Functional memory of drought affects leaf chemical defenses and microbial interactions in aspen.

Drought is an important driver of tree mortality, but its long-term impacts on trait expression remain poorly understood. Leaves are a key interface at which trees respond to abiotic stress, and foliar traits such as chemical defenses and microbial communities may play a role in determining post-drought resilience. We tested whether prior-year drought leaves a persistent imprint on leaf traits in Populus tremuloides Michx. Using a 3-year common garden experiment with controlled water limitation, we measured two classes of defensive phenolics-salicinoid phenolic glycosides (SPGs) and condensed tannins (CTs)-alongside foliar fungal community composition. SPG concentrations increased and CTs declined in response to prior-year drought, with no evidence of a trade-off between increased allocation to defensive chemistry and growth. Higher SPG concentrations were associated with reduced canopy damage across treatments, while CT-damaged canopy relationships depended on prior-year drought conditions. The relative abundance of pathogen-capable fungi increased under drought but declined with increasing SPG concentrations, consistent with the possibility that drought legacy effects on foliar chemistry extend to microbial colonization. These results indicate that drought-induced shifts in foliar chemistry can persist beyond the drought period and may continue to influence plant performance and biotic interactions, with implications for post-drought resilience.