Portland State University PDXScholar Dissertations and Theses Dissertations and Theses Spring 6-14-2019 Forest Structure, Composition, and Regeneration after High-Severity and Rapidly Repeated Wildfires in the Central Cascade Range Sebastian Upton Busby Portland State University Follow this and additional works at: https://pdxscholar.library.pdx.edu/open_access_etds Part of the Geography Commons Let us know how access to this document benefits ou.y Recommended Citation Busby, Sebastian Upton, "Forest Structure, Composition, and Regeneration after High-Severity and Rapidly Repeated Wildfires in the Central Cascade Range" (2019). Dissertations and Theses. Paper 5127. https://doi.org/10.15760/etd.7006 This Thesis is brought to you for free and open access. It has been accepted for inclusion in Dissertations and Theses by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. Forest Structure, Composition, and Regeneration after High-Severity and Rapidly Repeated Wildfires in the Central Cascade Range by Sebastian Upton Busby A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geography Thesis Committee: Andrés Holz, Chair Jeff Gerwing Melissa Lucash Kevan Moffett Portland State University 2019 Abstract Within mid-to-high elevation conifer forests in the Cascade Range, wildfire extent, severity, and frequency are expected to rise due to increasingly drier forest fuels under climate change. Considering dominant species composition, existing forests may be poorly adapted to absorb stress and recover following altered wildfire patterns. We tested the hypothesis that increased fire activity may disrupt the recovery of upper- montane and subalpine forest types by quantifying post-fire forest structure and conifer regeneration after spatially large, severe, and rapidly repeated wildfires in the Central Cascade Range. A stratified random sampling design was used to select field plots (n=122) and drivers of conifer regeneration were modeled using logistic and negative binomial regression models. Median conifer regeneration was very poor among sample plots that experienced either a single high-severity fire (49 seedlings/ha) or rapid reburn (14-28 seedlings/ha). Distance to seed source primarily drove seedling abundance, with shade- tolerant species abundance being most sensitive to increasing seed source distances and dry, exposed, post-fire environmental conditions. Rapidly repeated fire increased the size of high-severity patches by killing live seed source trees spared during an initial fire, with chronological sequence of burn severity promoting regeneration of all conifer species or primarily fast growing, fire-adapted pines. Low-seedling densities, a general lack of seed source, and future warming trends suggest these forests affected by expansive high-severity and/or repeated wildfire will transition into a patchy, low- i density forest state. This early-seral state ecosystem will be composed of fire-adapted pines farther from live seed source and incorporate a patchwork of shrubby grassland that in turn, may be more resilient to frequent wildfire than prior forests. If future wildfire patterns manifest as expected in the Cascade Range, recovering mid-to- high elevation forests may begin resembling their drier, lower-elevation mixed-conifer counterparts in structure and composition. ii Acknowledgements Funding for this research was provided by the National Science Foundation (NSF EAR- 1738104). Access to conduct fieldwork within our study areas was graciously provided by the Gifford Pinchot National Forest (USFS) and the Confederated Tribes of the Yakima and Warm Springs Nations. This work wouldn’t have been possible without help from our dedicated field assistants. Thank you Emily Sykes, Brian Michie, Anthony Lefort, and Christine (Crissy) Johnson for the many hours and days spent hiking up and down rugged terrain, through smoke and sweat, to search for tiny (or not so tiny) conifer seedlings. A great thanks to my advisor, Andrés Holz, for providing the opportunity to conduct this exciting research project and for providing helpful thoughts and feedback. An additional thanks to my thesis committee (Jeff Gerwing, Melissa Lucash, and Kevan Moffett) for taking time out of their busy schedules to provide feedback and assess the quality of this work. A final thanks to my partner, Caryn, for her excellent editing skills, non-expert feedback, and general support. iii Table of Contents Abstract ................................................................................................................................ i Acknowledgements ............................................................................................................. iii List of Tables ....................................................................................................................... vi List of Figures ..................................................................................................................... vii Introduction ........................................................................................................................ 1 Assessing SIFs in the Central Cascade Range .................................................................. 7 Study Areas: Geology, Climate and Ecology.................................................................... 9 Study Areas: Wildfire Regimes and Recent Wildfire Patterns ...................................... 12 Field Sampling Methodology ............................................................................................ 18 Site Selection: Limiting Human Impacts and Insect Influence ...................................... 18 Site Selection: Burn Severity and Fire History ............................................................... 19 Site Selection: Topographic Setting & Final Selection .................................................. 21 Field Sampling Design ................................................................................................... 25 Data Analysis ..................................................................................................................... 34 Post-Fire Forest Structure ............................................................................................. 34 Post-Fire Seedling Presence and Abundance ................................................................ 35 Logistic Regression Models for Seedling Presence/ Absence ....................................... 38 Negative Binomial Count Models for Seedling Abundance .......................................... 39 Seedling Probability Curves: Distance to Seed Source and Fire History ....................... 41 Seedling Composition and Establishment Across Fire Histories ................................... 41 Distance from High-Severity to Lower Severity Patches ............................................... 43 Seedling Stress (Fv/Fm) ................................................................................................. 43 Results ............................................................................................................................... 45 Fire Effects on Forest Structure .................................................................................... 45 Pre and Post-Fire Species Composition ........................................................................ 51 iv Seedling Density and Establishment Across Fire Histories ........................................... 53 Drivers of Post-Fire Conifer Regeneration: Binary Models ........................................... 58 Drivers of Post-Fire Conifer Regeneration: Count Models ........................................... 63 Distance from High-Severity to Lower Severity Patches ............................................... 67 Conifer Seedling Stress (Fv/Fm) .................................................................................... 68 Discussion.......................................................................................................................... 69 Fire Severity and Frequency Alter Seed Source Availability, Conifer Regeneration ..... 70 Secondary Controls on Conifer Regeneration ............................................................... 74 Mechanisms Promoting Persistent Seed Source: Refugia ............................................ 78 Repeated Wildfire Alters Stand Structure, Species Composition ................................. 80 Forest Ecosystem Transitions, Migrations, and Future Resilience ............................... 86 Management Implications ............................................................................................ 91 Limitations and Future Directions .................................................................................... 93 Seed Source Distance and Delayed Mortality ............................................................... 93 Compound Disturbance Interactions: Insect Outbreak ................................................ 94 Impacts of Woody Charring on Snowmelt .................................................................... 95 Soil Moisture and Post-Fire Forest Regeneration ......................................................... 96 Improving Seedling Stress Sampling Protocols ............................................................. 98 References .......................................................................................................................