Causes and Consequences of Conifer Invasion into Pacific Northwest Grasslands Ryan D. Haugo A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2010 Program Authorized to Offer Degree: Forest Resources University of Washington Graduate School This is to certify that I have examined this copy of a doctoral dissertation by: Ryan D. Haugo and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Chair of the Supervisory Committee _______________________________________ Jonathan D. Bakker Reading Committee: _______________________________________ Jonathan D. Bakker _______________________________________ Charles B. Halpern _______________________________________ Joseph A. Antos Date: __________________________________ In presenting this dissertation in partial fulfillment of the requirements for the doctoral degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of the dissertation is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Requests for copying or reproduction of this dissertation may be referred to ProQuest Information and Learning, 300 North Zeeb Road, Ann Arbor, MI 48106-1346, 1-800-521- 0600, to whom the author has granted “the right to reproduce and sell (a) copies of the manuscript in microform and/or (b) printed copies of the manuscript made from microform.” Signature ________________________ Date ____________________________ University of Washington Abstract Causes and Consequences of Conifer Invasion into Pacific Northwest Grasslands Ryan D. Haugo Chair of the Supervisory Committee: Assistant Professor Jonathan D. Bakker Forest Resources Widespread invasion of conifers during the 20th century is threatening the biological diversity and ecosystem services sustained by grassland ecosystems throughout the Pacific Northwest. Understanding the causes and consequences of these invasions is essential to predicting future changes in these ecosystems. In this dissertation I present the results of field experiments, retrospective analyses, and long- term observations that explore this theme. First, I examined how biotic relationships regulate conifer invasion. This study, conducted in low elevation prairies, involved experimental manipulations of above- ground competition, below-ground competition, and soil origin. I found that conifer establishment was strongly regulated by simultaneous positive and negative interactions between it and the recipient grassland community, but that the hierarchy of effects varied with life stage (germination, early survival, and seedling growth). Second, I used a retrospective approach to demonstrate that the cumulative influences of invading conifers on resident grassland species can vary in direction and magnitude over time and with the traits of dominant conifer species. In a western Cascade meadow I quantified the effects of individual Pinus contorta and Abies grandis (18-73 years old) by comparing vegetation structure and composition under and adjacent to each tree. Although trees generally exerted negative effects on meadow species, cover was elevated under 33% of trees (mostly younger Pinus). Effects on forest herbs increased steeply with age under Abies, but not under Pinus. Finally, I used three measurements over a 26-year period (1983-2009) to assess vegetation dynamics across forest-meadow boundaries representing diverse physical environments in the Cascade Range, Oregon. Over this period, declines in meadow species were greater in ecotones in which soil moisture was seasonally limiting. Forest understory species increased in montane, but not in subalpine ecotones (where depauperate forest understories limit potential for dispersal). The magnitude of change was not related to change in overstory structure, but to vegetation characteristics (overstory structure and ground vegetation) at initial measurement. These studies contribute to an increased understanding of the biotic and abiotic factors that regulate conifer invasions of grassland ecosystems in the Pacific Northwest and the consequences of these invasions—both in time and space—for the resident vegetation. TABLE OF CONTENTS List of Figures ........................................................................................................... ii List of Tables ........................................................................................................... iii Acknowledgements .................................................................................................. iv Chapter 1: A general overview of causes and consequences of conifer invasion into Pacific Northwest grasslands.................................................................................1 Chapter 2: Positive and negative biotic interactions regulate conifer invasion of prairie ecosystems .................................................................................................5 Chapter 3: Tree age and tree species shape positive and negative interactions in a montane meadow .................................................................................................34 Chapter 4: Landscape context and tree influences shape the long-term dynamics of forest-meadow ecotones in the central Cascade Range, Oregon ................... 57 References ............................................................................................................... 94 Appendix A: Species at Bunchgrass Ridge, OR. ................................................. 117 Appendix B: Tree canopy cover along TSBR forest-meadow ecotones .............. 122 Appendix C: Photographs of TSBR forest-meadow ecotones .............................. 128 Appendix D: Species along TSBR forest-meadow ecotones ................................ 168 i LIST OF FIGURES Figure Number Page 2.1 Environmental conditions ........................................................................... 19 2.2 Pseudotsuga germination and germinant survival ..................................... 22 2.3 Survival, growth and biomass of Pseudotsuga seedlings .......................... 25 3.1 Relationship between tree age and tree size .............................................. 46 3.2 Total cover and species richness ................................................................. 48 3.3 Relationship between tree age and changes in ground vegetation .............. 51 4.1 Location of TSBR forest-meadow transects .............................................. 65 4.2 NMDS ordination of landscape context ..................................................... 74 4.3 Climate variation ......................................................................................... 78 4.4 Changes in tree structure ............................................................................. 81 4.5 Changes in ground-layer vegetation ........................................................... 82 4.6 Climate in species composition .................................................................. 83 4.7 Relationship between landscape context and change in ecotone ground-layer vegetation .............................................................................. 86 ii LIST OF TABLES Table Number Page 2.1 PERMANOVA models of environmental conditions................................. 20 2.2 PERMANOVA models of Pseudotsuga germination and germinant survival ........................................................................................................ 23 2.3 PERMANOVA models of survival, growth, and biomass of Pseudotsuga seedlings ...................................................................................................... 26 2.4 Summary of treatment effects on Pseudotsuga performance ..................... 28 3.1 Effects of Pinus contorta and Abies grandis on species cover and richness ..................................................................................................................... 49 3.2 General linear models of tree effects on vegetation responses ................... 52 4.1 Characteristics of TSBR forest-meadow transects ................................ 66-67 4.2 Principle components analysis of tree structural variables ......................... 77 4.3 Multiple regression models of change in ecotone ground-layer vegetation .................................................................................................................... 85 iii ACKNOWLEDGMENTS First, I would like to thank my co-advisors, Jon Bakker and Charlie Halpern. Both have been incredibly generous with their time and support throughout these projects. My other committee members, Joe Antos, Peter Dunwiddie, and Janneke Hille Ris Lambers, have also provided much valuable feedback. Chapter 2: Thanks to my intrepid field assistant, Joanne Pontrello, as well as Jon Bakker, Charlie Halpern, Peter Dunwiddie, Eric Delvin, Laura Bloom, Laura Six, Rachel Mitchell, Matt Davies, and Elaine Boyd for field help. The Nature Conservancy and the WA Department of Fish and Wildlife generously provided access to their lands. The WA Department of Natural Resources donated the seeds used in this study. Eric Delvin, Sarah Hamman, and Grace Diehl (TNC); David Hays (WDFW); and Boyd Johnson (WA DNR) provided valuable logistical and technical assistance. Funding was provided