United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

Research Paper RMRS-RP-78

October 2010

Response of Six Non-Native Invasive Plant Species 

to Wildfires in the Northern Rocky Mountains, USA

Dennis E. Ferguson and Christine L. Craig

2001 2003

2005 2007
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Ferguson, Dennis E.; Craig, Christine L. 2010. Response of six non-native plant 

species to wildfires in the northern Rocky Mountains, USA. Res. Pap. RMRSRP-78 Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station. 12 p.

Abstract

This paper presents early results on the response of six non-native invasive plant 

species to eight wildfires on six National Forests (NFs) in the northern Rocky 

Mountains, USA. Stratified random sampling was used to choose 224 stands based 

on burn severity, habitat type series, slope steepness, stand height, and stand 

density. Data for this report are from 219 stands (875 plots) that have repeated 

measures 1 to 7 years post-fire. Six invasive plant species are abundant enough to 

analyze for early indications of response to burning. Spotted knapweed occurrence 

is highest on Douglas-fir and ponderosa pine habitat types on the Bitterroot NF. 

Canada thistle occurs on most of the sampled wildfires but at low occurrences 

and percent plot coverage. Bull thistle has rapid increases on the Bitterroot, 

Flathead, Kootenai, and Malheur NFs, generally with increasing occurrence at 

higher burn severities, but average percent coverage is low. Orange hawkweed 

has low occurrences (<5 percent) and never more than 1 percent coverage on a 

plot. Meadow hawkweed has its highest occurrence on the Bitterroot NF in low 

burn severities on Douglas-fir and ponderosa pine habitat types. Prickly lettuce is 

found on most NFs, the highest occurrences being on the Malheur and Panhandle 

NFs, with increasing occurrence at higher burn severities; however, average 

percent cover of prickly lettuce is low. Populations of the six species (especially 

spotted knapweed, bull thistle, and prickly lettuce) need continued monitoring to 

determine if occurrence and cover continue to change.

Keywords: Centaurea stoebe L. (= C. biebersteinii = C. maculosa), Cirsium arvense, 

Cirsium vulgare, Hieracium aurantiacum, Hieracium caespitosum, Lactuca 

serriola

Authors

Dennis E. Ferguson is a Research Silviculturist and, at the time of this study, 

Christine L. Craig was a Biological Science Technician, both with the USDA Forest 

Service, Rocky Mountain Research Station in Moscow, Idaho.

Acknowledgments

We are grateful to Kate Schneider and John Byrne for assistance in collecting data 

and preparing datasets and to field crews who diligently collected the data. We also 

thank Carolyn Sieg, Linda Pietarinen, and John Byrne for reviewing earlier drafts of 

this manuscript.

Cover photos: Plot 060, Bitterroot National Forest, 2001, 2003, 2005, and 2007.
USDA Forest Service RMRS-RP-78. 2010. 1

Introduction

Forest ecosystems in the Interior West are adapted 

to wildfire as a periodic disturbance agent (Arno 1980, 

Roe and others 1971, Wellner 1970). Indeed, many forest plant species are fire dependent (see Fischer and 

Bradley 1987, Smith and Fischer 1997). Recent wildfires in dry Interior West forests have increasingly been 

crown fires that kill much of the overstory, while historic low-severity ground fires have become less common 

(Covington and Moore 1994, Steele and others 1986). 

Fire exclusion and suppression, which have resulted in 

increased fuel load and fuel connectivity (Auclair and 

Bedford 1994, Hessburg and others 2000), may explain 

the increase in wildfire severity.

Establishment and growth of non-native invasive 

plant species are a major concern for western U.S. 

forest managers because invasive species alter these 

forests, resulting in ecological and economic losses 

(Evans 2003, Perrings and others 2002). Disturbances 

caused by wildfires create the potential for establishment and growth of non-native species, especially 

when coupled with the pre-fire presence of these species along roadsides and trails; in meadows; and in 

highly disturbed sites such as campsites, logging landings, and landslides. Disturbances create microsites for 

non-native species establishment and reduce competition from native plants. Therefore, it is important to 

understand secondary succession by non-native invasive plant species following wildfires.

This study was conducted in Montana, Idaho, 

and Oregon on wildfires that occurred from 2000 to 

2003. The study area encompasses forest types from 

Douglas-fir/ponderosa pine (Pseudotsuga menziesii/Pinus ponderosa) to grand fir/western redcedar/

western hemlock (Abies grandis/Thuja plicata/Tsuga 

heterophylla) to subalpine fir/lodgepole pine forests 

(Abies lasiocarpa/Pinus contorta).

