Vegetation Composition and Response to Fire of Native Willamette Valley Wetland Prairies
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AN ABSTRACT OF THE THESIS OF Kathy L. (Connelly) Pendergrass for the degree of Master of Science in Rangeland Resources presented on May 4, 1995. Title: Vegetation Composition and Response to Fire of Native WillametteValley Wetland Prairies. Abstract approved: Fires set by Native Americans were important in shaping and maintaining Willamette Valley prairie plant communities. With fire exclusion after Euro- American settlement in the 1840's, composition of the remaining prairies shifted toward shrub and tree dominance with a concomitant decline in endemicprairie plant populations.I hypothesize that by restoring fire as an ecosystem process native prairie flora would be enhanced by invigorating vegetative growth of natives, by controlling exotic species, and by controlling successional invasion of woody species. Additionally, I hypothesize that native prairie compositionmight be better enhanced by multiple burns compared to a single burn. Initial vegetation composition, fuels, fire behavior, and subsequent changes in plant composition following fire were quantified in a total of five plant communities at Rose Prairie and Fisher Butte study sites near Eugene, Oregon. Three treatments were assessed over three years (unburned control, and once burned, and twice burned). A total of 205 species were observed. Species richness was dominated by native forbs in all five plant communities. In contrast, vegetative cover was dominated by native perennial graminoids in all communities except one where exotic perennial graminoidsdominated.Rosa nutkana was the most prevalent woody species in allcommunities.Initial preburn R. nutkana densities ranged from 30,519 to 50,963 plants ha -1 at Rose Prairie andfrom 17,156 to 53,067 ha -1 at Fisher Butte. Fisher Butte had more tree encroachment relative to Rose Prairie. Most trees at both sites were seedlings andsaplings,ranging between3 to 70 cm in height. Total tree density initially ranged from 0 to 568 trees ha -1 and from 430 to 1,071 ha-1 at Fisher Butte where Fraxinus latifolia was invading the wet Rosa/Juncus community. Total aboveground biomass of WillametteValleywetland prairies was greater than that of manyother North Americangrasslands ranging from6,958 to 12,038 kgha-'. The firstseason following oneburn,above ground biomass decreased significantly in one community (-25.4%) and increased significantly in another(+68%). Response to burning was often inconsistent and variedgreatly byspecies among plantcommunities.Native as well as exotic species increased in frequency or cover with burning. Eight native perennials and three native annuals established or significantly increased frequency (p = 0.1) in burned areas in two or more plant communities. Four exotic perennials andone exotic annual increased significantly in frequency following fire. Although frequency of Deschampsia cespitosaincreased in burn treatments, cover declined significantly in the first postfire year following one burn, indicating a shift from fewer large plants to a greater number of smaller plants immediately following burning. Significant increases in vegetative cover occurred in burned areas in two Rose Prairie communities and in one Fisher Butte community, due primarily to changes in cover of perennial graminoids. Total cover of exotic species increased following one or two burns in all but one community. Significant increases in exotic forb cover occurred in two communities. After two burns, relative frequency of native perennial forbs increased significantly in three communities. Relative frequency of exotic perennial graminoids declined significantly in burn treatments in all but one community where they increased in the control treatment. Native annual forbs increased in burns in all but one community. Increases of woody biomass, frequency, and cover following burning suggest that Willamette Valley wetland prairie woody plants are highly resilient to burning and quickly recover and increase following fire. However, height of woody plants significantly declined following burning leading to an improved prairie structure. In addition, the number ofFraxinus latifoliaseedlings and saplings were significantly reduced in burn treatments in theRosa/Juncuscommunity. Fire behavior and fuel conditions in relation to vegetation response are required to develop optimal frequency and intensity of burn prescriptions. Information on the response of native, woody, exotic plants, and aboriginal food plants will assist land managers in determining optimal fire frequency for burning Willamette Valley wetland prairies to maintain and enhance desired species. This research initiates a long-term assessment of fire effects on Willamette Valley vegetation to determine optimal burn prescriptions for wetland prairies. ©Copyright by Kathy L. Pendergrass May 4, 1995 All Rights Reserved VEGETATION COMPOSITION AND RESPONSE TO FIRE OF NATIVE