Fire Resistance in Planted Western Larch Stands under Tribal Stewardship on the Colville Reservation

Abstract

Wildfire severity in the interior Pacific Northwest has increased over recent decades due to interacting climatic, ecological, and management drivers, resulting in longer fire seasons, warmer temperatures, and altered fuel structures. Understanding how forest composition, stand structure, and fuelbed composition influence fire behavior and post-fire resilience is critical for managing semi-arid conifer forests under a changing climate. We examine these dynamics through two complementary studies of planted western larch stands on the Colville Reservation, where Tribally led forest management and long-term investment in western larch reforestation provide a unique landscape for evaluating wildfire resilience. Chapter 1 establishes the ecological, historical, and epistemological foundations of this research. We review historical fire regimes of the inland Pacific Northwest, the role of Tribal fire stewardship in influencing landscape heterogeneity, forest structure, and resource availability prior to colonial disruption, and the consequences of fire exclusion and climate change for contemporary fire behavior. We also introduce western larch ecology, its fire-adaptive traits, and its dual functional role as both a fire resister and a fire impeder. Finally, we situate this research within a framework of co-produced inquiry under Tribal authority and leadership, in which we developed research questions, analytical priorities, and interpretive frameworks in partnership with the Confederated Tribes of the Colville Reservation. Chapter 2 evaluates the fire resistance of planted western larch stands across approximately 137 units spanning three wildfire footprints: the 2015 North Star Fire, the 2021 Summit Trail Fire, and the 2024 Swawilla Basin Fire. We quantified burn severity within planted western larch stands and adjacent unplanted forests using satellite-derived spectral indices and a validated Random Forest model implemented in Google Earth Engine (Parks et al. 2019). To assess structural controls on fire effects, we measured understory structure and surface fuel loads across 10 burned planted western larch plots, 9 unburned planted western larch plots, 6 untreated mixed-conifer plots within the Summit Trail Fire footprint, and 6 unburned high-elevation mixed-conifer reference plots. Our results show that planted western larch stands exhibited distinct fire effects relative to surrounding forests across the three fire footprints. Landscape-scale analyses showed that planted western larch stands generally experienced lower modeled burn severity (Composite Burn Index [CBI]) than adjacent unplanted forests, with significantly lower severity observed in two of the three fires examined. Field measurements indicated that adjacent mixed-conifer forests contained significantly greater dead woody fuel loads, whereas planted western larch stands supported greater live shrub cover. These patterns suggest that differences in fuel structure and composition may contribute to observed differences in fire severity. We detected no significant difference in burn severity within the Swawilla Basin Fire, highlighting that the influence of planted stand conditions on fire outcomes varies with local species composition, fuel characteristics, stand structure, and fire context. Chapter 3 investigates the fuelbed properties of western larch and co-occurring pine species through controlled laboratory assessments of moisture retention dynamics and flammability. Whereas Chapter 2 evaluates fire resistance at the stand and landscape scale, Chapter 3 isolates the surface fuel component of these systems to examine how species composition influences fuel moisture dynamics and combustion behavior. We reconstructed fuelbeds from field-collected needle litter across nine compositional gradients spanning pure larch, pure lodgepole pine, pure ponderosa pine, and mixed larch-pine combinations. We evaluated these fuelbeds under both oven-dry and equilibrium moisture content (EMC) treatments. Results indicate that larch fuelbeds retained significantly more moisture, reaching more than five times the saturation moisture content of ponderosa pine litter, and exhibited substantially lower flammability across all five metrics measured: flame height, fuel consumption, flaming duration, smoldering duration, and total burn duration. Mixed fuelbeds containing larch showed intermediate but consistently dampened burning behavior relative to pure pine fuelbeds, with the strongest dampening effects occurring at larch proportions of approximately 50% or greater by mass. These findings provide a mechanistic basis for interpreting the landscape- and stand-scale burn severity patterns documented in Chapter 2 and suggest that western larch restoration may influence surface fuel flammability across the range of forest types present on the Colville Reservation. Chapter 4 synthesizes the findings of Chapters 2 and 3 within the broader ecological and management context established in Chapter 1. We integrate landscape-scale burn severity patterns with laboratory-scale flammability mechanisms to develop a multi-scale interpretation of western larch fire performance. We identify the structural and compositional conditions under which western larch’s fire-moderating influence is most likely to be expressed and discuss implications for Tribal forest restoration planning on the Colville Reservation. Chapter 4 also addresses key limitations of the current evidence base and identifies priority directions for future co-produced research. Together, these chapters demonstrate that fire resistance in planted western larch stands emerges from interactions between stand structure and fuel composition, even before the full development of mature fire-adapted traits. This work provides mechanistic insight into how planted western larch stands may contribute to fire resilience and climate-adaptive forest restoration in semi-arid landscapes. Our findings provide additional ecological evidence supporting the Confederated Tribes of the Colville Reservation’s long-term investment in western larch restoration and contribute to ongoing Tribal-led efforts to manage resilient forests under changing climate and disturbance regimes.