Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-1 

APPENDIX D PNW Causal Paper Potential 

Potential Influences of Forest Service Land Management on Invasive Plant 

Species in Pacific Northwest Forests and Rangelands: A Review 

___________________________________________________________ 

D.N. Kimberling1

, C.G. Parks2,B.J. Shanafelt3, D.E. Knecht4, E.J. DePuit3

1 (Formally of) Department of Forest Science, Oregon State University, Corvallis, OR. 

2

USDA Forest Service, Pacific Northwest Research Station, La Grande, OR. 

3

USDA Forest Service, Pacific Northwest Research Station, Wenatchee Forestry Sciences 

Laboratory, Wenatchee, WA. 

4

USDA Forest Service, Pacific Northwest Region, Portland, OR. 

Introduction 

In the Pacific Northwest, conflict and debate have emerged in recent years over how to 

balance ecological, economic, and social values of national forest lands. A primary concern 

relates to the compatibility of land management objectives and practices with the preservation 

or enhancement of native biological diversity. Nonnative invasive plant species are central to 

this issue because land uses and management practices may influence the introduction, 

establishment, and spread of invasive plants. 

The expansion of invasive plants on western federal lands is occurring at an alarming rate. At 

least 17 million acres of western Federal lands were reportedly infested by 1996, more than 

quadrupling their range between 1985 and 1995 (Westbrooks 1998). The Pacific Northwest 

Region (Region 6) of the Forest Service includes 19 national forests, one grassland, and one 

scenic area. These public lands total approximately 26 million acres. Estimates of infested 

area range from 400,000 to 926,000 acres (1.5 to 3.5 percent) that are currently infested by 

one or more of 95 species of invasive plants. Estimates are based on incomplete inventories, 

however, and likely understate the extent of infestations. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-2 

Definitions 

The increase of invasive plants on public lands has brought national attention to the potential 

effect of these species on native ecosystems (see Appendix for effects and literature). In 

tandem with this increase in awareness has been confusion over the broad array of terms used 

to describe and discuss the problem. The terms used most frequently to describe these plants 

include noxious weed, invasive, nonnative, exotic, alien, and introduced. 

The term noxious weed is a legal designation that may be assigned at both the state and 

federal levels. Noxious weed lists differ by state and until recently focused on species that 

had a negative impact on commercial agriculture or rangelands. States have developed laws 

that require the control or elimination of noxious weeds by landowners; however, not all 

invasive plants are designated as state or federal noxious weeds. 

Historically, the Forest Service was directed to deal only with those species officially 

designated as noxious weeds. A recent amendment to the Federal Noxious Weed Act of 1974 

and issuance of Executive Order 13112 on invasive species has broadened that direction to 

include other undesirable plants that threaten native ecosystems. Executive Order 13112 

provides the most current legal direction for managing invasive species on federal lands. For 

consistency, this paper will use the term invasive plant as defined in the Executive Order: “a 

non-native plant whose introduction does, or is likely to, cause economic or environmental 

harm or harm to human health.” Nonnative means, with respect to a particular ecosystem, any 

species that is not indigenous to that ecosystem. 

Objective

Direction for management of invasive plants in national forest and grasslands of the Pacific 

Northwest was included in a 1988 environmental impact statement (EIS) that focused 

primarily on managing competing vegetation in forest plantations, and provided limited 

analysis of invasive plant management (USDA 1988). The EIS was recently challenged in a 

lawsuit brought against the Malheur National Forest in Oregon. One portion of the final 

district court decision in that lawsuit directed the Forest Service in Region 6 to evaluate “the 

causes and increasing spread of noxious weeds.” This document has been prepared to inform 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-3 

the Region 6 response to that directive, as well as future land management planning and 

practices that relate to invasive plants. 

The objective of this paper is to summarize current science-based understanding of the 

interactions between land management practices and the introduction, establishment, and 

spread of invasive plants. Primary land management and use activities in forests that may 

influence introductions and spread of invasive plants include road building and maintenance, 

recreation, logging, and prescribed burns. Grazing as a management action and land use is 

covered in a separate paper (Parks et al. 2003). Together, these two syntheses can be used by 

Forest Service managers to more effectively evaluate the potential impacts of proposed 

actions on the spread of invasive plants in the Pacific Northwest. Knowledge gaps are also 

identified. The information will be incorporated into an Invasive Plant Management EIS 

currently being prepared by Region 6 of the Forest Service. This new EIS is intended to 

replace the 1988 document mentioned above. 

Disturbance and invasions 

Some type of disturbance usually precedes, or in fact, may be an essential precursor to the 

establishment and spread of invasive plants in native forest communities (Fox and Fox 1986; 

Hobbs 1989; Hobbs and Huenneke 1992). Disturbance creates patches of open ground or 

increases the availability of one or more limiting resources. Even when invasions proceed 

without continuing disturbance (e.g., along river and stream corridors), there is often an initial 

disturbance event (Hobbs 1989). 

Definitions of disturbance abound in ecological literature (Hobbs 1991). For this paper, the 

definition of Pickett and White (1985) is applicable: "any relatively discrete event in time that 

disrupts ecosystem, community, or population structure and changes resources, substrate 

availability, or the physical environment." Disturbance regimes are characterized by type, 

size, intensity, timing, and frequency. Our intent is to generally restrict our discussion to 

human-caused disturbance and not include natural disturbance events such as flooding, 

drought, or defoliation by insects or diseases, although these may be additional factors that 

contribute to the spread of invasive plant species. We include discussions on fire because fires 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-4 

can be either natural or human-caused. Further, we review disturbances that occur in 

Northwest forest systems, and how human activities and land-use management may influence 

the probability of plant invasions. 

Invasive plant biology and ecology 

Understanding the invasion process includes knowing invasive species biology, especially 

those traits that make a species a successful invader, and recognizing which environmental 

factors (e.g., type of disturbance) make a habitat susceptible to invasion. Here, we focus first 

on a short review of invasive species biology, followed by a more lengthy discussion on the 

causes of invasive species establishment and spread. 

One of the best known descriptions of an "ideal weed" was developed by Baker (1965). He 

described the ideal weed as an adaptable perennial (e.g., persisting for several years) that is 

tolerant of a wide range of physical conditions, grows quickly, flowers early, is selfcompatible, produces many seeds that disperse widely, can also reproduce vegetatively, and is 

a good competitor. MacArthur and Wilson's r- and K-selected traits further contributed to the 

idea that certain plant life-history strategies lead to invasion success (MacArthur and Wilson 

1967). The problem lies in the fact that there are no consistent life-history traits and 

reproductive behaviors for all invasive plants (Perrins et al. 1992; Williamson and Fitter 

1996). Although invading species often share attributes that allow them to take advantage of 

disturbed areas (e.g., roads), including rapid germination and growth rates, shorter life cycles, 

increased flowering times, and generalist dispersal mechanisms (Hobbs 1991; Perrins et al. 

1992), there are many exceptions. 

Another approach to characterizing plant species with enhanced invasion potential has been to 

examine functional groups. This involves the grouping of invasive species based on common 

growth traits. Noble (1989) suggested that a large flowering or fruiting effort, pulsed 

flowering upon establishment, and good short-distance dispersal abilities are important for 

invasion success. But, he also emphasized that it is the invaded environment as much as the 

characteristics of the invading species that determines invasive success, and that a list of 

broadly based plant life-history strategies is not sufficient to identify invasive species. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-5 

In a related plant-based strategy for understanding the invasion process, McIntyre et al. (1995) 

examined life-history attributes (life form, dispersal unit morphology, and capacity for 

vegetative reproduction) of herbaceous species associated with different disturbance types 

(soil disturbance, grazing, and water enrichment). In particular, soil disturbance associated 

with vehicles, machinery, and roads was a factor in reducing species richness, especially for 

some lifeforms. Plants with persistent buds on stems above the ground (up to 30 cm) and 

those with persistent buds in the ground were most vulnerable to ground disturbance. 

However, the number of species of annual plants increased with increasing soil disturbance 

and the proportion of rosettes with horizontal leaf architecture increased (the horizontal leaf 

architecture may increase light interception and be more competitive for ground space in 

contrast to erect rosettes). The proportion of wind-dispersed species increased with increasing 

soil disturbance, but the number and proportion of species with vegetative reproduction 

declined. The attributes that contribute to success in soil-disturbed sites are the ability to 

colonize open space (e.g., annual plants, wind dispersal) and the capacity to capture resources 

effectively (e.g., flat architecture of rosettes) (McIntyre et al. 1995). 

Many exotic species have been intentionally introduced into North America for commercial 

purposes in agriculture, horticulture, and forestry. Some of these have become invasive. 

