Cutting a course: examining the effects of historical thinning treatments on directing forest response to bark beetle outbreaks

Abstract

Bark beetle outbreaks are major natural disturbances in temperate forests across the northern hemisphere, contributing to extensive tree mortality and driving forest change. Recent widespread and severe outbreaks—and projected climate-driven changes to disturbance activity—have raised concerns about forest resilience and the interaction of future outbreaks with other disturbances (e.g., fire). Thinning (i.e., density reduction) treatments may promote resilience to bark beetles by fostering resistance or bolstering forest capacity to respond post outbreak, but opportunities to test treatment efficacy and longevity have been rare. Further, forest managers must consider the effects of thinning on additional objectives such as tradeoffs between fire hazard mitigation and carbon storage. Here, I used a replicated study of old-growth lodgepole pine stands thinned in 1940 and affected by a recent (early 2000s) severe mountain pine beetle outbreak to examine the effects of thinning on (1) components of resilience to outbreak, and (2) post-outbreak fire hazard and carbon storage. I measured stand structural attributes in the field ~8 years post-outbreak and compared resistance to beetle attack (tree- and stand-scale survival), successional trajectories, fuel profiles, and aboveground carbon biomass between uncut (control) and thinned treatment units. Thinning six decades prior to mountain pine beetle outbreak had limited effects on resistance, but additional effects of thinning on stand trajectories, surface and canopy fuel profiles, and aboveground carbon storage persisted following severe outbreak. This study broadens the temporal extent of our understanding of thinning effects on directing forest response to bark beetle outbreaks, with important implications for developing effective management decisions in the face of global change.