We obtained a list of 32 non-native invasive plant species in Montana and Idaho (table 1) from the PLANTS 

database (USDA NRCS 2001). Five to 7 years after the 

wildfires, six species were abundant enough to warrant 

analyses and are the subject of this report. These species are spotted knapweed (Centaurea stoebe L. [= C. 

biebersteinii DC, = C. maculosa Lam.]), Canada thistle (Cirsium arvense (L.) Scop.), bull thistle (Cirsium 

vulgare (Savi) Ten.), orange hawkweed (Hieracium 

aurantiacum L.), meadow hawkweed (Hieracium 

caespitosum Dumort. = H. pratense Tausch), and 

prickly lettuce (Lactuca serriola L.).

Ecology of Invasive Species

Spotted Knapweed

Spotted knapweed is a short-lived perennial composite that is native to central Europe and Asia Minor 

(Sheley and others 1998). It primarily invades semiarid pastures and rangelands (Jacobs and Sheley 1998) 

and low-elevation forests (Forcella and Harvey 1983, 

Roche and Roche 1988), but is not a serious threat 

in subalpine fir forests following wildfires (Ferguson 

and others 2007). Knapweed does not compete well 

Table 1—Selected non-native invasive plant species in Montana and Idaho (extracted from PLANTS Database, http://

plants.usda.gov).

Scientific name Common name Scientific name Common name

Acroptilon repens Russian knapweed Hieracium caespitosum Meadow hawkweed

Cardaria draba Whitetop Hyoscyamus niger Black henbane

Carduus nutans Nodding plumeless thistle Hypericum perforatum Common St. Johnswort

Centaurea stoebe Spotted knapweed Isatis tinctoria Dyer’s woad

Centaurea diffusa White knapweed Lepidium latifolium Broadleaved pepperweed

Centaurea solstitialis Yellow star-thistle Linaria dalmatica	 Dalmation toadflax

Chondrilla juncea Rush skeletonweed Linaria vulgaris	 Yellow toadflax

Cirsium arvense Canada thistle Lythrum salicaria Purple loosestrife

Cirsium vulgare Bull thistle Onopordum acanthium Scotch thistle

Conium maculatum Poison hemlock Potentilla recta Sulfur cinquefoil

Convolvulus arvensis Field bindweed Ranunculus acris Tall buttercup

Crupina vulgaris Common crupina Senecio jacobaea Tansy ragwort

Cytisus scoparius Scotchbroom Sonchus arvensis Field sowthistle

Euphorbia dentata Toothed spurge Tamarix sp. Saltcedar

Euphorbia esula Leafy spurge Tribulus terrestris Puncturevine

Hieracium auranthiacum Orange hawkweed Zygophyllum fabago Syrian beancaper
2 USDA Forest Service RMRS-RP-78. 2010.

in moist areas such as near wet microsites or irrigated 

fields (Harris and Cranston 1979, Powell and others 

1997).

Disturbances are associated with establishment of 

knapweed populations (Ferguson and others 2007, 

Forcella and Harvey 1983, Watson and Renney 1974). 

Expansion of spotted knapweed colonies occurs 

through germination of seed along the periphery of 

stands, which may be aided by allelopathic compounds 

(Bais and others 2003). Burning can reduce population 

growth (Emery and Gross 2005), but it does not control knapweed (Sheley and others 1998). Burning can 

also facilitate colonization by decreasing competition 

and providing microsites for seedling establishment 

(Emery and Gross 2005, Ferguson and others 2007, 

Sheley and others 1998). Measures to reduce knapweed 

populations include hand weeding, plowing, selective 

animal grazing, herbicides, biological control, and revegetation (Sheley and others 1998).

Canada Thistle

Canada thistle is a native species of southeastern 

Europe and the eastern Mediterranean that is considered naturalized worldwide (Bayer 2000, Mitich 1988, 

Morishita 1999, Parish and others 1996, Whitson and 

others 1996). It is an early successional, slender taprooted, perennial composite (Parish and others 1996, 

Whitson and others 1996). In the northern Rocky 

Mountains at all elevations and habitat types, it 

invades disturbed areas—roadsides, fence lines, clearings, meadows, ditches, and stream banks (Meier and 

Weaver 1997, Morishita 1999, Parish and others 1996, 

Weaver and others 1990). Canada thistle does not compete well in full shade or dry, poor soils (Bayer 2000, 

Morishita 1999).

Growth of Canada thistle populations occurs by 

vegetative creeping roots and seed production, possibly aided by allelopathic compounds (Bayer 2000, 

Friedli and Bacher 2001, Morishita 1999, Stachon and 

Zimdahl 1980, Thomas and others 1994). Two characteristics that promote population growth are the ability 

to sprout from root fragments and seeds that can persist 

in the soil up to 22 years (Bayer 2000, Mitich 1988, 

Morishita 1999). Results of burning Canada thistle are 

mixed—burning initially decreases population size but 

enhances population establishment and growth. The 

roots are capable of surviving severe fires, thus allowing for sprouting from roots and seed establishment 

on bare soil, though it can be delayed up to 2 years 

(Travnicek and others 2005). Common methods of population control are hand weeding, mowing, selective 

animal grazing, chemical application, and biological 

control agents. Canada thistle is able to resist any one 

method of treatment, including many types of biological controls (Bayer 2000, Morishita 1999, Thomas and 

others 1994, Travnicek and others 2005).