Reichard and Hamilton (1997) examined the hypothesis that certain traits of introduced 

woody plants are positively correlated with invasiveness. Although their study was focused on 

intentional introductions, the results may logically be extended to unintentional introductions. 

Vegetative reproductive ability was considered an important trait that promoted invasiveness. 

This result is in contrast to the study by McIntyre et al. (1995) in which the proportion of 

species with vegetative reproduction declined in areas with increasing soil disturbance. 

Although the two studies were focused on different plant growth forms, they both looked at 

potential for invasiveness in terms of life-history traits. These differences in analytical 

outcome emphasize that the traits expressed by different taxonomic groups can be very 

different, and predictive analyses need to be carefully applied. For example, vegetative 

reproduction is rare in annual plants, although it is present in herbaceous perennials (Reichard 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-6 

and Hamilton 1997). Seed production, however, is present in both groups and also can be a 

significant means of dispersal for perennials. 

Grime (1974) argues that a plant responds to stress, competition, and disturbance in its 

environment, and these factors determine the survival and invasion strategies that might be 

successful. Since Grime's research almost three decades ago, many researchers have 

continued to attempt to characterize the traits of invasive species that make them successful in 

disturbed areas (Hobbs 1991; Perrins et al. 1992; Williamson and Fitter 1996). We employ 

this strategy in the following sections with a review of disturbances that can increase the 

likelihood of plant invasions in Pacific Northwest Forests. 

Disturbance by roads and associated activities 

Review of Forest Service road policy 

The Forest Service manages more than 380,000 miles of roads in the National Forest System. 

There have been legal and political debates about the importance of roadless areas in 

wilderness areas for over three decades. (eg., Roadless Area Review and Evaluation, RARE I 

(1972); RARE II (1979). An interim rule, issued in 1999 (64 FR 7289), temporarily 

suspended permanent and temporary road construction and reconstruction in certain unroaded 

areas of National Forest System lands (USDA Forest Service 2000). 

A portion of the Pacific Northwest Forests within the range of the northern spotted owl is 

exempt from the national moratorium, as these forests are managed under the Northwest 

Forest Plan, a regional assessment plan implemented in 1994. However, roadless areas in key 

watersheds are still protected from new road building under RARE II, and a watershed 

analysis must be conducted before any new roads are built in roadless areas in nonkey 

watersheds. In addition, for each new mile of road constructed, another mile of road is to be 

decommissioned (USDA USDI 1994). 

The Forest Service presented a new national road management policy in January 2001. This 

policy includes a science-based roads analysis process that examines the environmental, 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-7 

social, and economic impacts of proposed road construction, reconstruction, maintenance, and 

decommissioning (USDA Forest Service 2001). Once again, the policy is facing legal 

challenges. In addition, a new congressional bill, the National Forest Roadless Act (2002), is 

also being considered. 

Ecological effects of roads in forests 

Roads can be a primary vector for plant invasion because they provide transportation routes to 

remote areas (Tyser and Worley 1992). Although livestock grazing is often cited as an 

important vector for plant invasions, roads provide corridors for invasive species even in areas 

where livestock grazing has been limited. The relationship between transportation systems 

and the spread of invasive species in forest ecosystems is best understood from published 

literature. Roads can serve as corridors for establishment and dispersal, and the distribution of 

some exotic plants (e.g, knapweeds, Centaurea L. spp.) provides evidence of this association 

(Roché and Roché 1988; Tyser and Key 1988; Tyser and Worley 1992). The development of 

new roads and increasing levels of traffic for recreation and commercial purposes may greatly 

increase the potential for seed dispersal of invasive species (Hodkinson and Thompson 1997). 

The network of roads is one important cause of forest fragmentation, and forest fragmentation 

can increase the susceptibility of areas to biological invasions (Hobbs 1989; Reed et al. 1996). 

Fragmentation creates an interface between natural and human-modified systems, and the 

intentional and unintentional transport of invasive species often provides a means for such 

species to disperse into forest systems (Hobbs 1989. Road effects extend outward from roads 

and can be many times wider than the road and roadside surface (Forman 1998; Reed et al. 

1996). The spatial distribution of roads is also important, and roads that are more evenly 

distributed across a landscape can have more impact in forested watersheds than roads that are 

more aggregated (Tinker et al. 1998). Therefore, the actual number of roads (density) in a 

watershed may have less effect on forest fragmentation than how they are distributed (Tinker 

et al. 1998). 

Although most Forest Service roads are usually unpaved, narrow, and remote, they still 

remove native vegetation, disturb soil profiles, and increase erosion and runoff from road 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-8 

surfaces. These conditions can enhance opportunities for plant invasion and reduce the ability 

of native species to compete (Greenberg et al. 1997). Forest Service roads disrupt native 

species distributions and increase human access for logging and recreation. They change 

landscape spatial patterns and, if constructed through closed-canopy forests, negatively affect 

species adapted to interior habitats. “Road ecology,” with its inherent links between spatial 

patterns and ecological processes, is an emerging field with many problems to address 

(Forman 1998). 

Roads and roadside habitats 

Roads and roadside habitats may be particularly susceptible to plant invasions for a number of 

reasons. Roads eliminate some of the physical and environmental barriers that prevent plant 

invasions by increasing light availability and opportunities for dispersal. Microenvironmental 

changes along roads can provide opportunities for invasion because many invasive plants are 

adapted to open, disturbed habitats. Parendes and Jones (2000) found that nonnative species 

were most prevalent in areas with high light levels and high road use in the H.J. Andrews 

Experimental Forest in Oregon. In Australian eucalyptus forests, nonnative plants declined 

with increasing distance from a road, and were also correlated with diffuse light (Amor and 

Stevens 1976). Light levels were also important in a study in central Indiana, where exotic 

species richness and frequency dropped significantly from forest edges to forest interiors. In 

this study, agricultural fields delineated the edges of the forest instead of roads (Brothers and 

Spingarn 1992). 

Because light availability is much lower in forest interiors than along roads, interior sites may 

be somewhat buffered from invasions, unless shade-tolerant nonnative species are present. 

Some invasive species, such as Microstegium vimineum (Trin.)A. Camus, an exotic C4 grass 

in the eastern United States, occur in a variety of habitats from open moist fields, to roadsides 

and forest edges, to deep understory sites. This species can easily establish a monoculture and 

crowd out native species (Horton and Neufeld 1998). 

Roads also change soil density, temperature, and moisture. The exposed surfaces and greater 

sediment transport capacity result in higher erosion rates and increased flooding (Forman and 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-9 

Alexander 1998; Trombulak and Frissell 2000). These abiotic changes can greatly influence 

the ability of native plants to survive and compete with nonnative invasive species. Roadside 

mowing may both reduce plant species richness and favor exotic plants (Forman and 

Alexander 1998). Modified soil properties (e.g., pH) and nutrient availability (e.g., nitrogen 

and phosphorous) can also favor nonnative invasive species with growth requirements better 

matched to such changed conditions than native species (Greenberg et al. 1997). 

Other types of disturbance closely associated with roads and the establishment and spread of 

invasive plants are vehicular traffic and maintenance activities such as road grading, roadside 

mowing, and keeping roads free of fallen or overhanging vegetation. These activities can 

increase exotic plant introductions because vehicles can carry seeds on tires. Although there 

are no studies on these potential effects that are specific to the Northwest, there is one from 

Glacier National Park in Montana. There, the number of nonnative invasive species has 

steadily increased since 1920 and is correlated with the increasing number of visitors who 

travel by automobile (Lesica et al. 1993). Several studies from Australia and Europe indicate 

that vehicles are responsible for transporting some seeds of exotic species along roadsides 

(Hodkinson and Thompson 1997; Lonsdale and Lane 1994; Schmidt 1989). In another study 

of 41 southern Africa nature reserves, there was a significant correlation between the number 

of nonnative invasive species and the number of visitors (Macdonald et al. 1989). They 

hypothesized that the increase in exotic species was due to either direct introduction by 

visitors (e.g., mud on footwear) or propagules being carried on their vehicles. 

An important point from these studies is that unsurfaced roads, such as forest roads, may 

provide greater opportunity for invasive plant dispersal than surfaced roads. The samples 

collected by Lonsdale and Lane (1994) and Hodkinson and Thompson (1997) were from 

vehicles that were driven mainly on surfaced roads. In these cases, the number of seeds per 

vehicle ranged from 0.9 to 2.51 (Hodkinson and Thompson 1997). The number of seeds 

collected from a single vehicle during four sampling times (May, July, September, and 

November) in the Schmidt study ranged from 513 to1330 (Schmidt 1989). Approximately 

one-third of the mileage accumulated by this vehicle was from unsurfaced field and forest 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-10 

roads. In all cases, there was seasonal variation in the number of seeds collected per sample 

on vehicles, and species with small seeds were the most represented. 

