Interactions among disturbance agents in conifer forests : does fire injury increase susceptibility of lodgepole pine to mountain pine beetles and influence their population dynamics?

Powell, Erinn

Wildfire and bark beetle outbreaks are important and widespread natural disturbances in North American conifer ecosystems, yet little is known about how and to what extent they interact. We addressed whether fire injury increased the susceptibility of lodgepole pine (Pinus contorta Douglas var. latifolia Engelmann) trees to colonization by the mountain pine beetle (Dendroctonus ponderosae Hopkins), and whether the beetle’s population phase affected this interaction. I also addressed whether fire injury affected the per-capita brood production of D. ponderosae and how wildfire affects composition of coleopteran species. Eight sites that experienced wildfire and 8 adjacent paired control sites were examined in the Greater Yellowstone Ecosystem. Half of the sites were in areas in which D. ponderosae populations were in an outbreak phase and half were where populations were at low densities. A total of 2056 trees was sampled the following year, with each categorized accordingly to a range of fire injury levels and the presence or absence of D. ponderosae. Percent basal injury, percent bole injury, percent canopy injury, and cambium kill rating were used to assign each tree into its fire injury category. I also deployed 72 baited 12-funnel flight traps the year following each fire to sample the flying insects within the 8 burned sites. Nine traps were at each site, 3 each baited with myrcene, exo-brevcomin and trans-verbenol, (-) ipsdienol and lanierone, and (+) [alpha]-pinene and EtOH. One hundred and six trees within the 4 D. ponderosae outbreak burn sites and 2 corresponding non-burned sites were noted for level of fire injury and grouped into fire injury categories (unburned, low-, moderate-, and high-severity). I examined these trees for D. ponderosae attack density, and the density of exit holes by emerging adult offspring to determine per-capita brood production. Entry holes were counted after beetle flight, one year post-fire, and emergence holes were counted after beetle flight, at least two years post-fire. Seventy nine different trees within these 6 sites were also noted for level of fire injury and grouped into the previously specified fire injury categories. I measured the total average ovipositional or larval gallery area of D. ponderosae and other additional coleopteran subcortical herbivores under the bark of two 8 cm2 diameter disk shaped areas on the bole of each of the 79 trees. -- First paragraph.