Bull Thistle

Bull thistle is a native composite of Europe, western Asia, and northern Africa (Klinkhamer and de Jong 

1993, Mitich 1998, Parish and others 1996, Randall 

2000, Whitson and others 1996) and is becoming 

naturalized (Mitich 1998, Randall 2000). It is an early 

successional, tap-rooted biennial that prefers moist 

soils (Beck 1999, Mitich 1998, Parish and others 1996, 

Randall 2000, Whitson and others 1996). The common 

areas of invasion are mid to low elevations; recent or 

repeatedly disturbed forests, roadsides, ditches, and 

fences; or undisturbed grasslands, meadows, and forest 

openings (Beck 1999, Mitich 1998, Parish and others 

1996, Randall 2000). The disturbance can be as small 

as gopher mounds (Randall 2000). Bull thistle does not 

compete well in densely shaded or waterlogged soil 

(Klinkhamer and de Jong 1993).

Bull thistle can only expand its population through 

seed germination (Beck 1999, Klinkhamer and de Jong 

1993, Randall 2000). A highly debated issue is the longevity of seeds in soils, which can possibly be up to 

5 years but, depending on conditions, may be far less 

(Beck 1999, Downs and Cavers 2000, Mitich 1998). 

Burning has mixed results but generally promotes 

establishment (Arno 1999). Common methods of population control are hand pulling, tilling, mowing, and 

using chemical and biological controls (Beck 1999, 

Randall 2000). Bull thistle requires multiple control 

applications, and the biological controls are problematic because of their potential to attack native thistles 

(Louda and others 1997, Randall 2000).

Orange and Meadow Hawkweed

There are two non-native invasive hawkweeds—

orange and meadow hawkweed—that are nearly 

identical species, differing only by the extent of their 

origin and floret color (Mangold and Kittle 2009, 

Wilson and Callihan 1999). Orange hawkweed is native to restricted areas in northern and central Europe, 

whereas meadow hawkweed is native to northern, central, and eastern Europe (Gleason and Cronquist 1991, 

Mangold and Kittle 2009, Parish and others 1996, 

Wilson and Callihan 1999). Orange hawkweed has orange florets, and meadow hawkweed has yellow florets 
USDA Forest Service RMRS-RP-78. 2010. 3

(Gleason and Cronquist 1991, Mangold and Kittle 

2009, Parish and others 1996, Whitson and others 

1996). Hawkweeds are mat-forming, fibrous-rooted, 

stolon-producing perennial composites that prefer 

well-drained soils (Wilson and Callihan 1999, Wilson 

and others 1997). They invade low to mid elevations 

in mountain meadows, forest clearings, logged areas, 

roadsides, and other disturbed areas (Gleason and 

Cronquist 1991, Parish and others 1996, Wilson and 

Callihan 1999, Wilson and others 1997).

Hawkweed populations expand through vegetative growth from rhizomes, stolons, adventitious root 

buds, and seed germination (Mangold and Kittle 2009, 

Wilson and Callihan 1999), which may be aided by 

allelopathic compounds (Wilson and Callihan 1999). 

Plants can sprout from root fragments, and the seeds 

are viable in soil up to 7 years (Mangold and Kittle 

2009, Wilson and Callihan 1999). The most effective 

methods of control are hand pulling, digging, and applying chemicals. Multiple applications of chemical 

control are the most effective treatment, as well as 

digging small infestations to completely remove roots 

(Mangold and Kittle 2009, Wilson and Callihan 1999, 

Wilson and others 1997).

Prickly Lettuce

Prickly lettuce is a native of Europe, western Asia, 

and northern Africa (Parish and others 1996, Prince 

and others 1978, Weaver and Downs 2003). It is a taprooted biennial composite (Parish and others 1996, 

Weaver and Downs 2003) that is naturalized throughout the United States (Gleason and Cronquist 1991). 

Prickly lettuce invades low- to mid-elevation disturbed 

sites such as waste areas, roadsides, and open sites 

(Gleason and Cronquist 1991, Parish and others 1996, 

Prince and others 1978, Weaver and Downs 2003).

Expansion of prickly lettuce populations occurs 

through seed germination (Meier and Weaver 1997, 

Weaver and Downs 2003). A short-term seed bank of 

1 to 3 years can develop (Weaver and Downs 2003). 

The primary methods of control are chemical and biological. However, there have been mixed results with 

chemical application due to natural genetic resistance 

to some chemicals (Weaver and Downs 2003).

