We reviewed forest management in the mountainous regions of several northwestern states and California in the United States and how it has impacted current issues facing these forests. We focused on the large-scale activities like fire suppression and logging which resulted in landscape level changes. We divided the region into two main forests types; wet, like the forests in the Pacific Northwest, and dry, like the forests in the Sierra Nevada and Cascade ranges. In the wet forests, the history of intensive logging shaped the current forest structure, while fire suppression played a more major role in the dry forests. Next, we looked at how historical management has influenced new forest management challenges, like catastrophic fires, decreased heterogeneity, and climate change. We then synthesized what current management actions are performed to address these issues, like thinning to reduce fuels or improve structural heterogeneity, and restoration after large-scale disturbances. Lastly, we touch on some major policies that have influenced changes in management. We note a trend towards ecosystem management that considers a forest’s historical disturbance regime. With expected climate induced changes in fire frequency, it is suggested that fuel treatments be implemented in dry forests to ensure an understory fire regime is restored in these forest systems. With respect to wet forests in this region, it is suggested that there is still a place for stand-replacing fire regimes. However, these forests will require structural changes incorporating heterogeneity to improve their resiliency and health.

In the past, the dry mixed conifer forests of California’s Sierra Nevada mountains experienced frequent low to mixed severity fires. However, due to fire suppression and past management, forest structure has changed, and the new fire regimes are characterized by large, high severity fires which kill a majority of the overstory trees. These new disturbance patterns require novel approaches to regenerate the forest as they are not adapted to large, high severity fires. We forecasted growth and fire behavior of young plantations for 100 years into the future using the Forest Vegetation Simulator (FVS) and its Fire and Fuels Extension (FFE). In these simulations, we tested combinations of different fuel treatments (mastication only, mastication with prescribed burning, and no fuels treatments) with different overstory thinning intensities (residual densities of 370 SDI (stand density index), 495 SDI, 618 SDI (TPH), and no overstory thinning) on stand growth and potential fire behavior using analysis of variance. We compared growth and crowning index at the end of the simulation and the simulation age when the flame length, basal area mortality, and fire type reached low severity between fuel treatment, thinning intensity, and original management of stands (plantation with PCT [precommercial thinning], plantation without PCT, and natural regenerating stands). These comparisons are essential to identify which fuel treatment categories reduce fire risk. We found an overall pattern of decreasing crown fire occurrence and fire induced mortality across all simulations due to increasing canopy base height and decreasing canopy bulk density. In particular, stands with mastication and prescribed burning transitioned from crown fire types to surface fires 10 years earlier compared to mastication only or no fuel treatment. Furthermore, pre-commercially thinned stands transitioned from crown fire states to surface fires 10 years earlier in the simulations compared to un-thinned and naturally regenerating stands. Stands with mastication and burning went below 25% reference threshold of basal area mortality 11 and 17 years earlier before the mastication only and no fuel treatment, respectively. In addition, pre-commercially thinned stands went below 25% basal area mortality 9 and 5 years earlier in the simulation compared to un-thinned or naturally regenerated stands, respectively. Mastication with prescribed burning (MB) was the most effective treatment for quickly reducing fire behavior by consuming surface fuels, thus drastically lowing flame length (e.g., surface flame length of MB was 0.6 m compared to mastication only [1.3 m] and no treatment [1.4 m]). Furthermore, intensive thinning reduced risk of active crown fires spreading through the stand. Prioritizing prescribed burning, when possible, and thinning (both pre-commercially and from below) are the most effective ways to quickly improve fire resistance in mixed conifer plantations. Our results highlight the different stressors that post-fire planted forests experience and how different silvicultural treatments interact over time to reduce fire risk, which demonstrates the importance of treating stands early and the effectiveness of surface fuel treatments.

Thank you to my advisor Dr. Steve Chhin for guidance during my three years of working for him. His advice, patience, and thoughtfulness helped keep me from getting too overwhelmed by graduate school. Thank you to my project's Co-PI, Dr. Jianwei Zhang and the United States Forest Service for providing funding. Dr. Zhang has also dedicated a lot of his time and resources into making this project successful. The other members of my graduate committee, Dr. Kirsten Stephan and Dr. Jamie Schuler, also deserve thanks for providing helpful feedback and project assistance. A big thank you to Natalie Pawlikowski for assisting me with building and calibrating my FVS simulations and providing very constructive edits to my thesis. Thank you to Dr. Michael Premer, Mukti Subedi, and Dr. James McGraw for providing me with guidance in study design and statistical analysis. There is no way this project could have been completed without help in the field from Stephi Dickinson, Kaelyn Finley, Mukti Subedi, Simeon Spottswood, and Clint Albright. Thank you for dealing with hot weather and steep slopes and for all being wonder field partners. Thank you to the staff at the Amador ranger station of the Eldorado National Forest for providing housing and general assistance during my two field seasons. Lastly, thank you to my many mentors during my scientific career, especially Dr. Brian Smithers. Thank you for inspiring my love of field work and forest ecology, challenging me to become a better scientist, showing me how to be a good leader, and the countless letters of recommendation. List of Tables .