Invasive Plant Species 

and the Joint Fire 

Science Program

Heather E. Erickson and Rachel White

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical 

Report

PNW-GTR-707

November 2007
AUTHORS

Heather 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 photo

Kerry Metlen, University of Montana.

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equal opportunity provider and employer.
Invasive Plant

Species and 

the Joint Fire 

Science Program

Heather E. Erickson 

Rachel White

U.S. Department of Agriculture

Forest Service

Pacific Northwest Research Station

Portland, Oregon

November 2007
have the potential to

spread relatively quickly

over large areas. Invasive

plants may be responsible for serious, long-term

ecological impacts; they

can cause the decline of

native plant species, disrupt nutrient cycling and

hydrology, and alter fire

regimes. 

Disturbances, such as

wildfire and prescribed

fire, may promote plant

invasions via a number of mechanisms. On the

other hand, fire can be used to control some invasives. Determining how to successfully manage

invasives is crucial, but to do so, much more

information is needed on the ecology of species

invasions, the interactions between fire and invasives, and the responses of invasives to different

management practices. 

Several government-funded research programs and initiatives are addressing ways to 

better understand the biology, ecology, and management of invasive species. Since 1999, the Joint

Fire Science Program (JFSP) has funded over 25

projects related to invasive plant species and fire.

In this paper we summarize completed and ongoing JFSP research related to fire and invasives. 

INTERACTIONS BETWEEN FIRE

AND INVASIVES

Fire, like many disturbances, may promote

the invasion of nonnative plant species by

providing canopy openings, reducing cover of

competing vegetation, and creating favorable soil

1

ABSTRACT

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

plant species and the Joint Fire Science

Program. Gen. Tech. Rep. PNW-GTR-707.

Portland, OR: U.S. Department of Agriculture,

Forest Service, Pacific Northwest Research

Station. 18 p.

Invasive nonnative plants may be responsible for

serious, long-term ecological impacts, including

altering fire behavior and fire regimes. Therefore,

knowing how to successfully manage invasive

plants and their impacts on natural resources is

crucial. We present a summary of research on

invasive plants and fire that has been generated

through the Joint Fire Science Program—focusing

specifically on ecology of species invasions, the

interactions between fire and invasives, and the

responses of invasives to different management

practices. Selected findings include (1) prescribed 

fire may increase invasive species in some ecosystems; (2) fuel treatments that leave some overstory canopy, minimize exposure of bare ground,

and target sites that already host species capable

of resprouting may be less likely to promote 

invasives; and (3) postfire seeding should be

approached with caution, as it can increase 

invasives. 

Keywords: Invasive plants, fire management, 

cheatgrass, fuel treatments, postfire seeding, fire

regimes, exotic species.

INTRODUCTION

I

nvasive nonnative plants are plants introduced

into areas outside of their natural ranges, usually as a result of human activities. Plants that are

successful invaders often have high growth rates

and typically reproduce prolifically. Thus, they

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Orange hawkweed

(Hieracium aurantiacum),

particularly problematic

in the Northwestern

United States, is an

example of an aggressive

invader that thrives in

disturbed areas. 

Photo by Steve Sutherland, USFS,

Rocky Mountain Research Station
2

conditions such as newly exposed soil surfaces

and increased nutrient availability. Prescribed fire,

now used in many ecosystems to remove vegetation that has accumulated as a result of decades

of fire suppression, may have the unintended

effect of spreading invasive species. Similarly, fire

management activities such as fuel reductions can

often promote plant invasions, potentially replacing one type of fire hazard represented by existing fuels with another type of hazard from new

types of nonnative fuels. Moreover, fire exclusion

may promote the invasion of woody species into

grasslands. 

Invasive plants also may affect fire behavior

and fire regimes, often by increasing fuel bed

flammability, which increases fire frequency. A

classic example of a widespread invader that has

greatly altered fire in the Western United States

and Canada is cheatgrass (Bromus tectorum L.).

Cheatgrass is a winter annual, growing rapidly

during late winter and early spring and dying in

early summer when most other plants are still

green. Dead cheatgrass provides a continuous bed

of highly flammable fuel that can readily carry a

fast-moving fire. When the native flora is poorly

adapted to a more frequent fire regime, the cheatgrass is able to gain a competitive advantage,

resulting in a grass/fire cycle (D’Antonio and

Vitousek 1992). However, not all invasive plants

enhance an aspect of a fire regime; a few may

actually make sites less flammable (D’Antonio

2000). Documenting and explaining the causes

behind the sometimes complex interactions

between fire and invasive species is a major goal 

of JFSP research. 