Study Design

The data reported are part of an overall study 

to quantify secondary plant succession following 

wildfires in the northern Rocky Mountains. A stratified 

random sample was used to select stands within burn 

perimeters. Stratification insured a range of conditions 

was sampled, using combinations of:

• Three pre-fire cover types (Douglas-fir/ponderosa 

pine; grand fir/western redcedar/western hemlock; 

and subalpine fir/lodgepole pine);

• Two burning index classes (<75 and ≥75), an index of 

predicted fire spread and energy release (Bradshaw 

and others 1984);

• Two slope steepness classes (≤35 percent and >35 

percent);

• Two pre-fire canopy heights (≤40 ft tall and >40 ft 

tall); and

• Two pre-fire stand densities (≤35 percent and >35 

percent conifer canopy cover).

Within each of the combinations, three low-density stands were randomly selected for sampling. The 

location of the stand center and the first plot was determined using aerial photographs. Transect lines were 

drawn, and the intersection at the approximate stand 

center was assigned a GPS point. At this GPS point, 

field crews installed a four-point cluster of plots. A 

second four-point cluster was installed in an adjacent 

higher-density stand within the burn perimeter. This 

second stand was chosen to represent a change in prefire density, cover type, or forest structure. There were 

only a few instances when an acceptable adjacent stand 

was unavailable.

The four-point cluster plot design was similar to the 

sample design used by Forest Inventory and Analysis 

(Bechtold and Scott 2005). The center of plot 1 was 

the basis for the location of three satellite plots, which 

were 120 ft at azimuths of 0°, 120°, and 240°. Each of 

the four points had three associated plots. First was a 

1/300-acre plot to record slope, aspect, and burn severity to the forest floor, low shrubs, and tall shrubs. 

Second was a 1/24-acre plot to record habitat type 

(Cooper and others 1991, Johnson and Clausnitzer 

1992, Pfister and others 1977) and burn severity to 

overstory trees. The third plot was a variable radius 

plot for sampling overstory trees.

Elevation, the only variable recorded strictly at the 

stand level, was recorded at plot 1. All other variables 

were recorded at the plot level to make plots as independent as possible. Burn severity classes were used to 

characterize the effects of fire on vegetation and soil. 

Classes were recorded for each of four strata (forest 

floor, short shrubs, tall shrubs, and overstory trees) using the following categories: 0 = unburned, 1 = light 

(blackened duff, scorched foliage, and overstory trees 
4 USDA Forest Service RMRS-RP-78. 2010.

predominately green and/or brown needles), 2 = moderate (duff consumed, shrubs mostly consumed but 

stubs remaining, and overstory trees predominately 

brown and/or burned needles), and 3 = severe (mineral 

soil colored orange; shrubs consumed, leaving holes 

in the soil; and overstory trees predominately black). 

Each burn severity class was recorded by percent of the 

plot that it occupied; for example, 70 percent severe 

burn and 30 percent moderate burn to the forest floor.

Identification of habitat types can be difficult following wildfires, but it is not impossible. Species of 

fire-killed trees could usually be identified, and all 

vegetation was not consumed by the fire or it often 

sprouted from the roots. Field crews examined adjacent unburned stands to determine common habitat 

types in the area. Also at each re-measurement, crews 

reassessed habitat types and provided evidence for 

changes.

Vegetation abundance was estimated using 

Daubenmire’s (1959) ocular estimate of canopy coverage method, but two differences are that we used 

1/300-acre circular plots instead of rectangular plots, 

and we recorded vegetation by percentages rather than 

by broad cover classes. A value of 1 percent was used 

to note occurrence only (trace); otherwise, percent cover classes were 5 through 100 by 5 percent increments. 

As lifeforms, percent cover was recorded for shrubs, 

forbs, grasses, and ferns. For non-native invasive 

species, percent cover was recorded and, as of 2004, 

heights (nearest 0.5 ft) were also recorded. Occurrence 

is defined as any aerial part of a species being within 

the vertical cylinder of the 1/300-acre circular plot.

Vegetation sampling was done in late June through 

September, beginning at low elevations where the 

growing season starts earlier and progressing upward 

in elevation. Vegetation dried early in 2001 on the 

Bitterroot National Forest (NF), so we did not record 

vegetation for 25 stands. For these 25 stands, burn severity was recorded and overstory trees were measured 

in 2001, and vegetation was recorded in 2002.

Overstory trees were sampled to quantify pre-fire 

overstory density and post-fire overstory competition. 

Basal area factors of 10, 20, or 40 ft2

/acre were used to 

sample five to seven trees or snags per plot, if present. 

The same basal area factor was used on all plots within 

a stand.

