Abstract Societal Impact Statement Butternut is yet another North American tree species undergoing rapid mortality due to the globalization of forest pathogens. Pairing tree health surveys and genetic diversity metrics, we captured a 7‐year snapshot of butternut's path to extirpation from its distinct range in New Brunswick, Canada. We recorded substantial mortality and vigour decline in these genetically unique populations, where a major proportion of genetic diversity was contributed by dying trees. We discuss feasible actions on a tight deadline for species under range‐wide pathogen pressure and highlight a blind spot in conservational policy: the risk of substantial genetic depletion far prior to extirpation. Summary Butternut ( Juglans cinerea ) populations in New Brunswick, Canada, are rapidly declining due to the fungal pathogen Ophiognomonia clavigignenti‐juglandacearum . Mitigation strategies, including tree breeding, rely on access to genetically diverse populations and take time to implement. It is therefore vital to account for the rate of mortality and genetic diversity decline. Two main objectives: (I) characterize the rate of mortality and diversity decline; (II) assess if visually classed symptoms reflect the tree's state of decline towards mortality; and two exploratory objectives: (III) examine the contributions of trees grouped by health to the total allelic richness of the population; and (IV) project the timeline to mortality of the measured trees based on two timepoints (2016 and 2023). We examined tree health and diversity indices (allelic richness and expected heterozygosity) for 428 individuals (6 populations) at 2 time points: 2016 and 2023. Out of 428 butternut trees 22% died between 2016 and 2023, the median vigour declined in all but one population. Where the majority of the total allelic richness was contributed by vigorous trees in 2016, in 2023 this majority is conferred by dying trees. Qualitative phenotyping provided vital context to the rate of decline. Our results highlight the decreasing time window for the genetic conservation of Canadian butternut populations, and the urgency specifically for New Brunswick as a small, marginal population in steep decline.
Meer et al. (Thu,) studied this question.