Climate change has amplified eruptive bark beetle outbreaks over recent
decades. However, for projecting future bark beetle dynamics there is a
critical lack of evidence to differentiate how outbreaks have been
promoted by direct effects of warmer temperatures on beetle life cycles
vs indirect effects of drought on host susceptibility. To diagnose
whether drought-induced host-weakening was important to beetle attack
success we used tree death date demography during a spruce beetle
outbreak to differentiate early and late-dying trees and then determined
whether early-dying trees had greater sensitivity of tree-ring carbon
isotope discrimination to drought. Drought-sensitivity did not differ
among early- vs late-dying trees, suggesting proposed links between
spruce beetle outbreaks and drought primarily reflect warming- amplified
beetle life cycles rather than drought-weakened host defenses.
Additional iso-demographic studies are needed to diagnose the role of
direct vs indirect climate effects across wider regions and other
species.