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AN ABSTRACT OF THE DISSERTATION OF Craig A. Carr for the degree of Doctor of Philosophy in Rangeland Ecology and Management presented on October 16, 2007. Title: An Evaluation of Understory Vegetation Dynamics, Ecosystem Resilience and State and Transition Ecological Theory in an Eastern Oregon Ponderosa Pine Forest Abstract approved: William C. Krueger Relatively recent increases in ponderosa pine abundance have effected unprecedented changes to ecosystem structure and function. Efforts to restore ponderosa pine systems are often focused on the manipulation of tree structure and the re-introduction of a more natural fire regime. Successful restoration should also incorporate understory components but information addressing changes in understory species is minimal for Pacific Northwest ponderosa pine forests. Moreover, state and transition models (STM) derived from the concepts of non-equilibrium ecology may be appropriate for characterizing ecosystem dynamics in modern ponderosa pine forests, however this approach has not previously been used. The focus of this research was to evaluate ecosystem dynamics within a hypothesized STM framework and to experimentally assess the existence of model components with particular emphasis on understanding understory species dynamics. The analyses of vegetation, soil, and environmental attributes measured in a eastern Oregon ponderosa pine forest indicated that increased ponderosa pine occupancy modified the under-canopy environment through alterations in light, nitrogen availability, and soil temperature and was related to reductions in understory species diversity caused by a major shift in understory character away from the dominance of perennial bunchgrasses. Light availability appeared to be the most influential driver in understory species distribution. Similar results were observed in a greenhouse experiment that identified high light intensity as significant in generating increased Festuca idahoensis Elmer growth, biomass and vigor. A seed bank assessment to evaluate recovery potential from diminished understory conditions indicated that understory species dominant in open ponderosa pine forests do not form a persistent seed bank and can not be relied upon as a tool for their recovery. These data described ecosystem dynamics in accordance with the hypothesized STM model. Loss of ecosystem resilience associated with a fire loss threshold can initiate additional degradation in the understory component, however, pine influenced alterations to the under-canopy environment did not appear irreparable and restoration practices that reduce pine occupancy should enhance the conditions for perennial bunchgrass growth. Successful restoration will require the re-introduction of desired understory species seeds or other viable plant material when considering stands that have lost the understory component. © Copyright by Craig A. Carr October 16, 2007 All Rights Reserved An Evaluation of Understory Vegetation Dynamics, Ecosystem Resilience and State and Transition Ecological Theory in an Eastern Oregon Ponderosa Pine Forest by Craig A. Carr A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented October 16, 2007 Commencement June 2008 Doctor of Philosophy dissertation of Craig A. Carr presented on October 16, 2007. APPROVED: Major Professor, representing Rangeland Ecology and Management Head of the Department of Rangeland Ecology and Management Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Craig A. Carr, Author ACKNOWLEDGEMENTS Numerous people, individually and combined, helped make this Ph.D. endeavor a success. I must first recognize my wife Ruth who has been with me through this process in its entirety; the ups and downs, the missed family events, and the extended time away from home. Her love, support and encouragement were needed and her dedication to me was inspiring. My graduate committee consisted of Doctors Bill Krueger, Duane Lammers, Jeff Miller, Rick Miller, Bruce McCune, and Tamzen Stringham. All of whom provided sound advice as I started the program and challenged me to think about ecology both more broadly and more refined – I thank them all for their time and effort. Dr. John Selker agreed to stand-in as the graduate council representative for my defense and I thank him for his contribution. Bill Krueger spent numerous hours in discussions with me as I ‘crystallized my ecological theory’. Bill has taught me about ecology, about science, about education, and about myself and I truly appreciate the opportunity to pursue a Ph.D. with him and in the Department of Rangeland Ecology and Management. Tamzen Stringham first brought me to Oregon State University and it was her who suggested I might be stubborn enough to complete a PhD. My life certainly would not be as it is today were it not for an email I sent to OSU in hopes of procuring a Master’s degree that somehow landed in Tamzen’s inbox. I also wish to thank Duane Lammers for spending time with me in the field and lab looking at, talking about, playing with and classifying the soils of my study site. His interest in my project and assistance in helping me to better understand the relationships among soils, vegetation and landscapes were very much appreciated. My two good friends, Ted and Cindy McArthur, opened their home to me, my family, my dog, and even my field technicians. Ted and Cindy gave me a John Day home, were my contacts with the Forest Service, and even took stints as field techs and camp cooks. I truly appreciate their support and genuine interest in seeing me complete this project. Kyle Jackson and Michael Hansen assisted in field data collection. Jamie Kienzle and CiCi Brooks from the NRCS in John Day also spent several days with me in the field as we evaluated soils and stimulated good thoughts on ponderosa pine ecology. Jack Southworth provided a secure storage location for my camp trailer. The US Forest Service provided study sites and camp locations and their cooperation was appreciated. I thank Bob Gillaspy of the NRCS for his interest in this research and for providing funding to support the last year of the project. I would be remiss if I did not acknowledge the faculty and graduate students in the department of Rangeland Ecology and Management. Each one has greatly influenced my education and perspective of the world. The quality of my education would have suffered with out the contributions of these individuals. Lastly, I would like to thank my parents for creating an environment where curiosity, dedication and hard work were rewarded and for their words of encouragement and support always inserted when most needed. CONTRIBUTION OF AUTHORS Dr. William Krueger assisted in the development and study design of each research component described in this thesis and was instrumental in editing the associated manuscripts. Dr. Duane Lammers assisted in the soil classification and was instrumental in writing and editing the soils descriptions. TABLE OF CONTENTS Page Chapter 1: General Introduction ........................................................................... 1 Introduction......................................................................................................... 1 Historic Role of Fire in Ponderosa Pine Ecosystems.......................................... 3 Fire Frequency ................................................................................................ 5 Contemporary Ponderosa Pine Forests ............................................................... 8 Soils............................................................................................................... 11 Hydrology ..................................................................................................... 11 Understory Species Abundance .................................................................... 13 A Contemporary View of Ponderosa Pine Ecology.......................................... 16 Non-equilibrium Ecology.................................................................................. 17 The Threshold Concept................................................................................. 19 Detecting Thresholds in the Field ................................................................. 23 Project Rationale ............................................................................................... 25 Research Objectives.......................................................................................... 26 Research Approach ........................................................................................... 27 Literature Cited ................................................................................................. 29 Chapter 2: Site Description ................................................................................. 41 Location ............................................................................................................ 41 Climate .............................................................................................................. 42 Geology............................................................................................................. 44 Soils..................................................................................................................

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