The Role of Fuel Breaks in the 

Invasion of Nonnative Plants 

Inside front 

U.S. Department of the Interior 

Scientific Investigations Report 2006–5185

U.S. Geological Survey 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

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The Role of Fuel Breaks in the Invasion of Nonnative Plants 

The Role of Fuel Breaks in the Invasion of 

Nonnative Plants 

By Kyle E. Merriam1, Jon E. Keeley2, and Jan L. Beyers3 

1

U.S.D.A. Forest Service

Plumas National Forest 

Post Office Box 11500, Quincy, CA 95971 

email: kmerriam@fs.fed.us 

2

U.S. Geological Survey, Biological Resources Division

Sequoia and Kings Canyon Field Station 

47050 Generals Hwy #4, Three Rivers, CA 93271 

3

U.S.D.A. Forest Service, Pacific Southwest Research Station, Riverside Fire Laboratory 

4955 Canyon Crest Dr., Riverside, CA 92507 

U.S. Department of the Interior 

U.S. Geological Survey 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

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Suggested citation: 

Merriam, K.E., Keeley, J.E., and Beyers, J.L., 2007, The role of fuel breaks in the invasion of 

nonnative plants: U.S. Geological Survey Scientific Investigations Report 2006-5185, 69 p. 

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Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Executive Summary 

Fuel reduction projects have become an 

increasingly important component of state and 

federal fuels management programs. However, an 

unintended result of some pre-fire fuel manipulation 

projects may be the introduction of nonnative 

invasive plants. The establishment of nonnative 

plants within fuel breaks is a serious concern because 

the presence of invasive species in areas treated to 

reduce fuels could make adjacent wildland areas 

more susceptible to invasion, particularly following 

widespread disturbances such as fires. 

Fuel reduction projects have become an 

increasingly important component of state and 

federal fuels management programs. However, an 

unintended result of some pre-fire fuel manipulation 

projects may be the introduction of nonnative 

invasive plants. The establishment of nonnative 

plants within fuel breaks is a serious concern because 

the presence of invasive species in areas treated to 

reduce fuels could make adjacent wildland areas 

more susceptible to invasion, particularly following 

widespread disturbances such as fires. 

This report presents the results of a research 

project investigating the relationship between fuel 

reduction treatments and the invasion of nonnative 

plants. Throughout the rest of this document, we will 

collectively refer to these treatments as fuel breaks, 

although we sampled a range of fuel breaks described 

variously as fuel breaks, shaded fuel breaks, 

defensible fuel reduction zones, defensible fuel 

profile zones, fuel reduction projects, fuel 

management zones, wildfire protection zones, and 

community protection zones.

This report presents the results of a research 

project investigating the relationship between fuel 

reduction treatments and the invasion of nonnative 

plants. Throughout the rest of this document, we will 

collectively refer to these treatments as fuel breaks, 

although we sampled a range of fuel breaks described 

variously as fuel breaks, shaded fuel breaks, 

defensible fuel reduction zones, defensible fuel 

profile zones, fuel reduction projects, fuel 

management zones, wildfire protection zones, and 

community protection zones.

Antelope Border Defensible Fuel Profile Zone (DFPZ), 

Plumas National Forest 

___________________________________________________________________________________________

Executive Summary v
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

In Chapter One we discuss overall results 

compiled from 24 separate fuel breaks located across 

California. These fuel breaks represent a variety of 

different construction and maintenance methods, and 

different fire histories. Our primary findings were: 

• Fuel breaks may promote nonnative plants. 

• Fuel breaks with more canopy and ground 

cover may be less likely to be invaded. 

• Nonnative plants were more abundant 

adjacent to older fuel breaks and in areas that 

had experienced more numerous fires. 

• Native species may also be promoted by 

fuel breaks, particularly native annual forbs and 

grasses. 

Our data suggest that fuel breaks provide 

establishment sites for nonnative plants, and that 

surrounding areas may be susceptible to invasion, 

particularly after disturbances such as fire. Fuel break 

construction and maintenance methods that leave 

some overstory canopy cover and minimize exposure 

of bare ground may be less likely to promote 

nonnative plant invasion. 

 In Chapter Two we evaluate the association 

between nonnative plants and fuel breaks within 

vegetation types, including mixed coniferous forest, 

oak woodland, chaparral, and coastal scrub plant 

communities. We found that: 

• Nonnative abundance and species richness 

was significantly different among vegetation 

types: conifer forests had the lowest nonnative 

cover and richness while coastal sage scrub had 

the highest. 

• Differences in elevation, fire history, 

grazing, canopy cover, and disturbance history 

may explain some vegetation type differences. 

• Nonnatives were significantly more 

abundant on fuel breaks in all vegetation types. 

• Fuel break construction method was 

associated with nonnative abundance in all 

vegetation types. 

• Fire and grazing was positively associated 

with nonnative abundance in all vegetation 

types with adequate sample sizes to evaluate 

these factors. 

This analysis suggests that although different 

vegetation types experience varying degrees of 

invasion by nonnatives, our general 

recommendations described in Chapter One also may 

be applied within individual vegetation types. Fuel 

break construction and maintenance methods that 

retain some overstory canopy and minimize the 

exposure of bare ground will likely minimize the 

probability of nonnative invasion across vegetation 

types. 

Etz Meloy fuel break, Santa Monica Mountains 

National Recreation Area 

Executive Summary vi
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

In Chapter Three we provide a brief summary 

of each individual fuel break sampled in this study. 

Each site had a unique history, including various 

dates of construction and construction methods, 

different maintenance regimes, varying fire histories, 

and different land use histories. Our general findings 

were: 

• Individual sites differed greatly in nonnative 

cover, fire history, and environmental variables. 

• Nonnative cover was higher within fuel 

breaks than in adjacent wildlands at 19 of our 24 

study sites. 

• Fuel breaks that were not invaded by 

nonnatives had very deep duff layers, were 

constructed by methods other than bulldozing, 

were very young, or had very infrequent 

maintenance regimes. 

• Cheatgrass (Bromus tectorum) was the most 

common nonnative plant species at 8 of our 

study sites. 

• A number of different nonnative species 

dominated the other 16 sites, suggesting that 

many nonnative species may be well adapted to 

take advantage of the disturbed conditions 

associated with fuel breaks. 

Our site level review suggests that fuel breaks 

may create conditions favorable to a number of 

different nonnative plant species. The colonization of 

a fuel break by a particular species may reflect which 

species has a nearby seed source or is able to disperse 

into the treated area. Regardless of which nonnative 

species is of greatest concern in the region, retaining 

overstory canopy and ground cover will likely be the 

best way to prevent nonnative invasion on fuel 

breaks. 

When reviewing this report, please remember 

that our study was observational and additional 

research using controlled, replicated experiments will 

be necessary to fully understand the mechanisms that 

influence nonnative plant invasion within fuel 

reduction treatments. 

Aguanga fuel break, Cleveland National Forest 

Executive Summary vii
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Acknowledgments 

Canterbury bells (Phacelia minor) in the 

Cleveland National Forest 

We would like to thank the Joint Fire Science 

Program for funding this study (project #01B-3-2-

08). Sequoia and Kings Canyon National Parks were 

instrumental in supporting our research. This project 

would not have been possible without the generous 

cooperation of the Angeles, Cleveland, Lassen, Los 

Padres, Mendocino, Plumas, San Bernardino, 

Sequoia, Shasta-Trinity, Sierra, Six Rivers, and Lake 

Tahoe Basin Management Area USDA National 

Forests; the Whiskeytown and Santa Monica 

National Recreation Areas; the San Luis Obispo 

District of the California Department of Forestry and 

Fire Protection; the Bakersfield District of the Bureau 

of Land Management; the Los Angeles County Fire 

Department Division of Forestry; and the Orange 

County Department of Parks, Casper’s Wilderness 

Park. We also want to thank Katie VinZant, Lea 

Condon, Clara Arndt, Trent Draper, and Elizabeth 

Martin for their expert field assistance. 

Acknowledgments viii
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Table of Contents 

Executive Summary ............................................................................................................................................................................v

Acknowledgments ...........................................................................................................................................................................viii 

FIGURES .......................................................................................................................................................................................... ....x

Chapter One: Overall Results ............................................................................................................................................................1

METHODS ........................................................................................................................................................................................3

Sampling Protocol......................................................................................................................................................................4

Data Analysis ..............................................................................................................................................................................5

RESULTS...........................................................................................................................................................................................5

1. Nonnative cover, density, and species richness..............................................................................................................6

2. Differences among fuel breaks ...........................................................................................................................................7

3. Patterns adjacent to fuel breaks .......................................................................................................................................11

4. Species Diversity..................................................................................................................................................................12

5. Life history types ..................................................................................................................................................................12

6. Variation among nonnative species .................................................................................................................................13

7. Variation among native species ........................................................................................................................................14

DISCUSSION..................................................................................................................................................................................15

1. Canopy and ground cover ..................................................................................................................................................16

2. Fuel break age and fire history ..........................................................................................................................................17

3. Native species ......................................................................................................................................................................18

Chapter Two: Vegetation Type Differences .................................................................................................................................20

INTRODUCTION ...........................................................................................................................................................................20

CONIFEROUS FOREST.................................................................................................................................................................23

MIXED OAK WOODLANDS.........................................................................................................................................................25

CHAPARRAL .................................................................................................................................................................................27

COASTAL SCRUB.........................................................................................................................................................................29

DISCUSSION.................................................................................................................................................................................30

Chapter Three: Site Specific Results.............................................................................................................................................32

INTRODUCTION ...........................................................................................................................................................................32

Gasquet Shaded Fuel Break, Six Rivers National Forest ..................................................................................................36

Santa Monica Mountains National Recreation Area ........................................................................................................37

Oso Ridge Fuel Break, Casper’s Wilderness Park..............................................................................................................39

Whiskeytown National Recreation Area .............................................................................................................................40

Calf Canyon Fuel Break, Bakersfield BLM and San Luis Obispo CDF.............................................................................42

Sequoia and Kings Canyon National Park ...........................................................................................................................43

Sierra National Forest .............................................................................................................................................................45

Aguanga Fuel Break, Cleveland National Forest................................................................................................................48

Pilot Grove Fuel Break, Mendocino National Forest..........................................................................................................49

Rouse Ridge Fuel Break, San Bernardino National Forest...............................................................................................50

Sierra Pelona Fuel Break, Angeles National Forest...........................................................................................................51

Blacks Ridge Defensible Fuel Profile Zone, Lassen National Forest ..............................................................................52

Sierra Madre Ridge Fuel Break, Los Padres National Forest ..........................................................................................53

Palos Ranches Fuel Break, Sequoia National Forest and BLM.......................................................................................54

Antelope Border Defensible Fuel Profile Zone, Plumas National Forest .......................................................................55

Fallen Leaf Lake Shaded Fuel Break, Lake Tahoe Basin Management Unit .................................................................56

Oregon Fire Line, Shasta Trinity National Forest................................................................................................................57

McKenzie Ridge Fuel Break, Sequoia National Forest......................................................................................................58

DISCUSSION..................................................................................................................................................................................59

Chapter Four: Summary....................................................................................................................................................................62

Literature Cited ..................................................................................................................................................................................66

 

 ix
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

FIGURES

Figure 1-1. Map of 24 fuel breaks study sites across the State of California.........................................................................3 

Figure 1-2. Diagram of transect orientation along fuel break and quadrat locations along transects .............................4 

Figure 1-3. MANOVA found that relative nonnative cover, density, and species richness (expressed as the 

percentage of total plant cover, density, and richness) was significantly higher on fuel breaks 

compared with sites outside fuel breaks .................................................................................................................6 

Figure 1-4. MANOVA found that environmental variables associated with nonnative plant presence and 

abundance were significantly different on and off fuel breaks ...........................................................................7 

Figure 1-5. Differences in relative nonnative cover with fuel break construction method .................................................9 

Figure 1-6. Relationship between fuel break construction method and environmental variables significantly 

associated with nonnatives, including duff depth, litter cover, canopy cover, and bare ground were 

determined using MANOVA......................................................................................................................................10 

Figure 1-7. Two-way ANOVA found significant differences in relative nonnative abundance with distance 

from the fuel break and with fuel break age..........................................................................................................11 

Figure 1-8. Two-way ANOVA found significant differences in relative nonnative cover with distance 

from fuel break and with the number of fires ........................................................................................................11 

Figure 1-9. Both native and nonnative species diversity was significantly higher on fuel breaks according 

to paired t-tests ..........................................................................................................................................................12 

Figure 1-10. Differences between fuel breaks and adjacent wildland areas in the number of species 

from each life history type were evaluated using paired t-tests .......................................................................12 

Figure 2-1. ANOVA indicated relative nonnative cover differed significantly among vegetation types.........................20 

Figure 2-2. ANOVA found significant differences in nonnative species richness among vegetation types..................20 

Figure 2-3. ANOVA found significant differences in duff depth among vegetation types .................................................21 

Figure 2-4. ANOVA on canopy cover found significant differences among vegetation types..........................................21 

Figure 2-5. ANOVA determined that relative nonnative cover was significantly higher ON fuel breaks 

compared with OFF fuel breaks in all vegetation types .......................................................................................21 

Figure 2-6. ANOVA found that duff depth was lower on fuel breaks within each vegetation type ................................22 

 Figure 2-7. ANOVA found canopy cover was significantly lower on fuel breaks in each vegetation type ...................22 

Figure 2-8. ANOVA found significant differences in relative nonnative cover among different construction 

methods in coniferous forests..................................................................................................................................23 

 Figure 2-9. ANOVA found a significant interaction effect between fire number and distance from the 

fuel break on relative nonnative cover in coniferous forests...............................................................................24 

Figure 2-10. ANOVA found significant effect of construction method on relative nonnative cover in 

oak woodlands ............................................................................................................................................................26 

Figure 2-11. ANOVA found significant effects of fire number and distance from the fuel treatment on relative 

nonnative cover in oak woodlands..........................................................................................................................26 

Figure 2-12. ANOVA found significant effect of grazing and distance from the fuel treatment on relative 

nonnative cover in oak woodlands..........................................................................................................................26 

Figure 2-13. The effect of construction method on relative nonnative cover in chaparral vegetation types ..................28 

Figure 2-14. The effect of fire number and distance from the fuel treatment on relative nonnative cover in 

chaparral......................................................................................................................................................................28 

Figure 2-15. The effect of grazing and distance from the fuel treatment on relative nonnative cover in chaparral ......28 

Figure 2-16. ANOVA found significant effect of distance from the fuel treatment on relative nonnative cover 

in coastal scrub vegetation types............................................................................................................................29 

Figure 3-1. Variation in relative nonnative plant abundance between fuel breaks ............................................................34 

Figure 3-2. Variation in mean native species richness among fuel breaks..........................................................................34 

Figure 3-3. Variation in duff depth among fuel breaks .............................................................................................................34 

Figure 3-4. Variation in canopy cover among fuel breaks.......................................................................................................34 

Figure 3-5. ANOVA did not find significant differences in relative nonnative abundance on the Gasquet 

fuel break compared to outside of the fuel break.................................................................................................36 

 x 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Figure 3-6. ANOVA found significant differences between nonnative abundance on the Zuma Ridge and 

Etz Meloy fuel breaks compared with adjacent wildlands .................................................................................38 

Figure 3-7. ANOVA found significant differences in relative nonnative abundance on the Oso Ridge fuel break 

compared with adjacent wildlands .........................................................................................................................39 

Figure 3-8. ANOVA found a significant interaction between position on and off the fuel break and site ......................41 

Figure 3-9. ANOVA found significant differences in relative nonnative abundance on the Calf Canyon 

fuel break compared with adjacent wildlands ......................................................................................................42 

Figure 3-10. ANOVA found significantly higher relative nonnative abundance on the Shepard Saddle and 

Lookout Point fire lines compared with adjacent wildlands, but this pattern did not emerge on 

the Lewis Creek fire line ...........................................................................................................................................44 

Figure 3-11. ANOVA found significant differences between fuel breaks ...............................................................................47 

Figure 3-12. ANOVA found significant differences in relative nonnative abundance on the Aguanga 

fuel break compared with adjacent wildlands ......................................................................................................48 

Figure 3-13. ANOVA found significant differences in relative nonnative abundance on the Pilot Grove 

fuel break compared with adjacent wildlands ......................................................................................................49 

Figure 3-14. ANOVA found significant differences in relative nonnative abundance on the Rouse Ridge 

fuel break compared with adjacent wildlands ......................................................................................................50 

Figure 3-15. ANOVA found significant differences in relative nonnative abundance on the Sierra Pelona 

fuel break compared with adjacent wildlands ......................................................................................................51 

Figure 3-16. ANOVA found no difference in relative nonnative abundance on the Blacks Ridge DFPZ 

compared with adjacent wildlands .........................................................................................................................52 

Figure 3-17. ANOVA found significantly higher relative nonnative abundance on the Sierra Madre Ridge 

fuel break compared with the adjacent wildland .................................................................................................53 

Figure 3-18. ANOVA found significant differences in relative nonnative abundance on the Palos Ranches 

fuel break compared with adjacent wildlands ......................................................................................................54 

Figure 3-19. ANOVA found no significant differences in relative nonnative abundance within the 

Antelope Border DFPZ compared with adjacent untreated areas ....................................................................55 

Figure 3-20. ANOVA found significant differences in relative nonnative abundance within the Oregon 

fire line compared with adjacent untreated areas...............................................................................................57 

Figure 3-21. ANOVA found significant differences in relative nonnative abundance on the McKenzie Ridge 

fuel break compared with adjacent untreated areas ..........................................................................................58 

TABLES 

 

Table 1-1. Multiple linear stepwise regression analysis of the relationship between environmental variables 

and relative nonnative cover......................................................................................................................................7 

Table 1-2. Multiple linear stepwise regression analysis of the association between anthropogenic variables 

and relative nonnative plant cover............................................................................................................................8 

Table 1-3. Differences in mean cover on and off fuel breaks of the ten most frequently occurring nonnative 

plant species ...............................................................................................................................................................13 

Table 1-4. Native species that were significantly more common on fuel breaks..............................................................14 

Table 3-1. Selected characteristics of fuel breaks .................................................................................................................32 

Table 3-2. Characteristics of fuel breaks where relative nonnative cover did not differ between plots 

on the fuel break and plots in the adjacent wildland area..................................................................................59 

xi 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

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The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Chapter One: Overall Results

Reduction of hazardous fuels has become a 

priority for federal, state, local, and private land 

managers across the United States. High fuel loads 

resulting from fire suppression, population growth at 

the wildland-urban interface, and large catastrophic 

fires have focused unprecedented national attention 

on pre-fire fuel manipulation projects. As a result, a 

growing number of federal, state, and local fuel 

reduction programs have dramatically increased the 

number of hectares (acres) treated to reduce fuels 

nationwide, and extended the scope of these projects 

to include a wider range of vegetation types and 

treatment prescriptions. 

