Fire injury increases both host susceptibility and competitor load for the mountain pine beetle (Dendroctonus ponderosae Hopkins): Implications to population dynamics and outbreaks

The mountain pine beetle (Dendroctonus ponderosae Hopkins) is a bark beetle native to many pine dominated ecosystems throughout western North America. At low population densities, D. ponderosae is confined to living and reproducing within weakened or stressed trees. At high population densities, D. ponderosae has the ability to colonize healthy, vigorous trees. Fire is a common factor within these pine-dominated ecosystems. Frequently, trees are not killed by the fire, but are damaged or stressed to varying levels. The objectives of this study were to determine if fire damage predisposes lodgepole pine (Pinus contorta var. latifolia) trees to colonization by D. ponderosae, and if colonization of fire damaged trees is impacted by beetle abundance. Four sites in the Greater Yellowstone Ecosystem (Wyoming) that burned during summer 2006 were studied in the summer of 2007, and four sites that burned during summer 2007 were studied in the summer of 2008. Four 100m x 5m belt transects at each site were used to determine the number of lodgepole pines per fire injury category (low, moderate and high), the presence or absence of D. ponderosae in each tree, and the presence or absence of D. ponderosae competitors. Our results indicate that fire injury predisposes trees to D. ponderosae colonization, but the extent to which this occurs varies in a nonlinear fashion with the severity of injury, and that colonization is further modified by the presence of competitors and beetle population density.