Fuel Profiles and Biomass Carbon Following Bark Beetle Outbreaks: Insights for Disturbance Interactions from a Historical Silvicultural Experiment

Anticipating consequences of disturbance interactions on ecosystem structure and function is a critical management priority as disturbance activity increases with warming climate. Across the Northern Hemisphere, extensive tree mortality from recent bark beetle outbreaks raises concerns about potential fire behavior and post-fire forest function. Silvicultural treatments (that is, partial or complete cutting of forest stands) may reduce outbreak severity and subsequent fuel loads, but longevity of pre-outbreak treatment effects on outbreak severity and post-outbreak fuel profiles remains underexplored. Further, treatments may present tradeoffs for other management objectives focused on ecosystem services (for example, carbon storage). We measured structure in old-growth subalpine forests following a recent (early 2000s) severe mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak to examine effects of historical (1940s) cutting intensity on gray stage (~10 years after peak of outbreak) post-outbreak (1) fuel profiles and (2) aboveground biomass carbon. Compared to control (uncut) stands, historically cut stands subjected to the same MPB outbreak had approximately half the post-outbreak surface fuel loads, about 2-3x greater live canopy fuel loads, and greater within-stand spatial heterogeneity of dead canopy cover and available canopy fuel load. Post-outbreak total aboveground biomass carbon was similar across all stands, though historically cut stands had about 2x greater carbon in live biomass compared to uncut stands. These findings suggest tradeoffs with altered post-outbreak potential fire behavior and carbon storage in cut stands. Additional implications of historical silvicultural treatments for wildlife habitat, firefighting operations, and long-term carbon trajectories highlight temporal legacies of management on directing forest response to interacting disturbances.