Fuel reduction treatments are generally 

implemented to change fire behavior, to provide 

firefighter access, as an anchor point for indirect 

attack on wildland fires, or to contain prescribed fires 

(Agee and others, 2000). They range in configuration 

from narrow linear features along individual ridge 

lines, to large landscape-scale treatments spanning 

thousands of acres. The amount of surface and ladder 

fuels removed within these treatments can vary 

widely, with reduction of overstory canopy cover 

ranging from complete to less than 40%. 

Here, we will collectively refer to these 

treatments as fuel breaks, although they have been 

variously termed shaded fuel breaks, defensible fuel 

reduction zones, defensible fuel profile zones, fuel 

reduction projects, fuel management zones, wildfire 

protection zones, and community protection zones. 

Fuel break construction and maintenance methods 

have changed over time, and differ according to 

terrain, vegetation type, and implementing agency 

(Los Angeles County Fire Department 1962; Omi 

1979). For example, fuel break maintenance by aerial 

application of herbicides and seeding with nonnative 

grasses was common until the 1970s (Bentley 1967; 

Clark 1973; Edmunson & Cornelius 1961; San Diego 

County 1974; US Department of Agriculture 1959, 

1960). More recent fuel break construction and 

maintenance measures include selective thinning, 

mastication, and increased use of prescribed burning 

(Farsworth & Summerfelt 2002). 

Chapter One: Overall Results 1 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

An unintended consequence of large-scale fuel 

break construction of new fuel breaks, and increased 

maintenance of existing fuel breaks, may be the 

establishment of nonnative plant species. A number 

of studies have documented an association of 

nonnative plant species with disturbed areas similar 

to fuel breaks, such as logging sites, roads, trails, and 

pipeline corridors (e.g., D'Antonio and others, 1999). 

These disturbance corridors can promote the 

invasion of surrounding areas by providing a nearby 

seed source (Gelbard & Belnap 2003; Parendes & 

Jones 2000; Tyser & Worley 1992; Zink and others, 

1995). Wildland areas adjacent to fuel breaks might 

be particularly susceptible to invasion following 

landscape-scale disturbances such as fire, because 

fire has been shown to promote the invasion of 

nonnative plants in a number of habitats (D'Antonio 

2000). In many cases invading species are well 

adapted to fire and can invade fire prone ecosystems, 

particularly when natural fire regimes have been 

altered through fire suppression, increased 

anthropogenic ignitions, or by feedback effects with 

changes in plant species composition (D'Antonio and 

Vitousek 1992; Keeley 2001). 

In this study we evaluated the potential for fuel 

breaks to function as establishment sites for 

nonnative plants, and for nonnative species to invade 

surrounding wildland areas (see Text Box 1-1). The 

goals of this research were to provide fire and 

resource managers with information to develop fuels 

management strategies that both accomplish fuel 

hazard reduction goals and minimize the potential for 

nonnative plant invasion. 

Text Box 1-1: Our primary 

research questions were: 

• Do plant communities differ within 

fuel breaks compared with adjacent 

wildland areas? 

• What environmental and 

anthropogenic factors are correlated 

with the abundance of nonnatives? 

• Are nonnative species more abundant 

in areas adjacent to fuel breaks after 

disturbances such as fire? 

Chapter One: Overall Results 2 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

METHODS 

We investigated 24 fuel breaks across the State 

of California (Figure 1-1). Study sites were located 

primarily in the Sierra Nevada and coastal mountain 

ranges, including coastal scrub, chaparral, oak 

woodland, and mixed coniferous forest vegetation 

types ranging in elevation from 200 to 2000 m. 

The fuel breaks varied in age from those 

constructed by the Civilian Conservation Corps 

during the 1930s to fuel reduction projects 

implemented as recently as 2003. We attempted to 

include fuel breaks constructed by a variety of 

different methods such as bulldozers, mechanical 

equipment, and hand crews. Mechanical equipment 

included rubber tired and tracked vehicles used for 

thinning and mastication of forest and chaparral 

vegetation; hand crews relied primarily on hand tools 

such as shovels, Pulaskis, McCleods, and other hand 

line construction tools. 

The land use history of each study site was 

complex, and we could not control for many of the 

disturbance related factors that may have affected 

nonnative abundance on the fuel breaks we studied. 

Additional information about each fuel break is 

provided in Chapter Three. 

Figure 1-1. Map of 24 fuel breaks study sites across the State of 

California. Site codes are explained in Table 3-1. 

Chapter One: Overall Results 3 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Sampling Protocol 

Data were collected during the spring and 

summer of 2002 and 2003. At each site we 

established between 8 and 10 transects perpendicular 

to the fuel break, beginning at the origin nearest a 

road or urban interface, and continuing at discrete 

intervals of between 200 and 1500 m, depending on 

the total size of the treated area. Transects were 50 m 

in length, extending 10 m towards the center of the 

fuel break and 40 m into the surrounding vegetation. 

Two 1 m2

 plots were placed inside the fuel break, and 

four 1 m2

 plots were placed in the adjacent wildland 

area at discrete distances of 5, 10, 20, and 40 m from 

the edge of the fuel break (Figure 1-2). 

1X1 m 

plots

Road

50 m Transect

Distance between transects 

200-1500 m 

(not to scale) 

Fuel Break 

Figure 1-2. Diagram of transect orientation along fuel 

break and quadrat locations along transects. 

We collected a range of data at each fuel break 

(see Text Box 1-2). We estimated species cover by 

cover class according to Daubenmire (1959), and 

estimated the density of each species, including 

overstory trees and shrubs. All plants were identified 

according to Hickman (1993). Plant communities 

were identified according to categories developed by 

Sawyer and Keeler-Wolf (1995). 

Data on fire history was obtained from a statewide fire perimeter GIS data layer containing fires 

recorded since 1953 (California Department of 

Forestry and Fire Protection 2003), and from 

individual cooperators when available. Information 

about the distance and density of roads and urban 

interfaces to our plots was generated from GIS data 

layers using the ArcView Spatial Analyst extension 

(ESRI 2000). Information about fuel break age, 

construction and maintenance methods, and grazing 

history were obtained from GIS data, environmental 

and biological assessments, resource management 

plans, fire incident reports, and agency technical 

reports. Additional information was collected 

through personal communications with fuels and fire 

managers, botanists, range managers, and other staff 

familiar with each site. 

Text Box 1-2. Data Collected: 

At each plot: 

• Species composition, cover and density 

• Ground cover (bare ground, litter, etc.) 

• Overstory canopy cover 

• Litter and duff depth 

At the end points of each transect: 

• Soil nitrogen, carbon, moisture 

• Slope, aspect, elevation 

• Fire and grazing history 

• Distance to roads and Wildland Urban Interface 

(WUI) 

• Fuel break age, construction, and maintenance 

• Vegetation type 

Chapter One: Overall Results 4 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Data Analysis 

We used a number of different statistical 

methods to analyze the data we collected, including 

paired t-tests, Analysis of Variance (ANOVA), and 

multiple regressions. Data analysis was performed 

using SYSTAT version 10 (SPSS 2000). Residuals 

from each analysis were plotted to identify outliers 

and evaluate homogeneity of variance (Wilkinson 

and others, 1996). Percentage values were arcsine 

square root transformed to improve normality and 

homogeneity of variance. Sites were not equally 

distributed across all vegetation types, or with past 

land use such as grazing and fire history, and some 

data was missing, so N-values reported for different 

analyses below may vary. 

RESULTS 

We identified 736 plant species in our 1 m2 plots 

both in and adjacent to 24 separate fuel breaks. 

Approximately 85% were native, 11% were 

nonnative, and 4% could not be identified due to 

dessication, phenology, or lack of plant material. 

Fifty-one percent of our quadrats contained no 

nonnative plants. The most frequently occurring 

nonnative plant was cheatgrass. Our primary findings 

are summarized in Text Box 1-3, and more detailed 

results are provided in the sections that follow. 

Text Box 1-3. Our primary findings were: 

 

• Nonnative plants had higher relative cover, 

density, species richness, and diversity within fuel 

breaks than in adjacent wildlands. 

• Some fuel breaks are less likely to support 

nonnative species, particularly those with more 

overstory canopy and ground cover. 

• Nonnative plants are more abundant adjacent to 

older fuel breaks and in areas that have 

experienced more numerous fires. 

• Native species diversity was also higher in fuel 

breaks. 

• Both nonnative and native annual forbs and 

grasses were more abundant in fuel breaks than in 

adjacent wildlands. 

• Nonnative plants were generally more common 

on fuel breaks, but some species showed no 

response. 

• Native species were generally more common 

outside fuel breaks, but some species were more 

abundant in fuel breaks. 

Chapter One: Overall Results 5 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

1. Nonnative cover, density, and species 

richness 

Absolute and relative nonnative cover, density, 

and species richness were significantly higher on fuel 

breaks than in adjacent areas outside of fuel breaks 

(Figure 1-3). 

Cover Density Richness

Percentage of Total 

0

20

40

60

80

100

On Fuel Break 

Off Fuel Break 

Relative Nonnative

Figure 1-3. MANOVA found that relative nonnative 

cover, density, and species richness (expressed as 

the percentage of total plant cover, density, and 

richness) was significantly higher on fuel breaks 

compared with sites outside fuel breaks (N= 48, 

df=3, 44, F= 5.645, P<0.001). Error bars indicate 

+ 1 SE. 

Nonnatives occurred more frequently on fuel 

breaks, where they were found in 65% of quadrats, 

compared with outside of fuel breaks, where only 

43% of quadrats contained nonnative plants. 

Chapter One: Overall Results 6 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

2. Differences among fuel breaks 

Elevation, slope, duff depth, overstory canopy, bare ground, litter cover, and rock cover were significantly 

associated with the presence and abundance of nonnative plant species (Table 1-1). 

Table 1-1. Multiple linear stepwise regression analysis of the relationship between environmental variables and relative 

nonnative plant cover across all quadrats where nonnatives were found. 

Effect Standard Coefficient Tolerance T P 

Constant 0.000 29.25 <0.001 

Canopy cover (%) -0.354 0.763 -10.00 <0.001 

Elevation -0.229 0.895 - 7.02 <0.001 

Bare ground (%) -0.129 0.739 - 3.58 <0.001 

Slope (%) -0.097 0.722 - 2.66 0.008 

Litter cover (%) -0.094 0.651 - 2.46 0.014 

Transect 0.076 0.741 2.13 0.034 

Site 0.067 0.975 2.14 0.032 

Notes: Variables with P (probability)>0.15 were removed from the model, including rock cover, litter depth, and 

duff depth (N=754, adjusted multiple r2

=0.281, standard error of estimate=0.225). The T-value is the estimate 

divided by its standard error. 

Figure 1-4. MANOVA found that environmental 

variables associated with nonnative plant presence and 

abundance were significantly different on and off fuel 

breaks (N=48, df=4, 43, F=4.165, P=0.002). Error bars 

indicate + 1 SE. Univariate P-values shown on graphs. 

Several of these variables were significantly 

lower in quadrats on fuel breaks (Figure 1-4). 

Chapter One: Overall Results 7 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

The anthropogenic variables of fuel break 

construction method, maintenance method, 

maintenance frequency, fuel break age, and distance 

to roads were significantly associated with relative 

nonnative cover on fuel breaks (Table 1-2).

 

Table 1-2. Multiple linear stepwise regression analysis of the association between anthropogenic variables and relative 

nonnative plant cover on fuel breaks. 

[Data represent plots on the fuel break only, pooled to provide one value per transect] 

Effect Standard 

Coefficient 

Tolerance T P 

Constant 0.000 14.44 <0.001 

Construction Method -0.631 0.698 13.49 <0.001 

Maintenance Frequency 0.221 0.733 4.84 <0.001 

Site 0.121 0.821 2.82 0.005 

Fuel Break Age 0.084 0.711 1.82 0.071 

Maintenance Method 0.068 0.892 1.64 0.103 

Distance to Roads -0.062 0.925 1.53 0.127 

Notes: Variables dropped from the multiple stepwise linear regression as not being significant at P>0.15 included 

distance of the fuel break to urban interfaces, prescribed burning, and use of precautions against nonnative invasion 

such as washing equipment (N=220, adjusted multiple r2

=0.666, standard error of estimate= 0.207). 

Chapter One: Overall Results 8 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Fuel breaks constructed by bulldozers had 

significantly higher relative nonnative cover than fuel 

breaks constructed by hand crews, and fuel breaks 

thinned mechanically had significantly lower relative 

nonnative cover than fuel breaks constructed by other 

means, even with elevation and slope included as 

covariates (Figure 1-5). 

Construction Method

Bulldozers Hand Crews Mechanical

Relative Nonnative Cover (%)

0

10

20

30

40

50

60

Figure 1-5. Differences in relative nonnative cover 

with fuel break construction method. Error bars 

indicate + 1 SE. The effect of construction method 

was significant based on Analysis of Covariance 

(ANCOVA) with slope and elevation as covariates 

(N=220, effect of construction method: df=2, 217, 

F=118.655, P<0.001). 

Fuel break constructed by bulldozer, Santa 

Monica Mountains National Recreation 

Chapter One: Overall Results 9 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

 Environmental variables significantly associated 

with nonnative cover, including overstory canopy 

cover, litter cover, duff depth, and bare ground, also 

varied significantly among fuel breaks constructed by 

different methods (Figure 1-6). Fuel breaks 

constructed by bulldozers had the least duff depth, 

litter cover, and canopy cover of any fuel break 

construction method. 

Construction Method

Bulldozers

Hand Crews

Mechanical

Litter Cover (%)

0

20

40

60

80

p<0.001

Construction Method

Bulldozers

Hand Crews

Mechanical

Bare Ground (%)

0

10

20

30

40

P<0.001

Construction Method

Bulldozers

Hand Crews

Mechanical

Canopy Cover (%)

0

20

40

60

80

P<0.001 P=0.028

Bulldozers

Hand Crews

Mechanical

Duff Depth (cm)

0

2

4

6

8

10

12

14

16

18

20

P<0.001

Construction Method

Figure 1-6. Relationship between fuel break 

construction method and environmental variables 

significantly associated with nonnatives, including 

duff depth, litter cover, canopy cover, and bare 

ground were determined using MANOVA (N=220, 

df=4, 214, F=11.608, P<0.001). Univariate P-values 

are indicated on graphs. Error bars represent 

+ 1 SE. 

The presence of nonnatives also was associated 

with fuel break construction method; 49% of 

quadrats contained nonnatives on fuel breaks 

constructed by bulldozers, compared with 20% of 

quadrats on fuel breaks constructed by hand crews, 

and only 4% of quadrats on mechanically thinned 

fuel breaks contained nonnative species. 

This mechanically cleared fuel break in 

Whiskeytown National Recreation Area 

maintained some overstory canopy cover 

and ground cover. 

Chapter One: Overall Results 10 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

3. Patterns adjacent to fuel breaks 

Relative nonnative cover significantly declined 

with distance from the fuel break, and this effect was 

more significant with fuel break age category (Figure 

1-7). 

 

The relationship between relative nonnative 

cover and distance from the fuel break was also 

influenced by number of fires (Figure 1-8). In the 

absence of fire, relative nonnative abundance in 

wildland areas adjacent to fuel breaks did not change 

with increasing distance from the fuel break. 

However, in areas that had experienced one or more 

fires, relative nonnative abundance in wildland areas 

adjacent to fuel breaks significantly declined with 

increasing distance from the fuel break. 

0

10

20

30

40

50

Distance from fuel break (m)

< 10 years 

10-30 years 

>30 years

0 5 10 20 40

Relative Nonnative Cover (%)

Figure 1-7. Two-way ANOVA found significant 

differences in relative nonnative abundance with 

distance from the fuel break and with fuel break age 

(interaction effect with plot distance nested within 

transects, N=1543, df=12, 1523, F = 15.377, P<0.001). 

Error bars represent + 1 SE. 

Distance from fuel break (m)

0 10 20 30 40 50

Relative Nonnative Cover (%)

0

10

20

30

40

50

60

70

No Fires

One Fire

Two Fires

Three or More Fires

Figure 1-8. Two-way ANOVA found significant 

differences in relative nonnative cover with 

distance from fuel break and with the number of 

fires (interaction effect with plot distance nested 

within transects, N=793, df=12, 773, F=2.366, P<0.001). 

Error bars represent + 1 SE. 

Chapter One: Overall Results 11 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

4. Species Diversity 

Both native and nonnative species diversity was 

higher within fuel breaks than in adjacent wildland 

areas (Figure 1-9), and native and nonnative diversity 

were significantly positively correlated (Pearson 

correlation=0.326, P=0.024). 

