Chemical Ecology of Willows ( <i>Salix</i> L.): Ecological Roles, Evolutionary Dynamics, and Phytochemical Diversity

BACKGROUND: The genus Salix comprises approximately 450 species that are known for rich and distinctive specialized chemistry, dominated by phenolic compounds such as salicinoids, flavonoids, and tannins, as well as diverse volatile organic compounds. These metabolites mediate interactions with herbivores, pathogens, mutualists, and the abiotic environment. Owing to their species richness, frequent hybridization, polyploidy, and diverse ecological interactions, willows represent an exceptional model for studying the evolution and ecological functions of plant specialized metabolites. SCOPE: We synthesize current knowledge on the ecological and evolutionary roles of willow specialized metabolites, integrating evidence from chemical ecology, metabolomics, phylogenetics, and evolutionary biology. We examine how non-volatile and volatile chemistry shapes willow interactions with antagonists and mutualists, and how these interactions vary across environmental gradients. We focus on evolutionary trends in willow chemistry in response to various selection pressures and explore the effects of hybridization, introgression, and polyploidization on willow metabolomic profiles. We discuss emerging insights into chemical trait syndromes, metabolic constraints, and the role of abiotic filtering versus biotic selection in shaping phytochemical variation in willows. CONCLUSIONS: Willows combine extraordinary chemical diversity with high evolutionary flexibility, which likely drives their success across contrasting environments. Variation in their specialized chemistry emerges from the interplay between selection imposed by herbivores, pathogens, and mutualists, and environmental filtering along climatic and resource gradients. These selective forces act on both metabolite concentration and structural variation in willow metabolomes, while hybridization, polyploidy, and constraints within metabolic pathways modulate both the evolutionary trajectories and the impact of willow chemistry on their ecological interactions. Advances in untargeted metabolomics and phylogenomics now enable a more holistic understanding of willow chemical strategies beyond a small set of canonical compounds. As such, willows remain a powerful model system for uncovering general principles governing the evolution, function, and diversification of plant specialized metabolites.