MAJOR QUESTIONS AND 

FINDINGS

Joint Fire Science Program research projects are 

addressing many important questions regarding

invasives and fire. We grouped projects according

to these four broad questions: 

1. What Factors Influence Community 

Susceptibility to Invasion by Nonnative

Species 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 descriptions

of the projects, and some of the findings to date. 

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Prescribed fire may have the unintended effect of increasing

invasive species in some ecosystems. 

Photo by Tom Iraci, USFS
3

Question 1:

What Factors Influence Community

Susceptibility to Invasion by

Nonnative Species After Fire?

Community structure and disturbances have long 

been studied as factors contributing to how easily 

a community (community susceptibility, community invasibility) may be invaded by nonnative plants. Recently, invasibility after fire has

been 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 to

postfire invasion by nonnative plants at three

mixed-conifer forests in the Rocky Mountains.

Using a survey approach, they selected plots

across several strata defined by spatial layers of

elevation, aspect, vegetation, fuel treatments, and

fire severity and progression. Their main finding

was that fire severity was a consistent predictor of

nonnative species cover and was a more important predictor of nonnative species establishment

than other abiotic variables. These results suggest

that high-severity wildfires may be one of the

more important mechanisms for continued spread

of 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 Mitigation

Treatments and Future Predictions.”

Lisa Floyd-Hanna and colleagues also used a 

survey approach in the Mesa Verde National Park

on the Colorado Plateau to identify plant communities susceptible to postfire weed invasion.

Extensive fires occurred in the park in the 1990s

resulting in major plant invasions. The team

found that invasive species were prevalent in the

seed banks in pinyon-juniper woodlands, the

community type most vulnerable to invasions

after fire. Also, the pinyon-juniper woodlands had

a relatively sparse cover of native species capable

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

JFSP research has shown several factors that can contribute

to whether an ecosystem is vulnerable to the spread of 

invasive plants after a fire.

Photo by Jamie Barbour, USFS, Focused Science Delivery Program

1 The final reports and more information about each project are

available on the Joint Fire Science Program’s Web site, www.

firescience.gov. Search under project title or name of scientist.
4

of rapid postfire sprouting, potentially giving the

invasives a competitive advantage. The least vulnerable type of vegetation, mountain shrubland,

had a native vegetation that resprouted and

quickly grew back, outcompeting the invasives.

Soil properties differed substantially in burned

versus unburned areas, supporting the idea that

nutrient conditions are more favorable for plant

growth (especially weeds) in burned areas. In 

particular, nonnative plant density after fire was

weakly correlated with soil nitrate and percentage

of silt, suggesting these variables may be important 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 approaches

to examine specific mechanisms behind invasion

of cheatgrass in sagebrush ecosystems, Jeanne

Chambers and colleagues surveyed and then

burned and removed vegetation on sites at different elevations in Utah and Nevada and compared

responses of vegetation and soil to untreated

plots. Naturally, cheatgrass had different patterns

of establishment and growth across elevations.

Cheatgrass had low establishment, biomass, and

seed production on high-elevation sites, probably

owing to ecophysiological limitations resulting

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Postfire treatments that encourage high vegetative cover, without introducing new nonnative

species in contaminated seed mixes, can help prevent further spread of nonnative species. 

Photo by Steve Sutherland, USFS, Rocky Mountain Research Station
5

from cold temperatures. Cheatgrass had variable

establishment, biomass, and seed production at

low elevations, probably due to soil characteristics and spatial and temporal variations in soil

water. Annual variation in soil water was greater

at lower than at upper elevation sites. Soil water

and nitrate availability increased with increasing

elevation (Wyoming sagebrush to Vasey sagebrush to mountain brush communities), however,

unlike soil moisture, nitrate availability was not

more variable at lower elevations. In the experiment, the researchers found that removal of herbaceous perennials increased soil water and nitrate

availability. Cheatgrass biomass and seed production increased 2 to 3 times following vegetation

removal, 2 to 6 times after burning, and 10 to 30

times following removal and burning. This

research showed that invasibility by cheatgrass

varies across elevation gradients and appears

closely related to temperature at higher elevations

and soil water availability at lower elevations.