Nonnative species diversity at the site level was 

most strongly correlated with duff depth and 

elevation, and together these variables explained over 

half of the variation in nonnative diversity (adjusted 

multiple R2

=0.521). Duff depth and bare ground were 

most strongly correlated with native diversity at the 

site level, although the explanatory power of these 

variables was lower (adjusted multiple 2

 = 0.211). 

5. Life history types 

Fuel breaks were characterized by higher 

numbers of annual plant species than adjacent 

wildland areas, including native and nonnative 

grasses and forbs, which were significantly more 

common in treated areas than in adjacent wildlands 

(Figure 1-10). Native perennial forbs and nonnative 

perennial grasses also were significantly more 

common on fuel breaks. Native trees, shrubs, and 

subshrubs were more common outside of fuel breaks. 

We did not encounter any species of nonnative trees, 

and only two species of nonnative shrubs and five 

species of nonnative perennial forbs were sampled in 

this study. 

Species Diversity

Nonnative Native

0.0

0.2

0.4

0.6

0.8

On Fuel Break

Off Fuel Break 

Shannon's Index (D)

Life History Type

NAG

NNAG

NPG

NNPG

NAF

NNAF

NPF

Shrubs

Trees

Number of species/m2 0

1

2

3

4

5

On Fuel Breaks

Off Fuel Breaks

*

*

*

*

*

*

*

*

*

Figure 1-10. Differences between fuel breaks and 

adjacent wildland areas in the number of species 

from each life history type were evaluated using 

paired t-tests (NAG=Native annual grass, 

NNAG=nonnative annual grass, NPG=native 

perennial grass, NNPG= nonnative perennial 

grass, NAF=native annual forb, NNAF= nonnative 

annual forb, NPF= native perennial forb). Error 

bars indicate + 1 SE. Significantly different means 

indicated with an asterisk and determined by 

Bonferroni adjusted method with significance level 

at P=0.006 for nine comparisons. 

Figure 1-9. Both native and nonnative species 

diversity was significantly higher on fuel breaks 

according to paired t-tests. (native species 

diversity: t=2.983, df=216, P=0.003, nonnative 

species diversity: t=10.284, df=217, P<0.001) 

Chapter One: Overall Results 12 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

6. Variation among nonnative species 

 

Of the 79 nonnative species we identified, 21 

(26.6%) only occurred on fuel breaks, and 9 (11.4%) 

were restricted to plots in wildland areas. Of the 32 

nonnative species found in 10 or more plots, 17 were 

statistically equivalent in cover both on fuel breaks 

and in adjacent wildland areas, and 15 were more 

common on fuel breaks. Six of the ten most common 

nonnatives were equally common on fuel breaks and 

in adjacent wildlands (Table 1-3). The nonnative 

species we observed were primarily annual grasses 

and forbs. 

Table 1-3. Differences in mean cover on and off fuel breaks of the ten most frequently occurring nonnative plant species. 

[Differences between mean cover in wildland areas (OFF), and inside areas treated to reduce fuels (ON) were 

evaluated using paired t-tests, and significance was determined using Bonferroni adjusted P value of 0.005. Species 

with no significant difference in abundance are shown in bold]

Scientific Name # of 

plots 

Mean 

OFF 

Mean 

ON 

P 

Bromus tectorum 244 1.4169 1.8157 0.041 

Bromus madritensis 223 1.9604 2.3 0.237 

Vulpia myuros 217 1.2763 2.2591 <0.001 

Bromus hordeaceus 184 1.1588 2.5721 <0.001 

Bromus diandrus 177 1.9572 2.136 0.570 

Erodium cicutarium 150 0.6409 3.1544 <0.001 

Torilis arvensis 122 1.6898 1.4028 0.313 

Centaurea melitensis 99 1.613 2.375 0.051 

Aira caryphyllea 82 1.303 1.876 0.229 

Avena barbata 79 0.816 2.645 <0.001 

Chapter One: Overall Results 13 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

7. Variation among native species 

Of the 551 native species observed in this study, 

142 (25.8%) only were found outside of fuel breaks, 

while 108 (19.4%) only occurred inside fuel breaks. 

Of the 178 native species that were found in ten or 

more plots, 46 were statistically more common 

outside of fuel breaks, 12 species were more 

common inside fuel breaks and 111 native species 

did not differ in abundance in fuel breaks compared 

with adjacent wildland areas. Native species that 

were more common on fuel breaks were found in all 

vegetation types and represented a range of life 

history types, including annual grasses, annual forbs, 

perennial forbs, subshrubs, and shrubs (Table 1-4). 

Table 1-4. Native species that were significantly more 

common on fuel breaks, based on paired t-tests. 

[Life history abbreviations are AG=annual grass, 

AF=annual forb, PF= perennial forb, SS=subshrub, 

and S=shrub. Vegetation type abbreviations are 

CF=coniferous forest, OW=oak woodland, 

CH=chaparral, and CS=coastal sage scrub. 

Significance assessed with Bonferonni adjusted P

value of 0.002 for 10 comparisons] 

 Scientific Name Life 

History 

Vegetation 

Type 

P 

Vulpia microstachys AG CF, OW, CH <0.001 

Lupinus bicolor AF OW, CH, CS <0.001 

Lotus strigosus AF CH, CS 0.002 

Lotus argophyllus PF CF, CH 0.001 

Epilobium 

brachycarpum 

AF CF, OW, CH <0.001 

Trifolium 

albopurpureum 

AF OW, CH, CS 0.001 

Lotus purshianus AF CF, OW, CH 0.001 

Lotus wrangelianus AF CH, CS 0.002 

Eriophyllum lanatum SS CF, OW, 0.002 

Micropus californicus AF OW, CH, 0.002 

Arctostaphylos 

glandulosa 

S CH 0.001 

Madia glomerata AF OW, CH 0.001 

Both native and nonnative species may be 

found on fuel breaks, such as this one 

in the Sierra National Forest. 

Chapter One: Overall Results 14 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

DISCUSSION 

Pre-fire fuel manipulations, such as fuel breaks, 

are an important component of fire management 

programs, particularly in areas where prescribed fire 

or wildland fire use is impractical. Our study 

demonstrates that fuel breaks have the potential to 

promote the establishment and spread of nonnative 

plants. The management implications of our findings 

are summarized in Text Box 1-4. 

Text Box 1- 4. Some management 

implications of our findings are: 

• Fuel breaks may promote nonnative 

plants. 

• Fuel breaks with more canopy and ground 

cover may be less likely to be invaded. 

• Nonnative plants are more abundant 

adjacent to older fuel breaks and in areas 

that have experienced more numerous 

fires. 

• Native species may also be promoted by 

fuel breaks, particularly native annual 

forbs and grasses. 

• Managers will have to balance 

maintaining natural disturbance regimes 

against the potential risk of nonnative 

invasion. 

Blacks Ridge DFPZ, Lassen 

National Forest 

Chapter One: Overall Results 15 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

1. Canopy and ground cover 

We found that fuel breaks had significantly less 

overstory canopy, litter cover, and duff depth than 

adjacent wildland areas. These environmental 

variables were significantly associated with 

nonnative species presence and abundance. 

Removing overstory canopy within fuel breaks may 

benefit nonnative plants by reducing competition 

with natives and changing light, nutrient, and water 

levels (Berlow and others, 2003; McKenzie and 

others, 2000; Parendes & Jones 2000). Removing 

litter and duff and disturbing soils on fuel breaks 

could provide sites for nonnative plant establishment, 

stimulate seed germination, eliminate native seed 

banks, and disrupt soil profiles (Burke & Grime 

1996; D'Antonio 1993; Hobbs & Atkins 1988). 

Removing litter and duff also may change the 

physical characteristics of the soil, such as 

temperature, moisture, and nutrient availability, in 

ways that benefit nonnative plants (Parker and others, 

1993; Reynolds and others, 2001; Shaw & Diersing 

1990). These findings suggest that fuel break 

construction and maintenance strategies that retain 

some overstory canopy and ground cover may reduce 

the establishment and widespread invasion of 

nonnative plants. 

Fuel breaks might be constructed and maintained 

to retain overstory canopy and ground cover. We 

found that fuel breaks constructed by selective 

thinning had significantly lower nonnative cover and 

higher canopy and ground cover than those 

constructed by bulldozers. Fuel breaks with on-site 

chipping or mastication of fuels had deeper layers of 

litter cover and less exposed bare ground, which may 

have reduced nonnative germination and 

establishment at these sites. Increasing the time 

between fuel break maintenance might allow ground 

and canopy cover to increase and also lower the 

probability of nonnative invasion. 

Altering the type of machinery used to construct 

and maintain fuel breaks might also influence 

patterns of invasion by nonnative plants. We found 

that even in relatively uninvaded vegetation types 

such as coniferous forests, use of bulldozers 

significantly increased the abundance of nonnative 

plants. Bulldozers have large blades specifically 

designed to remove surface soil layers, and may be 

more likely to introduce nonnative seeds into fuel 

breaks by disrupting soil seed banks and transporting 

seeds between sites. 

The presence of overstory canopy cover and 

ground cover in a fuel break may be more important 

to preventing nonnative species invasion than 

employing specific methods of fuel break 

construction. For example, we might have expected 

that fuel breaks built by hand crews would be the 

least disturbed, but instead we found that these fuel 

breaks had significantly lower overstory canopy 

cover, litter cover, and duff depth than fuel breaks 

constructed by mechanical equipment. This result is 

consistent with the emphasis of hand crews on 

exposing bare mineral soil to construct effective fire 

lines. These fuel breaks had significantly higher 

cover of nonnatives than fuel breaks constructed by 

mechanical thinning. 

Chapter One: Overall Results 16 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

2. Fuel break age and fire history 

We found that nonnative cover decreased with 

distance from the fuel break, suggesting that fuel 

breaks act as sources of nonnative plant propagules 

in the invasion of adjacent areas. Fuel breaks may act 

as nonnative seed reservoirs because they support 

higher numbers of nonnative plants, and they also 

receive external inputs of seeds through vehicles, 

equipment, or humans traveling on them (Lonsdale & 

Lane 1994; Schmidt 1989; Tyser & Worley 1992). 

Wildland areas adjacent to fuel breaks were 

more likely to be invaded by nonnative species when 

the wildlands had been subject to recurrent fires. 

Numerous studies have found that fire can promote 

nonnative plant invasion, even in fire adapted 

vegetation types (Brooks and others, 2004; Keeley 

2001). Increased fire frequencies can kill native 

plants in fire prone ecosystems because native 

species develop life histories in response to specific 

fire frequencies; these native species may be 

extirpated when fires occur more frequently (Keeley 

& Fotheringham 2003; Moreno & Oechel 1991). The 

establishment of nonnative annual grass species, the 

most common nonnatives in our study, has been 

found to alter fuel characteristics such that fires 

become less intense and more frequent in many areas 

(Brooks and others, 2004; D'Antonio & Vitousek 

1992; Keeley 2001). Reduced fire intensity on fuel 

breaks may increase the survivorship of nonnative 

seeds (Keeley & Fotheringham 2003). The 

establishment of nonnatives in fuel breaks could lead 

to feedback effects with fire that increase the 

abundance of nonnatives in fuel breaks and promote 

the invasion of surrounding areas. 

We found nonnative plant abundance on fuel 

breaks and in adjacent wildlands continued to 

increase with fuel break age. Although some authors 

have suggested that dispersal does not limit alien 

plant abundance in later stages of invasion (e.g., 

Wiser and others, 1998), we found that fuel breaks 

may continue to provide inputs of alien propagules 

even after periods of 30 years. Giessow (1997) found 

a similar pattern in fuel breaks 80 years of age and 

older in coastal scrub habitats in southern California. 

Post-fire regeneration along the 

Rouse Ridge fuel break, 

San Bernardino National Forest 

Chapter One: Overall Results 17 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

3. Native species 

Fuel management treatments may also promote 

native plants, particularly native annual forbs. Other 

studies have found that the native annual forbs 

respond positively to disturbances such as grazing 

(Hayes & Holl 2003) and fire (Safford & Harrison 

2004). Annual plants may be better able to colonize 

disturbed areas because of their short life cycles and 

dormant seed banks. Six of the twelve native species 

that were more abundant on fuel breaks than in 

adjacent wildlands were in the Fabaceae family, 

which is known to have long-lived seed banks (Auld 

1996; Holmes & Newton 2004). 

We found that native and nonnative diversity 

were significantly correlated. Although it has been 

suggested that native diversity should be negatively 

related to nonnative diversity because diverse native 

plant communities may be more resistant to invasion 

by nonnative plant species, our results support the 

idea that natives and nonnatives respond similarly to 

habitat conditions such as heterogeneity, resource 

availability, and disturbance history (Levine & 

D'Antonio 1999; Stohlgren and others, 2003). 

Disturbance related variables, including fire 

number, overstory canopy cover, and duff depth 

significantly influenced both native and nonnative 

plant species at our study sites, and both native and 

nonnative diversity was most strongly negatively 

related to the depth of the duff layer. The depth of 

duff and litter layers have been found to be important 

indicators of plant species richness, probably by 

controlling plant establishment and germination 

(Battles and others, 2001; Facelli 1991; Hayes & 

Holl 2003; Xiong & Nilsson 1999). Our data suggest 

that deep duff layers suppress both native and 

nonnative plant species. 

Although many native trees and shrubs were 

more common outside of fuel breaks, there were 

native trees and shrubs that were either more 

common in fuel breaks, or equally common within 

fuel breaks and outside of fuel breaks. This may 

reflect selective application of treatments to avoid 

specific tree and shrub species of interest, such as 

Ponderosa pines (Pinus ponderosa). Some shrub 

species may have adaptations such as stump 

sprouting that would allow them to persist in treated 

areas. Pine and other tree seedlings may be able to 

utilize germination sites and high light availability 

created in fuel breaks. 

Many native species, particularly annual forbs, 

require periodic disturbance to persist in natural 

ecosystems. The best management regime for 

maintaining native plant diversity is likely one that 

restores natural disturbance processes of the 

frequency, intensity, and duration with which native 

species evolved. 

It is important to emphasize that the relative 

abundance of nonnative species was significantly 

higher on fuel breaks, indicating that native plants 

represented a smaller proportion of the total plant 

community on fuel breaks. Land managers may have 

to weigh the benefits of maintaining natural 

disturbance regimes to restore some ecosystem 

processes against the potential risks of promoting 

nonnative invasives. 

Chapter One: Overall Results 18 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

This study was observational, and there were 

many environmental and anthropogenic factors that 

we could not control for. Research using controlled, 

replicated experiments will be necessary to fully 

understand the mechanisms that influence nonnative 

plant establishment within pre-fire fuel manipulation 

projects and invasion into adjacent wildland areas. 

Here we have identified potentially important 

variables influencing patterns of nonnative 

abundance with respect to fuel breaks and suggested 

ways that the probability of nonnative invasion might 

be minimized. If these methods are strategically 

implemented as part of a long-term fuel reduction 

program, it may be possible to both achieve fuel 

management goals and reduce the probability of 

nonnative plant invasion on fuel breaks and in 

surrounding wildland areas. 

Rouse Ridge fuel break, San Bernardino 

National Forest 

 

Chapter One: Overall Results 19 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Chapter Two: Vegetation Type Differences 

INTRODUCTION 

We examined fuel breaks in a number of 

different plant communities throughout the California 

floristic province. Each study site was dominated by 

different plant species, which we classified into four 

major vegetation types; mixed coniferous forest, 

mixed oak woodland, chaparral, and coastal scrub 

communities. 

We examined fuel breaks in a number of 

different plant communities throughout the California 

floristic province. Each study site was dominated by 

different plant species, which we classified into four 

major vegetation types; mixed coniferous forest, 

mixed oak woodland, chaparral, and coastal scrub 

communities. 

We found that a number of variables differed 

significantly among these four vegetation types. For 

example, the relative cover of nonnative plants was 

lowest in coniferous forests, greater in oak 

woodlands, even higher in chaparral vegetation 

types, and the highest in coastal scrub vegetation 

types (Figure 2-1). 

We found that a number of variables differed 

significantly among these four vegetation types. For 

example, the relative cover of nonnative plants was 

lowest in coniferous forests, greater in oak 

woodlands, even higher in chaparral vegetation 

types, and the highest in coastal scrub vegetation 

types (Figure 2-1). 

Nonnative species richness was also 

significantly different among vegetation types. 

Coniferous forests had the lowest numbers of 

nonnative species of any vegetation type, followed 

by oak woodlands and chaparral. Coastal scrub 

vegetation types had the highest nonnative species 

richness of any vegetation type we studied 

(Figure 2-2). 

Nonnative species richness was also 

significantly different among vegetation types. 

Coniferous forests had the lowest numbers of 

nonnative species of any vegetation type, followed 

by oak woodlands and chaparral. Coastal scrub 

vegetation types had the highest nonnative species 

richness of any vegetation type we studied 

(Figure 2-2). 

Nonnative Species Richness

by Vegetation Type

Conifer Forest Oak Woodland Chaparral Coastal Scrub

Number of Species/m2 0

1

2

3

4

5

Figure 2-2. ANOVA found significant 

differences in nonnative species richness 

among vegetation types (N=1543,df=3, 1539, 

F = 124.726, P<0.001). 

Error bars represent + 1 SE. 

Nonnative Cover

by Vegetation Type

Conifer Forest Oak Woodland Chaparral Coastal Scrub

Relative Nonnative Cover (%) 0

10

20

30

40

50

60

Figure 2-1. ANOVA indicated relative nonnative 

cover differed significantly among vegetation 

types (N=1543, df=3, 1539, F=150.7, P<0.001). 

