Interactions of white pine blister rust, host species, and mountain pine beetle in whitebark pine ecosystems in the Greater Yellowstone

The current mountain pine beetle (Dendroctonus ponderosae Hopk.) activity in whitebark pine (Pinus albicaulis Englem.) ecosystems in the Greater Yellowstone Ecosystem (GYE) is historically unprecedented in extent and severity. In addition, a non-native pathogen, white pine blister rust (Cronartium ribicola Fisch.) is widespread and infection rates continue to intensify. Interactions between the beetle and blister rust are placing this species in a precarious state. I recorded stand- and tree-level data on four biogeographically variable sites in the GYE to quantify how four variables; severity of white pine blister rust; the presence of the alternate host lodgepole pine (Pinus contorta var. latifolia Englem.); whitebark pine density; and diffusion by non-alternate host species influence probability of selection by the mountain pine beetle for individual whitebark pine. Summary data show that 52% of the whitebark pine sampled in this study were dead, 70% attacked by mountain pine beetle, 85% infected with blister rust, and 61% were afflicted with both. Chi-square tests indicated that beetle activity was lower than expected in whitebark pine with light blister rust and increased significantly in whitebark pine with heavy blister rust. Habitat use-availability selection ratios (HSRs) indicated that, on sites with two potential host species, mountain pine beetle preferentially select whitebark pine over lodgepole pine. In addition, HSR analyses indicated that mountain pine beetle preferentially select whitebark pine with heavy blister rust over those with light rust. Whitebark pine diameter, rust severity, and overstory tree species composition were significant, not mutually exclusive predictors in logistic regression models. This work reveals that blister rust increases whitebark pine probability of selection by, and that lodgepole pine are not the preferred host of, the mountain pine beetle. Concurrently, climate change has resulted in the expansion of habitats thermally favorable to bark beetle reproductive success in whitebark pine ecosystems. This research suggests that interactions among these disturbance agents will enhance whitebark pine mortality, widespread population decline, and alter ecological functions and processes to which these trees are critical. However, mountain pine beetle selection preference for whitebark pine with severe blister rust infection accelerates the rate of change in the proportion of whitebark pine with severe rust in the remaining population.