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

Abstract Drought is an increasingly 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 an important role in determining post-drought resilience. Here, we tested whether prior-year drought leaves a persistent imprint on leaf traits in Populus tremuloides. Using a three-year common garden experiment with controlled water limitation, we measured two major 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 these effects persisting across growing seasons. While overall fungal community composition remained relatively stable, we detected shifts in the relative abundance of individual amplicon sequence variants (ASVs), particularly within potentially pathogenic lineages, associated with drought history. These findings provide evidence that drought leaves a legacy in leaf chemistry and microbial colonization, with potential consequences for how trees tolerate future abiotic and biotic stress. Our results highlight the importance of incorporating foliar trait legacies into models of forest resilience under climate change. Significance Statement Trees are known to exhibit long-term responses to drought, but whether such legacy effects extend to annually renewed tissues like leaves remains unclear. Our findings reveal that prior-year drought alters both chemical defenses and microbial associations in newly formed leaves, demonstrating a form of physiological memory. This work uncovers a novel mechanism by which trees integrate past environmental stress into future interactions, with implications for understanding forest resilience under climate change.