Error bars represent + 1 SE. 

Chapter Two: Vegetation Type Differences 20 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Many environmental variables also varied 

among vegetation types. For example, the depth of 

the duff layer was highest in coniferous forests, 

lower in oak woodlands and chaparral vegetation 

types, and duff depth was lowest in coastal scrub 

vegetation types (Figure 2-3). 

 

Other environmental variables such as overstory 

canopy cover were significantly different in some 

vegetation types but not in others (Figure 2-4). 

Canopy cover was similarly high in coniferous forest 

and oak woodland plant communities, but lower in 

chaparral and coastal scrub vegetation types. 

Canopy Cover by Vegetation Type

Conifer Forest Oak Woodland Chaparral Coastal Scrub

Percentage

0

20

40

60

80

Figure 2-4. ANOVA on canopy cover found 

significant differences among vegetation types 

(N=1502, df=3, 1498, F=60.203, P<0.001). 

Error bars represent + 1 SE. 

Duff Depth

by Vegetation Type

Conifer Forest Oak Woodland Chaparral Coastal Scrub

Centimeters

0

5

10

15

20

25

30

Figure 2-3. ANOVA found significant 

differences in duff depth among vegetation 

types (N=1498, df=3, 1493, F=31.955, P<0.001). 

Error bars represent + 1 SE. 

Relative nonnative cover was much higher 

on fuel breaks in all vegetation types 

(Figure 2-5). 

 

Figure 2-5. ANOVA determined that relative 

nonnative cover was significantly higher ON fuel 

breaks compared with OFF fuel breaks in all 

vegetation types (interaction effect: N=1543, df=3, 

1535, F = 27.699, P<0.001). 

Error bars represent + 1 SE. 

Chapter Two: Vegetation Type Differences 21 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

 Environmental variables that were significantly 

related to nonnative cover were associated with fuel 

breaks in each vegetation type. For example, duff 

depths were significantly higher outside of fuel 

breaks in each vegetation type (Figure 2-6). Plots on 

fuel breaks had significantly lower canopy cover in 

each vegetation type as well (Figure 2-7). 

 These general results suggest that different 

vegetation types vary in abundance of nonnative 

plants, as well as environmental variables such as 

canopy and ground cover. However, despite these 

differences, the general pattern we found of increased 

nonnative abundance on fuel breaks, and associated 

patterns of decreased canopy cover and duff depth 

were evident within each vegetation type. 

Figure 2-6. ANOVA found that duff depth was 

lower on fuel breaks within each vegetation type 

(interaction effect of vegetation type and position 

on or off fuel break: N=1498, df=3, 1490, F=29.72, 

P<0.001). Error bars represent + 1 SE. 

These results suggest that nonnative species may 

respond to fuel breaks in similar ways across 

vegetation types. Patterns of nonnative abundance 

within individual vegetation types are discussed in 

more detail below. 

Canopy Cover On vs. Off Fuel Breaks

by Vegetation Type

Conifer Oak Woodland Chaparral Coastal Scrub

Percentage

0

20

40

60

80

100

On Fuel Breaks

Off Fuel Breaks

Figure 2-7. ANOVA found canopy cover was 

significantly lower on fuel breaks in each 

vegetation type (interaction effect of vegetation 

type and position on or off fuel break N=1502, 

df=3, 1494, F=19.116, P<0.001). Error bars 

represent + 1 SE. 

Chapter Two: Vegetation Type Differences 22 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

CONIFEROUS FOREST 

There were six fuel breaks which contained 

mixed coniferous forest vegetation types, typically 

dominated by Ponderosa pine (Pinus ponderosa), 

Jeffrey pine (Pinus jeffreyi), incense cedar 

(Calocedrus decurrens) and white fir (Abies 

concolor). 

Coniferous forest sites included the McKenzie 

Ridge fuel break in the Sequoia National Forest, the 

Pilot Grove fuel break in the Mendocino National 

Forest, the Fallen Leaf Lake fuel break in the Lake 

Tahoe Basin Management Area, portions of the Palos 

Ranches fuel break in the Sequoia National Forest, 

the Antelope Border defensible fuel profile zone in 

the Plumas National Forest, and the Blacks Ridge 

defensible fuel profile zone in the Lassen National 

Forest. 

Our study sites within mixed coniferous forest 

vegetation types ranged in elevation from 1194 to 

2017 m, and were constructed by mechanical 

thinning, hand crews, and bulldozers. These sites 

ranged in age from 1 to 42 years, and had 

experienced between 0 and 2 fires. At the time of our 

study, most of these sites were being grazed, and had 

been subject to logging in the past 50 years. 

Coniferous forests had the lowest relative 

nonnative cover of the four vegetation types we 

identified (mean = 4.0% of total plant cover). This 

vegetation type was also characterized by 

significantly greater duff depths than other vegetation 

types, while canopy cover and litter depth were 

equivalent to oak woodlands, but higher than 

chaparral and coastal scrub habitats. We identified a 

total of 185 plant species in our 1m2

 plots in 

coniferous forest sites. 

We found that fuel break construction method 

was significantly associated with relative nonnative 

cover in coniferous forests. Fuel breaks that had been 

constructed by bulldozers had significantly higher 

relative nonnative cover than sites constructed by 

other means. However, in coniferous forests, 

mechanically constructed fuel breaks and those 

constructed by hand crews had statistically similar 

cover of nonnative species (Figure 2-8). 

Coniferous forest vegetation type 

in the Plumas National Forest 

Fuel Break Construction Method 

in Conifer Forest

Bulldozer Hand Crew Mechanical

Relative Nonnative Cover (%)

0

5

10

15

20

25

Figure 2-8. ANOVA found significant 

differences in relative nonnative cover among 

different construction methods in coniferous 

forests (N=39, df=2, 35, F=5.30, P=0.01). Error 

bars represent + 1 SE. 

Chapter Two: Vegetation Type Differences 23 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

 Coniferous forest vegetation types were the only 

plant communities where there was not a significant 

decline in relative nonnative cover outside of the fuel 

break when only unburned sites were considered. 

However, this pattern was significantly altered at 

sites that had experienced one or two fires (Figure 2-

9). Conversely, areas that had experienced one or two 

fires had a significant increase of relative nonnative 

abundance, particularly in plots located closer to fuel 

breaks. 

 

Fire Number and Plot Distance

in Coniferous Forests

Distance from treated area (m)

0 10 20 30 40 50

Relative Nonnative Cover (%) 0

5

10

15

20

No Fires

One Fire

Two Fires

Figure 2-9. ANOVA found a significant interaction 

effect between fire number and distance from the 

fuel break on relative nonnative cover in 

coniferous forests (N=339, df=12, 325, F=3.789, P

<0.001). Error bars represent 1 SE. 

Chapter Two: Vegetation Type Differences 24 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

MIXED OAK WOODLANDS 

We sampled nine fuel breaks that included 

mixed oak woodland vegetation types. These sites 

were typically dominated by black oak (Quercus 

kelloggii), canyon live oak (Quercus chrysolepis), 

and tan oak (Lithocarpus densiflorus) species. 

Fuel breaks within oak woodland plant 

communities included the Shepard Saddle and 

Lookout Point fire lines in Sequoia and Kings 

Canyon National Parks, the Oregon fire line in the 

Shasta Trinity National Forest, the Gasquet fuel 

break in the Six Rivers National Forest, the Cascadel 

Point and Burrough Mountain fuel breaks in the 

Sierra National Forest, the Shasta Divide fuel break 

in Whiskeytown National Recreation Area, the Pilot 

Grove fuel break in the Mendocino National Forest, 

and the Sierra Pelona fuel break in the Angeles 

National Forest. 

Fuel breaks in oak woodland vegetation types 

spanned a wide range of elevations, including sites 

from 200 to 1400 m. They included fuel breaks 

constructed by mechanical thinning, hand crews, and 

bulldozers, and ranged in age from 1 to 82 years. 

Oak woodland sites had experienced between 0 and 3 

fires. At the time of our study, about half (6 of 13 

transects) of these sites were being grazed. 

Oak woodlands had the second lowest relative 

nonnative cover of the four vegetation types we 

identified (mean = 25.0% of total plant cover), as 

well as the second lowest nonnative species richness. 

We identified a total of 313 plant species in our oak 

woodland sites. Oak woodlands had significantly 

higher species richness (at the 1m2 scale) than other 

vegetation types. Oak woodlands were comparable to 

coniferous forests in canopy cover, litter cover, and 

bare ground, but had significantly lower duff depth 

and litter depth than coniferous forest vegetation 

types. Oak woodlands had higher duff depth, litter 

depth, canopy cover, and litter cover than either 

chaparral or coastal scrub vegetation types. 

Oak woodland vegetation type 

in the Shasta Trinity National Forest 

Chapter Two: Vegetation Type Differences 25 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

We found that fuel break construction method 

was significantly associated with relative nonnative 

cover in oak woodland vegetation types (Figure 2-

10). Fuel breaks that had been constructed by 

selective thinning and mastication in oak woodlands 

had significantly lower relative nonnative cover than 

fuel breaks constructed by other means. 

Relative nonnative cover in oak woodland 

vegetation types was significantly higher in areas that 

had experienced more numerous fires, and in plots 

that were closer to fuel breaks (Figure 2-11). 

Relative nonnative cover also was significantly 

greater in oak woodland vegetation types that had 

been grazed, and this effect was significant at 

distances up to 40 m from the fuel break 

(Figure 2-12). 

Fuel Break Construction Method 

in Oak Woodland

Bulldozer Hand Crew Mechanical

Relative Nonnative Cover (%) 0

10

20

30

40

Fire Number and Plot Distance

in Oak Woodlands

Distance from treated area (m)

0 10 20 30 40 50

Relative Nonnative Cover (%)

0

10

20

30

40

50

No Fires

One Fire

Two Fires

Three Fires

Figure 2-11. ANOVA found significant effects of 

fire number and distance from the fuel treatment 

on relative nonnative cover in oak woodlands 

(N=385, number of fires, df=3, 365, F=11.827, 

P<0.001; distance, df=4, 365, F=6.588, P<0.001). 

Error bars represent + 1 SE. 

Figure 2-10. ANOVA found significant effect of 

construction method on relative nonnative cover in 

oak woodlands (N=57, df=2, 54, F=20.432, P<0.001). 

Error bars represent + 1 SE. 

Distance from fuel break (m)

0 10 20 30 40 50

Relative Nonnative Cover (%)

0

5

10

15

20

25

30

35

40

Grazed

Ungrazed

Figure 2-12. ANOVA found significant effect of 

grazing and distance from the fuel treatment on 

relative nonnative cover in oak woodlands (N=385, 

grazing effect: df=1, 375, F=29.991, P<0.001, plot 

distance effect: df=4, 375, F=13.018, P<0.001). Error 

bars represent + 1 SE. 

Chapter Two: Vegetation Type Differences 26 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

CHAPARRAL 

We studied 15 fuel breaks that contained 

chaparral vegetation types, typically dominated by 

chamise (Adenostoma fasciculatum), manzanita 

species (Arctostaphylos spp.), and scrub oak species 

(Quercus spp.). 

Fuel breaks located in chaparral type plant 

communities included the Shepard Saddle, Lewis 

Creek, and Lookout Point fire lines in Sequoia and 

Kings Canyon National Park, the Oregon fire line in 

the Shasta Trinity National Forest, the Tower fire line 

and Whiskey Creek fuel breaks in the Whiskeytown 

National Recreation Area, the Pilot Grove fuel break 

in the Mendocino National Forest, the Sierra Pelona 

fuel break in the Angeles National Forest, the Calf 

Canyon Fuel Break administered by the Bakersfield 

District of the Bureau of Land Management, the 

Palos Ranches fuel break in the Sequoia National 

Forest, the Etz Meloy and Zuma Ridge fuel breaks in 

the Santa Monica National Recreation Area, the 

Sierra Madre Ridge in the Los Padres National 

Forest, the Rouse Ridge fuel break in the San 

Bernardino National Forest, and the Aguanga fuel 

break in the Cleveland National Forest. 

Chaparral study sites ranged in elevation from 

300 to 1600 m, and included fuel breaks constructed 

by mechanical thinning, hand crews, and bulldozers. 

These sites ranged in age from 1 to 51 years, and 

included areas that had experienced between 0 and 4 

fires. About half (17 of 32 transects) of these sites are 

currently being grazed. 

Chaparral had the second highest relative 

nonnative cover of the four general vegetation types 

we identified (mean=39.0% of total plant cover). 

Chaparral sites had significantly more exposed bare 

ground than any other vegetation type, and less 

ground cover than oak woodland or coniferous forest 

vegetation types. However, ground and canopy cover 

were significantly higher in chaparral than in coastal 

scrub vegetation types. We identified a total of 438 

plant species in our 1m2

 plots in chaparral vegetation 

types. Species richness at the 1m2

 scale was 

statistically similar to coniferous forest and coastal 

scrub vegetation types. 

Chaparral vegetation type in the Mendocino 

National Forest 

Chapter Two: Vegetation Type Differences 27 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

We found that fuel break construction method 

was significantly associated with differences in 

relative nonnative cover in chaparral vegetation types 

(Figure 2-13). Fuel breaks constructed by bulldozers 

had significantly higher relative nonnative cover than 

fuel breaks constructed by other means. 

Wildland areas adjacent to fuel breaks in 

chaparral vegetation types were more likely to have 

higher relative nonnative cover when they had 

experienced grazing or recurrent fires (Figures 2-14 

and 2-15). There was a significant interaction effect 

between fire number and distance from the fuel 

break, while there was no interaction between 

grazing and distance from the fuel break. Distance to 

the fuel break was an important factor at all chaparral 

sites, and plots closer to fuel breaks had higher 

relative nonnative cover, even when those sites had 

experienced multiple fires or had been grazed. 

Figure 2-14. The effect of fire number and distance from 

the fuel treatment on relative nonnative cover in 

chaparral. ANOVA found significant interaction effect 

(N=720, df =12, 700, F=13.030, P <0.00), main effect of fire 

number (N=720, df=2, 700, F=15.365, P<0.001) and main 

effect of plot distance from fuel break (N=720, df=4, 700, 

F=48.314, P<0.001). 

Error bars represent + 1 SE. 

Fuel Break Construction Method 

in Chaparral

Bulldozer Hand Crew Mechanical

Relative Nonnative Cover (%) 0

10

20

30

40

50

60

Figure 2-13. The effect of construction method on 

relative nonnative cover in chaparral vegetation types. 

ANOVA found significant differences (N=108, df=2, 105, 

F=58.294, P<0.001). Error bars represent + 1 SE. 

Grazing and Plot Distance 

in Chaparral

Distance from fuel break (m )

0 10 20 30 40 50

Relative Nonnative Cover (%)

0

1 0

2 0

3 0

4 0

5 0

Grazed

Ungrazed

Figure 2-15. The effect of grazing and distance 

from the fuel treatment on relative nonnative cover 

in chaparral. ANOVA found significant main effects 

(grazing: N=719, df=1, 710, F=11.723, P<0.001, plot 

distance: N=719, df=4, 710, F=52.187, P<0.001). Error 

bars represent + 1 SE. 

Chapter Two: Vegetation Type Differences 28 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

COASTAL SCRUB

There were only three fuel breaks that contained 

coastal scrub vegetation types, typically dominated 

by California sage (Artemisia californica), black sage 

(Salvia mellifera) and California buckwheat 

(Eriogonum fasciculatum). 

Coastal scrub plant communities were found at 

the Oso Ridge fuel break in Casper’s Wilderness 

Park, Zuma Ridge fuel break in the Santa Monica 

National Recreation Area, and on a small portion of 

the Rouse Ridge fuel break in the San Bernardino 

National Forest. 

Coastal scrub study sites were limited in 

elevation to below 700 m. All of these fuel breaks 

were constructed by bulldozers, and ranged in age 

from 19 to 55 years. We did not sample any 

unburned areas in coastal scrub vegetation types; all 

fuel breaks sampled had experienced between 1 and 

5 fires. 

Coastal scrub vegetation types had the highest 

relative nonnative cover of the four vegetation types 

we identified (mean=68.3% of total plant cover). 

Coastal scrub sites had significantly less duff depth, 

litter depth, canopy cover, and litter cover than other 

vegetation types. 

We identified a total of 108 plant species in our 

1m2

 plots at the three coastal scrub study sites we 

sampled in this study. Native species richness at the 

1m2

 scale was statistically similar to coniferous forest 

and chaparral vegetation types. While many of the 

coastal scrub sites we sampled had experienced one 

or more fires during the past 50 years, very few of 

these sites were grazed. Therefore, we were not able 

to evaluate the effect of grazing within coastal scrub 

vegetation types; we found that fire number had no 

effect on relative nonnative cover. However, relative 

nonnative cover declined significantly with distance 

from the fuel breaks in coastal scrub vegetation types 

(Figure 2-16). 

Coastal scrub vegetation type in 

Casper’s Wilderness Park 

Figure 2-16. ANOVA found significant effect of 

distance from the fuel treatment on relative 

nonnative cover in coastal scrub vegetation types 

(N=95, df=4, 90, F=5.695, P <0.001). Error bars 

represent + 1 SE. 

Chapter Two: Vegetation Type Differences 29 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

DISCUSSION 

We found significant differences in relative 

nonnative cover and nonnative species richness 

among vegetation types. Vegetation type has been 

found to be among the most important factors 

influencing nonnative plant abundance, either 

because of the life history characteristics of the 

dominant plant community, or because of the 

association of different vegetation types with specific 

resources, such as soil nutrients or water availability 

(Aragon & Morales 2003; D'Antonio 1993; Larson 

and others, 2001; Mack 1989; Stohlgren and others, 

2001). 

