The Lodgepole × Jack Pine Hybrid Zone in Alberta, Canada: A Stepping Stone for the Mountain Pine Beetle on its Journey East Across the Boreal Forest?
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Historical data show that outbreaks of the tree killing mountain pine beetle are often preceded by periods of drought. Global climate change impacts drought frequency and severity and is implicated in the range expansion of the mountain pine beetle into formerly unsuitable habitats. Its expanded range has recently reached the lodgepole × jack pine hybrid zone in central Alberta, Canada, which could act as a transition from its historical lodgepole pine host to a jack pine host present in the boreal forest. This field study tested the effects of water limitation on chemical defenses of mature trees against mountain pine beetle-associated microorganisms and on beetle brood success in lodgepole × jack pine hybrid trees. Tree chemical defenses as measured by monoterpene emission from tree boles and monoterpene concentration in needles were greater in trees that experienced water deficit compared to well-watered trees. Myrcene was identified as specific defensive compound, since it significantly increased upon inoculation with dead mountain pine beetles. Beetles reared in bolts from trees that experienced water deficit emerged with a higher fat content, demonstrating for the first time experimentally that drought conditions benefit mountain pine beetles. Further, our study demonstrated that volatile chemical emission from tree boles and phloem chemistry place the hybrid tree chemotype in-between lodgepole pine and jack pine, which might facilitate the host shift from lodgepole pine to jack pine.
KeywordsMountain pine beetle Range expansion Drought Tree defenses Beetle condition
We acknowledge Adriana Arango, Jeremiah Bolstad, Janice Cooke, Christina Elliott, Matt Ferguson, Andrew Ho, Ed Hunt, Brad Jones, Jean Linsky, Boyd Mori, and William Sperling for their help before and during the field season; Miles Dyck for providing us with the TDR equipment; Celia Boone for sharing β-phellandrene. We particularly acknowledge Adrianne Rice for providing fungal culture and knowledge; Tim McCready from Millar Western Forest Products Ltd. for providing a suitable field site; Jörg Bohlmann for comments on the manuscript. Funding for this research was provided through grants from the Government of Alberta through Genome Alberta, the Government of British Columbia through Genome BC and Genome Canada in support of the Tria 1 and Tria 2 projects (http://www.thetriaproject.ca) of which MLE and NE are co-investigators.
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