High variability in soil water and lower average

perennial herbaceous cover may increase invasion

potential in lower elevation Wyoming sagebrush

sites. Soil water and nitrate availability increase

following either fire or removal of vegetation, but

on intact sites native perennials typically increase

following fire—limiting cheatgrass growth and

reproduction. Thus, following these types of

resource fluctuations, invasibility is lowest on

sites with relatively high cover of perennial

herbaceous species, i.e., sites in high ecological

condition (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 the

complex relationships among fire, soil nutrient

availability, and plant invasions. Annual grass

invasions are commonly attributed to fire, yet

there are areas of minimal disturbance where

alien annual grasses occur and areas of high 

disturbance that remain invasion-free. Using a

combination of extensive field surveys, and field

and laboratory experiments, this group is examining some of the mechanisms behind the wideranging patterns. The field surveys (across 432

sites) showed that soil characteristics differed

between invaded and uninvaded patches and

depended on climate. Invaded patches in regions

with low winter precipitation had higher available phosphorus than uninvaded patches, but in

regions with higher winter precipitation, other

nutrients (e.g., manganese and potassium)

became important as well. Intensive field experiments are ongoing in three ecosystems in the

Western United States: sagebrush steppe, ponderosa pine, and ponderosa-sagebrush ecotone.

At the ponderosa pine (Pinus ponderosa Dougl. ex

Laws.) site in Kings Canyon National Park, a plotscale experiment was initiated to examine the

effects of burn season and nine other treatments

simulating fire and fire-free conditions on soils

and invasives. Early findings show that soil nitrogen was positively related to cheatgrass biomass,

and that this occurred independently of fire season. The scientists conclude that where cheatgrass

is established in the forest, management using

short-return-interval fires is likely to maintain

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM
6

cheatgrass indefinitely. Laboratory studies

showed that cheatgrass grew better in soils that

were burned compared to unheated soils, and

that soil manganese may also play a role in cheatgrass performance, supporting the field surveys.

More information on the project is available at

http://www. werc.usgs. gov/fire/lv/ 

fireandinvasives (JFSP 2006). 

“Fire Effects on Seed Banks and Vegetation in 

the Eastern Mojave Desert: Implications for

Postfire Management.”

Matt Brooks and colleagues recently began a 

project to evaluate the immediate effects of fire on

seed banks and vegetation in the Mojave Desert.

This project is in response to a specific need

expressed by land managers to understand how

fire affects seed banks in hot desert regions, especially contrasting native versus nonnative species.

This information is needed to help determine if

postfire seeding treatments are warranted, especially if they are focused on reducing dominance

of invasive nonnative plants (Matt Brooks 2006,

pers. comm.).

“Rapid Response to the 2003 Fires in Southern 

California: Impact of Fuel Age on Fire Behavior

and Recovery.”

Jon Keeley, Tess Brennan, and Anne Pfaff found 

that fire severity in chaparral shrublands is not a

major determinant of alien plant invasion. But

past fire history, in particular fire frequency, is a

critical factor because it reduces the capacity for

the native shrublands to recover, a necessity for

inhibiting alien plant invasion (Keeley et al. 2005).

“Predicting the Invasion and Survival of the 

Exotic Species Paulownia tomentosa Following

Burning in Pine and Oak-Pine Forests.”

Dane Kuppinger, Peter White, and Michael 

Jenkins are examining landscape, watershed, and

stand-scale conditions that influence the establishment of Paulownia after fire. Paulownia has recently invaded extensive areas following fire in the

Great Smoky Mountain National Park. Initial

analyses show that Paulownia occurs in landscape 

positions associated with high fire intensity and

that a useful predictor for Paulownia at the stand

scale is the amount of bare soil. Additional studies are examining the survivorship of Paulownia

with time since burning (JFSP 2006).

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Paulownia (Paulownia tomentosa (Thunb.) Sieb. 

& Zucc. ex Steud.) a new invader after fire in the

Great Smoky Mountains, is like many invaders in

that it is strongly associated with bare exposed

soil.

Photo by Dane Kuppinger, University of

North Carolina
7

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 has

consequences for invasives.

“Pre-Fire Fuel Manipulation Impacts on Alien 

Plant Invasion of Wildlands.”

In this project, Jon Keeley and colleagues examined 24 fuel breaks across California in a range of

vegetation types including chapparal, oak woodland, coniferous forest, and coastal scrub. They

found that fuel breaks provide establishment sites

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

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 as

those using bulldozers. 