The effect of vegetation type on nonnative 

abundance may be related to elevation. Our study 

sites in coniferous forests occurred at relatively high 

elevations (above 1000 m) and had the lowest 

relative cover of nonnatives. Conversely, the coastal 

scrub plant communities we sampled were located at 

relatively low elevations (generally below 300 m), 

and had the highest relative nonnative cover. 

Elevation has been found to be strongly negatively 

correlated with nonnative plant invasion in 

California, and most nonnative plants are not adapted 

to the climate at high elevations (Keeley and others, 

2003; Schwartz and others, 1996). 

The chaparral and oak woodland vegetation 

types we sampled in this study spanned a wide range 

of elevations (400–1600 m). We found that chaparral 

plant communities had higher nonnative plant cover 

and species richness than oak woodland sites. The 

difference in nonnative abundance between oak 

woodland and chaparral vegetation types may reflect 

differences in land use and disturbance history 

(Keeley 2000). For example, the chaparral sites we 

sampled had experienced more frequent fires than 

oak woodland sites. Chaparral sites also had 

significantly less duff depth and canopy cover, and 

more exposed bare ground than oak woodland sites. 

Chapter Two: Vegetation Type Differences 30 
The R

C

ole of Fuel Breaks in the Invasion of Nonnative Plants 

hapter Two: Vegetation Type Differences 31 

Despite the strong influence of vegetation type 

on nonnative abundance and species richness, we 

found that a number of variables, including fuel 

break construction method, distance to the fuel break, 

fire number, and grazing still had significant effects 

on nonnative abundance when evaluated within 

individual vegetation types separately. This suggests 

that although important, vegetation type alone does 

not determine the abundance of nonnatives. We 

found that fuel breaks appear to promote nonnative 

abundance in all vegetation types, and this pattern 

may be compounded by more numerous fires or by 

grazing. 

Text Box 2-1: Summary of Findings 

Vegetation Type Differences

• Nonnative abundance and species richness 

differed among vegetation types. 

• Elevation, fire, grazing, and disturbance 

history may have explained some differences 

among vegetation types. 

• Nonnatives were significantly more common 

on fuel breaks in all vegetation types; 

• Environmental variables such as canopy 

cover and duff depth were significantly lower 

outside of fuel treatments in all vegetation 

types. 

• Fuel break construction method altered 

nonnative abundance in all vegetation types 

where it could be examined. 

• Fire and grazing were associated with higher 

abundance of nonnatives in most vegetation 

types. 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Chapter Three: Site Specific Results INTRODUCTION 

We sampled 24 different fuel breaks in this study, representing a wide range of vegetation types, construction and maintenance reg

histories. Table 3-1 provides a brief overview of the different fuel breaks sampled in this study. 

Table 3-1. Selected characteristics of fuel breaks included in this study ordered from lowest to highest elevation. [A range is shown for some variables because there were multiple transects on each fuel break, and individual transects often varied in 

history, construction method, and vegetation type. Abbreviations are: NF = National Forest; SMNRA = Santa Monica National Recrea

Whiskeytown National Recreation Area; CDF = California Department of Forestry and Fire Protection; BLM = Bureau of Land ManagKings Canyon National Parks, LTBMU= Lake Tahoe Basin Management Unit; MT= mechanical thinning, BD= bulldozer, HC= hand OW=Oak woodland, CP=Chaparral, CS=Coastal Scrub] [A range is shown for some variables because there were multiple transects on each fuel break, and individual transects often varied in 

history, construction method, and vegetation type. Abbreviations are: NF = National Forest; SMNRA = Santa Monica National Recrea

Whiskeytown National Recreation Area; CDF = California Department of Forestry and Fire Protection; BLM = Bureau of Land ManagKings Canyon National Parks, LTBMU= Lake Tahoe Basin Management Unit; MT= mechanical thinning, BD= bulldozer, HC= hand OW=Oak woodland, CP=Chaparral, CS=Coastal Scrub] 

 Site 

Code Site Name Managing Agency Elev. (m) Year Constructed Number of Fires Construction Method Vegetation Type Gr

L

GA Gasquet Six Rivers NF 202 1995 0-1 MT OW None ZU Zuma Ridge SMNRA 244 1952 3-5 BD CS/CP None CW Oso Ridge Casper’s Park 273 1963 2-3 BD CS None WC Whiskey Creek WNRA 390 2001-2003 0 MT CP None TO Tower WNRA 447 1980 1-4 BD CP GrazCC Calf Canyon CDF and BLM 474 1965-2002 1-3 BD CP None SD Shasta Divide WNRA 492 1985 0 BD OW None EM Etz Meloy SMNRA 652 1957 2-3 BD CP None OR Oregon Shasta-Trinity NF 922 2001 0 BD OW None SS Shepard Saddle SEKI 983 1960 2-3 BD CP/OW Graz

32 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

33 

 Site 

Code Site Name Managing Agency Elev. (m) Year Constructed Number of Fires Construction Method Vegetation Type Gr

L

BM Burrough Mtn Sierra NF 1109 1935 1 HC OW GrazAG Aguanga Cleveland NF 1189 1974 0-2 HC CP GrazPG Pilot Grove Mendocino NF 1194 1960 1-2 BD CP/OW/CF GrazCP Cascadel Point Sierra NF 1294 1920 2-3 HC OW LoggRR Rouse Ridge San Bernardino NF 1298 1984 1-2 BD CS/CP GrazSP Sierra Pelona Angeles NF 1302 1960 1-2 BD OW/CP GrazLC Lewis Creek SEKI 1461 1981 0-2 HC CP None BR Blacks Ridge Lassen NF 1533 2002-2003 0-1 MT CF Graz

Logg

SM Sierra Madre Los Padres NF 1535 1962-1966 0-2 BD CP GrazPR Palos Ranches BLM 1544 1977-2001 0 HC/BD CF/CP LoggLP Lookout Point SEKI 1579 1997 0 HC OW/CP None AB Antelope-Border Plumas NF 1590 2001 0 MT CF Graz

Logg

MC McKenzie Ridge Sequoia NF 1646 1960 0-2 MT CF Graz

Logg

FL Fallen Leaf LTBMU 1899 1995 0 MT CF Graz
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Many of the variables we measured differed greatly among fuel breaks. For example, mean relative nonnative 

cover ranged from 0 to over 50% (Figure 3-1). Native species richness also varied among sites ranging from 

between almost 2 to over 6 species per m2

 (Figure 3-2).

We found that duff depth and canopy cover together explained over 70% of the variation in relative nonnative 

cover (R2

=0.721, P<0.001). These environmental variables also varied considerably among sites. Duff depth ranged 

from less than 2 cm to over 50 cm (Figure 3-3). Overstory canopy cover ranged from 5 to 88% as shown in Figure 

3-4. 

Site Variation in Canopy Cover

Site Name Abbreviation

CW

RRCC

PG

ZU

AG

TO

SP

SM

BM

EM

SS

PR

LC

BR

OR

AL

CP

LP

WC

SD

MC

GA

FL

Overstory Canopy (%)

0

20

40

60

80

100

Figure 3-4. Variation in canopy cover among fuel 

breaks. N=1543. Error bars indicate + 1 SE. 

Site Variation in Duff Depth

Site Name Abbreviation

CCTOPG

CWORSS

RR

SM

WCBMLC

EM

SDSPPR

AG

MC

BRZU

CP

GALP

ALFL

Duff Depth (cm)

0

10

20

30

40

50

60

70

Figure 3-3. Variation in duff depth among fuel breaks. 

N=1543. Error bars indicate + 1 SE.

Site Variation in Native Species Richness

Site Name Abbreviation

FL

ZU

TO

LP

LC

EM

AL

RR

SM

PG

SD

WC

SP

CW

MC

OR

SS

PR

BR

CP

GA

BM

CC

AG

Number Native Species/m2

0

2

4

6

8

Figure 3-2. Variation in mean native species richness 

among fuel breaks. N=1543. Error bars indicate + 1 SE. 

Site Variation in Nonnative Plant Cover

FL

GA

BR

AL

MC

LP

WC

AG

SD

LC

PG

PRCP

ORSM

BM

EM

CC

SP

RR

ZU

TO

SS

CW

Percent of Total Plant Cover

0

20

40

60

80

Site Name Abbreviation

Figure 3-1. Variation in relative nonnative plant 

abundance between fuel breaks. N=1543. The mean 

value of 18% is shown as a dashed line. Error bars 

indicate + 1 SE. 

Chapter Three: Site Specific Results 34 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

We have described some of the factors that may 

explain differences in relative nonnative cover 

among fuel breaks, including construction methods 

and environmental factors (Chapter One), and the 

potential role of vegetation type (Chapter Two). In 

this chapter we focus on patterns observed at 

individual sites. A description of each fuel break is 

given separately below, arranged from lowest to 

highest elevation. Only a general description of the 

location of each study site is provided here. See 

Figure 1-1 for general location of each site. More 

specific information, including GIS shapefiles of plot 

locations, is available upon request. 

This fuel break near Lake Tahoe had the 

highest duff depth and overstory 

canopy of any of our sites. 

Chapter Three: Site Specific Results 35 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Gasquet Shaded Fuel Break, Six Rivers 

National Forest 

The Gasquet shaded fuel break is located in the 

Six Rivers National Forest near the town of Gasquet 

in the Smith River National Recreation Area of Del 

Norte County. This shaded fuel break was 

constructed in 1995 by mechanical thinning and 

prescribed burning. Only two transects at the Gasquet 

fuel break contained nonnative plants, and these were 

located within the perimeter of the 1996 Panther Fire. 

The Gasquet shaded fuel break is located in a tan 

oak and Douglas fir (Pseudotsuga menziesii) forest. 

Other dominant species include incense cedar 

(Calocedrus decurrens) and black huckleberry 

(Vaccinium ovatum). Our plots on the Gasquet 

shaded fuel break were located predominately on the 

Jayel soil series. These are moderately deep, well 

drained soils formed in material weathered from 

serpentinized peridotite. 

We identified 60 plant species in our 1m2

 plots at 

the Gasquet shaded fuel break. Only 3 of these 

species were nonnative. The most common nonnative 

plant in our plots was Scotch broom (Cytisus 

scoparius). 

Nonnative plants occupied 12% of the total plant 

cover at the Gasquet shaded fuel break in plots where 

they occurred, which represented only 10% of all 

plots. The mean relative nonnative cover (1%) on the 

Gasquet shaded fuel break was much lower than the 

mean (18%) of the 24 sites in our study. Relative 

nonnative plant cover was not significantly higher on 

the Gasquet fuel break than in the adjacent wildland 

off of the fuel break (Figure 3-5). 

Gasquet Shaded Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%) 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Gasquet shaded fuel break, 

Six Rivers National Forest

Figure 3-5. ANOVA did not find significant 

differences in relative nonnative abundance on 

the Gasquet fuel break compared to outside of the 

fuel break. (N=62, df=1, 60, F=0.022, P=0.883). 

Error bars indicate + 1 SE. 

Chapter Three: Site Specific Results 36 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Santa Monica Mountains National 

Recreation Area 

We sampled two fuel breaks in the Santa Monica 

Mountains National Recreation Area, near the city of 

Thousand Oaks in Los Angeles County. These were 

the Zuma Ridge and Etz Meloy fuel breaks. 

The Etz Meloy fuel break was constructed in 

1957 by bulldozers, and was maintained by 

bulldozers on an annual or biennual rotation by the 

Los Angeles County Fire Department until 1985. 

Portions of this fuel break were also maintained by 

goat grazing on an experimental basis. The Etz 

Meloy fuel break is not currently maintained. The 

area around the fuel break burned in the 1993 Green 

Meadows fire. 

The Etz Meloy fuel break is located in a 

chaparral habitat, dominated by black sage (Salvia 

mellifera), bigpod ceanothus (Ceanothus 

megacarpus), and redshank (Adenostema 

sparsifolium). Our plots were located primarily on 

soils from the Hambright series. These are shallow, 

well drained soils formed in material weathered from 

basic igneous rocks. 

We found 49 species in our 1m2

 plots at the Etz 

Meloy fuel break, including 13 nonnatives. The most 

common nonnative species in our plots on the Etz 

Meloy fuel break was red brome (Bromus 

madritensis). 

The Zuma Ridge fuel break was constructed in 

1952 by bulldozers. The fuel break was maintained 

by bulldozers on an annual or biennial rotation by the 

Los Angeles County Fire Department until 1985. Our 

plots along the Zuma Ridge fuel break were located 

in areas that have burned up to five times between 

1935 and 1995. 

Zuma Ridge fuel break, Santa Monica 

Mountains National Recreation Area 

Etz Meloy fuel break, Santa Monica 

Mountains National Recreation Area 

The Zuma Ridge fuel break is located in a 

coastal scrub type plant community, dominated by 

bigpod ceanothus, California encelia (Encelia 

californica), and purple sage (Salvia leucophylla). 

Our plots were located primarily on sandy soils 

classified as beaches. We identified 51 plant species 

in our 1m2

 quadrats at the Zuma Ridge fuel break, 

including 17 nonnative species. The most common 

nonnative species on the Zuma Ridge fuel break was 

wild oats (Avena fatua). 

Chapter Three: Site Specific Results 37 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Nonnative plants were found in 100% of our 

1 m2

 plots on both the Etz Meloy and Zuma Ridge 

fuel breaks, while nonnatives were only present in 

22% and 45% of plots outside of these fuel breaks, 

respectively. Nonnative plant cover was significantly 

higher on both the Etz Meloy and Zuma Ridge fuel 

breaks than in the adjacent wildland. Zuma Ridge 

had higher nonnative plant cover than Etz Meloy 

both on and off the fuel break (Figure 3-6). 

Figure 3-6. ANOVA found significant differences 

between nonnative abundance on the Zuma Ridge 

and Etz Meloy fuel breaks compared with adjacent 

wildlands. (Difference between fuel breaks, N=106, 

df=1, 102, F=6.528, P=0.012; and position on or off fuel 

breaks, N=106, df=1, 102, F=103.8, P<0.001). Error 

bars represent + 1 SE. 

Chapter Three: Site Specific Results 38 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Oso Ridge Fuel Break, Casper’s 

Wilderness Park 

The Oso Ridge fuel break is located within 

Casper’s Wilderness Park, near the town of San Juan 

Capistrano, in Orange County. Casper’s Wilderness 

Park is administered by the Orange County Parks and 

Recreation Department. The Oso Ridge fuel break 

was constructed in the 1960s by bulldozers, and has 

been maintained annually. A number of fires have 

occurred in the vicinity of this fuel break, including 

the 1993 Ortega fire. 

The Oso Ridge fuel break is located in a coastal 

scrub plant community, dominated by California 

sage, black sage, and California buckwheat. Our 

plots on Oso Ridge were located primarily in the Alo 

soils series. The Alo series consist of moderately 

deep, well drained soils formed in material weathered 

from shale or sandstone on mountains. 

We identified 80 species in our 1m2

 quadrats 

along Oso Ridge, including 18 nonnative species. 

The Oso Ridge fuel break had the highest relative 

cover (51%) of nonnative plant species of the 24 sites 

in our study. The most common nonnative was black 

mustard (Brassica nigra). Nonnative species were 

found in 100% of our plots both on and off the Oso 

Ridge fuel break. Nonnative plant cover was 

significantly higher on the Oso Ridge fuel break than 

in the adjacent wildland off of the fuel break 

(Figure 3-7). 

Error!

Oso Ridge Fuel Break

Oso Ridge fuel break, 

Casper’s Wilderness Park 

0

20

40

60

80

100

Relative Nonnative Cover (%)

On Fuel Break Off Fuel Break

Figure 3-7. ANOVA found significant differences in 

relative nonnative abundance on the Oso Ridge fuel 

break compared with adjacent wildlands (N=60, df=1, 

58, F=4.812, P=0.032). Error bars represent + 1 SE. 

Chapter Three: Site Specific Results 39 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Whiskeytown National Recreation Area 

We studied three separate fuel breaks within the 

Whiskeytown National Recreation Area, located west 

of the town of Redding in Shasta County. These 

included the Tower fire line, the Shasta Divide fuel 

break, and the Whiskey Creek fuel break. 

The Tower fire line was constructed in 1980 by 

bulldozers to contain the Tower wildfire. The fire 

line has not been maintained and was seeded with 

native plants after 1980. At the time we conducted 

our sampling in 2002, the area surrounding the 

Tower fire line had experienced four separate fires 

since 1959, the most recent in 1999. This area was 

burned again during the 2004 French fire. 

The Tower fire line is located in a chaparral type 

plant community dominated by chamise 

(Adenostoma fasciculatum), interior live oak 

(Quercus wislizenii), and yerba santa (Eriodictyon 

californicum). Our plots were located primarily on 

Boomer soils series. These are very deep, well 

drained soils that formed in material weathered from 

metavolcanic rock. We identified 51 species in our 

1m2

 plots at the Tower fire line, 19 of which were 

nonnative. The most common nonnative species on 

the Tower fire line was foxtail fescue (Vulpia 

myuros). 

The Shasta Divide fuel break was constructed in 

1985 by bulldozers and prescribed burning. The fuel 

break has been periodically maintained by prescribed 

burning and hand crews. The Shasta Divide fuel 

break is located primarily in an oak woodland 

vegetation type dominated by canyon live oak 

(Quercus chrysolepis), black oak (Quercus 

kelloggii), and whiteleaf manzanita (Arctostaphylos 

viscida). We identified 56 species in our 1m2

 plots at 

the Shasta Divide fuel break, 10 of which were 

nonnative. The most common nonnative species on 

the Shasta Divide fuel break was foxtail fescue. 