In contrast, a more recent study in Utah examined the effect of road improvement on the 

spread of nonnative invasive plants in a semiarid landscape (Gelbard and Belnap submitted). 

In this study, nonnative species richness was nearly twice as great on roadsides adjacent to 

paved, improved surfaces and graded roads than adjacent to four-wheel-drive tracks. Road 

improvement increased the cover and richness of nonnative species in both roadside verges 

and interior communities 50 m from roads. Specifically, the results suggest that improving 10 

km of four-wheel-drive tracks to paved roads converted an average of 12.4 ha of interior 

habitat to roadside plant communities. These communities contained a substantially greater 

richness and cover of nonnative species than the habitat they replaced. 

There are several other pathways for introduction of invasive plants that are indirectly related 

to roads. Recreational use by visitors includes a number of activities that can negatively affect 

the integrity of native plant communities. Although there is not a significant amount of 

published literature about the impact of offroad vehicles (ORVs) or all-terrain vehicles 

(ATVs), one outcome is certain. These vehicles carry the potential to extend vehicular traffic 

effects on ecosystems, including those on invasive plants, much further into forests than 

before their use. Because they leave established roads and trails, they create new 

disturbances in forest and grassland communities. The potential for ORVs to spread invasive 

plants is demonstrated by one study in Montana that showed one ORV is capable of 

distributing 2,000 knapweed seeds in one 10-mile trip (Lacey et al. 1997). Stout (1992) 

documented dispersal of several invasive species by ORVs in West Virginia. The ORVs 

disturb the soil and create seedbeds where new species can establish. Moisture-retaining 

depressions created by tires may further enhance germination probabilities. Resource 

managers and ecologists have noted increases in annual and invasive plants in ORV tracks 

through a variety of habitats (Defenders of Wildlife 2002). 

Vehicular disturbance may also negatively impact native species and subsequently favor 

establishment of nonnative invasive species. Offroad vehicular disturbance of a prairie in 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-11 

western Washington facilitated the replacement of a diverse native bunchgrass community by 

a less diverse community composed of sod-forming grasses and nonnative invasive forbs 

(Clampitt 1993). The prairie had also been modified by alteration of the historical fire regime. 

A regional rare endemic, white-top aster (Aster curtus Cronq.), was also threatened by the loss 

of the native bunchgrass community. If the native bunchgrass cover was reduced below 32 

percent, it was predicted that this native species and others might be permanently replaced by 

weedy exotics. 

The use of trails by hikers or horseback riders is another vector for dispersing invasive plant 

seeds. Although there is little information in the literature on the significance of such 

dispersal, horses and pack animals can disperse seeds in several ways. They can carry seeds 

for many miles along trails that penetrate forest interiors (Marcus et al. 1998). Anecdotal 

information suggests that hay and straw imported into the forest for feed or bedding may lead 

to expansion of invasive plant species. Back-packers can also inadvertently carry seeds on 

their clothing or shoes along trails. 

Logging

Logging and clearcutting are activities that can follow road building in forests. Clearcutting is 

associated with forest fragmentation, and can result in a decrease in core areas, increase in 

edge density, and fundamental changes in biotic and abiotic components of forests (Tinker et 

al. 1998). As habitats are altered, new generalist species or edge-adapted species can be 

favored. The gaps in forest canopy created by roads and clearcuts and the greater amount of 

light reaching the forest floor may increase seed germination and seedling establishment of 

invasive plants previously not adapted to forest interior conditions. For example, an influx of 

seeds of non-native species was detected in clear cuts of a eucalyptus forest in Australia, but 

not in the adjacent, unlogged forests (Appleby 1998). It was suggested that vehicles and 

roadside verges were the likely sources of the seeds of these species. 

The intensity and size of logging activities can influence susceptibility to invasion. 

Reader and Bricker (1994) examined the effects that plot size (area of tree removal), increased 

light, and exposed mineral soil had on the establishment of nonnative forest species in a 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-12 

deciduous forest in southern Ontario, Canada. Nonforest plant species were able to establish 

when 8% irradiance occurred at floor level. An increasing number of nonforest species 

established in larger plots (sizes of 0.01, 0.05 or 0.20 ha) and in plots with greater logging 

intensity (uncut, 33% tree basal area removed, or 66% tree basal area removed). More 

nonforest species established in plots with higher percentages of exposed mineral soil, but 

there was no single minimum area found for establishment. Nonforest plants established in all 

of the plots except uncut plots and in the smallest plots (0.01 ha) where 33% of tree basal area 

was removed. Reader and Bricker (1994) did not distinguish between exotic and native 

species of nonforest invaders, so it is not known what proportion of the species were exotic 

invasive species. They concluded that small disturbances may not lead to nonnative plant 

invasions. Deciduous forests may respond in different ways to disturbance (e.g., logging) 

than coniferous forests, but the relationship between disturbance and plant invasions is 

consistent among many studies (Fox and Fox 1986; Hobbs 1991; Hobbs and Huenneke 1992; 

Hodkinson and Thompson 1997; Mack et al. 2000; Pickett and White 1985). 

Conclusion 

The most thorough understanding of the relationship between roads and invasive plant species 

will come from studies of both environmental and biological attributes (Noble 1989). Because 

roads create new open spaces with more light availability, invasive plants can follow roads by 

natural dispersal mechanisms or be transported along them by vehicles, animals, or humans. 

For this reason, roads can be a primary vector for the spread of invasive species. Logging 

leads to further forest fragmentation and creates patches of open habitat that are also 

susceptible to invasion. 

Disturbance by fire 

The intensity and number of wildfires are increasing in the United States. Relationships 

between fire and invasions by specific exotic plants have been documented and continue to be 

explored (D’Antonio and Vitousek 1992). After a fire, a site can often be more susceptible to 

exotic plant invasions (Milberg and Lamont 1995). Burned areas also are more susceptible to 

spread of invasive plants where invasive plant populations already exist and where there have 

been poor management practices either before or after fire (Goodwin and Sheley 2001). The 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-13 

intent of this discussion is to provide a basic understanding of fire, exotic plant invasions, and 

Forest Service management practices. 

The most important environmental requirements for successful establishment of many 

invasive plants are increased light, open ground, reduced competition, available water, and 

nutrients. Fire provides these conditions, thus providing an enhanced place for invasive plants 

to establish (Goodwin and Sheley 2001; McDaniel et al. 2000; Morghan et al. 1999; Ojima et 

al. 1990; Trabaud 1990). 

Interactions among fire, native communities, and invasive plants 

Successional trajectories in plant communities can be ultimately changed when an invasive 

plant species colonizes a large area (Sheley et al. 1996). When a stand-replacing fire sweeps 

through an area, secondary succession in a plant community is initiated and invasive plants 

are often the pioneer species in this process (Koniak 1985). Three general types of species are 

found after fire occurrences: annuals or biennials that invade the area immediately after the 

fire and disappear with fire exclusion, truly invasive species that persist once established, and 

endemic species that increase with fire (Trabaud 1991). Pioneer species that inhabit a site for 

only a short period after fire are not as much of a concern as some other species, such as 

cheatgrass (Bromus tectorum L.) (Keeley et al. 1981; Keeley and Keeley 1984). 

Cheatgrass is a persistent species throughout much of its range in the West because of the 

synergistic relationship it has with fire (Mack and D’Antonio 1998). Once a fire has occurred, 

cheat grass can move into a burned area and become established. The cheatgrass provides 

ample fuel for future fires because it is highly flammable, especially in summer. Subsequent 

fires become more frequent and more intense, and a positive feedback mechanism develops. 

Permanent conversions of native communities to exotic annual grass communities have 

occurred where this cycle develops (Brooks and Pyke 2001; D’Antonio and Vitousek 1992). 

There is an interaction between the characteristics of fire (e.g., fire interval, season of 

occurrence, and intensity) and the growth characteristics of the plants in an area (Hobbs and 

Huenneke 1992; Keith and Tozer 1997; Rowe 1983). Fire affects individual species in 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-14 

different ways. Lifeform function and adaptation dictate how well a species can escape or 

survive a fire and how quickly it can colonize available niches (Rowe 1983). 