Photo by Tom Iraci, USFS

for nonnative plants, especially when combined

with livestock grazing and in areas with repeated

burning. They also found that 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 as those using bulldozers (Merriam et al.

2006). 

“Fuels Management and Nonnative Plant Species:

An Evaluation of Fire and Fire Surrogate

Treatments in Chaparral Plant Community.”

Similarly, Jennifer Gibson and colleagues found 

that fuel treatments in shrub (primarily whiteleaf 
8

manzanita [Arctostaphylos viscida Parry]) communities that retain greater levels of overstory

shading and litter or surface cover greatly mitigate risk of increasing exotic plant cover. They

took advantage of manipulative treatments implemented in the Whiskeytown National Recreation

Area in northern California where native brush

was masticated to various degrees and examined

the responses of invasive and native plant cover.

Invasive plants increased most in masticated plots

that were also burned, suggesting that this treatment should be used discriminantly (JFSP 2006). 

“Fuel Reduction in Oak Woodlands, Shrublands, 

and Grasslands of SW Oregon: Consequences for 

Native Plants and Invasion by Nonnative

Species.”

Using a retrospective analysis in thinned chapparal and oak communities in southwestern

Oregon, Keith Perchemlides and Patricia Muir

found that although the relative dominance of

exotic and native species did not change between

areas thinned 4 to 7 years previously and areas

never thinned, among the exotic species, annual

grass cover increased strongly in response to thinning (nearly doubling to almost 20 percent cover),

whereas exotic annual forb cover decreased.

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Canada thistle (Cirsium arvense), an invasive forb widely distributed across North America, increased after

burning 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. 

Photo by Kerry Metlen; Lubrecht Experimental Forest, College of Forestry and

Conservation, University of

Montana
9

These changes occurred across treatments (hand-piled and burned and

mechanical mastication), suggesting that

fuel reduction in these systems may have

the unintended effect of changing fuel

dynamics and increasing exotic annual

grasses (Pat Muir 2006, pers. comm.). 

“Fire Effects and Fuels Management in 

Blackbrush (Coleogyne ramosissima

[Torr.]) Shrublands of the Mojave

Desert.”

Matt Brooks and colleagues examined

the effect of using fire to control blackbrush. Blackbrush is considered a hazardous fuel and is often burned, especially at urban-wildland interfaces. Using 

a field sampling approach, they examined vegetation at three paired (burned and

unburned) sites and found that as fire removed

the blackbrush, the cover of nonnatives increased.

They caution that because blackbrush is slow to

recover after fire, invasive annual grasses may

replace blackbrush and change the fire cycle. Past

land use also may play a role in invasive establishment in that if lands were previously grazed,

native herbaceous perennial grasses and forbs

may be depleted and invasive grasses may

already be present. Rangelands were historically

burned across much of the West to replace poor

forage species with annual grasses that often 

provided superior forage (Brooks and Matchett

2003, JFSP 2006). 

“Fire and Fire Surrogate Program.”

As part of the national Fire and Fire

Surrogate Program (http://www.fs.fed.us/ffs),

Andrew Youngblood and colleagues at the

Hungry Bob Site in the Blue Mountains of

Oregon and Kerry Metlen and Carl Fiedler at 

the Lubrecht Site in western Montana examined

how understory vegetation, including nonnatives,

responded to thinning, burning, and thinning

with burning in mixed ponderosa pine/Douglasfir (Pseudotsuga menziesii (Mirb.) Franco) forests.

In Oregon, three invasive grasses were found in

areas that were thinned and burned, although

two of these were present before the treatments

were implemented and all had very low levels of

abundances (< 4 percent). Continued monitoring

will determine whether these invasives will

increase in the future. In Montana, exotic forbs

increased 2 and 3 years after treatment with the

greatest response in the thin-and-burn treatment.

According to the authors, this increase may be an

unavoidable short-term consequence of making

forests more resilient to exotic invasion should

wildfire occur (Metlen and Fiedler 2006,

Youngblood et al. 2006). 

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

JFSP scientists have found that light or moderate-severity burns that

keep exposure of bare soil to a minimum are least likely to promote

plant invasions. 

Photo by Tom Iraci, USFS
10

“Fire and Fire Surrogate Program.”

