Tree phenotypes vary because of genotype–climate interactions, and this variation influences host selection by tree-killing bark beetles. As climate-driven bark beetle outbreaks intensify, identifying phenotypic traits that best predict resistance or susceptibility is critical. We examined genetic variation, secondary chemistry, growth rates, and climate sensitivity in interior ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) at two sites in the Black Hills—Devils Tower National Monument (DETO), Wyoming, and Wind Cave National Park (WICA), South Dakota—experiencing low-moderate levels of mountain pine beetle (Dendroctonus ponderosae Hopkins) activity. Genetic structure differed between sites. At DETO, large and small trees formed a single genetic cluster, whereas at WICA, two clusters emerged, one consisting of large trees and another comprising both small and large trees. The concentrations of some terpenes also differed between sites. Compared to beetle-killed trees, surviving trees exhibited distinct lifelong growth patterns and greater sensitivity to climate. Notably, surviving trees showed significant correlations of growth with climate variables, while beetle-killed trees were relatively insensitive. Long-term responsiveness of growth to climate was a stronger predictor of tree susceptibility to beetles than responses in years just before attacks occurred. These findings suggest trees with lower sensitivity to climate may be more vulnerable to beetle attack under changing climatic conditions.
Six et al. (Fri,) studied this question.