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UC Riverside UC Riverside Electronic Theses and Dissertations Title Invasive Plants, Fire Succession, and Restoration of Creosote Bush Scrub in Southern California Permalink https://escholarship.org/uc/item/2c31j28z Author Steers, Robert Jeremy Publication Date 2008 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA RIVERSIDE Invasive Plants, Fire Succession, and Restoration of Creosote Bush Scrub in Southern California A Dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Plant Biology by Robert Jeremy Steers December 2008 Dissertation Committee: Dr. Edith B. Allen, Chairperson Dr. Jodie S. Holt Dr. Richard A. Minnich Copyright Robert Jeremy Steers 2008 ACKNOWLEDGEMENTS This dissertation was influenced by the work of Matthew Brooks who has produced a large amount of research regarding invasive annual grasses and fire in creosote bush scrub. It also builds upon work by Richard Minnich and his past students that carefully studied desert shrubland wildfires within the study area. Finally, the restoration treatments utilized in the dissertation were largely influenced by Edith Allen, Jodie Holt, Robert Cox, and other colleagues. I especially want to thank my advisor, Edith Allen, for her constant support, encouragement, and confidence in this effort as well allowing me freedom to pursue my own interests within her larger research program. Finally, my wife, Kelly, has been an enormous blessing and I couldn’t have done it without her. Chapter 1 Acknowledgements: Thanks to Greg Hill (BLM) for supporting this effort and to Matt Brooks (USGS) for his comments on an earlier draft of this manuscript. Jodie Holt and Richard Minnich also provided helpful comments. Chris True helped collect field data. Andy Sanders provided assistance with plant identification. This research was funded by grants to R. J. Steers from the Community Foundation of Riverside and San Bernardino Counties and to E. B. Allen by the NSF (DEB 04-21530). Chapter 2 Acknowledgements: Thanks to Greg Hill (BLM) for supporting this effort and to Matt Brooks (USGS) for helping develop the seedbank portion of the experimental design and for his comments on an earlier draft of this manuscript. Jodie Holt and Richard Minnich also provided helpful comments. Mike Bell, Tom Bytnerowicz, Ryan Chien, Heather Schneider, Chris True, and Lynn Wihbey helped weed plots, collect field data, or enter data, especially Chris True. Andy Sanders provided assistance with plant iv identification. This research was funded by grants to R. J. Steers from the Community Foundation of Riverside and San Bernardino Counties and to E. B. Allen by the NSF (DEB 04-21530). Chapter 3 Acknowledgements: Much thanks to Greg Hill (BLM), Dee and Betty Zeller (BMCP), Ray Yeager (BMCP) and other staff at BMCP for their assistance in helping this experiment take place. Mike Bell, Tom Bytnerowicz, Ryan Chien, Sarah Huels, Robin Marushia, Heather Schneider, Kelly Steers, and Chris True helped weed plots, collect field data, or enter data, especially Chris True. Andy Sanders provided assistance with plant identification. Thanks to Jodie Holt and Richard Minnich for helpful comments. This research was funded by grants to R. J. Steers from the Southern California Botanists and California Native Plant Society, and grants to E. B. Allen by the NSF (DEB 04-21530). Chapter 4 Acknowledgements: Thanks to Cameron and Katie Barrows for helping us obtain permission to use these field sites. Mike Bell, Ryan Chien, Sara Jo Dickens, Sarah Huels, Robin Marushia, Leela Rao, Heather Schneider, Chris True, and Kris Weathers helped weed plots, collect field data, or enter data, especially Chris True. Andy Sanders provided assistance with plant identification. Thanks to Robert Cox, David Crowley, Jodie Holt, and Richard Minnich for helpful comments. This research was funded by grants to E. B. Allen by the University of California, Integrated Pest Management (UC IPM) - Exotic/Invasive Pest and Disease Research Program, and the NSF (DEB 04- 21530). v ABSTRACT OF THE DISSERTATION Invasive Plants, Fire Succession, and Restoration of Creosote Bush Scrub in Southern California by Robert Jeremy Steers Doctor of Philosophy, Graduate Program in Plant Biology University of California, Riverside, December 2008 Dr. Edith B. Allen, Chairperson Exotic annual plant species have invaded large regions of southern California deserts. Certain areas have been especially impacted, such as the western edge of the Colorado Desert adjacent to Mt. San Gorgonio, Banning Pass, and Mt. San Jacinto. This landscape is highly invaded due to relatively high winter rainfall compared to interior desert locales and elevated anthropogenic nitrogen deposition