Susceptibility of High-Elevation Forests to Mountain Pine Beetle (Dendroctonus ponderosae Hopkins) Under Climate Change
Creators: name: Soderberg, David N.
Publisher: name: Utah State University
Year: 2021

description: Across western North America, pine forests are important for timber, wildlife habitat, and at high elevations are important for water retention and yield from rain and snowmelt. The mountain pine beetle (MPB) is one of the most significant disturbance agents shaping pine forests, and like all insects, temperature is a major driver of its population success and the dynamics of the landscapes that they inhabit. Changing temperature regimes can therefore directly influence MPB population persistence at a particular location, in addition to potential shifts in the range boundaries that they inhabit. MPB is currently expanding its range northward in British Columbia and Alberta, Canada in parallel with warming climates, however, the potential impact of climate change on southern populations of mountain pine beetle is unknown. As the climate warms, the future distribution of MPB will be dictated by the ability to adapt to new and changing environments, in addition the availability and susceptibility of the pine trees that they feed upon. Pine species are known to vary in susceptibility to MPB, which is largely attributed to differences in the production of chemical (e.g., terpenes and their derivatives) and physical (e.g., resin ducts) defenses. Among pines, chemical defenses have been shown to confer defense against MPB, however, the nature of these defenses following biotic incitation has not been evaluated in many pine species. Moreover, lignification within bark beetle feeding tissues (e.g., bark, phloem) has been shown to confer defense within related conifers, but its defensive efficacy has yet to be assessed within pines.
To assess MPB response to a changing climate and the relative susceptibility of their pine hosts, I employed a variety of experimental approaches to assess the role of climate on MPB persistence and southern range expansion, in addition to the growth and defense strategies employed within and among high-elevation pine hosts that vary in resistance to MPB. The results from this work suggests that in a warming climate, MPB populations will not only persist, but increase in population. In addition, the MPB southern range boundary is likely limited by biotic interactions, rather than direct temperature effects. Among pines that differ in susceptibility to MPB, the concentration and composition of chemical defenses, as well as concentrations of lignin within the phloem were inversely correlated, with less MPB-susceptible pine species (e.g., Great Basin bristlecone pine) displaying higher concentrations of chemical defenses, but lower concentrations of phloem lignin, relative to more MPB-susceptible species (e.g., limber pine). These findings provide supporting evidence for evolved differences among pine species in investment between growth and defenses, where the concentration and composition of various chemical defenses, but not phloem or bark lignification, are adaptive traits for resisting MPB attack and brood development. My dissertation research advances our understanding of the interactions between MPB and its high-elevation, five-needle Pinus hosts, contributing to the adaptive management of high-elevation forests.