P-330 - Multi-year drought affects aspen chemical defenses against pests and pathogens

Quaking aspen (Populus tremuloides) is a keystone species that has declined across the western United States over the past two decades. This decline is instigated by prolonged, severe drought events, however drought alone may not explain the decline. Preliminary evidence suggests that drought also influences the pathogen and broader microbial community, resulting in altered disease pressure in drought-stressed stands. To defend against pests, aspen can allocate over 30% of their total leaf dry weight to secondary metabolites, primarily salicinoid phenolic glycosides (SPGs) and condensed tannins (CTs). How drought influences pathogen abundance and aspen defenses is poorly understood. We conducted a multi-year common garden drought experiment, tracking stress response, secondary metabolite production, and leaf pathogen communities to investigate how drought stress affects the production of chemical defenses in aspen and influences pathogen interactions. Metabolomic results show a 9% mean increase in SPGs in years following drought, with a simultaneous decrease in CT abundance. 16S and ITS sequencing reveal that drought-stressed plants are colonized by a distinct set of potentially pathogenic bacteria and fungi. This study indicates that drought alters how aspen allocate resources to the production of SPGs versus CTs, changing the chemical environment in the leaf and potentially shifting which microbes are able to colonize. These findings highlight the complex role of abiotic stress in shaping plant-pathogen interactions, with implications for managing forest health.