from urban areas to the west. Invasive annual grasses, in particular, are abundant here and have fueled disastrous wildfires in creosote bush scrub (CBS) since the 1970s. Invasive annual plants and fire pose major threats to the sustainability of CBS. The purpose of this dissertation was to document the impact of fire on CBS perennial and annual plant components. The ability of various restoration treatments to remove invasive annual plants and to promote native species in both burned and unburned contexts was also tested. Fire disturbance was shown to dramatically reduce shrub species richness and diversity. Fire also altered the vertical and horizontal vegetation structure of CBS. Total shrub density returned to unburned vi levels within a decade or two after fire due to recruitment by Encelia farinosa, a relatively small, short-lived shrub. Fire also increased the abundance of invasive annual plants. These changes may result in a vegetation type that is more fire prone than unburned CBS. Post-fire increases in invasive annual plants were also shown to decrease native annual plant cover and species richness. However, if invasive annuals were removed after fire, then native annual plant abundance and richness increased greatly, resembling unburned, pristine stands. Positive responses to invasive plant removal were also documented by native annual plant assemblages in unburned CBS. Clearly, the combination of invasive annual plants and fire has a large negative impact on native plant components of CBS. Fortunately, there is great potential to reestablish native annuals if invasives can be controlled. vii Table of Contents Introduction………………………………………………………………………1 References…………………………………………………………………6 Chapter 1 Abstract……………………………………………………………………10 Introduction………………………………………………………………..11 Methods…………………………………………………………………....13 Results……………………………………………………………………..19 Discussion………………………………………………………………….26 Conclusions………………………………………………………………..37 References………….……………………………………………..……….38 Figures and Tables………………………………………………………...44 Chapter 2 Abstract……………………………………………………………………56 Introduction………………………………………………………………..57 Methods……………………………………………………………………60 Results……………………………………………………………………..68 Discussion…………………………………………………………………76 Conclusions………………………………………………………………..83 References…………………………………………………………………84 Figures and Tables…………………………………………………………90 Chapter 3 Abstract……………………………………………………………………103 viii Introduction………………………………………………………………..104 Methods……………………………………………………………………107 Results……………………………………………………………………..114 Discussion………………………………………………………………....120 Conclusions…………………………………………………………..........129 References………………………………………………………………....131 Figures and Tables………………………………………………………...136 Chapter 4 Abstract…………………………………………………………………....145 Introduction………………………………………………………………..146 Methods…………………………………………………………………....150 Results……………………………………………………………………..160 Discussion…………………………………………………………………164 Conclusions………………………………………………………………..173 References…………………………………………………………………174 Figures and Tables…………………………………………...………..…..183 Conclusion…………………………………………………………………….…..189 References………………………………………………………………....191 Appendix A……………………………………………………………………….193 Appendix B……………………………………………………………………….196 ix List of Figures Figure 1.1. Map of study area in Riverside County, CA………………………….44 Figure 1.2. Shrub vertical structural heterogeneity……………………………….45 Figure 1.3. Shrub horizontal structural heterogeneity…………………………….46 Figure 1.4. Shrub relative density ……………………...…………………………47 Figure 1.5. Shrub relative cover ……….…………………………………………48 Figure 1.6. Shrub species richness and diversity indices …………………………49 Figure 1.7. Relative density of shrub seedlings, resprouts, and fire survivors…....50 Figure 2.1. Map of study area in Riverside County, CA………………………….90 Figure 2.2. Mod-NAWMA plot configuration……………………………………91 Figure 2.3. Relative cover of annual plants……………………………………….92 Figure 2.4. Species richness of annual plants and shrubs…………………………93 Figure 2.5. Twice burned vegetation parameters…………………………………94 Figure 2.6. Seedbank assays from burned and unburned stands………………….95 Figure 3.1. Recorded precipitation during experiment…………………………...136 Figure 3.2. Native annual species richness by treatment…………………………137 Figure 3.3. Abundance of annual plants at treatment application time…………..138 Figure 3.4. Cover of annual plants by treatment………………………………….139 Figure 3.5. Density of annual plants by treatment………………………………..140 Figure 3.6. Principle components analyses on annual plant communities………..141 Figure 4.1. Sampling design plot configuration…………………………………..183 x Figure 4.2. Interspace annual plant parameters…………………………………...184 Figure 4.3. Understory annual plant parameters………………………………….185 Figure 4.4. Initial