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Chapter 16: Fire and Nonnative Plants— Summary and Conclusions Jane Kapler Smith Kristin Zouhar Steve Sutherland Matthew L. Brooks Chapter 16: Fire and Nonnative Plants— Summary and Conclusions This volume synthesizes scientific information about The complexity of this subject makes it difficult to interactions between fire and nonnative invasive plants identify trends and implications for management. In in wildlands of the United States. If the subject were this chapter we summarize the patterns (and lack of clear and simple, this volume would be short; obviously, patterns) currently demonstrated by research regard- it is not. Relationships between fire and nonnative ing fire effects on nonnatives and the use of prescribed species are variable and difficult to interpret for many fire to reduce invasions, and we suggest some manage- reasons: ment implications. We also present some of today’s • Fire and invasions are both inherently com- burning questions about relationships between plant plex, responding to site and climate factors invasions and fire. Background for the assertions in this and the condition of the plant community. In chapter can be found in Parts I and III of this volume. addition, the nationwide scope of this volume Readers interested in specific regional problems and incorporates great variation across ecosystems, issues should refer to Part II. climates, and regions. • Fire effects and invasions interact with other Nonnative Invasive Species ecosystem processes and land use history and patterns, and these interactions and effects and Wildland Fire ________________ can vary over time. The literature shows that fire in many cases favors • Research tends to focus on highly successful nonnative species over natives and thereby may lead invasions even though comparisons to failed to postfire invasions—that is, cases where ecosystems, or marginally successful invasions could be habitats, or species are threatened because fire has instructive (Beyers and others 2002). promoted the establishment and spread of invasive • To date, research on fire and nonnative invasives plants (chapter 1). In some cases, nonnative species has been limited, with few studies covering alter the native plant community and fuel character- more than 1 year after fire (chapter 12). istics to the extent that the fire regime is altered, and USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 6. 2008 293 the altered fire regime favors further dominance of severe with increasing time since fire in closed-canopy the plant community by the nonnative invader. This forests and chaparral, but there are exceptions, and positive feedback loop is sometimes referred to as few long-term studies have investigated this pattern. a grass/fire cycle or invasive plant/fire regime cycle Postfire invasions tend to be more severe with time (chapter 3). since fire in shrub/grass ecosystems invaded by trees The potential for nonnatives to negatively impact where native understory species (often sprouters) wildland ecosystems after fire suggests that manag- have been reduced. Postfire invasions are less likely in ers should give priority to (1) controlling nonnative high-elevation than low-elevation ecosystems. Where species known to be invasive after fire in the area human-caused disturbance occurs in high-elevation burned or similar areas, especially if they are likely ecosystems, however, postfire invasions are more likely to alter the fire regime; (2) preventing new invasions than in similar undisturbed systems. through early detection and eradication of likely in- vaders; and (3) long-term monitoring and adaptive Management Implications management after fire to control or reduce invasions. Tables provided at the beginning of each bioregional Scientific study of the relationship between fire and chapter in Part II may be helpful for identifying po- nonnative invasive species is a relatively young field tential invaders. However, postfire invasion cannot be of investigation (Klinger and others 2006a) with many assumed for every ecosystem or for every nonnative uncertainties. Many studies describe postfire invasions, species—even those mentioned in the bioregional but our scientific knowledge base is not yet extensive tables. Invasion potential varies with prefire plant enough in space and time to explain or predict patterns community condition, fire characteristics, and climate, of invasion across a range of ecosystems (Rejmánek and and it depends on which plants and propagules (native others 2005a), with or without fire. Information about and nonnative) are present within and near the burn. fire effects on specific plant communities with specific Invasions can be exacerbated by other disturbances and invasive species provides the best knowledge base for management activities, and they can be transient or management decisions regarding those communities persistent (chapter 2). While postfire invasion cannot and species. Knowledge of nonnative species biology, be assumed, neither can a burn that is not invaded ecology, and responses in similar environments may immediately after fire be assumed “safe” from inva- also be useful for directing management decisions, sion. Nonnative species may persist at low density for although the effect of the particular environment must years before becoming invasive. Postfire disturbances, be considered (Rejmánek and others 2005a). One of such as grazing or logging, may “tip the scales” toward the few consistent predictors of a nonnative species’ invasion by altering resource availability, increasing potential to invade is its success in previous invasions nonnative propagules, and stressing native plants. (Daehler and Carino 2000; Kolar and Lodge 2001; In addition, wildlands are constantly exposed to new Reichard and Hamilton 1997; Williamson 1999). Yet, nonnative species with unknown invasive potential. as Williamson (1999) comments, “…we know that that Generalizations commonly made about fire and non- can fail badly.” This approach may be useful for pre- native species are supported by the literature under dicting which species are “risky” but not for predicting some circumstances but not others (chapter 2). For ex- which species are “safe”. The most useful predictions ample, nonnative species establishment may increase likely require the integration of several approaches with increasing fire severity, but this pattern can also (Rejmánek and others 2005a). be influenced by condition of the prefire plant commu- While more knowledge is needed on fire and invasive nity, postfire response of onsite species (both native and species, research will never eliminate uncertainty, so nonnative), propagule pressure, and the uniformity and scientists and managers must integrate many kinds size of high-severity burn patches. Plant communities of knowledge while remaining aware of their appli- dominated by native species that sprout after fire may cations and limitations. The more the prefire plant be more resistant to invasion than communities where community and conditions in a burned area diverge desired natives must regenerate from seed. Invasions from conditions described in published research, the are more likely in some plant communities when the less reliable predictions based on that research will baseline fire regime is disrupted, including locations be. This is why the location and scope of research proj- where a nonnative grass/fire cycle has developed and ects should be presented clearly in publications and native grasslands from which fire has been excluded read with care by managers. Management actions for periods exceeding the baseline fire-return interval. in general, including those based on extrapolation Unfortunately, current conditions can diverge from of research results, should be implemented with presettlement conditions in so many ways that this caution, monitored, and adapted as new knowl- generalization may not be helpful for predicting post- edge develops. Partnerships between scientists fire responses. Postfire invasions tend to become less and managers are likely to increase the pace and 294 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 6. 2008 effectiveness of adaptive management (Beyers and and vegetative sprouting) (chapters 2 and 14). Where others 2002). Hobbs and Mooney’s (2005) comment on fire is used, incorporate precautions mentioned above nonnative species invasions in light of global change (“During Fire Suppression”). applies well to fire/invasive interactions: “Scientists Addressing Invasive-Caused Changes in Fire need to become smarter at considering potential sce- Regimes—If a species has already changed one or narios based on multiple levels of uncertainty. Even more fire regime characteristics, evaluate the altered qualitative analyses of likely outcomes can provide regime and prioritize species control based on poten- useful input to decision-making processes.” tial for negative effects on native species diversity, Many strategies for evaluating and addressing ecosystem processes, natural resources, public safety, potential postfire invasions by nonnative species are property, and local economies. In some cases, it may described in this volume and the supporting literature. not be possible to restore communities to their pre- A brief summary is provided here. invasion state, and managers may need to establish During Fire Suppression—Wildfire managers communities of native species that can coexist with should include training for crews on identifying non- the nonnatives (chapters 3 and 11). native invasive plants and preventing their spread.
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