Native plants are often at a disadvantage once a fire has occurred. As fire intensity increases, 

native survivorship declines and invasion potential increases (Goodwin et al. 2002). More 

intense fires reduce litter more completely, increase short-term nutrient availability and 

turnover, and alter soil surface characteristics. Native species that are adapted to cooler fires 

may not survive (Brooks 2002). Also, shorter fire intervals resulting from established 

nonnative annual grasses adversely affect fire-sensitive native species (Brooks 2002; Brooks 

and Pyke 2001; Kucera and Koelling 1964; Wycherley 1984). 

Seedbanks are an important factor in fire ecology. Some native shrub species tend to decrease, 

at least initially, with fire, and if there is opportunity, invasive plants will usurp the 

aboveground space and seedbank potential of those shrubs (Allen 1999; Anderson and Inouye 

2001; Knick and Rotenberry 1997). Regeneration after disturbance is closely tied to the soil 

seedbank, and if a fire degrades the native seedbank, invasive species can be favored, 

(Chambers and MacMahon 1994; Lunt 1990; Shanafelt 2000). 

Fire and management 

Historically, natural fires were an important influence in many forest ecosystems, as they 

reduced fuel buildup in forest understories and promoted healthy stands of fire-adapted 

species. Because fire suppression activities during the 1900s resulted in dangerously high fuel 

loads and high-density stands of even-aged smaller trees, the Forest Service now uses 

prescribed burns to reduce fuel loading and restore structural elements, plant communities, 

and ecosystem properties that characterized forests prior to fire suppression activities. 

However, these man-made fires can have unintended consequences. One of these is creating 

conditions that potentially can promote the spread of invasive species. For example, early 

spring prescribed fires may provide the open space, greater light, and darker surface area 

(with greater heat absorption) that favor winter annuals (Hulbert 1988; Sans and Masalles 

1995). Hobbs and Atkins (1990) found that fall burns also promote seed germination, and 

Lunt (1990) found an increase in exotic species after fall-burning a pasture with an 80-year 

grazing history. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-15 

Conversely, prescribed burns also are used to control invasive species. Some species, such as 

Canada thistle (Cirsium arvense L. Scop.) in tall grasslands, can be controlled by fire 

(Morghan et al. 1999). Research has also shown that annual grasses can be managed, at least 

in the short-term, by careful fire planning so that the plants are burned before dropping seed 

and after the biomass has dried enough to sustain burning conditions (Brooks and Pyke 2001). 

However, this opportunity is very short-lived because of the short life cycles associated with 

annual grasses. 

Predicting which invasive species can be controlled by fire requires knowledge about the life 

cycle and dispersal abilities, as well as understanding how growth tissue responds to flame 

proximity. There is potential to use fire to control some species (e.g., yellow star thistle, 

Centaurea solstitialis L., and some annual grasses), but information is incomplete or 

ambiguous for most species (Brooks and Pyke 2001). The most successful responses include 

some combination of burning and herbicide application (Brooks and Pyke 2001). More 

research is needed on individual invasive species to better understand their responses to fire. 

Prescribed fire alone may not be sufficient to control invasive plants and promote native plant 

success (Lunt 1990). Restoration after fire, especially with respect to reducing potential for 

invasive species, requires an understanding of fire effects, fire history of the specific site, 

existing populations at the site, and behavior of potential new species. Replanting an area with 

fire-resistant native plants may help to stem the flow of invasive species into an area, but 

careful selection of these species and knowledge of their biology is necessary to prevent 

unforeseen problems. For example, kochia (Kochia prostrate L. Schrad) has been considered 

as a living fuelbreak because it tolerates fire and competes with cheatgrass. However, it also 

exhibits invasive behavior itself (Brooks and Pyke 2001). 

Some management practices after fire may increase the probability of invasive species 

establishment. Caling and Adams (1999) studied the effects of bulldozing and wildfire on 

plant species richness and found more exotic species in gaps created by bulldozing than by 

fire. They suggest this might be caused by heavy equipment carrying in seeds and creating 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-16 

open seedbeds for establishment. However, invasive plant seeds germinated more rapidly and 

grew more quickly after fire than when soil was disturbed by tractor scalping in another study 

(Floyd 1966). It is important to assess the impacts of current fire management practices and 

how they may be influencing the likelihood of invasion (Harrod 2001). 

In summary, there is a synergistic relationship between certain invasive plants and fire, 

whereas fire relations of many other invasive species have yet to be firmly demonstrated. 

Some invasive species, particularly annual grasses, can increase the frequency and intensity of 

fire. Fire, in turn, increases the presence and abundance of some invasive species. 

Monitoring and assessment after fires and postfire restoration activities are extremely 

important. Because the biology of many invasive species is poorly understood and there is 

still much to learn about the relationship between fire and specific invasive species, postfire 

restoration monitoring can be critical to understanding plant species responses. One 

management approach to prevent invasions after fire may be to remove invasive species at 

fire edges. This may allow a release of native plants without competition by nonnative plants, 

and the invasion process can be stopped or slowed (Hester and Hobbs 1992). 

It is still not clear whether seeding after fire has positive, neutral, or negative effects on 

nonnative plant invasions during the recovery process after fire, and this is an area of needed 

research. The practice of seeding with nonnative grasses to reduce erosion after fire can have 

some negative consequences. Native plant cover was reduced by 47% in areas that were 

seeded with nonnative grasses and a legume, versus areas that remained unseeded in an 

eastern Cascades forest (Schoennagel and Waller 1999). In addition to providing nonnative 

competition, the artificial seeding disrupted the natural recolonization by native species. 

Conifer seedlings were only half as abundant on seeded plots. As seeding with nonnative 

species did not enhance total plant cover and inhibited reestablishment of native plants and 

conifers, the benefits of this practice were questioned. Whether these negative effects might 

be reversed via artificial seeding of native species after fires is an important research question. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-17 

Summary 

Disturbance can be a precursor to many biological invasions, and we have summarized a few 

of the anthropogenic and natural disturbances that are pertinent to Pacific Northwest forest 

management. The single factor for invasion risk most common to all of these disturbances is 

ground disturbance. The greater the extent and intensity of ground disturbance, the more 

likely an invasive plant will be successful in a native plant community (Crawley 1987; Evans 

and Young 1972; Hobbs 1989). 

Many forests experience multiple disturbances, and the effects are not merely additive. 

Management practices can have a synergistic effect in altering native plant communities and 

enhancing probabilities of plant invasions. Natural disturbances can also add to the synergy. 

Roads and trails can interact with fire to increase susceptibility to plant invasions by 

increasing the amount of exposed mineral soil (Greenberg et al. 1997; Hobbs 1991; Harrod 

2001). Invasive roadside plants have established in newly burned areas that were previously 

intact native communities (Milberg and Lamont 1995). 

Despite the known linkages between disturbance and exotic plant invasions, much remains to 

be learned about invasion success. Some disturbance types can be managed (grazing regimes, 

fire, road construction) while others cannot (floods, drought, storms). Understanding the 

interaction between environmental (or physical) factors and the biology of invasive species 

will be critical for not only managing current infestations, but in predicting where the next 

invasion will occur. 

The issues of nonnative invasive plants must be addressed from both short-term and long-term 

perspectives. Short-term efforts may strive to eradicate or contain existing populations of 

exotic species. These efforts also may strive to implement daily management actions in ways 

that do not encourage new introductions or exacerbate the spread of existing populations. 

Long-term needs include research, monitoring, and adaptive management projects that focus 

on restoration of landscapes that are currently infested, or at risk to infestation, by invasive 

plants. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-18 

Literature cited 

Allen, C. 1999. USGS studies of wildlife ecology in the western United States. United States 

Department of the Interior, USGS, Reston, VA. 

Amor, R.L. and P.L. Stevens. 1976. Spread of weeds from a roadside into sclerophyll forests 

at Dartmouth, Australia. Weed Research 16:111-118. 

Anderson, J.E. and R.S. Inouye. 2001. Landscape-scale changes in plant species abundance 

and biodiversity of a sagebrush steppe over 45 years. Ecological Monographs 71:531-

556. 

Appleby, M.W.A. 1998. The incidence of exotic species following clearfelling of Eucalyptus 

regnans forest in the Central Highlands, Victoria. Australian Journal of Ecology 

23:457-465. 

Asher, J E. and D. W. Harmon. 1995. Invasive exotic plants are destroying the naturalness of 

U.S. Wilderness Areas. International Journal of Wilderness 1(2):35-37. 

Baker, H.G. 1965. Characteristics and modes of origin of weeds. pp. 147-169. In H.G. Baker 

and C.L. Stebbins (editors). The Genetics of Colonizing Species. Academic Press, New 

York. 

Bautista, S. 1998. A comparison of two methods for controlling Arundo donax. In 

Proceedings of the Arundo and Saltcedar Workshop; Ontario, California 49-52. 