Tower fire line, Whiskeytown 

National Recreation Area 

Shasta Divide fuel break, 

Whiskeytown National Recreation Area 

Chapter Three: Site Specific Results 40 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

The Whiskey Creek shaded fuel break was 

created in 2001 and 2003 by selective thinning and 

prescribed burning. Fuels were masticated and 

remain on the ground within the fuel break. The 

Whiskey Creek shaded fuel break is located in a 

chaparral habitat dominated by whiteleaf manzanita, 

black oak, and chamise. Our plots were located 

primarily on Boomer soils at this site, which are very 

deep, well drained soils that formed in material 

weathered from metavolcanic rock. 

We identified 56 species in our 1m2

 plots at the 

Whiskey Creek fuel break, 8 of which were 

nonnative. The most common nonnative species on 

the Whiskey Creek shaded fuel break was barren 

brome (Bromus sterilis). 

 

The Whiskey Creek and Shasta Divide fuel 

breaks had lower mean relative nonnative cover (7% 

and 12%, respectively) than most other sites in our 

study (18%) including the Tower fire line (39%). 

Relative nonnative cover was higher on the fuel 

break than in the adjacent wildland at all three sites 

in the Whiskeytown National Recreation Area, and 

this effect differed among fuel breaks (Figure 3-8). 

Whiskeytown Study Sites

Site Name 

Creek Divide Tower

Relative Nonnative Cover (%)

0

10

20

30

40

50

60

70

On Fuel Break 

Off Fuel Break 

Figure 3-8. ANOVA found a significant 

interaction between position on and off the fuel 

break and site (N=173, df=2, 167, F=11.743, 

P<0.001). Error bars represent + 1 SE. 

Whiskey Creek fuel break, 

 Whiskeytown National Recreation Area 

Chapter Three: Site Specific Results 41 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Calf Canyon Fuel Break, Bakersfield BLM 

and San Luis Obispo CDF 

The Calf Canyon fuel break is located near the 

town of Santa Margarita in San Luis Obispo County. 

This fuel break is managed by the Bakersfield 

District of the Bureau of Land Management (BLM), 

and maintained by the San Luis Obispo Unit of the 

California Department of Fire and Forestry (CDF). 

This fuel break was first constructed in 1965 using 

bulldozers, and has been maintained periodically by 

hand crews. The areas adjacent to the fuel break 

burned during the 2002 Highway 58 fire. 

Calf Canyon is a chamise dominated chaparral 

habitat. Other shrub species we observed at Calf 

Canyon included wedgeleaf ceanothus (Ceanothus 

cuneatus) and black sage. Our plots at Calf Canyon 

were located primarily on Cieneba soils. These are 

very shallow, somewhat excessively drained soils 

that formed in material weathered from granitic rock. 

Calf Canyon had the second highest species 

richness of our 24 study sites; we identified 97 

species in our 1m2

 plots at Calf Canyon, 21 of which 

were nonnative. The most common nonnative species 

was cheatgrass (Bromus tectorum). 

The Calf Canyon fuel break had higher mean 

relative nonnative cover (30%) than the mean (18%) 

of 24 sites in our study. Nonnative plants were found 

in 100% of our 1m2

 plots on Calf Canyon fuel break, 

and in 76% of plots outside the fuel break. Nonnative 

plant cover was significantly higher on the Calf 

Canyon fuel break than in the adjacent wildland 

(Figure 3-9). 

Calf Canyon Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative 

Cover (%)

0

10

20

30

40

50

60

70

Figure 3-9. ANOVA found significant differences in 

relative nonnative abundance on the Calf Canyon 

fuel break compared with adjacent wildlands 

(N=60, df=1, 58, F=55.639, P<0.001). Error bars 

represent + 1 SE. 

Calf Canyon fuel break, 

San Luis Obispo CDF and BLM. 

Chapter Three: Site Specific Results 42 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Sequoia and Kings Canyon National Park 

We studied three fuel breaks at Sequoia and 

Kings Canyon National Parks, the Shepard Saddle 

fire line, the Lookout Point fire line, and the Lewis 

Creek fire line. All of our plots in Sequoia and Kings 

Canyon were located primarily on soils of the 

Holland series. These are very deep, well drained 

soils that formed in material weathered from granitic 

rock. 

The Shepard Saddle fire line is located near 

Sequoia National Park’s southwestern boundary near 

the town of Three Rivers in Tulare County. This fire 

line was constructed in 1960 by bulldozers, and has 

not been maintained. Areas in the vicinity of this fire 

line were burned during the 1996 Kaweah fire. 

Unlike most other areas in Sequoia National Park, the 

Shepard Saddle fire line is occasionally subject to 

grazing. The Shepard Saddle fire line includes black 

oak woodlands and chaparral habitats. 

The most common nonnative species on the 

Shepard Saddle fire line was soft chess (Bromus 

hordeaceus). We observed 80 species in our 1m2

plots on Shepard Saddle, including 20 nonnative 

species. 

The Lookout Point fire line is located in the 

Mineral King area of Sequoia National Park in 

Tulare County. This fire line was constructed in 1997 

by hand crews to contain the Redwood fire. The 

Lookout Point fire line is located in oak woodland 

and chaparral habitats, dominated by canyon live oak 

(Quercus chrysolepis) and indian manzanita 

(Arctostaphylos mewukka). The most common 

nonnative species on the Lookout Point fire line was 

cheatgrass. We observed 42 species in our 1m2

 plots 

at Lookout Point, including three nonnative species. 

The Lewis Creek fire line is located in the Cedar 

Grove area of Kings Canyon National Park, Fresno 

County. This fire line was constructed in 1981 by 

hand crews, in preparation for a prescribed burn. 

Portions of the Lewis Creek fire line were burned in 

the 1980 Lewis Creek fire. The Lewis Creek fire line 

is located in a chaparral habitat, dominated by 

whiteleaf manzanita, wedgeleaf ceanothus, and 

birchleaf mountain mahogany (Cercocarpus 

betuloides). The most common nonnative plant found 

on the Lewis Creek fire line was cheatgrass. We 

observed a total of 55 species in our 1m2

 plots at 

Lewis Creek, three of which were nonnative. 

Shepard Saddle fire line in 

Sequoia National Park 

Chapter Three: Site Specific Results 43 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Shepard Saddle had much higher relative 

nonnative cover (39%) than either the Lookout Point 

or Lewis Creek fire lines (6% and 12% respectively). 

Relative nonnative plant cover was significantly 

higher on the Shepard Saddle and Lookout fire lines 

than in the adjacent wildland areas. However, there 

was no difference in relative nonnative plant cover 

on the Lewis Creek fire line compared with adjacent 

wildlands (Figure 3-10). 

SEKI Sites

Site Name

Saddle Lewis Creek Lookout 

Relative Nonnative Cover (%)

0

10

20

30

40

50

60

On Fuel Break 

Off Fuel Break 

Figure 3-10. ANOVA found significantly higher relative 

nonnative abundance on the Shepard Saddle and 

Lookout Point fire lines compared with adjacent 

wildlands, but this pattern did not emerge on the Lewis 

Creek fire line (interaction between site and position 

on fuel break, N=173, df=2, 167, F=41.113, P<0.001). 

Error bars represent + 1 SE. Significance assessed 

with Bonferroni adjusted P value of 0.015 for 3 posthoc comparisons. 

Chapter Three: Site Specific Results 44 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Sierra National Forest 

We studied two fuel breaks in the Sierra 

National Forest, the Burrough Mountain fuel break 

and the Cascadel Point fuel break. The Burrough 

Mountain fuel break is located in the Sierra National 

Forest near the town of Toll House in Fresno County. 

The Cascadel Point fuel break is located east of the 

town of North Fork in Madera County. 

The Burrough Mountain fuel break was first 

constructed in 1935, and has been maintained 

regularly through clearing and prescribed burning. 

This fuel break is located in mixed oak woodland. 

The most common tree species we observed included 

interior live oak, black oak, and California buckeye 

(Aesculus californica). Our plots on the Burrough 

Mountain fuel break were located on soils from the 

Ahwahnee, Chaix, and Holland series. These are 

deep, well to somewhat excessively well drained 

soils formed in material weathered from igneous rock 

such as granite and granodiorite. 

The Burrough Mountain fuel break had the 

highest site level species diversity of any of our study 

sites. We detected over 100 species in our 1m2

 plots 

at Burrough Mountain. Twenty of these species were 

nonnative. The most common nonnative plant we 

encountered at this site was tocalote (Centaurea 

melitensis). 

Burrough Mountain fuel break, 

Sierra National Forest 

Chapter Three: Site Specific Results 45 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

The Cascadel Point fuel break is not currently 

maintained, although prescribed burning was 

conducted in the vicinity of the fuel break in 1980. 

The Cascadel Point fuel break is located within a 

mule deer winter range habitat area and is currently 

excluded from cattle grazing. 

The fuel break is located in a mixed oak 

woodland, characterized by black oak (Quercus 

kelloggii), ponderosa pine (Pinus ponderosa), canyon 

live oak (Quercus chrysolepis), and whiteleaf 

manzanita (Arctostaphylos viscida). Our plots at 

Cascadel Point were located primarily on soils of the 

Holland series. These are very deep, well drained 

soils that formed in material weathered from granitic 

rock. 

The most common nonnative plant in our plots 

on the Cascadel Point fuel break was field hedge 

parsley (Torilis arvensis). We recorded 80 species in 

our 1m2

 plots on the Cascadel Point fuel break, 20 of 

which were nonnative. 

Cascadel Point fuel break, 

Sierra National Forest 

Chapter Three: Site Specific Results 46 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

The Burrough Mountain fuel break had higher 

mean relative nonnative cover (23%) than the mean 

(18%) of 24 sites in our study, while the Cascadel 

Point fuel break had slightly lower mean relative 

nonnative cover (17%). Nonnative plants occurred in 

100% of the plots on the Burrough Mountain fuel 

break, and 53% of plots in the adjacent wildland. 

Nonnative plants occurred in 75% of the plots on the 

Cascadel Point fuel break, and 56% of plots in the 

adjacent wildland. 

Nonnative plant cover was significantly higher 

on both the Burrough and Cascadel Point fuel breaks 

than in the adjacent wildlands, and there were also 

significant differences between fuel breaks 

(Figure 3-11). 

Burrough Mtn. and Cascadel Pt. 

Burrough Mtn. Cascadel Pt. 

Relative Nonnative Cover (%)

0

20

40

60

80

100

On Fuel Break 

Off Fuel Break 

Figure 3-11. ANOVA found significant differences 

between fuel breaks (N=110, df=1, 106, F=4.674, 

P=0.033), and with position on or off fuel break 

(N=110, df=1, 106 F=18.772, P<0.001). Error bars 

represent + 1 SE. 

Chapter Three: Site Specific Results 47 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Aguanga Fuel Break, Cleveland National 

Forest 

The Aguanga fuel break is located in the 

Cleveland National Forest, near the town of Oak 

Grove in San Diego County. This fuel break was 

constructed in 1974 by hand crews, mechanical 

thinning, and prescribed fire. It has been maintained 

on an approximately five year rotation by hand crews 

and prescribed burning. Portions of the fuel break 

were most recently prescribed burned in 2003. 

The Aguanga fuel break is located in a chaparral 

vegetation type, dominated by chamise, red shank, 

and cupleaf ceanothus (Ceanothus greggii). 

The most common nonnative species on the 

Aguanga fuel break was red brome. We identified 80 

species in our 1m2

 plots at this site, 9 of which were 

nonnative. The Aguanga fuel break had lower mean 

relative cover (9%) than the mean (18%) of 24 sites 

in our study. Nonnative plants occurred in 80% of 

plots on the Aguanga fuel break, and 40% of plots in 

the adjacent wildland. 

Relative nonnative cover was significantly 

higher on the Aguanga fuel break than in the adjacent 

wildland area (Figure 3-12). 

Aguanga fuel break, Cleveland 

National Forest 

Aguanga Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%)

0

5

10

15

20

Figure 3-12. ANOVA found significant differences in 

relative nonnative abundance on the Aguanga fuel 

break compared with adjacent wildlands (N=60, df=1, 

58, F=14.016, P<0.001). Error bars represent + 1 SE. 

Chapter Three: Site Specific Results 48 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Pilot Grove Fuel Break, Mendocino 

National Forest 

The Pilot Grove fuel break is located in the 

Mendocino National Forest near Clear Lake in Lake 

County. This fuel break was constructed in 1960 by 

bulldozers, and has been maintained by prescribed 

burning on an approximately five-year rotation. The 

area surrounding this fuel break was burned during 

the 1996 Forks fire. 

The Pilot Grove fuel break includes a range of 

plant communities, including ponderosa pine forest, 

black oak woodlands, and chaparral habitats 

dominated by chamise. Portions of the fuel break 

were historically seeded with grasses, including 

Harding grass (Phalaris aquatica). Our plots on the 

Pilot Grove fuel break were located primarily on 

soils from the Neuns series. These are moderately 

deep, well drained soils that formed in slope alluvium 

and colluvium from metamorphosed igneous and 

sedimentary rocks. 

We identified 78 species in our 1m2

 plots at Pilot 

Grove, 17 of which were nonnative. The most 

common nonnative species on the Pilot Grove fuel 

break was foxtail fescue. Nonnatives were detected 

in 80% of the quadrats on the Pilot Grove fuel break, 

and only 20% of those outside the fuel break. The 

Pilot Grove fuel break had slightly lower mean 

relative nonnative cover (15%) than the mean (18%) 

of 24 sites in our study. Nonnative plant cover was 

significantly higher on the Pilot Grove fuel break 

than in the adjacent wildland off of the fuel break 

(Figure 3-13). 

Pilot Grove fuel break, 

Mendocino National Forest 

Pilot Grove Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%)

0

10

20

30

40

50

Figure 3-13. ANOVA found significant differences 

in relative nonnative abundance on the Pilot Grove 

fuel break compared with adjacent wildlands, 

(N=70, df= 1, 68, F=29.807, P<0.001). Error bars 

represent + 1 SE. 

Chapter Three: Site Specific Results 49 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Rouse Ridge Fuel Break, San Bernardino 

National Forest 

The Rouse Ridge fuel break is located east of the 

town of Hemet, Riverside County, in the San 

Bernardino National Forest. This fuel break was 

constructed in 1984 by bulldozers and prescribed 

burning. Portions of the fuel break were prescribed 

burned again during 2002 and 2003. 

The Rouse Ridge fuel break is located primarily 

in a chaparral vegetation type dominated by chamise, 

California buckwheat, and birchleaf mountain 

mahogany (Cercocarpus betuloides). Soils on the 

Rouse Ridge fuel break were primarily of the 

Chawanakee and Goulding series. Both series are 

shallow, somewhat excessively drained soils. The 

Chawanakee is formed in material weathered from 

granitic rock, while the Goulding series is formed in 

material weathered from metavolcanic or 

metasedimentary rocks. 

We identified 81 species in our 1m2

 plots at 

Rouse Ridge, 16 of which were nonnative. The most 

common nonnative species on the Rouse Ridge fuel 

break was cheatgrass. 

Nonnative species occurred in 65% of plots 

outside the fuel break, and 90% of plots on the fuel 

break. The mean relative nonnative cover at the 

Rouse Ridge study site (31%) was higher than the 

mean (18%) of 24 sites in our study. Nonnative plant 

cover was significantly higher on the Rouse Ridge 

fuel break than in the adjacent wildland 

(Figure 3-14). 

Rouse Ridge Fuel Break

On fuel break Off fuel break

Relative Nonnative Cover (%)

0

10

20

30

40

50

60

Figure 3-14. ANOVA found significant differences 

in relative nonnative abundance on the Rouse 

Ridge fuel break compared with adjacent wildlands 

(N=60, df=1, 58, F=12.195, P=0.001). 

Error bars represent + 1 SE. 

Rouse Ridge fuel break, 

San Bernardino National Forest 

Chapter Three: Site Specific Results 50 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Sierra Pelona Fuel Break, Angeles 

National Forest 

The Sierra Pelona fuel break is located in the 

Angeles National Forest east of the City of Santa 

Clarita, Los Angeles County. The Sierra Pelona Fuel 

Break was constructed in the 1960s by bulldozers. 

The fuel break has been annually maintained by 

cattle grazing, and is periodically prescribed burned. 

The Sierra Pelona fuel break occurs in oak 

woodland and chaparral type plant communities, 

dominated by scrub oak (Quercus berberidifolia), 

canyon live oak, chamise, and California buckwheat. 

Our plots on the Sierra Pelona fuel break were 

located on the Gaviota series. These consist of 

shallow, well drained soils that formed in material 

weathered from hard sandstone or meta-sandstone. 

The most common nonnative species we observed on 

the Sierra Pelona fuel break was cutleaf filaree 

(Erodium cicutarium). We identified 71 species in 

our 1m2

 plots at the Sierra Pelona fuel break, 16 of 

which were nonnative. 

Nonnative species were found in 75% of plots in 

the wildland area, and 100% of plots on the fuel 

break. The Sierra Pelona fuel break had higher 

relative cover of nonnative plant species (31%) than 

the mean (18%) of 24 sites in our study. 

Relative nonnative plant cover was significantly 

higher on the Sierra Pelona fuel break than in the 

adjacent wildland (Figure 3-15). 

 

Sierra Pelona Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%)

0

10

20

30

40

50

60

Figure 3-15. ANOVA found significant differences in 

relative nonnative abundance on the Sierra Pelona 

fuel break compared with adjacent wildlands (N=60, 

df=1, 58, F=25.518, P<0.001). 

Error bars represent + 1 SE. 

Sierra Pelona fuel break, 

Angeles National Forest 

Chapter Three: Site Specific Results 51 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Blacks Ridge Defensible Fuel Profile 

Zone, Lassen National Forest 

The Blacks Ridge Defensible Fuel Profile Zone 

(DFPZ), a shaded fuel break, is located in the Eagle 

Lake Ranger District of the Lassen National Forest, 

near the town of Little Valley in Lassen County. 

