United States Department of Agriculture Forest Service Pacific Northwest Research Station General Technical Report Invasive Plant Species PNW-GTR-707 October 2007 and the Joint Fire Science Program Heather E. Erickson and Rachel White The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of multiple use management of the Nation’s forest resources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and management of the National Forests and National Grasslands, it strives—as directed by Congress—to provide increasingly greater service to a growing Nation. 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AUTHORS Heather Erickson is a research ecologist, and Rachel White is a science writer, Forestry Sciences Laboratory, 620 SW Main St., Suite 400, Portland, OR 97205. Cover photo Kerry Metlen, University of Montana. Invasive Plant Species and the Joint Fire Science Program Heather E. Erickson Rachel White U.S. Department of Agriculture Forest Service Pacific Northwest Research Station Portland, Oregon October 2007 INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM ABSTRACT have the potential to Erickson, Heather E.; and White, R. 2007. spread relatively quickly Invasive plant species and the Joint Fire over large areas. Invasive Science Program. Gen. Tech. Rep. PNW- plants may be responsi- GTR-707. Portland, OR: U.S. Department of ble for serious, long-term Agriculture, Forest Service, Pacific Northwest ecological impacts; they can cause the decline of Research Station. 18 p. Photo by Steve Sutherland,Rocky USFS, Mountain Research Station native plant species, dis- Invasive nonnative plants may be responsible for Orange hawkweed rupt nutrient cycling and (Hieracium aurantiacum), serious, long-term ecological impacts, including hydrology, and alter fire particularly problematic altering fire behavior and fire regimes. Therefore, in the Northwestern regimes. United States, is an knowing how to successfully manage invasive Disturbances, such as example of an aggressive plants and their impacts on natural resources is invader that thrives in wildfire and prescribed disturbed areas. crucial. We present a summary of research on fire, may promote plant invasive plants and fire that has been generated invasions via a number of mechanisms. On the through the Joint Fire Science Program—focusing other hand, fire can be used to control some inva- specifically on ecology of species invasions, the sives. Determining how to successfully manage interactions between fire and invasives, and the invasives is crucial, but to do so, much more responses of invasives to different management information is needed on the ecology of species practices. Selected findings include: prescribed invasions, the interactions between fire and inva- fire may increase invasive species in some ecosys- sives, and the responses of invasives to different tems, fuel treatments that leave some overstory management practices. canopy, minimize exposure of bare ground, and Several government-funded research pro- target sites that already host species capable of grams and initiatives are addressing ways to resprouting may be less likely to promote inva- better understand the biology, ecology, and man- sives, and postfire seeding should be approached agement of invasive species. Since 1999, the Joint with caution, as it can increase invasives. Fire Science Program (JFSP) has funded over 25 Keywords: invasive plants, fire management, projects related to invasive plant species and fire. cheatgrass, fuel treatments, postfire seeding, fire In this paper we summarize completed and on- regimes, exotic species. going JFSP research related to fire and invasives. INTRODUCTION INTERACTIONS BETWEEN FIRE nvasive nonnative plants are plants introduced AND INVASIVES into areas outside of their natural ranges, usu- ire, like many disturbances, may promote Ially as a result of human activities. Plants that are the invasion of nonnative plant species by successful invaders often have high growth rates Fproviding canopy openings, reducing cover of and typically reproduce prolifically. Thus, they competing vegetation, and creating favorable soil 1 INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM conditions such as newly exposed soil surfaces green. Dead cheatgrass provides a continuous bed and increased nutrient availability. Prescribed fire, of highly flammable fuel that can readily carry a now used in many ecosystems to remove vegeta- fast-moving fire. When the native flora is poorly tion that has accumulated as a result of decades adapted to a more frequent fire regime, the cheat- of fire suppression, may have the unintended grass is able to gain a competitive advantage, effect of spreading invasive species. Similarly, fire resulting in a grass/fire cycle (D’Antonio and management activities such as fuel reductions can Vitousek 1992). However, not all invasive plants often promote plant invasions, potentially replac- enhance an aspect of a fire regime; a few may ing one type of fire hazard represented by exist- actually make sites less flammable (D’Antonio ing fuels with another type of hazard from new 2000). Documenting and explaining the causes types of nonnative fuels. More-over, fire exclusion behind the sometimes complex interactions may promote the invasion of woody species into between fire and invasive species is a major goal grasslands. of JFSP research. Invasive plants also may affect fire behavior and fire regimes, often by increasing fuel bed MAJOR QUESTIONS AND flammability, which increases fire frequency. A FINDINGS classic example of a widespread invader that has JFSP research projects are addressing many greatly altered fire in the Western United States important questions regarding invasives and fire. and Canada is cheatgrass (Bromus tectorum L.). We grouped projects according to these four Cheatgrass is a winter annual, growing rapidly broad questions: during late winter and early spring and dying in 1. What Factors Influence Community early summer when most other plants are still Susceptibility to Invasion by Nonnative Species After Fire? 2. How Do Fuel Reduction Treatments Affect Invasives? 3. How Do Plant Invasions Affect Fire Regimes? 4. What Are Effective Mitigation Strategies Against Invasive Plants? Following each question are the related research project titles (in italics), brief descriptions of the projects, and some of the findings to date. om Iraci, USFS Photo by T Prescribed fire may have the unintended effect of increasing invasive species in some ecosystems. 2 INVASIVE PLANT SPECIES AND THE JOINT FIRE SCIENCE PROGRAM Question 1: What Factors Influence Community Susceptibility to Invasion by Nonnative Species After Fire? Community structure and disturbances have long been studied as factors contributing to how easily a community (community susceptibility, com- munity invasibility) may be invaded by non- native plants. Recently, invasibility after fire has been examined more closely. “Spatial Interactions Among Fuels, Wildfire, and Nonnative Plants.” In this project, Philip Omi, Molly Hunter, and Photo by Jamie Barbour, USFS, Focused Science Delivery Program their team determined factors that contribute to JFSP research has shown several factors that can contribute postfire invasion by nonnative plants at three to whether an ecosystem is vulner-able to the spread of invasive plants after a fire. mixed-conifer forests in the Rocky Mountains. Using a survey approach, they selected plots across several strata defined by spatial layers of “Weed Invasions Following Fire in Southwestern elevation, aspect, vegetation, fuel treatments, and Colorado: Long-Term Effectiveness of Mitigation fire severity and progression. Their main finding Treatments and Future Predictions.” was that fire severity was a consistent predictor of Lisa Floyd-Hanna and colleagues also used a nonnative species cover and was a more impor- survey approach in the Mesa Verde National Park tant predictor of nonnative species establishment on the Colorado Plateau to identify plant com- than other abiotic variables. These results suggest munities susceptible to postfire weed invasion. that high-severity wildfires may be one of the Extensive fires occurred in the park in the 1990s more important mechanisms for continued spread resulting in major plant invasions. The team of nonnative species in the Western United States 1 found that invasive species were prevalent in the (Final report to the JFSP, Hunter et al. 2006). seed banks in pinyon-juniper woodlands, the community type most vulnerable