Pre-fire structure drives variability in post-fire aboveground carbon and fuel profiles in wet temperate forests

Biological legacies (i.e., materials that persist following disturbance; “legacies”) shape ecosystem functioning and feedbacks to future disturbances, yet how legacies are driven by pre-disturbance ecosystem state and disturbance severity is poorly understood—especially in ecosystems influenced by infrequent and severe disturbances. Focusing on wet temperate forests as an archetype of these ecosystems, we characterized live and dead aboveground biomass 2–5 years post-fire in western Washington and northwestern Oregon, USA, to ask: How do pre-fire stand age (i.e., pre-disturbance ecosystem state) and burn severity drive variability in initial post-fire legacies, specifically (1) aboveground biomass carbon and (2) fuel profiles? Dominant drivers of post-fire legacies varied by response variable, with pre-disturbance ecosystem state driving total legacy amounts and disturbance severity moderating legacy condition. Total post-fire carbon was ~3–4 times greater in mid- and late-seral stands compared to young stands. In unburned and low-severity fire stands, >70% of post-fire total carbon was live, and canopy fuel profiles were largely indistinguishable, suggesting greater continuity of structure and function following low-severity fire. Conversely, in high-severity stands, >95% of post-fire total carbon was dead and sparse canopy fuel remained. Regardless of burn severity, most biomass present pre-fire persisted following fire, suggesting high-carbon pre-fire stands lead to high-carbon post-fire stands (and vice versa). Persistence of legacy biomass in high-severity stands, even as it decays, will therefore buffer total ecosystem carbon storage as live carbon recovers over time. Further, all burned stands had considerable production of black carbon in charred wood biomass which can support ecosystem functioning and promote long-term carbon storage. Initial post-fire fuel profiles are likely sufficient to support fire in all stands, but reburn potential may be greater in high-severity stands due to rapid regeneration of flammable live surface vegetation and more exposed microclimatic conditions. Effects of fuel reduction from fire on mediating the occurrence and potential behavior of subsequent fires in high-productivity systems therefore appear short-lived. Our findings demonstrate the importance of pre-disturbance ecosystem state in dictating many aspects of initial post-disturbance structure and function, with important implications for managing post-fire recovery trajectories in some of Earth's most productive and high-biomass forests.