Defense traits in the long-lived Great Basin bristlecone pine and resistance to the native herbivore mountain pine beetle.

Barbara J Bentz,E Matthew Hansen,Karen E Mock,Sharon M Hood,James C Vandygriff

doi:10.1111/nph.14191

Abstract

Summary Mountain pine beetle (MPB, Dendroctonus ponderosae) is a significant mortality agent of Pinus, and climate‐driven range expansion is occurring. Pinus defenses in recently invaded areas, including high elevations, are predicted to be lower than in areas with longer term MPB presence. MPB was recently observed in high‐elevation forests of the Great Basin (GB) region, North America. Defense and susceptibility in two long‐lived species, GB bristlecone pine (Pinus longaeva) and foxtail pine (P. balfouriana), are unclear, although they are sympatric with a common MPB host, limber pine (P. flexilis).We surveyed stands with sympatric GB bristlecone–limber pine and foxtail–limber pine to determine relative MPB attack susceptibility and constitutive defenses. MPB‐caused mortality was extensive in limber, low in foxtail and absent in GB bristlecone pine. Defense traits, including constitutive monoterpenes, resin ducts and wood density, were higher in GB bristlecone and foxtail than in limber pine. GB bristlecone and foxtail pines have relatively high levels of constitutive defenses which make them less vulnerable to climate‐driven MPB range expansion relative to other high‐elevation pines. Long‐term selective herbivore pressure and exaptation of traits for tree longevity are potential explanations, highlighting the complexity of predicting plant–insect interactions under climate change.

Highlights

A well-documented consequence of recent climatic change is alteration in the geographic ranges of species (Lenoir et al, 2008; Mason et al, 2015), often causing complex, cascading, negative effects on biodiversity and community dynamics in newly invaded ecosystems (Forister et al, 2010)
We focused on areas in which Great Basin (GB) bristlecone pine grows in mixed stands with limber pine, a known susceptible MPB host, and where MPB activity was recorded
The GB bristlecone proportion of pine varied from 76% in the Snake Mountains South to 17% in the Ruby Mountains (Table S4)

Summary

Introduction

A well-documented consequence of recent climatic change is alteration in the geographic ranges of species (Lenoir et al, 2008; Mason et al, 2015), often causing complex, cascading, negative effects on biodiversity and community dynamics in newly invaded ecosystems (Forister et al, 2010). Variation in temperature-dependent developmental rates, life cycle timing and long-distance dispersal provide the capacity to track rapidly changing climatic conditions (Bale et al, 2002). In addition to appropriate thermal regimes, range expansion of herbivorous insects requires suitable host plant resources. Long-term contact and evolutionary history between specific plant and herbivorous insect species are expected to increase plant defenses (Herms & Mattson, 1992), and the resource availability hypothesis (RAH) predicts that slow-growing plants will invest heavily in defenses against herbivore attack and feeding because of the high cost of replacing tissue (Coley et al, 1985). Plants encountered by insects as a result of climate change-driven range expansion may or may not be suitable hosts, depending on the level and form of defense.

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