In another analysis from the Lubrecht Site in 

Montana, Erich Dodson and Carl Fieldler examined how invasive plants, in particular those

thought to alter ecosystem properties, (i.e., “transformers”) responded to the thin, burn, and thinand-burn restoration treatments. The thin-andburn treatment yielded the greatest number and

abundance (percentage of cover) of exotic plants

and transformer species, whereas the control

yielded the least. Multiple regression analysis

showed that the transformer species were related

to the reduction of overstory tree cover and

scorch height, suggesting that treatment intensity

was important in facilitating invasion. A companion study (Gundale et al. 2006) showed that soil

inorganic nitrogen increased most in the thinand-burn treatment, suggesting increased

resource availability may also have enhanced

invasion. Specific strategies, such as seeding of

native species, limiting grazing, and harvesting 

in a manner to minimize soil disturbances, may

be necessary to limit invasions in these systems

(Dodson and Fiedler 2006). 

“Effects of Fuel Management Treatments in 

Pinyon-Juniper Vegetation at a Site on the

Colorado Plateau, Evaluating the Effects of

Pinyon-Juniper Thinning Treatments at a

Wildland/Urban Interface.”

In two ongoing projects on the Colorado Plateau

and the southern Great Basin, Matt Brooks and 

colleagues are evaluating the effects of various

types of mechanical and chemical thinning 

treatments for pinyon and juniper woodlands.

Response variables are focused on fuel bed and

potential fire behavior in addition to general plant

community characteristics, especially the subsequent dominance of cheatgrass and other invasive

nonnative plants. Preliminary results indicate that

some of the treatments are clearly most effective

at shifting the woodland landscape towards the

desired native shrub-steppe condition. More

information is available at http://www.werc.

usgs.gov/fire/lv/pj/lakemead (2004 and 2005

annual reports to the JFSP, unpublished).

“Reducing Wildfire Risk by Integration of 

Prescribed Burning and Biological Control of

Invasive Saltcedar (Tamarix spp.).”

Another ongoing fuel reduction project seeks to

reduce hazardous fuels created by the invasive

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Cheatgrass (Bromus tectorum), a noxious invasive annual

grass, is found across North America and has exceptional

ecosystem altering potential. JFSP research has shown that

cheatgrass dominance often increases with fuel reduction

treatments. Here, cheatgrass is in both the foreground and

across the slope in the distance in the Great Basin desert,

north of Reno, Nevada.

Photo by Jon Keeley, USGS
11

nonnative tree, saltcedar (Tamarix spp.), in riparian zones in the southern Great Basin. The specific

treatment combinations are biocontrol to defoliate

and weaken saltcedar trees followed by prescribed 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 of

cheatgrass in low-elevation ponderosa pine

forests at Kings Canyon National Park. Their

findings suggest that increasing time between

prescribed fires may inhibit cheatgrass establishment. This is somewhat counter to the goal of

restoring historical fire regimes, but may be an

appropriate compromise between reducing serious fire hazards and exacerbating alien plant

invasions (Keeley and McGinnis 2007).

Question 3: 

How Do Plant Invasions Affect Fire

Regimes?

“Fire and Invasive Plant Ecology and 

Management: The Need for Integration to

Effectively Restore Ecosystems.”

Matt Brooks and colleagues organized a special 

session at an international scientific meeting to

present summaries describing the interrelationships between invasive plants and fire. This symposium reviewed the history of fire and invasive

plant research and management, summarized the

interrelationships between fire and invasive

species, highlighted the many complex interrelationships that remain unstudied, provided examples of promising new lines of research, and 

identified gaps in existing research programs. A

significant product from this symposium was a

description of the various ways that plant invasions can alter fuel bed characteristics, fire behavior, and fire regimes—which the authors called

the 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 frequencies below their natural range. Alison Dibble and

her team have shown that black locust (Robinia

pseudoacacia L.), a prominent invasive in eastern

pitch pine (Pinus rigida P. Mill.) forests, is not as

flammable as native vegetation and may actually

lengthen fire-return intervals (Dibble and Rees

2005). 

Question 4: 

What Are Effective Mitigation

Strategies Against Invasive Plants?

Invasives can be removed by several means 

including fire, biological control, herbicide use,

and mechanical cutting. These and followup seeding practices hold promise for mitigation, but few

studies have documented their effectiveness.

“The Use of Fire as a Tool for Controlling 

Invasive Plants.”

Fire is often used as part of integrated pest management programs to control invasive plants.