Bedunah, D. and J. Carpenter. 1998. Plant community response following spotted knapweed 

(Centaurea maculosa) control on three elk winter ranges in western Montana. In 

Proceedings of the 1998 Knapweed Symposium, Montana State University. Bozeman, 

MT 59717. 

Belcher, J.W and S D. Wilson. 1989. Leafy spurge and the species composition of a mixedgrass prairie. Journal of Range Management 42(2):172-175. 

Blossey, B. 1999. Before, during and after: the need for long-term monitoring in invasive 

plant species management. Biological Invasions 1(2/3):301-311. 

Bonnivier, G. 1999. Idaho field notes. The Nature Conservancy, Spring/Summer 8. 

Bridges, D.C. 1994. Impact of weeds on human endeavors. Weed Technology 8(2):392-395. 

Bright, C. 1996. Understanding the threat of bio-invasions. In State of the World: A 

Worldwatch Institute Report on Progress toward a Sustainable Society. L. Starke 

(editor). W. W. Norton and Company, New York: 95-113. 

Brooks, M.L. 2002. Peak fire temperatures and effects on annual plants in the Mojave Desert. 

Ecological Applications 12:1088-1102. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-19 

Brooks, M.L. and D.A. Pyke. 2001. Invasive plants and fire in the deserts of North America. 

Tall Timbers Research Station. Misc. Pub., Report no. 11, Allen Press, Inc. 

Tallahassee, FL. 

Brothers, T.S. and A. Spingarn. 1992. Forest fragmentation and alien plant invasion of central 

Indiana old-growth forests. Conservation Biology 6:91-100. 

Caling, T.M. and R. Adams. 1999. Ecological impacts of fire suppression operations in a 

small vegetation remnant. In Flammable Australia: the Fire Regimes and Biodiversity 

of a Continent, Australian Bushfire Conference. School of Environmental and 

Information Sciences, Charles Sturt University, Albury, New South Wales, Australia. 

Chambers, J.C. and J.A. MacMahon. 1994. A day in the life of a seed: movements and fates 

of seeds and their implications for natural and managed systems. Annual Review of 

Ecology and Systematics 25:263-292. 

Cheater, M. 1992. Alien invasion. Nature Conservancy. September/October 25-29. 

Clampitt, C.A. 1993. Effects of human disturbances on prairies and the regional endemic 

Aster curtus in western Washington. Northwest Science 67: 163-169. 

Cooperider, A. 1995. State of the biome uniqueness, biodiversity, threats and the adequacy of 

protection in the Sonoran bioregion. In: The Wildlands Project. Tucson, Arizona. 

Center for Plant Conservation (CPC). 2002. Under attack alien plants steal native flora 

habitat. St. Louis, Mo: Center for Plant Conservation 15(3):5. 

Cranston, R. 1980. British Columbia: Knapweed Action Committee, B.C. Ministry of 

Agriculture Field Crops Branch. 

Crawley, M.J. 1987. What makes a community invasible?. In Colonization Succession and 

Stability. A.J. Gray, M.J. Crawley, and P.J. Edwards (editors). Blackwell Scientific 

Publications, Oxford, England: 429-453 

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. 

Defenders of Wildlife. 2002. The impacts of off-road vehicles and roads on wildlife and 

habitat in Florida's National Forests. Defenders of Wildlife, Washington, D.C. 

Duncan, C.L. 1997. Environmental impacts of noxious weeds. Techline, Ag West 

Communications July:10. 

Evans, R.A. and J.A. Young. 1972. Microsite requirements for establishment of annual 

rangeland weeds. Weed Science 20:350-356.
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-20 

Flather, C.H., L.A. Joyce, and C.A. Bloomgarden. 1994. Species endangerment patterns in the 

United States. USDA Forest Service, General Technical Report RM-241. Ft. Collins, 

Colorado.

Floyd, A.G. 1966. Effect of fire upon weed seeds in the wet sclerophyll forests of northern 

New South Wales. Australian Journal of Botany 14:243-256. 

Forman, R.T. 1998. Road ecology: a solution for the giant embracing us. Landscape Ecology 

13:iii-v. 

Forman, R.T. and L.E. Alexander. 1998. Roads and their major ecological effects. Annual 

Review of Ecology and Systematics 29:207-231. 

Fox, M.D. and B.J. Fox. 1986. The susceptibility of natural communities to invasion. pp. 57-

66. In Ecology of Biological Invasions: an Australian Perspective. R.H. Groves and 

J.J. Burdon (editors). Australian Academy of Science, Canberra. 

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

landscape. Conservation Biology revised and resubmitted 

Gerlach, J.D. and K. Rice. 1996. Centaurea solstitialis (yellow starthistle) threatens 

California's ecosystems. Bull. Ecol. Society of America 77:160. 

Goodwin, K., R. Sheley, and J. Clark. 2002. Integrated noxious weed management after 

wildfires. Montana State University Extension Service, Report nr EB-160, Bozeman, 

MT., 45 pp. 

Goodwin, K.M. and R.L. Sheley. 2001. What to do when fires fuel weeds. Rangelands 23:15-

21. 

Gordon, D R. 1998. Effects of invasive, non-indigenous plant species on ecosystem 

processes: lessons from Florida. Ecological Applications 8(4):975-989. 

Greenberg, C.H., S.H. Crownover, and D.R. Gordon. 1997. Roadside soils: a corridor for 

invasion of xeric scrub by nonindigenous plants. Natural Areas Journal 17:99-109. 

Grime, J.P. 1974. Vegetation classification by reference to strategies. Nature 250:26-31. 

Harrod, R.J. 2001. The effect of invasive and noxious plants on land management in eastern 

Oregon and Washington. Northwest Science (Special issue)75:85-90. 

Harvey, S.J. and R.M. Nowierski. 1989. Spotted knapweed: allelopathy or nutrient depletion? 

In Proceedings of the 1989 Knapweed Symposium, Montana State University. 

Bozeman, MT 59717. 

Hester, A.J. and R.J. Hobbs. 1992. Influence of fire and soil nutrients on native and nonnative annuals at remnant vegetation edges in the Western Australian wheatbelt. 

Journal of Vegetation Science 3:101-108. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-21 

Hobbs, R.J. 1989. The nature and effects of disturbance relative to invasions. pp. 389-405 In 

Biological Invasions. A Global Perspective. SCOPE 37. J.A. Drake, H.A. Mooney, F. 

di Castri, R.H. Groves, F.J. Kruger, M. Rejmanek, and M. Williamson (editors). John 

Wiley & Sons, Chichester, Great Britain. 

Hobbs, R.J. 1991. Disturbance a precursor to weed invasion in native vegetation. Plant 

Protection Quarterly. 6:99-104. 

Hobbs, R.J. and L. Atkins. 1990. Fire-related dynamics of a Banksia woodland in 

southwestern Western Australia. Australian Journal of Botany 38:97-110. 

Hobbs, R.J. and L.F. Huenneke. 1992. Disturbance, diversity, and invasion: implications for 

conservation. Conservation Biology 6:324-337. 

Hobbs, R.J. and H.A. Mooney. 1993. Restoration ecology and invasions. pp. 127-133 In 

Nature Conservation 3: Reconstruction of fragmented ecosystems. D.A Saunders, R.J. 

Hobbs, and P.R. Ehrlich (editors). Surrey Beatty and Sons. 

Hodkinson, D.J. and K. Thompson. 1997. Plant dispersal: the role of man. Journal of Applied 

Ecology 34:1484-1496. 

Horton, J.L. and H.S. Neufeld. 1998. Photosynthetic responses of Microstegium vimineum

(Trin.) A. Camus, a shade-tolerant, C4 grass, to variable light environments. 

Oecologia 114:11-19. 

Huenneke L.F. 1996. Ecological impact of invasive plants in natural resource areas. 

Proceedings of Western Society of Weed Science 49:119-121. 

Hulbert, L.C. 1988. Causes of fire effects in tallgrass prairie. Ecology 69:46-58. 

Johnson, K.H., R.A. Olson, T.D. Whitson, R.J. Swearingen, and G.L. Jurz. 1994. Ecological 

implications of Russian knapweed infestation: small mammal and habitat associations. 

Proceedings of Western Weed Science Society 47: 98-101. 

Keeley, J.E. and S.C. Keeley. 1984. Postfire recovery of California coastal sagebrush. 

American Midland Naturalist 111:105-117. 

Keeley, S.C., J.E. Keeley, S.M. Hutchinson, and A.W. Johnson. 1981. Postfire succession of 

the herbaceous flora in southern California chaparral. Ecology 62:1608-1621. 