Data Analysis

Our analyses used techniques applicable to two-state 

modeling systems (Hamilton and Brickell 1983). The 

first step was to analyze the occurrence of species—a 

species either occurs on a plot or it does not. All plots 

were used to calculate percent occurrence. The second 

step looked at attributes on plots where the species 

occurs—in this case, the attribute was percent cover. 

Analyzing data in this sequence could detect if species were expanding by becoming established on more 

plots (occurrence) or expanding horizontally (cover). 

Conversely, species could decrease in occurrence or 

cover over time.

Although there were 224 stands in the study, only 

219 were used in this report because several stands 

were not re-measured at years 5 and 6 due to shortfalls 

in funding. Therefore, only stands that had a complete 

series of measurements were used in analyses. Also, 

we inadvertently missed measuring one plot in 2003 on 

the Bitterroot NF, so all data for this plot were withheld 

from the analyses.

Burn severity indices were calculated for each plot. 

The burn severity codes (0, 1, 2, and 3) were weighted 

by the proportion of the plot they occupied to calculate 

a burn severity index. For presentation of results, the 

burn severity index was then classified into three categories: low (0.0 to 1.0), moderate (1.0 to 2.0), and high 

(2.0 to 3.0). Preliminary analyses showed the forest 

floor burn severity was the best predictor of occurrence 

and cover compared to burn severity for low shrubs, 

tall shrubs, or overstory trees.

Results

Table 2 summarizes information about the 219 

stands used in this report. Table 3 summarizes data 

by important plot attributes, which shows how stratified random sampling incorporates variation into the 

data. A total of 290 plots were sampled on Douglas-fir/

ponderosa pine habitat types, 145 on grand fir/western redcedar/western hemlock habitat types, and 440 

on subalpine fir/lodgepole pine habitat types. When 

plots were summarized by forest floor burn severity, 

210 were in the low burn severity class, 369 were in the 

moderate class, and 296 were in the high burn severity 

class.

Occurrence

For the six species in this report, bull thistle had the 

highest overall occurrence at 23.0 percent, followed 

by prickly lettuce at 19.8 percent, spotted knapweed 

at 15.1 percent, Canada thistle at 8.8 percent, meadow 
USDA Forest Service RMRS-RP-78. 2010. 5

hawkweed at 2.2 percent, and orange hawkweed at 

0.5 percent. Figure 1 shows species occurrence by 

wildfire.

Spotted knapweed was most abundant on the 

Bitterroot NF, where it occurred on 19.4 percent of 

plots at 1 or 2 years, 26.1 percent at 3 years, 37.1 percent at 5 years, and 41.3 percent at 7 years (table 4). 

When these data are summarized by time and forest 

floor burn severity (table 5), there is a clear trend of decreasing occurrence with increasing burn severity at 1 

or 2 years, coupled with higher increases in occurrence 

over time on higher burn severities.

Canada thistle had low occurrences on most wildfires 

(table 4). The highest occurrences were 13.5 percent on 

the Flathead-Roberts NF at 3 years and 25.0 percent 

on the Malheur NF at 5 years; otherwise, occurrences 

were below 3.9 percent. There were no clear trends of 

occurrence versus burn severity on the Bitterroot NF 

(table 5).

Occurrence of bull thistle was virtually non-existent at 1 year but had rapid increases by year 3 or 5 

(table 4). The most rapid increase was on the Malheur 

NF where bull thistle occurred on 41.1 percent of 

plots at 3 years. High occurrences did not appear until 

5 years on the Bitterroot NF (15.2 percent), FlatheadMoose (19.5 percent), and Kootenai NF (17.5 percent) 

at 6 years. There were also higher occurrences of bull 

thistle with increasing burn severity for most habitat 

type series (table 5). For example, the highest occurrence of bull thistle was on Douglas-fir habitat types 

on the Malheur NF, where it increased from 6.9 percent 

in the low burn severity class to 41.1 percent in the 

moderate class to 74.3 percent in the high burn severity class.

Hawkweeds were found on the Bitterroot and 

Flathead NF wildfires, and occurrence was less than 

about 5 percent at 5 years (table 4). There was no clear 

trend for meadow hawkweed occurrence as a function 

of burn severity (table 5). By year 7, hawkweeds were 

no longer found on the Bitterroot NF plots.

Occurrence of prickly lettuce was quite low except 

on the Malheur and Panhandle NFs. Occurrence was 

highest (71.7 percent) on the Malheur NF at 3 years 

(table 4). It was also high (35.0 percent) on the 

Panhandle NF at 3 years, but this average represents 

only 40 plots. Occurrence increased rapidly with 

Table 2—Attributes of sample stands by wildfire.