Matt Brooks, additional collaborators, and the

California Invasive Plant Council convened a

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM
12

panel of experts to develop state-of-the-science

summaries and recommendations for using fire 

to manage invasive plants. This panel concluded

that annual species that produce seeds well after

the fire season begins, that have flowering structures embedded within the fuel bed, and that

have short-lived seed banks are most amenable 

to control using fire. In contrast, perennial species

with perennating tissue that is either belowground or well above the fuel bed (and thus protected from heating), and that resprout readily

because they are adapted to fire or some other

form of recurrent disturbance, are not generally

amenable to control by fire. Invasive plants that

alter the fuel bed structure making it less flammable may also be difficult to control with fire

because they produce fuel beds that are relatively

inflammable. In all cases, followup monitoring

and plans for retreatment are required, and in

most cases, fire should be integrated with other

control methods. The net effects of any treatment

plan on the entire plant community, higher tropic

levels, and ecosystem properties, need to be considered before treatments are implemented

(DiTomaso and Johnson 2006, DiTomaso et al.

2006, JFSP 2006). 

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Researchers have found that certain mitigation treatments can be effective at shifting the woodland landscape towards the

desired native shrub-steppe condition.

Photo by Tom Iraci, USFS
13

“Fire Management Options to Control Woody 

Invasive Plants in the Northeastern and the 

Mid-Atlantic U.S.”

Although burning is often used to control invasive shrubs in the Northeast and Mid-Atlantic

States, it may be counterproductive if burns are

conducted during dormancy. Using an experimental approach, Alison Dibble and colleagues

examined the effect of dormant-season burn,

growing-season cut with same-season burn, and

growing-season cut followed by a second cut

across three sites in the Northeast. For the seven

species that were studied, a single treatment,

whether applied during the dormant or early

growing season, decreased root reserves (essential

for resprouting) for less than one growing season.

With 2 years of treatment, resprouting still

occurred, but growth tended to be less vigorous

than for those treated only once. Across species,

the authors concluded that carefully applied cutting and prescribed fire treatments, in tandem or

alone, can reduce the fire hazard in invaded fuel

beds (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 grasses

in riparian zones at Zion National Park (ZNP): 

(1) biomass reduction (mowing vs. fire); (2) herbicide to target the grasses (different types, rates,

and timings); and (3) followup seeding of earlysuccessional native species to compete with invasive grasses that escaped the first two treatments.

Preliminary results indicate strong differences

among treatments, suggesting that definitive recommendations for future management of invasive

annual grasses at ZNP will emerge from this project. These early results were used by the National

Park Service to design Burned Area Emergency

Response treatments to manage invasive annual

grasses after the Kolob Fire of 2006, the largest

fire in the history of ZNP (Matt Brooks 2006, pers.

comm.). 

“Management of Fuel Loading in the ShrubSteppe.”

In field trials, Steven Link and his team tested 

two herbicides for use against invasives (mostly

cheatgrass) in a shrub-steppe community in eastern Washington. Imazapic2 reduced cheatgrass

cover by about 40 percent and increased native

vegetation cover by about 10 percent 2 years after

application, whereas glyphosphate reduced the

cover of both cheatgrass and native species.

Imazapic is recommended to reduce cheatgrass

but should be used with caution where rare

native mustards are present (the mustards were

reduced with treatment). The team found that

prescribed fire can be used in the fall to prepare

surfaces for preemergent herbicide application

where cheatgrass cover is less than 50 percent.

Link and colleagues also conducted a restoration

study and found that fire risk of two communities

8 years after establishment of large bunchgrasses

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

2 The use of trade or firm names in this publication is for reader

information and does not imply endorsement by the U.S.

Department of Agriculture of any product or service.
14

was approximately 77 percent, and decreased 

further to 66 percent 18 years after restoration.

The savings associated with reducing fire risk

from 100 percent to 66 percent may be enough 

to cover the costs of restoring shrub-steppe

ecosystems. More information can be found on

the 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 and

Reproduction in Recently Burned Sagebrush

Steppe.”