Keith, D.A. and M.G. Tozer. 1997. Experimental design and resource requirements for 

monitoring flora in relation to fire. In Proceedings Australian Bushfire Conference 8-

10 July 1997. B.J. McKaige, R.J. Williams, and W.M. Waggitt (editors). CSIRO 

Tropical Ecosystems Research Centre, Darwin, Northern Territory, Australia. 

Knick, S.T. and J.T. Rotenberry. 1997. Landscape characteristics of disturbed shrubsteppe 

habitats in southwestern Idaho (USA). Landscape Ecology 12:287-297. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-22 

Koniak, S. 1985. Succession in pinyon-juniper woodlands following wildfire in the Great 

Basin. Great Basin Naturalist 45:556-566. 

Kucera, C.I. and M. Koelling. 1964. The influence of fire on composition of central Missouri 

prairie. American Midland Naturalist 72:142-147. 

Lacey, J.R., C.B. Marlow, and J.R. Lane. 1989. Influence of spotted knapweed (Centaurea 

maculosa) on surface runoff and sediment yield. Weed Technology 3:627-631. 

Lesica, P., K. Ahlenslager, and J. Desanto. 1993. New vascular plant records and the increase 

of exotic plants in Glacier National Park, Montana. Madrono 40(2):126-131. 

Londsdale, W.M. and A.M. Lane. 1994. Tourist vehicles as vectors in weed seeds in Kakadu 

National Park, northern Australia. Biological Conservation 69:277-283. 

Lunt, I.D. 1990. Impact of an autumn fire on a long-grazed Themeda triandra (Kangaroo 

grass) grassland: implications for management of invaded, remnant vegetations. The 

Victorian Naturalist 107:45-51. 

MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton 

University Press, Princeton, New Jersey. 

Macdonald, I.A.W., L.L. Loope, M.B. Usher, and O. Hamann. 1989. Wildlife conservation 

and the invasion of nature reserves by introduced species: A global perspective. pp. 

215-255 In Biological Invasions. A Global Perspective. SCOPE 37. J.A. Drake, H.A. 

Mooney, F. di Castri, R.H. Groves, F.J. Kruger, M. Rejmanek, and M. Williamson 

(editors). John Wiley & Sons, Chichester, Great Britian. 

Mack, R.N. 1981. Invasion of Bromus tectorum into western North America : an ecological 

chronicle. Agroecosystems. 7:145-65. 

Mack, M.C. and C.M. D’Antonio. 1998. Impacts of biological invasions on disturbance 

regimes. Trends in Ecology and Evolution 13:195-198. 

Mack, R.N., D. Simberloff, W. M. Lonsdale, H. Evans; M. Clout, and F. Bazzaz. 2000. 

Biotic invasions: causes, epidemiology, global consequences and control. Ecological 

Applications 10:689-710. 

Marcus, W.A., G. Milner, and B. Maxwell. 1998. Spotted knapweed distribution in stock 

camps and trails of the Selway-Bitterroot Wilderness. Great Basin Naturalist 58(2): 

156-166. 

Masters, R.A and R.L. Sheley. 2001. Invited synthesis paper: principles and practices for 

managing rangeland invasive plants. Journal of Range Management 54:502-517. 

McDaniel, K.C., D.B. Carroll, and C.R. Hart. 2000. Broom snakeweed establishment 

following fire and herbicide treatments. Journal of Range Management 53:239-245. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-23 

McIntyre, S., S. Lavorel, and R.M. Tremont. 1995. Plant life-history attributes: their 

relationship to disturbance response in herbaceous vegetation. Journal of Ecology 

83:31-44. 

Milberg, G.P. and B.B. Lamont. 1995. Fire enhances weed invasion of roadside vegetation in 

southwestern Australia. Biological Conservation 73:45-49. 

Mooney, H.A. and E.E. Cleland. 2001. The evolutionary impact of invasive species. 

Proceedings of the National Academy of Science, Washington D.C. 

Morghan, K.J., T.R. Seastedt, P.J. Sinton. 1999. Frequent fire enables ungrazed tallgrass 

prairie to resist invasion by Cirsium arvense (Canada thistle). 

http://www.geocities.com/Rainforest/2579/thistlepub.html. 

Mullin, B.H., L.W. Anderson, J.M. Di'Tomaso, R.E. Eplee, and K.D. Getsinger. 2000. 

Invasive plant species. In: Council for Agricultural Science and Technology -Issue 

Paper 13. 

Noble, I.R. 1989. Attributes of invaders and the invading process: terrestrial and vascular 

plants. pp. 303-313 In Biological Invasions. A Global Perspective. SCOPE 37. J.A. 

Drake, H.A. Mooney, F. di Castri, R.H. Groves, F.J. Kruger, M. Rejmanek, and M. 

Williamson (editors). John Wiley & Sons, Chichester, Great Britain. 

Ojima, D.S., W.J. Parton, D.S. Schimel, C.E. Owensby. 1990. Stimulated impact of annual 

burning in prairie ecosystems. pp. 118-132. In Fire in North American Tallgrass 

Prairies. S.L. Collins and L.C. Wallace (editors). University of Oklahoma Press, 

Norman Oklahoma. 

Olson, B.E. 1999 Impacts of noxious weeds on ecologic and economic systems. In Biology 

and Management of Noxious Rangeland Weeds. R.L. Sheley and J. K. Petroff 

(editors). Corvallis, Oregon: OSU Press. 

Panetta, F.D. and J.M. Hopkins. 1991. Weed in corridors: invasion and management. pp.341-

351 In D.A. Saunders and R.J. Hobbs (editors). Nature Conservation 2: The Role of 

Corridors. Chipping Norton, Australia: Surrey Beatty & Sons. 

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 11:64-75. 

Parker, I.M., D. Simberloff, W.M. Lonsdale, K. Goodell, M. Wonham, P.M. Kareiva, M.H. 

Williamson, B. Von Holle, P.B. Moyle, J.E. Byers, and L. Goldwasser. 1999. pp.3-19

In Impact: toward a framework for understanding the ecological effects of invaders. 

The Netherlands: Kluwer Academic Publishers. 

Perrins, J., M. Williamson, and A. Fitter. 1992. Do annual weeds have predictable characters? 

Acta Œcologica 13:517-533. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-24 

Peters, E.F. and S.C. Bunting. 1994. Fire conditions pre- and post-occurrence of annual 

grasses on the Snake River plain. pp.31-36 In Proceedings: Ecology and Management 

of Annual Rangelands. S.B. Monson and S.G. Kitchen (editors). USDA FS 

Intermountain Research Station. INT-GTR-313. Ogden, UT. 

Pickett, S.T.A. and P.S. White. 1985. The Ecology of Natural Disturbance in Patch Dynamics. 

Academic Press, New York. 

Randall, J.M. 1995. Weeds and natural areas management. pp.23-28. In Proceedings of the 

Sixteenth Annual Vegetation Management Conference. Sacramento, CA. Jan 10-12: 

Shasta County Opportunity Center. 

Randall, J. M. 1996. Weed control for the preservation of biological diversity. Weed 

Technology 10:370-383. 

Randall, J.M. 1997. Defining weeds of natural areas. In Assessment and Management of Plant 

Invasions. J.O. Luken and W. Thieret (editors). Springer, New York, NY. 

Reader, R.J. and B.D. Bricker. 1994. Barrier to establishment of invading, non-forest plants in 

deciduous forest nature reserves. Environmental Conservation 21:62-66. 

Reed, R.A., J. Johnson-Barnard, and W.L. Baker. 1996. Contribution of roads to forest 

fragmentation in the Rocky Mountains. Conservation Biology 10:1098-1106. 

Reichard, S.H. and C.W. Hamilton. 1997. Predicting invasions of woody plants introduced 

into North America. Conservation Biology 11:193-203. 

Rice, P. M., C.A. Lacey, J.R. Lacey, and R. Johnson. 1991. Sulfur cinquefoil: Biology, 

Ecology and Management in Pasture and Rangeland. Extension Bulletin 109. 

Bozeman, MT: Montana State University, Extension Service. 9 p. 

Rice, P. M., D. Bedunah, and C. Carlson. 1992. Plant community diversity after herbicide 

control of spotted knapweed. USDA Forest Service, Intermountain Research Station. 

Research Paper INT-460. 

Roché, C.T. and B.F. Roché Jr. 1988. Distribution of four knapweed species in eastern 

Washington. Northwest Science 62:242-253. 

Rowe, J.S. 1983. Concepts of fire effects on plant individuals and species. pp. 135-154 In

The Role of Fire in Northern Circumpolar Ecosystems. R.W. Wein and D.A. Maclean 

(editors). John Wiley & Sons, New York. 

