in Californian plant communities.Ecological Applications. 16: 515–527.

Metlen, K.L.; Fiedler, C.E. 2006. Restoration treatment effects on the understory of pon-derosa pine/Douglas-fir forests in westernMontana, USA. Forest Ecology andManagement. 222: 355–369.

U.S. Geological Survey. 2004. Effects of fuel management treatments in piñon juniper vegetation at a site on the Colorado Plateau.http://www.werc.usgs.gov/fire/lv/pj/lakemead. (27 November 2006).

Youngblood, A.K.; Metlen, L.; Coe, K. 2006.Changes in stand structure and composition after restoration treatments in low elevation dry forests of northeastern Oregon. ForestEcology and Management. 234: 143–163.


Pacific Northwest Research Station

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INVASIVE PLANT SPECIES AND THE JOINT FIRE ...Invasive plants may be responsi-ble for serious, long-term ecological impacts; they can cause the decline of native plant species, dis-rupt

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