WILLAMETTE VALLEY WETLAND PRAIRIES by Kathy L. (Connelly)Pendergrass A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed May 4, 1995 Commencement June 1996 Master of Science thesis of Kathy (Connelly) Pendergrass presented on May 4, 1995 APPROVED: aj r essor, rep es n "9ngeland Resources ad of Department of Rangeland Resources I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. i ACKNOWLEDGEMENTS First and foremost, I would like to thank all of my friends and family for their love and support through the many long years and life crises that it has taken to bring this thesis to fruition. Thanks to my mother, Grace, and my now- deceased father, Paul, who always believed that the quest for knowledge was a valuable endeavor. Thanks to my ex-husband, Paul Connelly, for persevering the turmoil of life with me and this task for as long and as well as he could. We did the best that we could with the knowledge that we had at the time... I must thank my major professor, J. Boone Kauffman, and the Rangeland Resources Department for securing funding so I could survive materially during a portion of this project. Boone, thank you for your endurance in editing verbose draft chapters.I wonder if horrible experiences of graduate students must be perpetuated through successive generations of students - it takes very caring and courageous people to break these cycles. Good luck, Boone, on the paths that you choose in the future.I do intend to publish information in this thesis as well as other data collected during the course of this research endeavor (e.g. Lomatium bradshawiiandErigeron decumbensvar.decumbensdata). I wish to extend special thanks for the mentoring support of my friend, Patsy Miller, who inspired and helped me through Chapters 4 and 5 and beyond when I was ready to quit entirely. Oh, if only research were conducted as it should be conducted...I have been appalled by my own view of the scientific process - I shall henceforth have a healthy, critical eye. Thanks for those long 11 hours just before the defense; thanks for sticking by me and not allowing me to give up on myself - you are truly one of the world's fine people. I'd like to extend a special thanks to Dr. Robert Frenkel for serving on my committee, for his endless encouragement, for his help in organizing slides for several presentations, and for his invaluable comments and suggestions throughout this project. Thank you, Paul Doescher for serving on my committee and for trying to get me to pare down the project - I wish I would have followed that counsel. You're a good teacher, keep up the good work. Thanks, Doug Johnston, for your ever-enthusiastic support, for filling in at last moment in my defense, and especially for getting me the help of work-study students for data input; thanks Michelle and Ann. Doug, you exemplify what I think a teacher and advisor should be; thanks for the investment of yourself that you give to the people around you - it is valued far more than you may know. Thanks also to the guys in geography who transferred my rough maps into professional maps in Autocad. Thanks to Brett Jones for updating those maps and making them look even better for inclusion into this thesis - I look forward to supplying some more dinners and hikes for that endeavor. Thanks to Don Henshaw for help in SAS, Mack Barrington for statistics crunching, Carol Kubeck, Pam Sutton, and Dodi Reesman for table construction and clean up, and Susanna Saxton for all of the last-minute errands. iii This thesis would also not have been possible without the hard work of a number of individuals working for government agencies involved in this research endeavor. I would like to extend special thanks to Jim Beale with the U.S. Army Corps of Engineer for his persistence in procuring funding and his ever-helpful assistance throughout this project. Without the loan of field technicians I could never have completed the field work necessary for this project.I apologize for all that I was not able to complete. Thank you, MaryAnn Weber, for your many hours of patience in the hot sun and your doodles on the data sheets as reminders of some of the fun-field days. I'm grateful to Peter Zika for his friendly botanical assistance during that first frantic year and for his help in initiating involvement by Bureau of Land Management (BLM) in implementing the prescribed burns. Thank you, Phil Dills and Gene Chounnard and all of the people in the Eugene District BLM fire crews that helped make those burns a reality so this researcher could in fact look at the response of the plant community to fire. Much gratitude goes out to Nancy Wogen, BLM District Botanist, for her friendship, assistance, and precious moneys from the BLM. I especially thank her for encouraging me to stay away from work as long as necessary to get the thesis completed. Thanks are also extended to folks on staff at the Oregon Field Office of The Nature Conservancy, particularly Cathy Macdonald, for all of their efforts and contributions to this research endeavor.