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

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.

Anatomical defences against bark beetles relate to degree of historical exposure between species and are allocated independently of chemical defences within trees

“…longaeva (0.015 mm 2 ; ~2 ducts per year), and P . balfouriana (0.030 mm 2 ; ~5 ducts per year; Bentz et al, ; Ferrenberg et al, ). However, the range of sizes and production of resin ducts in both species at our sites overlapped with these other populations, indicating substantial between‐site variability due to potential environmental, genetic, or interacting factors (Ferrenberg et al, ; Hood & Sala, ).…”

“…These processes have linkages, in that resin cells extend throughout the plant to produce and deliver chemicals in attacked tissues (Björkman, Larsson, & Gref, ; Zas et al, ). Although some studies examined both anatomical and chemical components of tree defence (Bentz, Hood, Hansen, Vandygriff, & Mock, ), or within‐tree correlations within seedlings (Moreira, Zas, Solla, & Sampedro, ), to our knowledge, none have tested their relatedness within the same mature trees or compared relationships between species of different exposure histories.…”

Bark beetles as agents of change in social–ecological systems

“…However, although a beetle species is capable of moving into new landscapes, this does not mean that the new tree species it encounters will be a susceptible host (Bentz et al . ). Similarly, regionally synchronous outbreaks are not necessarily a given, and only a few studies have explored how past climate dynamics influenced outbreak synchrony and intensity at any spatial scale (Jarvis and Kulakowski ).…”

Contrasting Impacts of Climate and Competition on Large Sugar Pine Growth and Defense in a Fire-Excluded Forest of the Central Sierra Nevada

“…These differences indicate that the relationship between resin duct defense and climate likely varies depending on the species and the climatic regions examined. Recent research has observed substantial variation in defense characteristics in pines across sites of differing climate [65] and along elevational gradients [66], but more direct examination with climate across these scales is needed.…”