Wildfire Number Habitat type

Wildfire name National Year of size of stands seriesa

 and Years

and acronym Forest wildfire (acres) sampled (number of plots) sampled

Bitterroot Complex, Bitterroot 2000 356,075 71 DF/PP (154), 1 or 2, 3, 5, 7

BITT AF/LP (129)

Kootenai Complex, Kootenai 2000 34,600 30 DF/PP (12), 2, 4, 6

KOOT GF/RC/WH (15),

 AF/LP (93)

Ninemile Complex, Lolo 2000 38,700 18 GF/RC/WH (9), 2, 4, 6

LOLO AF/LP (63)

Moose, FLMO Flathead 2001 70,975 32 AF/LP (128) 1, 3, 5

Robert and Flathead 2003 24,125 13 RC/WH (28), 1, 3, 6

Crazy Horse, FLRO AF/LP (24)

Flagtail, MAHL Malheur 2002 7,250 45 DF/PP (120), GF 1, 3, 5

 (57), AF/LP (3)

Myrtle Creek, Panhandle 2003 3,450 10 DF/PP (4), RC (36) 1, 3, 6

PANH

a DF = Douglas-fir, PP = ponderosa pine, GF = grand fir, RC = western redcedar, WH = western hemlock, AF = subalpine fir,

and LP = lodgepole pine community type.

Table 3—Number of 1/300-acre plots sampled by forest 

floor burn severity and habitat type series (codes from

table 2).

Forest floor burn severity

Habitat

type series Low Medium High Total

DF/PP 88 121 81 290

GF/RC/WH 30 97 18 145

AF/LP 92 151 197 440

Total 210 369 296 875
6 USDA Forest Service RMRS-RP-78. 2010.

increased burn severity (table 5). For example, occurrence of prickly lettuce at 3 years on Douglas-fir habitat 

types on the Malheur NF increased from 62.1 percent 

in the low burn severity class to 89.3 percent in the 

moderate burn severity class to 97.1 percent in the high 

burn severity class. Also, the occurrence of prickly lettuce decreased after 5 years on the Bitterroot NF and 

after 3 years on the Malheur and Panhandle NFs.

Cover

Percent cover is an ocular estimate of the vertical 

projection of each species’ foliage on the horizontal 

plane of the plot, expressed as a percentage of the plot 

area. Data are shown only for the plots on which the 

species occurs.

Table 6 shows the number of plots where species occur by percent cover. This display of cover data shows 

where high coverage may be a concern. With one exception, coverage was fairly low for invasive species. 

The exception was for spotted knapweed on Douglasfir and ponderosa pine habitat types on the Bitterroot 

NF. There were more plots with higher percent cover 

compared to the other species and locations. For example, after 7 years, there were 42 plots with a trace 

of spotted knapweed cover, 14 with 5 percent cover, 

12 with 10 percent cover, 5 with 15 percent cover, 

8 with 20 percent cover, and continuing to 2 plots with 

≥50 percent cover (table 6).

In contrast, cover for the other species seldom exceeded 20 percent (table 6). Bull thistle on the Bitterroot 

NF showed a decline in cover over time, as did prickly 

lettuce on the Malheur and Panhandle NFs.

Figure 1—Percent occurrence of six non-native invasive plant species by wildfire. Occurrence for this figure is defined as the 

species being present on the plot at any of the measurements. Species abbreviations are CEST8 = spotted knapweed, CIAR4 

= Canada thistle, CIVU = bull thistle, HIAU = orange hawkweed, HICA10 = meadow hawkweed, and LASE = prickly 

lettuce. Wildfire acronyms are from table 2.

Wildfire
USDA Forest Service RMRS-RP-78. 2010. 7

Table 4—Percent occurrence of invasive plant species on 1/300-acre plots by wildfire and measurement.