Postfire seeding is often used to suppress the 

growth and reproduction of cheatgrass in lowelevation sagebrush steppe. Nonnative perennial

grasses are traditionally seeded to compete with

invasive annuals because of their presumed

greater growth rates compared with native perennials, yet few controlled studies have tested this

assumption. With a field experiment, Matt Brooks

and associates examined the response of cheatgrass and other invasive grasses to postfire seeding of native vs. nonnative perennial grass

species. Because of low rainfall, the growth of

seeded plants was low the first 2 years. Also,

across seeded plots, an invasive perennial grass

(bulbous bluegrass [Poa bulbosa L.]) increased in

dominance, presumably because of soil disturbance that occurred with the seeding technique 

(a mechanized drill). Researchers will continue to

evaluate the responses after seeded plants reach

maturity. A Web site with further information is

available 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 the

Colorado Plateau.”

A similar project evaluated the effects of seeding 

native perennial grasses after fire with and without seedbed preparation on cheatgrass dominance

in a basin big sagebrush community on the

Colorado Plateau. Preliminary results 3 years

posttreatment indicate that seedbed preparation

improved establishment of the seeded native

species, but also led to increased dominance of a

nonnative forb. Responses of cheatgrass among

treatments have been nonsignificant so far. This

delayed response is likely due to the timing of the

fire prior to seeding, which occurred when many

cheatgrass plants had emerged as seedlings and

were likely killed, thus reducing the onsite population 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 after

treatments were applied following a fire at Mesa

Verde National Park. Density of a persistent postfire invasive (muskthistle [Carduus nutans L.]) was

reduced by 66 percent, and density of a native

grass was doubled in areas that received seeding

treatments compared to areas that were not seeded. Although aerial application of native grass

seeds did not completely prevent weed invasion,

it greatly reduced postfire weed density and rapidly increased the native component of the plant

community. Mechanical treatment of muskthistle

(removing seed heads and digging plants out

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM
15

with a shovel) was effective only in areas

where native grasses were present: muskthistle was completely absent from these

areas 7 years later. Chemical treatment of

Canada thistle (Cirsium arvense (L.) Scop.)

(by using Curtail) for 2 years following fire

resulted in almost complete mortality of

Canada thistle, which was nearly absent

from treated areas 7 years later. Although

chemical treatment of Canada thistle was

very effective, it is labor-intensive and

probably 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 nonnative species, and their application on landscape

scales may introduce a large number of nonnative

weed seeds. Using research from study sites in

the Rocky Mountains of Colorado and New

Mexico, Philip Omi and colleagues showed that

where seed mixes were applied aerially, there was

a positive association between the cover of nonnative species and seeded grasses. However,

where seed mixes were applied by hand 

or a seed drill on a more limited scale, postfire

seeding successfully deterred nonnative species

establishment, similar to the effect of abundant

native cover. Postfire treatments that encourage

high vegetative cover, without introducing 

new nonnative species in contaminated seed

mixes, would best prevent further spread of nonnative species. These researchers suggest that

hand application of native grass seed mixes may

be preferable to broad application of nonnative

grass 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 colleagues

recommend against postfire seeding, particularly

in the shrublands and ponderosa pine forests of

California and the southwest owing to lack of

strong 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 team

that has completed detailed literature reviews on

60 nonnative invasive plant species in the Fire

Effects Information System (FEIS, at www.fs.fed.

us/database/feis). These reviews synthesize the

scientific literature on each species’ basic biology,

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM

Application of contaminated seed mixes on landscape scales may

introduce a large number of nonnative weed seeds. 

Photo by Tom Iraci, USFS
16

ecology, and relationship to fire. Reviews emphasize how fire affects each species, how the species

may alter fire regimes, and potential management

actions for specific ecosystems. A comprehensive

search of the scientific literature forms the basis

for each review. An additional 10 to 20 nonnative

invasives will be covered in FEIS under another

JFSP task entitled “Bringing the Fire Effects

Information System Up-to-Date and Improving

Service to Land Managers” (JFSP 2006, Jane

Kapler 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 and

synthesizing current knowledge about the ecology and management of invasive plants and fire in

the United States. This document includes three

sections: (1) overview chapters highlighting the

potential for nonnative species to increase after

fire, the effect of these species on fire regimes, and

the use of prescribed fire to control them; (2)

analysis of the relationship of fire to invasions

and invasible plant communities in seven regions

of the United States: the Northeast, Southeast,

Central, Interior West, Pacific Northwest and

Alaska, Southwest coast, and Hawaiian Islands;

and (3) issues of nationwide concern, including

knowledge gaps in relation to fire and invasives,

postfire rehabilitation, prescribed fire monitoring,

and effects of fire surrogate treatments on invasive species (Jane Kapler Smith 2006, pers.

comm.).