Sakai, A.K.; F.W. Allendorf, J.S. Holt, D.M. Lodge, J. Molofsky, K.A. With, S. Baughman, 

R.J. Cabin, J.E. Cohen, N.C. Ellstrand, D.E. McCauley, P. O'Neil, I.M. Parker, J.N. 

Thompson, and S.G. Weller. 2001. The population biology of invasive species. 

Annual Review of Ecology and Systematics 32:305-332. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-25 

Sans, F.X. and R.M. Masalles. 1995. Phenological patterns in an arable land weed community 

related to disturbance. Weed Research 35:312-332. 

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

Schmidt, K.A. and C.J. Whelan. 1999. Effects of exotic Lonicera and Rhamnus on songbird 

nest predation. Conservation Biology 13:1502-1506. 

Schoennagel, T.L. and D.M. Waller. 1999. Understory responses to fire and artificial seeding 

in an eastern Cascades Abies grandis forest, U.S.A. Canadian Journal of Forest 

Research 29:1393-1401. 

Shanafelt, B.J. 2000. Effects of control measures on diffuse knapweed, plant diversity, and 

transitory soil seed-banks in eastern Washington (unpublished dissertation). 

Washington State University, Pullman, WA. 

Sheley, R.L., and J.K. Petroff, eds. 1999. Biology and management of noxious rangeland 

weeds. Oregon State University Press, Corvallis, OR. 

Sheley, R.L., T.J. Svejcar, and B.D. Maxwell. 1996. A theoretical framework for developing 

successional weed management strategies on rangeland. Weed Technology 10:766-

773. 

Stalling, D. 1998. An exotic invasion of elk country. Bugle: Journal of Elk and the Hunt, 

Rocky Mountain Elk Foundation. July/August:18-27. 

Stevens, K. L. 1986. Allelopathic polyacetylens from Centaurea repens (Russian Knapweed). 

Journal of Chemical Ecology 12(6):1205-1211. 

Stout, B.M. III. 1992. Impact of off-road vehicle use on vegetative communities of northern 

Canaan Valley, West Virginia. Final report of the Canaan Valley Task Force, 

Wheeling Jesuit College, Department of Biology, Wheeling, W. VA. 

Tinker, D.B., C.A.C. Resor, G.P. Beauvais, K.F. Kipfmueller, C.I. Fernandes, and W.L. 

Baker. 1998. Watershed analysis of forest fragmentation by clearcuts and roads in a 

Wyoming forest. Landscape Ecology 13:149-165. 

Tisdall, J.M. and J. Oades. 1982. Organic matter and water-stable aggregates in soils. Soil 

Science 33:141-163. 

Trabaud, L. 1990. Fire as an agent of plant invasion? A case study in the French 

Mediterranean vegetation. pp. 417-437. In F. Di Castri, A.J. Hansen, and M. 

Debussche (editors). Biological invasions in Europe and the Mediterranean Basin. 

Kluwer Academic Press, Netherlands. 

Trabaud, L. 1991. Is fire an agent favoring plant invasions? pp. 170-190 In R.H. Groves and 

F. Di Castri (editors). Biogeography of Mediterranean Invasions. Cambridge 

University, New York. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-26 

Trombulak, S.C. and C.A. Frissell. 2000. Review of ecological effects of roads on terrestrial 

and aquatic plant communities. Conservation Biology 14:18-30. 

Tyser, R.W. and C.H. Key. 1988. Spotted knapweed in natural area fescue grasslands: an 

ecological assessment. Northwest Science 62:151-160. 

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

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

United States Department of Agriculture Forest Service. 1988. Managing competing and 

unwanted vegetation. Final environmental impact statement. Pacific Northwest 

Region, Portland, OR. 

United States Department of Agriculture Forest Service. 2000. Notice of proposed 

rulemaking; request for comment. Administration of the forest development 

transportation system; prohibitions; use of motor vehicles off Forest Service roads. 

Federal Register 45 (43) 11680-11683. 

United States Department of Agriculture Forest Service. 2001. Proposed rules. National 

Forest System land and resource management planning; special areas; roadless area 

conservation. Federal Register 66: 3244. 

United States Department of Agriculture Forest Service and United States Department of the 

Interior Bureau of Land Management. 1994. Record of decision for amendments to 

Forest Service and Bureau of Land Management Planning documents within the range 

of the northern spotted owl. Standards and guidelines for management of habitat for 

late-successional and old-growth forest related species within the range of the northern 

spotted owl, Attachment A. Washington, D.C. pp 74. 

Vitousek, P.M., C.M. D'Antonio, L.L. Loope, and R.Westbrooks. 1996. Biological invasions 

as global environmental change. American Scientist 84:468-478.

Westbrooks, R. 1998. Invasive plants, changing the landscape of America. Fact book. 

Federal Interagency Committee for the Management of Noxious and Exotic Weeds 

(FICMNEW), Washington, D.C. 109 pp. 

Whisenant, S. G. 1990. Changing fire frequencies on Idaho's Snake River plants: ecological 

and management implications. In Proceedings from Symposium on cheatgrass shrub 

die-off and other aspects of shrub biology and management; April 1989, Las Vegas, 

Nevada. 

Williamson, M.H. and A. Fitter. 1996. The characters of successful invaders. Biological 

Conservation 78:163-170. 

Wycherley, P. 1984. People, fire and weeds: can the vicious spiral be broken? pp. 11-17 In

The Management of Small Bush Areas in the Perth Metropolitan Area. S.A. Moore 

(editor). Department of Fisheries and Wildlife, Perth, Australia. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-27 

Appendix 

AUTHOR AND YEAR IMPACTS OF INVASIVE PLANTS 

Impacts to Native Plant Communities 

Bonnivier 1999 Some invasive plants (e.g., yellow starthistle) begin growing far 

earlier in the spring than the native community and deplete soil 

moisture before the natives can make use of it. 

CPC 2002 Invasive species may disrupt pollination by obscuring flowers, 

distracting pollinators, or interfering with fragrance cues. Seed 

dispersal of natives may be reduced if invasive plants obscure their 

seeds. 

D'Antonio and Vitousek 

1992; Tyser and Key 1988 

Some biological invasions alter ecosystem processes in invaded 

areas, thereby causing functional as well as compositional change. 

D'Antonio and Vitousek 

1992 

A major consequence of the competitive success of alien grasses is 

the slowing or alteration of succession. 

Gerlach and Rice 1996 Using deeper root systems, many invasive plants continue growth 

into summer after native cool-season grasses are dormant, possibly 

affecting next season's soil moisture and nutrient availability. 

Hobbs and Mooney 1993 Invasive species can form new metastable states which deflect or 

halt the normal dynamics of the community. 

Mack and D'Antonio 1998; 

Bright 1996 

Evidence suggests that invaders alter disturbance regimes and that 

alteration of disturbance regime may be the most profound effect 

that a species can have on an ecosystem. 

Mack et al. 2000; Randall 

1996; Parker et al. 1999 

Invasive plant species can eliminate native plants through 

hybridization. For example, the North American codgrass 

Spartina alterniflora has hybridized with the European S. 

maritima to produce a new polyploid species (S. anglica) that is 

more invasive in Great Britain than the original form. 

Mack et al. 2000; Randall 

1996; Belcher and Wilson 

1989; Rice et al. 1994; Tyser 

and Key 1988 

Invasive species can reduce vigor or eliminate native plants 

through competition. This can lead to alterations in plant 

community composition. 

Masters and Sheley 2001 Invading species may also cause minimal change to ecological 

processes, and may simply increase species richness. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-28 

Appendix 

AUTHOR AND YEAR IMPACTS OF INVASIVE PLANTS 

Mooney and Cleland 2001 Invasives can alter basic evolutionary processes. "Evolutionary 

trajectories may be altered if any of the following situations occur: 

native species adapt to or hybridize with invasives, invasive 

species cause behavioral or trait shifts in natives, native species 

are ecologically displaced or excluded by invasives, mutualistic 

relationships between native species are disrupted by invasives, or 

invasive species cause the extinction of native species." 

Mullin et al. 2000; Duncan 

1997 

Invasive plants such as purple loosestrife can replace or suppress 

native wetland communities beyond control capacity. Wetland loss 

to weeds is in addition to loss from urban, agricultural, and 

industrial uses. 

Olson 1999 Weeds may usurp soil water from native species by (1) greater use 

earlier in growing season, (2) more rapid uptake when soil 

moisture is replenished by rain, (3) more effective use of deep soil 

moisture. 

Olson 1999 Invasive plants may immobilize "more than their share of 

nutrients" thereby limiting opportunities for natives to establish 

and thrive. 

