Post-fire carbon, fuels, and vegetation dynamics in wet temperate forests: implications for future fire and management in the Pacific Northwest

Abstract

Forecasting ecosystem dynamics under warming climate and increasing fire activity is a critical priority for contemporary ecology and ecosystem management. However, fundamental understanding of fire effects is missing in ecosystems where fire is infrequent, including wet temperate forests west of the Cascade Range crest in Washington and northern Oregon, USA (“northwestern Cascadia”). In this dissertation, I combined empirical and simulation insights from recent fires in northwestern Cascadia to address research priorities for anticipating the effects of large infrequent fires on future forest dynamics. First, I characterized initial (2–5 years) post-fire aboveground biomass in long-term monitoring plots within five fires to explore the relative influence of pre-fire stand age and burn severity on two important post-fire disturbance legacies: carbon and fuel profiles. I found that pre-fire stand age drove total legacy amounts while burn severity modified legacy condition. Regardless of burn severity, most biomass present pre-fire persisted following fire. These findings suggest that, when burned, older stands may have greater potential than younger stands to support several ecosystem functions, due to more abundant and complex disturbance legacies. Next, I characterized the relative importance of bottom-up and top-down drivers on short-interval reburn potential. I used the Fire and Fuels Extension to the Forest Vegetation Simulator to model potential fire behavior and effects in each field plot under two fire weather scenarios. I found that initial fuel variability influenced some aspects of potential fire behavior and effects in reburns under moderate fire weather conditions. However, extreme fire weather is likely to override these effects and result in stand-replacing fire effects regardless of differences in initial fuel variability among stands. Microclimate responding to differences in stand structure buffered potential fire behavior and effects, particularly in unburned stands, but did not change overall patterns. These findings suggest the dominance of top-down drivers on influencing short-interval reburn potential in wet temperate forests. Finally, I examined potential tradeoffs for managing post-fire forest trajectories, specifically focusing on early-seral conditions, tree regeneration, and fuel profiles. I initialized each high severity field plot in the individual-based landscape model, iLand, and simulated 80 years of stand development under two future climate scenarios. I found that pre-fire stand age had lasting effects on forest recovery following stand-replacing fire, with older stands having longer persistence of early-seral conditions, more abundant and diverse live tree regeneration, and greater canopy and surface fuel loads. Post-fire trajectories were similar under both warming and current future climate scenarios. These findings suggest that post-fire recovery may be more robust in older stands, and negative effects of warming are unlikely through the end of the century in the absence of additional disturbance. Further, common post-fire management interventions present likely tradeoffs for post-fire forest structure and function. Collectively, this work builds understanding of the drivers and consequences of fire in forests shaped by long intervals between severe disturbances. Exploring the immediate and future effects of fire on ecosystem functions, disturbance interactions, and management outcomes supports our ability to manage post-fire recovery trajectories in some of the world’s highest biomass forests.