The Role of Fuel Breaks in the Invasion of Nonnative Plants Scientific Investigations Report 2006–5185 U.S. Department of the Interior U.S. Geological Survey Inside front The Role of Fuel Breaks in the Invasion of Nonnative Plants cover: This page intentionally left blank. The Role of Fuel Breaks in the Invasion of Nonnative Plants The Role of Fuel Breaks in the Invasion of Nonnative Plants By Kyle E. Merriam1, Jon E. Keeley2, and Jan L. Beyers3 1U.S.D.A. Forest Service Plumas National Forest Post Office Box 11500, Quincy, CA 95971 email: [email protected] 2U.S. Geological Survey, Biological Resources Division Sequoia and Kings Canyon Field Station 47050 Generals Hwy #4, Three Rivers, CA 93271 3U.S.D.A. Forest Service, Pacific Southwest Research Station, Riverside Fire Laboratory 4955 Canyon Crest Dr., Riverside, CA 92507 U.S. Department of the Interior U.S. Geological Survey The Role of Fuel Breaks in the Invasion of Nonnative Plants U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director U.S. Geological Survey, Reston, Virginia 2007 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS For additional information, please visit the project website: http://www.werc.usgs.gov/fire/seki/ffm/ Suggested citation: Merriam, K.E., Keeley, J.E., and Beyers, J.L., 2007, The role of fuel breaks in the invasion of nonnative plants: U.S. Geological Survey Scientific Investigations Report 2006-5185, 69 p. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. The Role of Fuel Breaks in the Invasion of Nonnative Plants Executive Summary Fuel reduction projects have become an This report presents the results of a research increasingly important component of state and project investigating the relationship between fuel federal fuels management programs. However, an reduction treatments and the invasion of nonnative unintended result of some pre-fire fuel manipulation plants. Throughout the rest of this document, we will projects may be the introduction of nonnative collectively refer to these treatments as fuel breaks, invasive plants. The establishment of nonnative although we sampled a range of fuel breaks described plants within fuel breaks is a serious concern because variously as fuel breaks, shaded fuel breaks, the presence of invasive species in areas treated to defensible fuel reduction zones, defensible fuel reduce fuels could make adjacent wildland areas profile zones, fuel reduction projects, fuel more susceptible to invasion, particularly following management zones, wildfire protection zones, and widespread disturbances such as fires. community protection zones. Antelope Border Defensible Fuel Profile Zone (DFPZ), Plumas National Forest ___________________________________________________________________________________________ Executive Summary v The Role of Fuel Breaks in the Invasion of Nonnative Plants In Chapter One we discuss overall results In Chapter Two we evaluate the association compiled from 24 separate fuel breaks located across between nonnative plants and fuel breaks within California. These fuel breaks represent a variety of vegetation types, including mixed coniferous forest, different construction and maintenance methods, and oak woodland, chaparral, and coastal scrub plant different fire histories. Our primary findings were: communities. We found that: • Fuel breaks may promote nonnative plants. • Nonnative abundance and species richness • Fuel breaks with more canopy and ground was significantly different among vegetation cover may be less likely to be invaded. types: conifer forests had the lowest nonnative • Nonnative plants were more abundant cover and richness while coastal sage scrub had adjacent to older fuel breaks and in areas that the highest. had experienced more numerous fires. • Differences in elevation, fire history, • Native species may also be promoted by grazing, canopy cover, and disturbance history fuel breaks, particularly native annual forbs and may explain some vegetation type differences. grasses. • Nonnatives were significantly more abundant on fuel breaks in all vegetation types. Our data suggest that fuel breaks provide • Fuel break construction method was establishment sites for nonnative plants, and that associated with nonnative abundance in all surrounding areas may be susceptible to invasion, vegetation types. particularly after disturbances such as fire. Fuel break • Fire and grazing was positively associated construction and maintenance methods that leave with nonnative abundance in all vegetation some overstory canopy cover and minimize exposure types with adequate sample sizes to evaluate of bare ground may be less likely to promote these factors. nonnative plant invasion. This analysis suggests that although different vegetation types experience varying degrees of invasion by nonnatives, our general recommendations described in Chapter One also may be applied within individual vegetation types. Fuel break construction and maintenance methods that retain some overstory canopy and minimize the exposure of bare ground will likely minimize the probability of nonnative invasion across vegetation types. Etz Meloy fuel break, Santa Monica Mountains National Recreation Area Executive Summary vi The Role of Fuel Breaks in the Invasion of Nonnative Plants In Chapter Three we provide a brief summary Our site level review suggests that fuel breaks of each individual fuel break sampled in this study. may create conditions favorable to a number of Each site had a unique history, including various different nonnative plant species. The colonization of dates of construction and construction methods, a fuel break by a particular species may reflect which different maintenance regimes, varying fire histories, species has a nearby seed source or is able to disperse and different land use histories. Our general findings into the treated area. Regardless of which nonnative were: species is of greatest concern in the region, retaining overstory canopy and ground cover will likely be the • Individual sites differed greatly in nonnative best way to prevent nonnative invasion on fuel cover, fire history, and environmental variables. breaks. • Nonnative cover was higher within fuel breaks than in adjacent wildlands at 19 of our 24 When reviewing this report, please remember study sites. that our study was observational and additional • Fuel breaks that were not invaded by research using controlled, replicated experiments will nonnatives had very deep duff layers, were be necessary to fully understand the mechanisms that constructed by methods other than bulldozing, influence nonnative plant invasion within fuel were very young, or had very infrequent reduction treatments. maintenance regimes. • Cheatgrass (Bromus tectorum) was the most common nonnative plant species at 8 of our study sites. • A number of different nonnative species dominated the other 16 sites, suggesting that many nonnative species may be well adapted to take advantage of the disturbed conditions associated with fuel breaks. Aguanga fuel break, Cleveland National Forest Executive Summary vii The Role of Fuel Breaks in the Invasion of Nonnative Plants Acknowledgments We would like to thank the Joint Fire Science Program for funding this study (project #01B-3-2- 08). Sequoia and Kings Canyon National Parks were instrumental in supporting our research. This project would not have been possible without the generous cooperation of the Angeles, Cleveland, Lassen, Los Padres, Mendocino, Plumas, San Bernardino, Sequoia, Shasta-Trinity, Sierra, Six Rivers, and Lake Tahoe Basin Management Area USDA National Forests; the Whiskeytown and Santa Monica National Recreation Areas; the San Luis Obispo Canterbury bells (Phacelia minor) in the District of the California Department of Forestry and Cleveland National Forest Fire Protection; the Bakersfield District of the Bureau of Land Management; the Los Angeles County Fire Department Division of Forestry; and the Orange County Department of Parks, Casper’s Wilderness Park. We also want to thank Katie VinZant, Lea Condon, Clara Arndt, Trent Draper, and Elizabeth Martin for their expert field assistance. Acknowledgments viii The Role of Fuel Breaks in the Invasion of Nonnative Plants Table of Contents Executive Summary ............................................................................................................................................................................v Acknowledgments ...........................................................................................................................................................................viii FIGURES..........................................................................................................................................................................................….x Chapter One: Overall Results ............................................................................................................................................................1