Panetta and Hopkins 1991; 

Blossey 1999 

Invasive plants dominate in part by suppressing recruitment of 

native plant species, especially slow growers and ones with small 

seeds (which could be most of the native flora in some areas). 

Panetta and Hopkins 1991 Invasion by nonnatives may move plant communities "to new 

metastable states, often containing fewer species." Simplified 

communities may be "more resilient in statistical terms," yet 

"more vulnerable to further, long term degradation." 

Rice et al. 1992 Invasive plants can reduce the vigor of native plant populations. 

Sakai et al. 2001 Invasive species may have both ecological and genetic impacts on 

the communities that they invade. This could lead to rapid 

evolutionary changes in the native species. 

Parker et al. 1999 The impact of an invader can be measured at five levels: effects on 

individuals, genetic effects, population dynamic effects, 

community effects, effects on ecosystem processes. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-29 

Appendix 

AUTHOR AND YEAR IMPACTS OF INVASIVE PLANTS 

Stevens 1986 Many invasive plants are allelopathic. For example, Russian 

knapweed "appears to produce chemicals that inhibit the growth of 

competing vegetation." 

Tyser and Key 1988 Species richness of "undisturbed" plant community declined as 

spotted knapweed invaded. 

Impacts to Fire Regime 

Bright 1996; Mack 1981; 

Mack et al. 2000; Whisenant 

1990; Mullin et al. 2000 

Invasive species can foster more frequent or intense fires, to which 

native species are not adapted. 

D'Antonio and Vitousek 

1992; Peters and Bunting 

1994 

The most significant effects of alien grasses on ecosystems result 

from interactions between grass invasion and fire. 

Wycherly 1984 Invasive plants alter "the nature and spatial array of the fuel 

particles in the fuel bed," which alters the fire regime and "leads 

inevitably to losses in native plant species" (such weed invasions 

are "self-promoting"). 

Impacts to Soils/Hydrology 

Blossey 1999 Impacts from invasive plants include alterations of 

biogeochemical cycling (Tamarix spp.), alterations of 

geomorphological processes (Ammophila arenaria), and changes 

in hydrological cycles (Melaleuca quinquenervia). 

Cooperider 1995 Tamarisk invasions can eliminate surface water in springs and 

streams. 

Gordon 1998 In Florida, 40 to 65 percent of invasive plant species affected 

ecosystem processes including soil structure, hydrology, 

biogeochemistry, or disturbance regimes. 

Harvey and Nowierski 1989 Invasive plants may deplete soil nutrient reserves. For example, 

knapweed infestation lowered potassium 44%, nitrogen 62%, and 

phosphorous 88% compared to adjacent grassland soil. 

Lacey et al. 1989 Invasive plants can increase sediment yields. For example, runoff 

was 56% higher, and sediment yield 192% higher for plots with 

knapweed cover as compared to bunchgrass cover. 

Mullin et al. 2000 Invasive plants such as tamarisk can increase soil salinity and 

displace riparian hardwoods. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-30 

Appendix 

AUTHOR AND YEAR IMPACTS OF INVASIVE PLANTS 

Olson 1999 Secondary compounds in soil at noxious weed sites may decrease 

activity and growth of aerobic soil microbial populations, slowing 

nutrient cycling. 

Olson 1999 Invasive plants with sparse canopies may allow increased 

evaporation from the soil surface, inhibiting seedling survival of 

shallower rooted species. Sparse canopies may also increase 

temperature extremes at the soil surface by allowing more sunlight 

to reach the ground and more heat loss at night. 

Olson 1999 Dense weed infestations can reduce infiltration and increase 

runoff, altering seasonal water flows. Adequate stream flows to 

support fish populations depend partly on subsurface water 

supplied by infiltration. 

Tisdall and Oades 1982 Tap-rooted weeds may reduce infiltration compared to dense, finerooted grasses that they displace. 

 Impacts to Wildlife 

Bautista 1998 Giant reed in California eliminates native streamside vegetation 

and dries up creeks that provide habitat for four endangered 

species; least bell’s vireo, southwestern willow flycatcher, 

California red-legged frog, and unarmored threespine stickleback. 

Duncan 1997 On elk winter range in Montana, forage (grass) loss from spotted 

knapweed can be as high as 1620 lbs. per acre. 

Duncan 1997 Displacement of native vegetation by invasive species 

significantly changes bird species composition and small mammal 

populations. 

Johnson et al. 1994 Russian knapweed infested areas had severely reduced populations 

of kangaroo rats and ground squirrels in Wyoming. 

Mullin et al. 2000 Invasive plants can cause injury to animals. Examples include 

yellow starthistle (Centaurea solstitialis) which causes "chewing 

disease" in horses and medusahead awns (Taeniatherum caputmedusae) which can get lodged in mouth and eyes. 

Panetta and Hopkins 1991 Lizards in an Australian study lost feeding habitat to invasive 

plants filling spaces between native perennials. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-31 

Appendix 

AUTHOR AND YEAR IMPACTS OF INVASIVE PLANTS 

Schmidt and Whelan 1999 Exotic bush honeysuckle and common buckthorn in Illinois allow 

significantly higher nest mortality to American robins and wood 

thrushes than for nests constructed in native plants. 

Stalling 1998 Spotted knapweed invasion in North Dakota reduced bison forage 

by 83% and deer and elk forage by 70% on parts of the Theodore 

Roosevelt National Park. 

Mack et al. 2000; Bedunah 

and Carpenter 1998; Mullin 

et al. 2000; Cranston 1980; 

Sheley and Petroff 1999 

Invasive plants displace native plant populations and limit the 

vegetative resources available to wildlife. For example, spotted 

knapweed has been shown to be an aggressive competitor that 

decreases animal forage by up to 88%. 

Economic Impacts 

Bridges 1994 The economic impact of weed control to the forestry industry is 

approximately 60 million dollars annually. The total economic 

impact of weed management on non-cropland sites approximates 

one billion annually. 

Mullin et al. 2000 Invasive plants decrease the economic value of infested land. For 

example, spotted knapweed impacts the livestock industry by 

>$42 million per year. 

Vitousek et al. 1996 Invasive species can impact crops, rangelands, and commercial 

forests, costing millions of dollars annually in lost yields and 

expensive efforts to control the invasions. 

Impacts to Threatened and Endangered Species 

Flather et al. 1994 Invasive species have adversely affected more than 50% of the 

species included on the Federal List of Threatened and 

Endangered Species. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-32 

Appendix 

AUTHOR AND YEAR IMPACTS OF INVASIVE PLANTS 

Flather et al. 1994; Lesica et 

al.1993; Huenneke 1996; 

Cheater 1992 

The spread of invasive species is second only to habitat 

destruction for endangering imperiled species. In Montana the 

Threatened species sapphire rockcress (Arabis fecunda) is losing 

out to spotted knapweed. In New Mexico, teasel (Dipsacus 

sylvestris) decreases growth and germination of the Threatened 

Sacramento thistle (Cirsium vinaceum). In Colorado, populations 

of butterfly plant (Gaura neomexicana), a federal listing 

candidate, have declined 40-50% in competition with Canada 

thistle (Cirsium arvense). 

Randall 1997 Invasives plants jeopardize threatened and endangered species by 

"reducing the quality of natural areas established to protect 

habitats critical to the survival of these desirable species." 

 General Impacts 

Bright 1996 The effects of invasive plants can ripple through an ecosystem, 

upsetting relationships that would appear to be far removed from 

the invader itself. 

Bright 1996; Mack et al. 

2000; Vitousek et al. 1996 

Human activity has accelerated the migration of species to an 

unprecedented level. 

Bright 1996; Mack et al. 

2000; Randall 1996; Asher 

and Harmon 1995 

As ecosystems lose their diversity, they tend to become more 

susceptible to pressures in general. Invasion can be so complete 

that entire landscapes can be profoundly altered. 

D'Antonio and Vitousek 

1992; Vitousek et al. 1996 

Biological invasions (of all types) have caused more species 

extinctions than have resulted from human-caused climatic change 

or the changing composition of the atmosphere. 

Mullin et al. 2000 Invasive plants decrease the carrying capacity of rangeland. 

Randall 1995 Impacts on Nature Conservancy reserves cited include: alteration 

of the water table; alteration of fire regimes; suppression of native 

species recruitment thereby affecting community structure; 

successful competition with native species; alteration or 

elimination of habitat for native animals; and the provision of food 

and cover for undesirable nonnative animals. 

Randall 1996 There is the risk that when one pest is eliminated another will take 

its place. The infestation is the symptom of a more fundamental 

problem.