National Forest, Years since fire

number of plots, and

wildfire acronym Species 1 or 2 3 5 7

Bitterroot, n = 283 Spotted knapweed 19.4 26.1 37.1 41.3

BITT Canada thistle 1.8 1.1 3.9 3.2

Bull thistle 0.0 0.7 15.2 12.0

Orange hawkweed 0.0 0.0 0.0 0.0

Meadow hawkweed 0.4 0.0 5.7 0.0

Prickly lettuce 0.0 0.0 5.3 3.9

1 3 6

Flathead-Roberts, n = 52 Spotted knapweed 0.0 1.9 3.8

FLRO Canada thistle 0.0 13.5 3.8

Bull thistle 0.0 1.9 5.8

Orange hawkweed 0.0 0.0 1.9

Meadow hawkweed 0.0 0.0 0.0

Prickly lettuce 3.8 5.8 0.0

1 3 5

Flathead-Moose, n = 128 Spotted knapweed 0.0 0.0 0.8

FLMO Canada thistle 1.6 3.9 2.3

Bull thistle 0.0 0.0 19.5

Orange hawkweed 0.0 0.0 2.3

Meadow hawkweed 0.0 0.0 0.0

Prickly lettuce 0.0 2.3 0.8

2 4 6

Kootenai, n = 120 Spotted knapweed 0.0 0.0 0.0

KOOT Canada thistle 0.0 0.8 0.8

Bull thistle 0.0 0.0 17.5

Orange hawkweed 0.0 0.0 0.0

Meadow hawkweed 0.0 0.0 0.0

Prickly lettuce 0.0 0.0 0.0

2 4 6

Lolo, n = 72 Spotted knapweed 0.0 0.0 1.4

LOLO Canada thistle 1.4 1.4 1.4

Bull thistle 0.0 0.0 4.2

Orange hawkweed 0.0 0.0 0.0

Meadow hawkweed 0.0 0.0 0.0

Prickly lettuce 0.0 0.0 0.0

1 3 5

Malheur, n = 180 Spotted knapweed 0.0 0.0 0.0

MAHL Canada thistle 0.0 0.0 25.0

Bull thistle 0.6 41.1 41.7

Orange hawkweed 0.0 0.0 0.0

Meadow hawkweed 0.0 0.0 1.1

Prickly lettuce 5.0 71.7 42.8

1 3 6

Panhandle, n = 40 Spotted knapweed 0.0 0.0 0.0

PANH Canada thistle 0.0 0.0 0.0

Bull thistle 0.0 2.5 2.5

Orange hawkweed 0.0 0.0 0.0

Meadow hawkweed 0.0 0.0 0.0

Prickly lettuce 15.0 35.0 10.0
8 USDA Forest Service RMRS-RP-78. 2010.

Table 5—Percent occurrence of invasive plant species on 1/300-acre plots by wildfire, measurement, and forest floor burn severity.

National Forest

Forest and Habitat floor Years since fire

wildfire type burn # of

Species acronym series severity 1-2 3-4 5-6 7 plots

Spotted knapweed Bitterroot DF/PP Low 51.9 46.3 59.3 63.0 54

BITT Medium 32.8 51.7 62.1 60.3 58

High 9.5 23.8 54.8 73.8 42

AF/LP Low 5.4 5.6 10.8 10.8 37

Medium 0.0 11.9 16.7 14.3 42

High 3.9 3.9 5.9 13.7 51

Canada thistle Bitterroot DF/PP Low 3.7 1.9 1.9 3.7 54

BITT Medium 1.7 0.0 5.4 3.4 58

 High 0.0 0.0 0.0 0.0 42

AF/LP Low 0.0 0.0 5.4 5.4 37

 Medium 4.8 4.8 9.5 4.8 42

 High 0.0 0.0 2.0 2.0 51

Bull thistle Bitterroot DF/PP Low 0.0 0.0 5.6 1.9 54

BITT Medium 0.0 1.7 19.0 10.3 58

High 0.0 0.0 33.3 28.6 42

AF/LP Low 0.0 0.0 5.4 0.0 37

 Medium 0.0 0.0 9.5 21.4 42

High 0.0 2.0 17.6 11.8 51

Bull thistle Flathead-Moose AF/LP Low 0.0 0.0 20.0 10

FLMO Medium 0.0 0.0 16.7 42

High 0.0 0.0 21.1 76

Bull thistle Kootenai AF/LP Low 0.0 0.0 21.7 23

KOOT Medium 0.0 0.0 21.4 28

High 0.0 0.0 14.3 42

Bull thistle Malheur DF/PP Low 0.0 3.4 6.9 29

MAHL Medium 1.8 42.9 41.1 56

High 0.0 57.1 74.3 35

GF Low 0.0 23.5 11.8 17

Medium 0.0 51.6 51.6 31

 High a a a

 9

Meadow hawkweed Bitterroot DF/PP Low 0.0 0.0 13.0 0.0 54

BITT Medium 0.0 0.0 10.3 0.0 58

 High 0.0 0.0 4.8 0.0 42

AF/LP Low 2.7 0.0 0.0 0.0 37

 Medium 0.0 0.0 2.4 0.0 42

 High 0.0 0.0 0.0 0.0 51

Prickly lettuce Bitterroot DF/PP Low 0.0 0.0 0.0 3.7 54

BITT Medium 0.0 0.0 3.4 0.0 58

High 0.0 0.0 28.6 21.4 42

AF/LP Low 0.0 0.0 0.0 0.0 37

 Medium 0.0 0.0 0.0 0.0 42

 High 0.0 0.0 2.0 0.0 51

Prickly lettuce Malheur DF/PP Low 3.4 62.1 34.5 29

MAHL Medium 5.4 89.3 51.8 56

High 8.6 97.1 68.6 35

GF Low 0.0 23.5 0.0 17

Medium 6.5 41.9 29.0 31

 High a a a

 9

Prickly lettuce Panhandle RC Low a a a

 2

PANH Medium 12.5 25.0 3.1 32

 High a a a

 2

a

 <10 plots, no average calculated.
USDA Forest Service RMRS-RP-78. 2010.

