Fire severity and cumulative disturbance effects in the post-mountain pine beetle lodgepole pine forests of the Pole Creek Fire

Abstract

Recent large scale mountain pine beetle (Dendroctonus ponderosae Hopkins, MPB) outbreaks have created concern regarding increased fuel loadings and exacerbated fire behavior and have prompted a desire to understand the effects of sequential disturbances on the landscape. However, previous research has focused on quantifying fuel loadings and using operational fire behavior models, rather than direct field measurements, to understand changes in fire severity following MPB. The 2012 Pole Creek Fire in central Oregon partially occurred in gray stage (8–15years post-MPB epidemic) lodgepole pine forests. We examined the combined effects of MPB and fire disturbances on stand structure, and investigated the influence of previous MPB severity and fire weather on subsequent fire severity and cumulative disturbance severity. We randomly selected and installed 52 plots over a gradient of MPB and fire severity combinations and measured stand structure and fire severity characteristics. Fire severity metrics representing both crown and surface fire decreased with increased MPB severity under extreme burning conditions, following expected trends for crown fire severity, but not surface fire severity. Cumulative basal area mortality increased with MPB severity under moderate burning conditions, while other cumulative disturbance severity metrics were unrelated or weakly related to MPB severity. High severity crown fire was common despite hypothesized low canopy fuel loadings during the gray stage, indicating the importance of understanding variable mortality density of MPB outbreaks. Although long-term studies are needed to understand ecosystem recovery trajectories over time, there was no indication that a loss of ecosystem resilience occurred as a result of two sequential disturbances in this landscape.