This shaded fuel break was constructed as part of 

the Herger-Feinstein Quincy Library Group Forest 

Recovery Act, Bill HR 858, passed in 1997. The 

entire Blacks Ridge DFPZ is planned to occupy 

almost 3642 hectares (ha) (9000 acres), including 

thinning and group selection treatments. As of 2003 

when we collected data, much of this DFPZ had not 

yet been constructed. We collected data on two 

portions of the DFPZ, where almost 101 ha (250 

acres) had been treated to reduce fuels using 

mechanical thinning and prescribed burning between 

2002 and 2003. 

The Blacks Ridge DFPZ is located in a 

ponderosa pine forest. Other common species we 

observed in our plots at the Blacks Ridge DFPZ 

included mountain mahogany (Cercocarpus 

ledifolius), mahala carpet (Ceanothus prostratus), 

and mule’s ear (Wyethia mollis). Our plots on the 

Blacks Ridge DFPZ were located on Trojan and 

Holland soils. Trojan soils are very deep, well 

drained soils that formed in colluvium and residuum 

derived from volcanic rocks or from schist and 

argillite. Holland soils are very deep, well drained 

soils that formed in material weathered from granitic 

rock. 

We identified 60 species in our 1m2

 plots at 

Blacks Ridge DFPZ, three of which were nonnative. 

The most common nonnative plant we found was 

cheatgrass. Nonnative species were found in 18% of 

plots in the untreated wildland area, and only 8% of 

plots within the DFPZ. The Blacks Ridge DFPZ had 

the third lowest relative cover of nonnative plant 

species (1.6%) of 24 sites in our study. 

Blacks Ridge DFPZ, 

Lassen National Forest 

Nonnative plant cover was not statistically 

different in the treated area of the Blacks Ridge 

DFPZ compared to the adjacent wildland 

(Figure 3-16). 

Blacks Ridge DFPZ

Relative Nonnative Cover (%)

0

20

40

60

80

100

On Fuel Break Off Fuel Break 

Figure 3-16. ANOVA found no difference in relative 

nonnative abundance on the Blacks Ridge DFPZ 

compared with adjacent wildlands (N=80, df=1, 78, 

F=25.518, P=0.1430). Error bars represent + 1 SE. 

Chapter Three: Site Specific Results 52 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Sierra Madre Ridge Fuel Break, Los 

Padres National Forest 

The Sierra Madre Ridge fuel break is located in 

the Los Padres National Forest, southwest of the 

Cuyama Valley, in Santa Barbara County. This fuel 

break was constructed in 1964 by bulldozers. The 

fuel break has been maintained regularly, initially 

through the use of herbicides and mechanical 

clearing, and more recently through prescribed 

burning. 

The Sierra Madre Ridge fuel break is located in 

a scrub oak habitat, dominated by Tucker’s oak 

(Quercus john-tuckeri), and scrub oak. The most 

common nonnative species on the Sierra Madre 

Ridge fuel break was cheatgrass. Our plots were 

located on Aramburu and Lodo soils. The Aramburu 

series are moderately deep, well drained soils that 

formed in material weathered from shale or 

sandstone. The Lodo series consists of shallow, 

somewhat excessively drained soils that formed in 

material weathered from hard shale and fine grained 

sandstone. 

We identified 80 plant species in our 1m2

 plots at 

the Sierra Madre Ridge, 11 of which were nonnative. 

Nonnative species occurred in 20% of plots outside 

the fuel break and 100% of plots within the fuel 

break. The Sierra Madre Ridge fuel break had 

slightly higher relative nonnative cover (22%) than 

the mean (18%) of 24 sites in our study. 

Nonnative plant cover was significantly higher 

on the Sierra Madre Ridge fuel break than in the 

adjacent wildland (Figure 3-17). 

Sierra Madre Ridge fuel break, 

Los Padres National Forest 

Sierra Madre Ridge

Relative Nonnative Cover (%)

0

20

40

60

80

100

On Fuel Break Off Fuel Break 

Figure 3-17. ANOVA found significantly higher relative 

nonnative abundance on the Sierra Madre Ridge fuel 

break compared with the adjacent wildland (N=60, df=1, 

58, F= 133.226, P<0.001). Error bars represent 

+ 1 SE. 

Chapter Three: Site Specific Results 53 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Palos Ranches Fuel Break, Sequoia 

National Forest and BLM 

This fuel break is located west of the town of 

Wofford Heights in Kern County. The fuel break 

consists of two separate portions, the Shirley portion 

and the Palos Ranches portion, constructed and 

managed by separate agencies, the USDA Forest 

Service and the Bureau of Land Management. The 

Shirley portion is located within the Sequoia National 

Forest. This portion of the fuel break was constructed 

in 1977 by mechanical thinning. The fuel break is 

located in a mixed conifer forest dominated by 

incense cedar, white fir, and black oak. The only 

nonnative plant found on the Shirley portion of the 

fuel break was cheatgrass. 

 

The Palos Ranches portion of the fuel break is 

located on land managed by the Bakersfield District 

of the Bureau of Land Management. This portion of 

the fuel break was constructed by bulldozers and 

hand crews in 1998 and 2001 for use during a 

prescribed burn. The Palo Ranches portion of the fuel 

break is located in a chaparral habitat, dominated by 

wedgeleaf ceanothus, birchleaf mountain mahogany, 

and greenleaf manzanita (Arctostaphylos patula). 

The most common nonnative species on the Palos 

Ranches portion of the fuel break was cheatgrass. 

Soils on both portions of the Palos Ranches fuel 

break were predominately Walong series. The 

Walong series consists of moderately deep, well 

drained soils that formed in material weathered from 

granitic rocks. 

We identified 85 species in our 1m2

 plots on the 

entire Palos Ranches fuel break, including both the 

Palos Ranches and Shirley portions. Eight of these 

species were nonnative. Nonnative species were 

found in 50% of our plots outside the fuel break and 

70% of our plots within the treated area. The Palo 

Ranches fuel break had slightly lower relative 

nonnative cover (16%) than the mean (18%) of 24 

sites in our study. 

Nonnative plant cover was significantly higher 

on the Palo Ranches fuel break than in the adjacent 

wildland (Figure 3-18). 

Palos Ranches fuel break, BLM 

Palos Ranches Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%)

0

5

10

15

20

25

30

Figure 3-18. ANOVA found significant differences in 

relative nonnative abundance on the Palos Ranches 

fuel break compared with adjacent wildlands (N=60, 

df=1, 58, F=5.094, P=0.028). 

Error bars represent + 1 SE. 

Chapter Three: Site Specific Results 54 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Antelope Border Defensible Fuel Profile 

Zone, Plumas National Forest 

The Antelope Border Defensible Fuel Profile 

Zone (DFPZ), a shaded fuel break, is located in the 

Plumas National Forest in the vicinity of the 

Antelope Recreation Area, Plumas County. This fuel 

reduction project was implemented as part of the 

Herger-Feinstein Quincy Library Group Forest 

Recovery Act, HR 858, passed in 1997. The entire 

Antelope Border DFPZ consists of 34 units, varying 

in size from 5 to 50 ha (11 to 127 acres). For this 

study we collected data on units 11 and 12, totaling 

about 40 ha (100 acres) in size. These units were 

constructed in 2001 using mechanical thinning and 

prescribed burning. 

The Antelope Border DFPZ is located in a 

ponderosa pine forest. Other common species we 

observed included antelope bitterbrush (Purshia 

tridentata) and mahala carpet. Soils on the Antelope 

Border DFPZ were from the Chaix and Cagwin 

series. The Chaix series consists of moderately deep, 

somewhat excessively drained soils that formed in 

material weathered from acid intrusive igneous rock, 

mainly granite or granodiorite. The Cagwin series 

consists of moderately deep, somewhat excessively 

drained soils that formed in material weathered from 

granite. 

The most common nonnative plant we 

encountered in our study plots at the Antelope Border 

DFPZ was cheatgrass. We identified 59 species in 

our 1 m2 plots at the Antelope Border DFPZ, three of 

which were nonnative. Nonnative species occurred in 

26% of our plots outside the fuel break and only 4% 

of our plots within the treated area. 

The Antelope Border DFPZ had much lower 

relative nonnative cover (2%) than the mean (18%) 

of 24 sites in our study. Relative nonnative plant 

cover tended to be higher outside of the Antelope 

Border DFPZ (P=0.076), (Figure 3-19). 

Antelope Border DFPZ, 

Plumas National Forest 

Figure 3-19. ANOVA found no significant 

differences in relative nonnative abundance within 

the Antelope Border DFPZ compared with adjacent 

untreated areas (N=80, df=1, 78, F=3.237, P=0.076). 

Error bars represent + 1 SE. 

Chapter Three: Site Specific Results 55 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Fallen Leaf Lake Shaded Fuel Break, 

Lake Tahoe Basin Management Unit 

The Fallen Leaf Lake shaded fuel break is 

located within the Lake Tahoe Basin Management 

Unit of the USDA Forest Service, near Lake Tahoe 

in El Dorado County. This shaded fuel break was 

constructed in 1995 by mechanical thinning and 

prescribed burning. The Fallen Leaf Lake shaded fuel 

break is located at an elevation of 2000 meters, the 

highest of any of our study sites. This site also had 

the greatest duff depth (57 cm) of any area we 

sampled. 

The Fallen Leaf Lake shaded fuel break is 

located in a mixed coniferous forest. Dominant trees 

species include white fir, canyon live oak, incense 

cedar, and Jeffrey pine. Soils at this fuel break were 

predominately from the Meeks series, a deep, well or 

somewhat excessively drained soil that formed in 

material weathered from glacial outwash. 

We did not find nonnative plants in any of our 

1m2

 plots at Fallen Leaf Lake. We identified 21 

native species at the Fallen Leaf fuel break. 

Fallen Leaf Lake shaded fuel break, 

Lake Tahoe Basin Management Unit 

Chapter Three: Site Specific Results 56 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Oregon Fire Line, Shasta Trinity 

National Forest 

The Oregon fire line is located in the Shasta 

Trinity National Forest near the town of Weaverville 

in Trinity County. This fire line was constructed by 

bulldozers in 1980 to suppress the Oregon fire. 

The Oregon fire line is located in an oak 

woodland, dominated by Oregon white oak (Quercus 

garryana). Other common species found in the 

vicinity of the Oregon fire line include grey pine 

(Pinus sabiniana) and greenleaf manzanita. Soils at 

the Oregon fire line were from the Musserhill series. 

The Musserhill series consists of moderately deep, 

well drained soils formed in materials weathered 

from weakly consolidated conglomerate. 

We identified 60 species in our 1m2 plots at the 

Oregon fire line, 11 of which were nonnative. The 

most common nonnative plant we found on the 

Oregon fire line was field hedge parsley. 

The Oregon fire line had slightly higher mean 

relative nonnative cover (19%) than the mean (18%) 

of 24 sites in our study. Nonnative species occurred 

in 78% of plots outside the fire line and 90% of plots 

within the fire line. 

Nonnative plant cover was significantly higher 

on the Oregon fire line than in the adjacent wildland 

(Figure 3-20). 

Oregon fire line, Shasta 

Trinity National Forest 

Oregon Fire Line

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%)

0

10

20

30

40

50

Figure 3-20. ANOVA found significant differences in 

relative nonnative abundance within the Oregon fire 

line compared with adjacent untreated areas (N=70, 

df=1, 68, F=7.190, P=0.009). Error bars represent 

+ 1 SE. 

Chapter Three: Site Specific Results 57 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

McKenzie Ridge Fuel Break, Sequoia 

National Forest 

The McKenzie Ridge fuel break is located in the 

Sequoia National Forest, west of Grant Grove in 

Kings Canyon National Park, Fresno County. This 

fuel break was constructed in the 1960s by 

mechanical thinning. It has been periodically 

maintained by prescribed burning, thinning, and 

discing. Portions of the McKenzie Ridge fuel break 

were burned in the 2001 Highway fire. The fuel 

break is located in a mixed conifer forest, dominated 

by incense cedar, ponderosa pine, and black oak. Our 

plots on McKenzie Ridge were found in Holland and 

Jocal soil series. The Holland series consists of very 

deep, well drained soils that formed in material 

weathered from granitic rock. The Jocal series 

consists of deep and very deep, well drained soils 

formed in material weathered from metasedimentary 

rocks. 

We identified 58 species in our 1m2 plots at 

McKenzie Ridge, 10 of which were nonnative. The 

most common nonnative plant we found on the 

McKenzie Ridge fuel break was cheatgrass. 

Nonnative species occurred in only 5% of plots 

outside the fuel break and 31% of plots within the 

fuel break. The McKenzie Ridge fuel break had 

much lower relative nonnative plant cover (4%), than 

the mean (18%) of 24 sites in our study. 

Relative nonnative plant cover was significantly 

higher in plots on the McKenzie Ridge fuel break 

than outside of the fuel break (Figure 3-21). 

McKenzie Ridge Fuel Break

On Fuel Break Off Fuel Break

Relative Nonnative Cover (%)

0

2

4

6

8

10

12

14

Figure 3-21. ANOVA found significant differences 

in relative nonnative abundance on the McKenzie 

Ridge fuel break compared with adjacent 

untreated areas (N=81, df=1, 79, F=42.771, P<0.001). 

Error bars represent + 1 SE. 

Chapter Three: Site Specific Results 58 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

DISCUSSION 

The 24 fuel breaks described in this chapter 

occurred across a wide range of vegetation types, soil 

series, elevations, and climates. Each site had a 

unique history, including various dates of 

construction, different construction and maintenance 

regimes, varying fire histories, and different land use 

histories. Despite this variation, we found that 19 of 

the 24 fuel breaks had significantly higher relative 

nonnative cover than the adjacent wildland areas. 

The six fuel breaks where nonnative cover was 

 

similar on the fuel break and in the adjacent wildland 

were the Blacks Ridge DFPZ, the Antelope Border 

DFPZ, the Lewis Creek fire line, the Cascadel Point 

fuel break, and the Gasquet shaded fuel break. These 

sites had several common characteristics (Table 3-2). 

None of them were constructed by bulldozers, and 

they had either never been maintained or had very 

long (i.e. 20 year) maintenance rotations. All of these 

sites, except the Lewis Creek fire line, had 

significantly deeper duff depths on the fuel break 

than the mean (11 cm) of all sites studied.

Table 3-2. Characteristics of fuel breaks where relative nonnative cover did not differ between plots on the fuel break and 

plots in the adjacent wildland area. 

[A range of values is given where individual transects at the same site vary. NA, not applicable, is given for 

maintenance frequency of fuel breaks that have not yet been maintained. Cm, centimeter] 

Site Name Construction 

Method 

Maintenance 

Frequency 

Age 

(years) 

Vegetation 

Type 

Number of 

Fires 

Duff 

Depth 

(cm) 

Blacks Ridge Mechanical NA 0-1 Coniferous 

Forest 

0 21.6 

Antelope 

Border 

Mechanical NA 1 Coniferous 

Forest 

0 17.0 

Lewis Creek Hand Crews 20 yrs 21 Chaparral 0-2 7.1 

Cascadel 

Point 

Hand Crews 20 yrs 80 Oak Woodland 2-3 17.9 

Gasquet Mechanical NA 7 Oak Woodland 0-1 21.0 

Chapter Three: Site Specific Results 59 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

In addition to sharing common features, these 

sites also differed from each other in several ways. 

They included three of the four vegetation types we 

identified, and they had experienced a range of fire 

histories. 

It is likely that a number of factors, including 

construction method and maintenance history as well 

as propagule availability and chance may explain the 

lack of an association between nonnative abundance 

and position relative to the fuel break that we 

observed at these sites. 

Continued monitoring would help determine if a 

pattern of increasing nonnative abundance appeared 

over time on these fuel breaks. Time since 

construction was strongly associated with nonnative 

abundance when we evaluated all of the fuel breaks 

together (See Figure 1-7 from Chapter 1). 

Chapter Three: Site Specific Results 60 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

 Cheatgrass was the most common nonnative 

species at eight of our study sites, and was both the 

most abundant and frequent species across all of our 

sites. This result supports the findings of others that 

cheatgrass is both widespread and common in 

California (Bossard and others, 2000). 

Foxtail fescue was the most common nonnative 

at three of the sites, red brome was the dominant 

nonnative at two sites, and field hedge parsley was 

the most common nonnative at two sites. The other 

nine study sites had different dominant nonnative 

species. The lack of one dominant nonnative at all 

sites suggests that fuel breaks may create conditions 

favorable to a number of different nonnative plant 

species or that local and site-specific factors may 

dictate differences between nonnatives in fuel breaks. 

The colonization of a fuel break by a particular 

species may reflect which species has a nearby seed 

source or is able to disperse into the treated area. 

Most of the common nonnatives encountered in 

this study are known to respond favorably to 

disturbance. Four of the five most common 

nonnatives were annual grasses from the genus 

Bromus. These annual grasses have been found to 

readily colonize disturbed sites across a wide range 

of environments (Bossard and others, 2000). These 

grasses have also been found to alter fuel conditions 

in the habitats they invade, and contribute to changes 

in fire behavior and frequency to the detriment of the 

native plant community (D'Antonio & Vitousek 

1992). The establishment of these species in fuel 

breaks may have serious implications for native plant 

communities and fire regimes in the areas they 

invade (Brooks and others, 2004). 

Text Box 3-1: Summary of Site Specific 

Findings: 

• Individual sites differed greatly in nonnative 

cover, fire history, and environmental 

variables. 

• Nonnative cover was higher within fuel 

treatments than adjacent wildlands at 19 of 

our 24 study sites. 