SUMMARY OF FINDINGS

Have we answered our questions? Partly, yes! 

We’ve learned (see detailed bullets below) about

several factors that can contribute to whether an

ecosystem is vulnerable to invasion, we’ve

learned how prescribed fire and fuel treatments

might actually increase invasive species, and

we’ve learned about some successful mitigation

strategies against invasives. Many of the projects

are ongoing or just beginning. More questions

will be answered and tools for managers developed and refined as the word continues to get

out. 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 affect

a community’s susceptibility to invasion

after fire. 

• Prescribed fire can be a problem:

Prescribed fire may have the unintended

effect of increasing invasive species in

some ecosystems, especially if the ecosystems are in poor ecological condition and

the native species capable of establishing

or resprouting following fire have been

depleted.

• Fuel treatments matter: Fuel treatments 

that leave some overstory canopy, minimize exposure of bare ground, and target

sites with species capable of establishing

or resprouting may be less likely to promote invasion by alien plant species.

• Fuel reduction treatments: Fuel treatments that encourage high vegetative

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM
17

cover of native species, without introducing new nonnative species (e.g., in contaminated seed mixes or by soil disturbance) may prevent further spread of 

nonnative species. 

• Mitigation strategies may be effective:

On sites that exhibit species invasions 

following wildfire, active intervention

with herbicides or other treatments

designed to control or eliminate the 

invasive can be highly effective.

• Postfire seeding: Seeding treatments 

should be used with caution, as studies

are showing these can increase invasives.

ACKNOWLEDGMENTS

We thank Jeanne Chambers, Matt Brooks, and 

Seth White for technical reviews; the JFSP staff in

Boise for helping us access project information;

and the scientists for providing feedback on their

findings.

FOR MORE INFORMATION, 

PLEASE CONTACT:

Tim Swedberg

Timothy_Swedberg@nifc.blm.gov

(208) 387-5865

FURTHER READING

Brooks, M.L.; D’Antonio, C.M.; Richardson, 

D.M.; Grace, J.; Keeley, J.J.; DiTomaso, J.M.;

Hobbs, R.; Pellant, M.; Pyke, D. 2004. Effects

of invasive alien plants on fire regimes.

BioScience. 54: 677–688. 

Brooks, M.L.; Matchett, J.R. 2003. Plant community patterns in unburned and burned

blackbrush (Coleogyne ramosissima Torr.)

shrublands in the Mojave Desert. Western

North 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 in

the control and spread of invasive species:

Proceedings of invasive species workshop.

Fire conference 2000: the first national congress on fire ecology, prevention and management. Misc. Publ. 11. Tallahassee, FL: Tall

Timbers Research Station: 1–14.

Chambers, J.C.; Roundy, B.A.; Blank, R.R.; 

Meyer, S.E.; Whittaker, A. 2007. What makes

great 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/fire

cycle, and global change. Annual Review of

Ecology and Systematics. 23: 63–87.

Dibble, A.C.; Rees, C.A. 2005. Does the lack of 

reference ecosystems limit our science? A case

study in nonnative invasion plants as forest

fuels. Journal of Forestry. 103: 329–338.

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM
18

DiTomaso, J.M.; Brooks, M.L.; Allen, E.B.; 

Minnich, R.; Rice, P.M.; Kyser, G.B. 2006.

Control of invasive weeds with prescribed

burning. Weed Technology. 20(2): 535–548.

DiTomaso, J.M.; Johnson, D.W., eds. 2006. The 

use of fire as a tool for controlling invasive

plants. 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 in

Pinus 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 weed

invasions to restore natural post-fire succession 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 heterogeneity influences understory diversity following restoration treatments in a ponderosa

pine/Douglas-fir forest, Montana. Ecological

Applications. 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 treatments, abiotic and biotic factors, and postfire

grass seeding treatments. International

Journal 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 cheatgrass

persistence in a Sierra Nevada ponderosa pine

forest. International Journal of Wildland Fire.

16: 96–106

Merriam, K.E.; Keeley, J.E.; Beyers, J.L. 2006.

Fuel breaks affect nonnative species abundance in Californian plant communities.

Ecological Applications. 16: 515–527.

Metlen, K.L.; Fiedler, C.E. 2006. Restoration 

treatment effects on the understory of ponderosa pine/Douglas-fir forests in western

Montana, USA. Forest Ecology and

Management. 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. Forest

Ecology and Management. 234: 143–163.

INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM
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