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Background
Mountain pine beetle (MPB) is the most significant mortality agent of pines throughout western North America, and climate-driven range expansion is occurring. All pine trees are considered suitable hosts for the beetle. Warming temperatures have resulted in extensive MPB-caused tree mortality, including in high-elevation forests containing whitebark and limber pines.
MPB is native to western North America, and native pines have evolved defenses against the insects attack. Pine defenses in recently-invaded areas, however, may be lower than in areas with longer-term MPB presence. MPB was recently observed in high-elevation forests of the Great Basin region. Great Basin bristlecone and foxtail pines occur at high elevations in the Great Basin region of the western United States and have the longest lifespan of all trees worldwide. Susceptibility of Great Basin bristlecone and foxtail pines to MPB was previously unknown. Defense and susceptibility in these two long-lived species are unclear, although they occur in the same forest stands with limber pine, a common MPB host. We surveyed stands with co-occurring Great Basin bristlecone-limber pine and foxtail-limber pine to determine relative MPB attack susceptibility and inherent defenses.
A live Great Basin bristlecone pine surrounded by mountain pine beetle-killed limber pines near Mount Moriah, Nevada. BJ Bentz photo.
Key Findings
- MPB-caused mortality was extensive in limber, low in foxtail, and absent in Great Basin bristlecone pine.
- Defense traits including resin terpenes, resin ducts, and wood density were higher in Great Basin bristlecone and foxtail than limber pine.
- The high level of inherent defenses in Great Basin bristlecone and foxtail, relative to limber pine, make them less vulnerable to climate-driven MPB range expansion. This is also potentially due to a co-option of traits that confer tree longevity, in addition to a long-term evolutionary relationship with phloem feeders like the mountain pine beetle.
- Our research shows that Great Basin bristlecone and foxtail pines have inherent defenses that make them less susceptible to climate-driven MPB-caused mortality. Understanding traits that confer resistance to MPB in these species will facilitate conservation efforts of keystone, high elevation pines, and tree protection in high value areas.
- The results highlight the complexity of predicting plant-insect interactions under climate change.