Long-term Fuel and Vegetation Responses to Mechanical Mastication in northern California and southern Oregon

Historical land use and changes in climate have altered fire behavior and severity in fire-prone ecosystems of western North America. A variety of fuels treatments are used to abate fire hazard, restore ecosystem processes, and increase forest resilience. Mechanical fuels treatments are increasingly used to alter forest structure and fuel continuity due to impediments to the use of prescribed fire. An increasingly common fuels treatment is mechanical mastication. Mastication does not remove fuels, but instead rearranges live and dead vertical woody fuels into a compacted layer on the forest floor. While mastication reduces potential fire intensity, these compacted fuels are flammable and capable of causing tree mortality and other negative ecological consequences when they burn in prescribed fires or wildfires. A current knowledge gap is quantitative information about the rate at which masticated fuels decompose and the rate at which vegetation reestablishes within sites previously masticated. Using 25 sites across northern California and southern Oregon, this thesis examines how masticated fuels change over time. Results from this study demonstrate that the majority of mass lost from masticated fuel beds occurred in the 1 and 10-hour woody fuel classes. Because surface fire behavior is driven by these fine fuels, these findings are valuable to the planning and retreatment of masticated fuels treatments and the corresponding fire suppression efforts in masticated sites. In combination with masticated wood surface fuels, shrubs and small trees play an important role in fire behavior, acting as ladder fuels that exacerbate surface fire behavior and threaten to ignite residual trees. A lack of understanding of how woody vegetation recovers following masticated fuel treatments gives rise to questions and challenges regarding treatment longevity. In this study, species with the ability to resprout tended to recover more quickly than obligate seeding species. Residual conifer saplings or trees that establish in masticated fuelbeds also recovered rapidly, reducing the efficacy of fuels treatments. Future implementation of masticated fuels treatments should consider both woody fuel decomposition and the corresponding recovery of shrubs and small trees to maximize treatment longevity.