Elevational range expansion of mountain pine beetle is driven by localized irruptive populations

Warming temperatures are allowing native insect herbivores to expand into regions that previously exceeded their thermal tolerance. Latitudinal and elevational range expansions are yielding new insect-plant interactions that can fundamentally alter ecosystem processes. Bark beetles cause the most widespread morality to conifers globally, but are limited in their range by host availability and minimum temperatures. Recently, several beetle species have expanded their geographic ranges to cause significant mortality to naïve and semi-naive tree species. However, it is unknown whether these outbreaks are currently driven by continued emigration from historical host species or local build-up within these newly accessed habitats.

We tested two non-mutually exclusive hypotheses by constructing landscape random forest models for mountain pine beetle expansion into endangered high-elevation whitebark pine communities in British Columbia. Our study area included all 10-ha hexagonal grid cells that contained any whitebark pine, as well as adjoining cells that contained mostly lodgepole pine or lodgepole - whitebark mixtures. Our models included geographic information, stand characteristics, regional bioclimatic data, cold suitability metrics, and beetle propagule pressure.