Blister rust severity and stand composition influence whitebark pine susceptibility to selection by mountain pine beetle in the Greater Yellowstone Ecosystem

The current mountain pine beetle (Dendroctonus ponderosae) outbreak in whitebark pine (Pinus albicaulis) ecosystems in the Greater Yellowstone Ecosystem (GYE) is unprecedented in extent and severity. The disturbance dynamics between the beetle and an exotic pathogen, white pine blister rust (Cronartium ribicola), are placing this foundation species in a precarious state. Stand and tree level data was recorded on four biogeographically variable sites in the GYE to quantify how severity of white pine blister rust, the presence of the alternate host lodgepole pine (Pinus contorta var. latifolia), tree density, and diffusion by non-alternate host species influence the susceptibility of whitebark to selection by MPB. Summary data show that 52% of the whitebark sampled in this study are dead, 70% attacked by mountain pine beetle, 85% infected with blister rust, and 61% are encumbranced with both. Non-parametric tests indicated that beetle activity was lower than expected in trees with low severity blister rust and increased significantly in trees with high blister rust severity. Habitat use selection ratios indicated that, on sites with two potential host species, whitebark are preferentially selected over lodgepole pine. Tree diameter, rust severity and overstory tree species composition were significant predictors in logistic regression models. This work reveals that blister rust increases whitebark pine susceptibility to selection by, and that lodgepole pine are no longer the preferred host of, the mountain pine beetle. The interactions of these disturbance agents will enhance whitebark pine mortality, widespread population decline, and the alteration of ecological processes to which these trees are critical.  Concurrently, habitats thermally favorable to bark beetle life history have expanded to encompass whitebark pine.