Climate indirectly modulates tree survival of spruce beetle attacks via effects on constitutive and induced secondary...

Despite many years of research, it remains challenging to determine why some trees survive bark beetle attacks while others do not. Current theory suggests that survival is predicated by interactions between tree defense systems and climatic drivers, but such interactions are poorly resolved at the scale of individual trees. Using a widespread conifer species (Engelmann spruce, Picea engelmannii) and a lethal phloem-feeding herbivore (North American spruce bark beetle, Dendroctonus rufipennis) as a study system, we characterized tree responses to mass attack across five populations representing a climate gradient in a field experiment. Over the course of a growing season, we measured variation in monoterpene concentrations in the phloem of 100 trees, half of which were challenged with mass attack using pheromone baits. Study trees were visited two years after baiting to record survival, and match survival to constitutive and induced monoterpenes. Our results revealed that regional climate correlated with constitutive, pre-attack concentration of monoterpenes in spruce phloem: a 1 kPa increase in vapor pressure deficit correlated with a doubling of monoterpene concentration. Higher constitutive phloem monoterpene concentrations predicted tree survival: a one-fold increase in concentration was associated with a three-fold increase in the odds of survival. Regional climate did not significantly affect the magnitude of induced response; however, early induction of monoterpenes in phloem distal to attack sites was associated with an average of twofold increase in survival odds, suggesting rapidity of monoterpene induction is a key resistance trait. These findings extend our understanding of climate-defense relationships in this system and indicate that Engelmann spruce tree populations in warmer and drier sites may have traits associated with higher constitutive resistance.