9

Table 6—Number of plots on which the species occurred by percent cover.

National

Forest Percent cover

and Habitat %

wildfire type occurSpecies acronym series Year 1 5 10 15 20 25 30 35 40 45 ≥50 rence

Spotted knapweed, Bitterroot DF/PP 1-2 22 7 10 3 1 2 2 2 2 33.1n = 154 BITT 3 32 14 2 5 1 3 1 2 3 1 1 42.2

5 49 9 9 6 7 2 3 1 1 1 3 59.17 42 14 12 5 8 4 6 3 3 1 2 64.9

Spotted knapweed, Bitterroot AF/LP 1-2 2 1 1 3.1

n = 129 BITT 3 7 2 7.0

5 10 2 2 10.9

7 13 2 1 1 13.2

Canada thistle, Bitterroot DF/PP, 1-2 4 1 1.8

n = 283 BITT AF/LP 3 2 1 1.1

5 8 1 1 1 3.97 6 1 1 1 3.2

Bull thistle, Bitterroot DF/PP, 1-2 0.0

n = 283 BITT AF/LP 3 2 0.7

5 42 1 15.2

7 31 3 12.0

Bull thistle, Malheur DF/PP, 1 1 0.6

n = 180 MAHL GF, 3 68 5 1 41.1

AF/LP 5 58 12 2 2 1 41.7

Meadow hawkweed, all all 1-2 1 0.1

n = 875 3-4 0.0

5-6 18 2.1

Prickly lettuce, Bitterroot DF/PP, 1-2 0.0

n = 283 BITT AF/LP 3 0.0

5 12 1 2 5.3

7 10 1 3.9

Prickly lettuce, Malheur, DF/PP, 1 15 6.8

n = 220 Panhandle GF/RC, 3 117 9 8 3 3 2 1 65.0

MAHL AF/LP 5/6 76 5 36.8

PANH
10 USDA Forest Service RMRS-RP-78. 2010.

Discussion

Spotted knapweed occurred primarily on the 

Bitterroot NF, especially on Douglas-fir and ponderosa 

pine habitat types. It was not particularly invasive on 

subalpine fir habitat types. Occurrence was initially 

lower at higher burn severities versus lower burn severities, but the rate of increase over time was greater 

at higher burn severities. It is not yet clear how knapweed occurrence and cover will continue to change as 

secondary succession continues.

Canada thistle occurred on most of the sampled 

wildfires but at low occurrences. Only 9 percent of the 

875 plots had Canada thistle, and average coverage 

was low. Therefore, Canada thistle was only a minor 

problem on these wildfires.

Bull thistle was not present immediately after wildfires but increased rapidly over time on several NFs. 

Occurrence usually increased with increasing burn severity. However, average percent cover was very low; 

where bull thistle did occur, cover was generally only 

a trace. Even though bull thistle readily invaded some 

areas, it did not dominate vegetation succession 3 to 

7 years after wildfire. Continued plot re-measurement 

will determine if bull thistle populations continue to 

expand.

Hawkweeds were found on the Bitterroot and 

Flathead NFs, with occurrences less than about 5 percent. In addition to low occurrences, percent cover was 

never more than 1 percent on any plot. Hawkweeds 

were only a minor problem.

Prickly lettuce was found on most NFs, generally 

with occurrences less than about 5 percent, except on 

the Malheur and Panhandle NFs. The Malheur NF had 

71.7 percent occurrence 3 years after the wildfire, but 

this decreased to 42.8 percent by year 5. The Panhandle 

NF had 35.0 percent occurrence 3 years after the 

wildfire, but this decreased to 10.0 percent by year 6. 

Occurrence generally increased rapidly with increasing burn severity. Even though prickly lettuce readily 

invaded some areas, percent cover was very low.

Results for these six invasive species provide information on responses to disturbance during the first 

7 years after wildfires. More insights will be gained 

through re-measurement of these plots. For example, 

will occurrence and coverage for some of these invasives continue to increase? Will development of native 

vegetation reduce or eliminate these invasive species?

Various approaches can be used to study non-native 

invasive plant species. Some investigators might locate 

areas that are being invaded and then study their size, 

density, expansion rate, and effects on native vegetation. In this study, the approach was to randomly select 

stands that were forested at the time of the wildfire and 

then study the vegetation that developed following the 

fire. We did not deliberately seek out heavily infested 

areas, nor did we sample meadows, logging landings, 

or along roads. Our results show what can be expected 

within previously forested areas.

Wildfires selected for this study were chosen to 

include a range of habitat types, overstory densities, 

slopes, and burn severities. However, the choice of 

study areas was dependent on finding suitable areas 

that had been recently burned. Results of this study 

may or may not extrapolate well to other areas. We feel 

that the presentation of data by habitat type series and 

burn severity will help with extrapolation of results to 

other areas.

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