The Role of Wildfire in the Establishment and Range Expansion of Nonnative Plant Species into Natural Areas A review of current literature MARA JOHNSON1, LISA J. REW1, BRUCE D. MAXWELL1, AND STEVE SUTHERLAND2 1 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717 2 USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT 59807 2006 FOREWORD Discussions of nonnative plant species and wildfire present excellent opportunities to integrate knowledge from different disciplines, but they also present significant challenges. As both wild- fire and nonnative plant species elicit emotionally charged reactions, researchers are challenged to provide quantitative information to replace current opinions and dogma based on emotion and con- jecture. The conventional wisdom is that many notorious nonnative plant species will dramatically increase and change future fire frequency following wildfire. A surprising number of papers draw conclusions about nonnative plant species and wildfire using this conventional wisdom as the null hypothesis, rather than using the more traditional and testable null hypothesis that the abundance of nonnative plants is not altered by wildfire. We set out to review the scientific literature on the response of nonnative plant species to wildfire. We were focused on an objective assessment of responses based on quantitative data. That is, we did not accept conclusions not well supported by data and statistical analysis. We intentionally did not use information from the grey literature; in fact, we used information only from quantitative stud- ies published in journals, proceedings, books, and websites. We held the null hypothesis that there would be no response by nonnative species following wildfire, and looked for evidence that would allow one to reject this null hypothesis. Thus, we may have erred on the side of conservatism with regard to integrating nondata-based knowledge that often finds its way into conclusions. However, one can be certain that our assessment was as objective as possible. We hope 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. —Mara Johnson, Lisa Rew, Bruce Maxwell, and Steve Sutherland Suggested citation: Johnson, M., L.J. Rew, B.D. Maxwell, and S. Sutherland. 2006. The Role of Wildfire in the Establishment and Range Expansion of Nonnative Plant Species into Natural Areas. Bozeman, MT: Montana State University Center for Invasive Plant Management. 3 CONTENTS INTRODUCTION . 7 Previous Literature Reviews . 8 APPROACH AND METHODS . 9 Literature Search . 9 Literature Selection . 9 Key Questions . 10 Synthesis of Relevant Literature . 10 RESULTS . 11 Southwestern Shrubsteppe (Desert Grasslands) . 11 Disturbance History . 11 Time Since Fire . 12 Response of Nonnative Plant Species . 12 Conclusions . 13 Chaparral-Mountain Shrub . 13 Fire Regime . 14 Fire Frequency and Disturbance History . 15 Fire Severity . 15 Response of Nonnative Plant Species . 16 Fire Management Activities . 19 Conclusions . .20 Desert Shrublands . .20 Sagebrush Desert Shrublands . 21 Disturbance History . .22 Time Since Fire . .22 Response of Nonnative Plant Species . .23 Native Plants . .25 Fire Severity . .25 Livestock Grazing . .25 Rehabilitation and Seeding . .25 Conclusions . .26 Other Desert Shrublands . .26 Fire Regime . .27 Disturbance History . .27 Time Since Fire . .28 Fire Severity . .28 Response of Nonnative Plant Species . .29 Conclusions . .32 Pinyon-Juniper . .33 Fire Regime . .33 Disturbance History . .34 Topographical and Environmental Variables . .35 Time Since Fire . .35 Pre-Fire Vegetation . .36 Responses of Nonnative Plant Species . .36 Fire Severity . .38 Conclusions . .38 4 Center for Invasive Plant Management, Montana State University Ponderosa Pine . .38 Disturbance History . .39 Spatial Variables . .39 Time Since Fire . .40 Pre-Fire Vegetation . .40 Fire Severity . .40 Response of Nonnative Plant Species . .42 Environmental Variables . .44 Conclusions . .44 Lodgepole Pine and Spruce/Sub-alpine Fir . .45 Time Since Fire . .46 Fire Severity . .46 Pre-Fire Vegetation . ..