AN ABSTRACT OF THE THESIS OF Adam J. Bouché for the degree of Master of Science in Forest Ecosystems and Society presented on March 11, 2020. Title: Scaling the Issue of Changing Disturbance Regimes: Implications of Novel Landscape Conditions for Black Stain Root Disease. Abstract approved: ________________________________________________________________ Klaus J. Puettmann David C. Shaw Changes to disturbance regimes resulting from shifts in forest management practices have created novel landscape conditions in the Pacific Northwest (PNW). I analyzed the implications of changes to landscape conditions caused by forest management for the spread of a native root disease: black stain root disease (BSRD) of Douglas-fir. BSRD spreads via root and insect vectors and causes rapid decline and mortality in young Douglas-fir (below age 30-40). Management practices including thinnings, clear-cut harvests, soil compaction, and roadside disturbance, e.g., tree damage from traffic and machinery, are associated with increased BSRD infection. In western Oregon and Washington, decreases in rotation lengths are altering Douglas-fir age class distributions, and the associated increased prominence of younger stands and harvest frequency have created concern about BSRD leading to increased Douglas-fir mortality. Management changes are influencing patterns of disturbance and Douglas-fir age distribution at both the stand and landscape scale. Therefore, for my research efforts, the probability of BSRD infection for each stand was hypothesized to be influenced by drivers at multiple scales, within and beyond stand boundaries. I used a spatially explicit model to simulate BSRD transmission in forest landscapes to evaluate whether forest management and the resulting stand and landscape conditions influence BSRD spread. Factors affecting the probability of infection, including their variability and uncertainty, were determined and quantified from literature, verified by expert opinion, and used to develop and parameterize the model. By comparing BSRD spread in different landscape scenarios, I analyzed the influence of management disturbances and stand age class distributions on BSRD infection, mortality, and spatial distribution at the stand and landscape scale. Critical knowledge gaps were identified regarding BSRD spread and impacts and highlighted as priorities for future research. In the simulations, infection spread was found to be highest in landscapes dominated by extensively managed stands, driven in part by higher tree densities and longer rotations. In contrast, infections were lowest in a landscape with all stands managed intensively. In landscapes with 50% intensively managed and 50% extensively managed stands, infection increased in intensively managed stands and decreased in extensively managed stands relative to the 100% intensive and extensive management scenarios, respectively, suggesting drivers of infection spread operate at the stand and landscape scales. Simulation results also suggested the hypothesis that the insects Pissodes fasciatus and Hylastes nigrinus account for the highest proportion of infections. This model was intended as a first step to connect management scenarios with disease outcomes and generate hypotheses to be examined in future research. The model also aims to serve as a tool to inform forest management practices regarding the impact of BSRD and stimulate future research on this disease. ©Copyright by Adam J. Bouché March 11, 2020 CC-BY-SA 4.0 Scaling the Issue of Changing Disturbance Regimes: Implications of Novel Landscape Conditions for Black Stain Root Disease by Adam J. Bouché A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented March 11, 2020 Commencement June 2020 Master of Science thesis of Adam J. Bouché presented on March 11, 2020. APPROVED: Co-Major Professor, representing Forest Ecosystems and Society Co-Major Professor, representing Forest Ecosystems and Society Head of the Department of Forest Ecosystems and Society Dean of the Graduate School 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. Adam J. Bouché, Author ACKNOWLEDGEMENTS Thank you to all of those who have assisted me during this journey and whose support has contributed to this achievement. First, thank you to my major professors. We struggled through these complex ideas and challenges together, and I have learned much under your guidance. Thank you to my committee, who helped improve my work. I want to express my appreciation for all the foresters and scientists who participated in the development of this project, especially Dr. Paul Hessburg and Dr. Rick Kelsey, whose experience and insight helped guide my decisions and understanding of this ecological system. Thank you to Mario Vega, whose wit, creativity, and dedication helped me move this project forward. I send my appreciation to Terralyn Vandetta and College of Forestry Computing Helpdesk as well as the Center for Genome Research and Biocomputing (CGRB) at Oregon State University for helping to make my research possible. I also express my thanks to Ariel Muldoon for her help through statistical consultation. During this as with every step in my life, I have deep appreciation for my family, whose love and commitment to my education throughout my life have helped me reach this height. A mi pareja, que me aseguró que tenía la capacidad de terminar este proyecto y hacerlo con calidad, que nos seguimos acompañando, retando, y construyendo. To all of my friends and loved ones, new and old, thank you for sharing this life with me, and let us work towards a better world. TABLE OF CONTENTS Page Chapter 1. Introduction ............................................................................................................................. 1 Context ................................................................................................................................................ 1 Research questions and hypotheses ............................................................................................... 2 Chapter 2. Literature Review ................................................................................................................... 6 Background information .................................................................................................................. 6 The BSRD-management connection ............................................................................................... 8 BSRD as a current forest health issue ............................................................................................. 9 Chapter 3. Methods.................................................................................................................................. 12 Study area ......................................................................................................................................... 12 Literature review and data collection ........................................................................................... 13 Model development and implementation ................................................................................... 14 Modeling environment and description .............................................................................. 14 Spatial scale .............................................................................................................................. 15 Temporal scale......................................................................................................................... 16 Forest management approaches ........................................................................................... 16 Model scheduling ................................................................................................................... 17 Infection processes .................................................................................................................. 18 Other factors ............................................................................................................................ 31 Simulations ....................................................................................................................................... 31 Landscape scenarios ............................................................................................................... 31 TABLE OF CONTENTS (Continued) Infection variants .................................................................................................................... 32 Analysis ............................................................................................................................................ 32 Verification ....................................................................................................................................... 33 Chapter 4. Results .................................................................................................................................... 48 Infection ............................................................................................................................................ 48 Stand-scale infection incidence ............................................................................................. 49 Infection spread as influenced by stand and landscape