White ash trees (Fraxinus americana) originally from colder, drier areas are more susceptible to emerald ash borer (Agrilus planipennis)

Trees are dying prematurely across the globe at alarming rates, often due to drought and/or insects. Insect attacks can range in severity with some tree species surviving (e.g., the American beech and beech bark disease) and some species becoming essentially extinct (e.g., the American chestnut and the chestnut blight). Tree responses may also vary because of genetic differences among “families” of trees (i.e., intraspecific variation). It is thus hard to predict how tree species will respond to insect attacks. To this avail, we examined tree canopy dieback (leaf loss), drought stress, wound healing, and chemical defense responses of 44 families of white ash (Fraxinus americana) from across the species range to a recent emerald ash borer (Agrilus planipennis) outbreak in an arid common garden in Kansas. At our site, white ash families originally from drier, colder areas (northern USA) exhibited more canopy dieback than families from wetter, warmer regions (southern USA). We also found that tree families that were sensitive to drought and/or slower to heal bark wounds were more likely to show canopy dieback. Overall, we are the first to demonstrate significant intraspecific variation in white ash canopy dieback due to emerald ash borer and connect it to temperature and precipitation conditions of original seed sources. If this canopy dieback pattern holds for mortality, southern white ash may be suitable for reintroduction to the American north, where 99% of white ash has already died.