• Sites without significant differences in 

nonnative cover in and out of fuels breaks 

were characterized by higher duff depths, 

construction methods other than bulldozing, 

and were newly constructed or had very 

infrequent maintenance. 

• Cheatgrass was the most common nonnative 

plant species at 8 of our study sites. 

• A number of different species assumed 

dominance at the other 16 sites, suggesting 

that many nonnative species may be well 

adapted to take advantage of the conditions 

provided by fuel treatments. 

Chapter Three: Site Specific Results 61 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Chapter Four: Summary

 ______________________________________________________________________________________

Chapter Four: Summary 62 

We found that fuel breaks have the potential to 

promote the establishment and spread of nonnative 

plants. However, fuel breaks with more canopy and 

ground cover may be less likely to be invaded. 

Varying construction methods to retain more litter 

cover, minimize the exposure of bare ground, and 

retain some canopy cover might reduce nonnative 

germination and establishment on fuel breaks. 

Increasing the time between fuel break maintenance 

also could allow ground and canopy cover to increase 

and also lower the probability of nonnative invasion. 

We found that fuel breaks have the potential to 

promote the establishment and spread of nonnative 

plants. However, fuel breaks with more canopy and 

ground cover may be less likely to be invaded. 

Varying construction methods to retain more litter 

cover, minimize the exposure of bare ground, and 

retain some canopy cover might reduce nonnative 

germination and establishment on fuel breaks. 

Increasing the time between fuel break maintenance 

also could allow ground and canopy cover to increase 

and also lower the probability of nonnative invasion. 

Even in relatively uninvaded vegetation types 

such as coniferous forests, the use of bulldozers 

significantly increased the abundance of nonnative 

plants. Bulldozers have large blades specifically 

designed to remove surface soil layers, and may be 

more likely to introduce nonnative seeds into fuel 

breaks by disrupting soil seed banks and transporting 

seeds between sites. 

Even in relatively uninvaded vegetation types 

such as coniferous forests, the use of bulldozers 

significantly increased the abundance of nonnative 

plants. Bulldozers have large blades specifically 

designed to remove surface soil layers, and may be 

more likely to introduce nonnative seeds into fuel 

breaks by disrupting soil seed banks and transporting 

seeds between sites. 

Some litter cover and overstory canopy 

were retained on the 

Whiskey Creek fuel break in 

Whiskeytown National Recreation Area. 

Bulldozers were used to construct this fuel 

break in the Mendocino National Forest. 

 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

We found that nonnative cover decreased with 

distance from the fuel break, suggesting that fuel 

breaks act as sources of nonnative plant seeds during 

the invasion of adjacent areas. Wildland areas 

adjacent to fuel breaks were more likely to be 

invaded by nonnative species when the wildlands had 

been subject to recurrent fires. Nonnative plant 

abundance on fuel breaks and in adjacent wildlands 

continued to increase with fuel break age. 

Native species may also be promoted by fuel 

breaks, particularly native annual forbs and grasses.

We found that native and nonnative diversity were 

significantly correlated. Many native species, 

particularly annual forbs, require periodic 

disturbance to persist in natural ecosystems. The best 

management regime for maintaining native plant 

diversity is likely one that restores natural 

disturbance processes of the frequency, intensity, and 

duration with which native species evolved. 

However, these disturbances may also promote 

nonnative plants. Land managers will have to weigh 

the benefits of maintaining natural disturbance 

regimes to native plant communities against the 

potential risks of promoting nonnative invasives. 

We found significant differences in relative 

nonnative cover and nonnative species richness 

among vegetation types. Vegetation type has been 

found to be among the most important factors 

influencing nonnative plant abundance, either 

because of the life history characteristics of the 

dominant plant community or because of other 

factors such as elevation land use and disturbance 

history (Keeley 2000). 

Native grasses were abundant on 

this fuel break in the 

Sierra National Forest. 

Recent burn adjacent to the Calf Canyon 

fuel break maintained by the San Luis 

Obispo Unit of CDF 

The Oso Ridge fuel break, and other coastal 

scrub sites, supported high numbers 

of nonnative plants. 

Chapter Four: Summary 63 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Despite the strong influence of vegetation type 

on nonnative abundance and species richness, we 

found that a number of variables, including fuel 

break construction method, distance to the fuel break, 

fire number, and grazing still had significant effects 

on nonnative abundance when evaluated within 

individual vegetation types separately. This suggests 

that although important, vegetation type alone does 

not determine the abundance of nonnatives. We 

found that fuel breaks appear to promote nonnative 

abundance in all vegetation types, and this pattern 

may be compounded by more numerous fires or by 

grazing. 

The 24 fuel breaks we sampled had unique 

histories, including various dates of construction, 

different construction and maintenance regimes, 

varying fire histories, and different land use histories. 

Despite this variation, we found that 19 of the 24 

sites had significantly higher relative nonnative cover 

within fuel breaks than in adjacent wildland areas. 

 Cheatgrass was the most common nonnative 

species at eight of our study sites. Cheatgrass was 

both the most abundant and frequent species across 

all of our sites, suggesting that this species is 

widespread and common in California. However, the 

other sixteen study sites had twelve different 

dominant nonnative species. The lack of one 

dominant nonnative at all sites suggests that fuel 

breaks may create conditions favorable to a number 

of different nonnative plant species or that local 

conditions may dictate nonnative occurrence. The 

colonization of a fuel break by a particular species 

may reflect which species has a nearby seed source 

or is able to disperse into the treated area. 

At most sites, such as the Tower fire line, 

there was a significant increase in 

nonnative abundance on the fuel break. 

Cheatgrass was the most common 

nonnative species at our sites. 

Chapter Four: Summary 64 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

 This study was observational, and there were Summary of our findings: 

many environmental and anthropogenic factors for 

which we could not control. Research using 

controlled, replicated experiments will be necessary 

to fully understand the mechanisms that influence 

nonnative plant establishment within pre-fire fuel 

manipulation projects and invasion into adjacent 

wildland areas. Here we have identified potentially 

important variables influencing patterns of nonnative 

abundance with respect to fuel breaks and suggested 

ways that the probability of nonnative invasion might 

be minimized. If these methods are strategically 

implemented as part of a long term fuel reduction 

program, it may be possible to both achieve fuel 

management goals and reduce the probability of 

nonnative plant invasion on fuel breaks and in 

surrounding wildland areas. 

• Fuel breaks may promote nonnative and 

native plants. 

• Fuel breaks with more canopy and ground 

cover may be less likely to be invaded. 

• Nonnative plants are more abundant 

adjacent to older fuel breaks and in areas 

that have experienced more numerous fires. 

• Native species may also be associated with 

fuel breaks, particularly native annual forbs 

and grasses. 

• Cheatgrass was the most common nonnative 

plant species at eight of our study sites. 

• A number of different species assumed 

dominance at the other 16 sites, suggesting 

that many nonnative species may be well 

adapted to take advantage of the conditions 

provided by fuel breaks. 

Oso Ridge fuel break, 

Casper’s Wilderness Park 

Chapter Four: Summary 65 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

___________________________________________________________________________________________

Literature Cited 66 

Literature Cited 

Agee, J., B. Bahro, M. Finney, P. Omi, D. Sapsis, C. Skinner, J. v. Wagtendonk, and C. P. Weatherspoon. 2000. 

The use of shaded fuelbreaks in landscape fire management. Forest Ecology and Management 127:55-66. 

Aragon, R., and J. M. Morales. 2003. Species composition and invasion in NW Argentinian secondary forests: 

effects of land use history, environment and landscape. Journal of Vegetation Science 14:195-204. 

Auld, T. D. 1996. Ecology of the Fabaceae in the Sydney region: fire, ants and the soil seedbank. Cunninghamia 

4:531-551. 

Battles, J. J., A. J. Shlisky, R. A. Barrett, R. C. Heald, and B. H. Allen-Diaz. 2001. The effects of forest 

management on plant species diversity in a Sierran conifer forest. Forest Ecology and Management 146:211-222. 

Bentley, J. 1967. Conversion of chaparral areas to grassland: techniques used in California. Pacific Southwest 

Research Station, USDA Forest Service, Berkeley, California, USA. 

Berlow, E. L., C. M. D'Antonio, and H. Swartz. 2003. Response of herbs to shrub removal across natural and 

experimental variation in soil moisture. Ecological Applications 13:1375-1387. 

Bossard, C. C., J. M. Randall, E. Marc, and C. Hoshovsky. 2000. Invasive plants of California's wildlands. 

University of California Press, Berkeley, California, USA. 

Brooks, M. L., C. M. D'Antonio, D. M. Richardson, J. B. Grace, J. E. Keeley, J. M. DiTomaso, R. J. Hobbs, M. 

Pellant, and D. Pyke. 2004. Effects of invasive alien plants on fire regimes. BioScience 54:677-688. 

Burke, M. J. W., and J. P. Grime. 1996. An experimental study of plant community invasibility. Ecology 77:776-

790. 

California Department of Forestry and Fire Protection. 2003. Fire Perimeters, Sacramento, California, USA. 

Clark, K. P. 1973. Fuelbreaks, fire and brushlands. Environmental Southwest:1-2. 

D'Antonio, C. 2000. Fire, plant invasions, and global changes. Pages 65-93 in H. A. Mooney, and R. J. Hobbs, 

editors. Invasive species in a changing world. Island Press, Covelo, California, USA. 

D'Antonio, C. M. 1993. Mechanisms controlling invasion of coastal plant communities by the alien succulent 

Carpobrotus edulis. Ecology 74:83-95. 

D'Antonio, C. M., T. L. Dudley, and M. Mack. 1999. Disturbance and biological invasions: direct effects and 

feedbacks. Pages 413-452 in L. R. Walker, editor. Ecosystems of disturbed ground. Elsevier, New York, New 

York, USA. 

D'Antonio, C. M., and P. M. Vitousek. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global 

change. Annual Review of Ecology and Systematics 23:63-87. 

Daubenmire, R. F. 1959. A canopy-coverage method of vegetation analysis. Northwest Science 33:43-64. 

 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Edmunson, G., and D. Cornelius. 1961. Promising grasses for southern California fuel breaks. Pacific Southwest 

Forest and Range Experiment Station, Berkeley, California, USA. 

ESRI. 2000. ArcView. ESRI Limited, Redlands, California, USA. 

Facelli, J. M. 1991. Plant litter: its dynamics and effects on plant community structure. Botanical Review 57:1-32. 

Farsworth, A., and P. Summerfelt. 2002. Flagstaff interface treatment prescription: results in the wildland-urban 

interface. Fire Management Today 62:13-18. 

Gelbard, J. L., and J. Belnap. 2003. Roads as conduits for exotic plant invasions in a semiarid landscape. 

Conservation Biology 17:420-432. 

Giessow, J. 1997. Effects of fire frequency and proximity to firebreak on the distribution and abundance of nonnative herbs in coastal sage scrub. Biology. San Diego State University, San Diego, California, USA. 

Hayes, G. F., and K. D. Holl. 2003. Cattle grazing impacts on annual forbs and vegetation composition of mesic 

grasslands in California. Conservation Biology 17:1694-1702. 

Hickman, J. C., editor. 1993. The Jepson manual: higher plants of California. University of California Press, 

Berkeley, California, USA. 

Hobbs, R. J., and L. Atkins. 1988. Effect of disturbance and nutrient addition on native and introduced annuals in 

plant communities in the western Australian wheatbelt. Australian Journal of Ecology 13:171-180. 

Holmes, P. M., and R. J. Newton. 2004. Patterns of seed persistence in South African fynbos. Plant Ecology 

172:143-158. 

Keeley, J. E. 2000. Chaparral. Pages 204-253 in M. Barbour, and W. Billings, editors. North American Terrestrial 

Vegetation. Cambridge University Press, Cambridge, UK. 

Keeley, J. E. 2001. Fire and invasive species in mediterranean-climate ecosystems of California. Pages 81-94 in K. 

Galley, and T. Wilson, editors. Fire conference 2000: the first national congress on fire ecology, prevention, and 

management. Tall Timbers Research Station, San Diego, California, USA. 

Keeley, J. E., and C. J. Fotheringham. 2003. Impact of past, present, and future fire regimes on north American 

mediterranean shrublands in T. T. Veblen, W. L. Baker, G. Montenegro, and T. W. Swetnam, editors. Fire and 

climatic change in temperate ecosystems of the western Americas. Springer, New York, New York, USA. 

Keeley, J. E., D. Lubin, and C. J. Fotheringham. 2003. Fire and grazing impacts on plant diversity and alien plant 

invasions in the southern Sierra Nevada. Ecological Applications 13:1355-1374. 

Larson, D. L., P. J. Anderson, and W. Newton. 2001. Alien plant invasion in mixed-grass prairie: effects of 

vegetation type and anthropogenic disturbance. Ecological Applications 11:128-141. 

Levine, J., and C. M. D'Antonio. 1999. Elton revisited: a review of evidence linking diversity and invasibility. 

Oikos 87:15-26. 

Lonsdale, W. M., and A. M. Lane. 1994. Tourist vehicles as vectors of weed seeds in Kakadu National Park, 

northern Australia. Biological Conservation 69:277-283. 

Los Angeles County Fire Department. 1962. A fuel break has been tested. Los Angeles County Fire Department, 

CDF, USDA Forest Service, Los Angeles, California, USA. 

Literature Cited 67 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

Mack, R. M. 1989. Temperate grasslands vulnerable to plant invasions: characteristics and consequences. Pages 

155-179 in J. A. Drake, H. A. Mooney, F. d. Castri, R. H. Groves, F. J. Kruger, M. Rejmanek, and M. 

Williamson, editors. Biological invasions: a global perspective. Wiley and Sons, New York, New York, USA. 

McKenzie, D., C. B. Halpern, and C. R. Nelson. 2000. Overstory influences on herb and shrub communities in 

mature forests of western Washington, USA. Canadian Journal of Forest Research 30:1655-1666. 

Moreno, J., and W. Oechel. 1991. Fire intensity effects on germination of shrubs and herbs in southern California 

chaparral. Ecology 72:1993-2004. 

Omi, P. 1979. Planning future fuelbreak strategies using mathematical modeling techniques. Environmental 

Management 3:73-80. 

Parendes, L. A., and J. A. Jones. 2000. Role of light availability and dispersal in exotic plant invasion along roads 

and streams in the H.J. Andrews Experimental Forest, Oregon. Conservation Biology 14:64-75. 

Parker, I. M., S. K. Mertens, and D. W. Schemske. 1993. Distribution of seven native and two exotic plants in a 

tallgrass prairie in southeastern Wisconsin: the importance of human disturbance. American Midland Naturalist 

130:43-55. 

Reynolds, S. A., J. D. Corbin, and C. M. D'Antonio. 2001. The effects of litter and temperature on the germination 

of native and exotic grasses in a coastal California grassland. Madroño 48:230-235. 

Safford, H. D., and S. Harrison. 2004. Fire effects on plant diversity in serpentine vs. sandstone chaparral. Ecology 

85:539-548. 

San Diego County. 1974. Fuel reduction and brushland management in San Diego County, California. Watershed 

Resources Advisory Commission, County of San Diego, USA. 

Sawyer, J. O., and T. Keeler-Wolf. 1995. A manual of California vegetation. California Native Plant Society, 

Sacramento, California, USA. 

Schmidt, W. 1989. Plant dispersal by motor cars. Vegetatio 80:147-152. 

Schwartz, M. W., D. J. Porter, J. M. Randall, and K. E. Lyons. 1996. Impact of nonindigenous plants. Pages 1203-

1226. Sierra Nevada Ecosystem Project: Final Report to Congress, vol. II. University of California, Centers for 

Water and Wildland Resources, Davis, California, USA. 

Shaw, R. B., and V. E. Diersing. 1990. Tracked vehicle impacts on vegetation at the Pinon Canyon maneuver site, 

Colorado, USA. Journal of Environmental Quality 19:234-243. 

SPSS. 2000. Systat. SPSS, Chicago, Illinois, USA. 

Stohlgren, T. J., D. T. Barnett, and J. T. Kartesz. 2003. The rich get richer: patterns of plant invasions in the United 

States. Frontiers in Ecology and the Environment 1:11-14. 

Stohlgren, T. J., Y. Otsuki, C. A. Villa, M. Lee, and J. Belnap. 2001. Patterns of plant invasions: a case example in 

native species hotspots and rare habitats. Biological Invasions 3:37-50. 

Tyser, R. W., and C. A. Worley. 1992. Alien flora in grassland adjacent to road and trail corridors in Glacier 

National Park, Montana (USA). Conservation Biology 6:253-262. 

US Department of Agriculture. 1959. Four Corners- a cooperative fuel break demonstration project. Pacific 

Southwest Forest and Range Experiment Station, Berkeley, California, USA. 

Literature Cited 68 
The Role of Fuel Breaks in the Invasion of Nonnative Plants 

US Department of Agriculture. 1960. Trabuco Oaks, a cooperative fuel break demonstration project. Pacific 

Southwest Forest and Range Experiment Station, Berkeley, California, USA. 

Wilkinson, L., G. Blank, and C. Gruber. 1996. Desktop data analysis with SYSTAT. Prentice Hall, Upper Saddle 

River, New Jersey, USA. 

Wiser, S. K., R. B. Allen, P. W. Clinton, and K. H. Platt. 1998. Community structure and forest invasion by an 

exotic herb over 23 years. Ecology 79:2071-2081. 

Xiong, S., and C. Nilsson. 1999. The effects of plant litter on vegetation: A meta-analysis. Journal of Ecology 

87:984-994. 

Zink, T. A., M. F. Allen, B. Heindl-Tenhunen, and E. B. Allen. 1995. The effect of a disturbance corridor on an 

ecological reserve. Restoration Ecology 3:304-310. 

Literature Cited 69