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THE ROLE OF WILDFIRE IN THE ESTABLISHMENT AND RANGE EXPANSION OF
INVASIVE PLANT SPECIES INTO NATURAL AREAS
Lisa Rew (Montana State University) et al.
Introduction
Non-native, invasive plants are one of the greatest threats to natural
ecosystems worldwide (Vitousek et al., 1996). In fact, their spread has been
described as “a raging biological wildfire” (Dewey et al., 1995). Disturbances
tend to create conditions that are favorable for germination and establishment
of plant species. Non-native plant species are often characterized as weeds,
exotics and invasives that can exploit such conditions (Rejmanek, 1996) and
many of them possess traits such as rapid germination, high fecundity and
effective means of seed dispersal (Stohlgren et al., 1998).
Managers responding to a 1995-6 survey of 21 National Forests in the Northern
Rocky Mountains estimated that 2 million out of approximately 46 million acres
of those Forests had been invaded by non-native plants (Markin, 2004).
Respondents of a survey of all U.S.A. Fish and Wildlife Service Wilderness Areas
identified invasive plants as a major problem in 12 out of the 68 responding
areas. In the western United States, numerous anthropogenic disturbances
such as grazing, vehicle use, logging and development have been linked to nonnative plant species introductions and population expansion, especially since
Euro-American settlement (Kemp & Brooks, 1998; Young et al., 1972).
Wildfire, a natural disturbance in many areas of the western United States, has
recently received considerable attention for its purportedly significant role in
non-native plant species introductions and subsequent invasions (Asher &
Spurrier, 1998; Asher et al., 2001; Mason, 2002; Ririe, 2001).
Wildfire is a natural process in many ecosystems of the western United States
and many native plants are adapted to fire; that is they have characteristics
that allow them to persist with repeated burning (Agee, 1993; Wright & Bailey,
1982) they say this it is not data supported there????. However, the impact,
frequency and scale of wildfires have changed in some ecosystems as a result
of anthropogenic presence, manipulation and management. Prior to EuroAmerican settlement, Native Americans used fire to alter vegetation in many
areas of the West (Barrett & Arno, 1982; Humphrey, 1958; Samuels &
Betancourt, 1982) }. With Euro-American settlement many anthropogenic
disturbances intensified including land management activities (e.g. logging and
grazing) and fire suppression. Because of this, fuels have been accumulating
for the past century and have resulted in unusually large and severe wildland
fires in certain ecosystems in recent years (USDA, 2000b). In 1999, wildfires
burned 1.6 times the ten year average of acres burned; in 2000; wildfires
burned 2.3 times the ten year average; in 2001, wildfires burned the ten year
average; and in 2002, wildfires burned 2 times the ten year average (Center).
The Federal Wildland Fire Management Policy recognizes that fire is a critical
natural process in many ecosystems and further contends that it should be
reintroduced where it has been suppressed (Glickman & Babbitt, 1995).
Natural areas as defined for this review are those areas that are composed of
predominantly native vegetation and have not been or have been minimally
impacted by anthropogenic disturbances, particularly post-European
settlement disturbances. More recently, maintaining natural processes has
been considered a critical part of preserving natural areas. Wildfire is
considered a natural process necessary for maintaining many natural
ecosystems in the western United States. The National Management Plan for
Invasive Species identifies prevention (Council, 2001). It is often easier to
prevent introduction of species or control small populations than to eradicate
established populations. Maintaining native vegetation and the natural process
of wildfire in these systems is complicated by the potential for non-native plant
species invasions following wildfire. In addition, suppressing wildfires in
natural areas also represents a possible avenue for non-native plant dispersion
into natural areas (Ririe, 2001). A clearer understanding of the relationship
between wildfire and non-native plant responses in natural areas will assist
managers in allocating limited resources to control or monitor for non-native
plants following wildfires. In addition, clarifying the scope and gaps of
available research on non-native plant responses to wildfire will help direct
future research.
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Final Remarks
Wildfire is a natural process in many ecosystems in the Western United States.
In the last century, numerous anthropogenic disturbances have altered how this
natural process operates in the forests, shrublands and grasslands of the West.
The intentional and/or unintentional introduction of non-native plant species
and the ongoing invasion of these species into natural areas is one of the
anthropogenic disturbances that has caused grave concern. In particular, the
potential response of non-native plants to wildfire has alarmed resource
managers in the West. Unfortunately, the overall conclusion of this review is
that little information is available on the response of non-native plants to
wildfire in natural areas. Of what little information exists, the majority of
research has occurred in semiarid vegetation of the West, though much of this
has been conducted in areas impacted by other anthropogenic disturbances.
Little information on non-native plant responses to wildfire is available for
forested landscapes of the West. Fortunately, although little published
information is currently available, several studies are being conducted
throughout the West on this topic and should be available in the near future.
As previously stated, each study provides a piece of the puzzle in
understanding the relationship of non-native plants and wildfire. This review
has resulted in identifying numerous missing pieces and several important
issues that warrant future research. Future research should focus on more
completely integrating aspects of invasion biology and fire ecology in
formulating hypotheses and in study designs. Specifically, research should
address not only introductions of non-native species and short-term responses
but issues of persistence and dominance of these species and the impacts on
native species following wildfire. Borrowing from fire ecology, researchers
should address not only fire severity effects but the effect of wildfire
frequency, extent and pattern on non-native plants. Standard measures of
these characteristics also need to be used in order to determine patterns
among studies. In addition, because the Western United States has
experienced numerous anthropogenic disturbances that affect the invasion of
non-native plants, site disturbance histories need to be explicitly documented
in studies of wildfire and non-native plants. Where possible, these disturbance
effects should be integrated into the study designs along with the effect of
wildfire. Finally, the relationship between fire management activities, such as
fire suppression and rehabilitation, requires far more research. Quantitative
research on this topic was notably underrepresented in the literature located
for this review. Considering the extent of these ongoing activities and the
intuitively great potential for them to affect non-native plant invasion
immediate attention is required.
The issue of non-native plants and wildfire presents an excellent opportunity to
integrate knowledge from different disciplines but also presents significant
challenges. Because wildfire and non-native plants both illicit emotionally
charged reactions, researchers are challenged to provide quantitative
information to replace opinions currently based on emotion and conjecture.
Also, considering the extent of natural areas in the West, researchers are
challenged to provide information on non-native plant responses to wildfire
that will help land managers prioritize activities to manage non-native plants
following wildfire. Hopefully, this review will provide context for researchers
to determine which issues need to be studied and how best to conduct those
studies to further contribute to this important issue.