FOREST COVER CHANGE AND POST-FIRE RESPONSE IN THE SOUTHERN ROCKY MOUNTAINS by Kyle C. Rodman B.A., University of Colorado Colorado Springs – 2011 M.S., Northern Arizona University – 2015 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Geography 2019 This thesis entitled: Forest Cover Change and Post-Fire Response in the Southern Rocky Mountains Written by Kyle C. Rodman has been approved by the Department of Geography _____________________________________ Thomas T. Veblen _____________________________________ Barbara P. Buttenfield _____________________________________ Stefan Leyk Date ____________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above- mentioned discipline. ii ABSTRACT Rodman, Kyle C. (Ph.D., Geography) FOREST COVER CHANGE AND POST-FIRE RESPONSE IN THE SOUTHERN ROCKY MOUNTAINS Thesis directed by Distinguished Professor Thomas T. Veblen. Forests are some of the Earth’s most important ecosystems. They are sanctums of biodiversity, play a key role in nutrient cycling, and provide countless ecosystem services to humans. Recent studies have identified broad-scale changes in forest cover across the globe. A richer understanding of these changes can be developed by identifying specific causal mechanisms, placing recent shifts in forest cover in a historical context, and by using knowledge of the past and present to make inferences about the future. The goal of our research was to better understand the past, present, and future of forested ecosystems in the northern Front Range of Colorado (NFR) and in the Southern Rocky Mountains Ecoregion (SRME). In particular, this dissertation focused on the roles of historical land use, wildfire activity, and post-fire recovery in observed and projected changes in conifer forests. To quantify the extent to which forest cover in the NFR changed 1938-2015 and to provide insight into possible potential drivers, we used high-resolution historical (c. 1938) and contemporary (2015) aerial photography to classify forested area across a 2932 km2 study area. We found that historical patterns of land use, as well as historical and contemporary fire activity iii and subsequent post-fire recovery, played key roles in 20th-century changes in forest cover across the NFR. To better characterize the drivers of limited conifer recovery in low-elevation forests, we surveyed post-fire conifer seedling abundance, tree establishment, and seed cone production across 15 wildfires in southern Colorado and northern NeW Mexico. We then used these data to quantify potential bottlenecks to post-fire conifer recovery. Lastly, we combined these data with similar surveys in the SRME performed by three former graduate students in Colorado and Arizona. Using the combined dataset, we modeled the extent to which a warming climate is limiting post-fire recovery of tWo widespread conifer species and projected that further declines in resilience are likely through 2100. Our findings reinforce the historical importance of Wildfire, anthropogenic impacts, and abiotic factors in the dynamics of forested landscapes in the southern Rocky Mountains. These same factors will remain important throughout the upcoming century. iv DEDICATION This dissertation is dedicated to Teresa Cotten, Lee Rodman, Dori Rodman, Howard Lankford, Jamie Cotten-Walker, Carita Rodman, Hubert Rodman, and Ana Miller-Ter Kuile. I wouldn’t be here without your unwavering support, love, and inspiration. v ACKNOWLEDGEMENTS I owe many thanks to my advisor Tom Veblen for sharing his vast knowledge of forests in North and South America and for pushing me intellectually and academically. I have been blessed to work in a lab with a number of tremendously talented, interesting, and kind people including Robbie Andrus, Rachel Chai, Teresa Chapman, Brian Harvey, Julia Kelly, Jennifer Landesmann, and Sara Saraceni. Thanks to my committee members Mike Battaglia, Babs Buttenfield, Stefan Leyk, and Carol Wessman for helpful feedback and keen scientific insight. Tania Schoennagel, Rosemary Sherriff, Alan Tepley, and Solomon Dobrowski provided additional feedback or contributed data for various portions of this dissertation. Teresa Chapman, Sara Saraceni, Monica Rother, Andreas Wion, Miranda Redmond, Mike Battaglia, Marin Chambers, Paula Fornwalt, Zack Holden, Jessi Ouzts, and Tom Kolb have all been helpful collaborators in this work. Katie Lage helped in accessing and understanding the aerial photography collections at the University of Colorado Boulder. The Department of Geography has been an amazing and supportive place to work and I value each of the friendships I have developed with students, faculty, and staff. Darla Shatto and Karen Weingarten provided crucial support at many steps throughout this process. Geography is where it’s at. Thanks to AndreW Sánchez Meador, David Huffman, Kristen Waring, Margaret Moore, Dave Havlick, Eric Billmeyer, Tom Huber, Curt Holder, and Mark Hesse for teaching me about the natural world and for pulling me in the right direction. Thanks also to my many friends and former co-Workers at RMFI for helping shape the way I think about science, nature, and the land around us. Over the last four years, Ryan Lima, Tim Bryant, Brent Nelson, and the Flagstaff creW kept me sane by getting me out on some amazing adventures. Adam Mahood, Tasha Snow, vi Joe Tuccillo, Sarah Posner, Josh Vonloh, Dan Allen and many others helped to do the same here in Boulder. The work in this dissertation would not have been possible without financial support from the Australian Research Council, the National Science Foundation, the Joint Fire Science Program, Boulder Open Space and Mountain Parks, the John Marr Ecology Fund, the Colorado Mountain Club Foundation, and the University of Colorado. Many employees of the U.S. Forest Service, the Vermejo Park Ranch, the Tercio Ranch, and the Trinchera-Blanca ranch gave permission to access and sample our research sites. A special thanks to Amanda Benton, Craig Taggart, Aaron SWallow, Lora Arciniega, Gabriel Romero, Gretchen Fitzgerald, Frank Nemick, Paul Crespin, Dennis Carril, and Alex Rudney. For help with field data collection and laboratory Work, I am indebted to Kelsey Bohanon, Aaron Broughton, Kristina Cowell, Molly Day, Katherine Feldmann, Mike Greene, Angela Gonzalez, Ana Miller-ter Kuile, Josh Vonloh, Katie Weimer, Brenna Wider, and Jake Zatz. Lastly, thanks to each member of my large and complicated family. We’ve been through a lot together, but you all helped me get through it. vii CONTENTS Chapter 1 – Introduction ................................................................................................................. 1 1.1 Justification ...................................................................................................................... 1 1.2 Background ...................................................................................................................... 2 1.2.1 Forests and Disturbance Dynamics in the Southern Rocky Mountains .................. 2 1.2.2 Human Land Use Change and Wildfire in the Southern Rocky Mountains ........... 4 1.3 Research Aims ................................................................................................................. 6 Chapter 2 – Wildfire Activity and Land Use Drove 20th Century Changes in Forest Cover in the Colorado Front Range ................................................................................................................... 10 2.1 Introduction .................................................................................................................... 10 2.2 Study Area ..................................................................................................................... 15 2.2.1 Climate and Vegetation .......................................................................................... 15 2.2.2 Disturbance History ................................................................................................ 16 2.2.3 Land Use History .................................................................................................... 17 2.3 Methods.......................................................................................................................... 19 2.3.1 Interannual and Interdecadal Trends in Climate, Wildfire, and Tree Growth ....... 19 2.3.2 Image Acquisition and Digitization ....................................................................... 20 2.3.3 Image Processing .................................................................................................... 21 2.3.4 Image Segmentation and Classification ................................................................. 22 2.3.5 Data Aggregation ................................................................................................... 24 2.3.6 Accuracy Assessment and Uncertainty Modeling .................................................. 25 2.3.7 Potential Drivers of Change in Forest Cover ......................................................... 27 2.3.8 Comparison with Fire History ................................................................................ 30 2.4 Results ............................................................................................................................ 33 2.4.1