Climate Shapes Elevational Gradients in Concentration, Composition, and Functional Diversity of Willow Metabolomes

Plants produce broad spectra of metabolites comprising primary metabolites involved in core physiological processes and specialized metabolites mediating responses to biotic and abiotic stress. Understanding how these various components of the metabolome respond to different environmental pressures can help us to understand drivers of phytochemical diversity and its role in plant ecology and evolution. We investigated how climatic stress, soil resources, and herbivory influence various measures of phytochemical diversity in four willow species along an elevational gradient (950-2250 m a.s.l.), focusing on flavonoids, salicinoids, and primary metabolites. Harsher climates increased metabolite concentrations, particularly salicinoids, but reduced flavonoid richness and overall structural α-diversity. Similar responses in primary metabolites suggest that these shifts extend beyond defence chemistry. Chemical properties showed the strongest shifts in flavonoids and salicinoids and weaker changes in primary metabolites. β-diversity generally increased with elevation and was most strongly associated with climate, while soil nutrients contributed to shifts in β-diversity of salicinoids and primary metabolites. Willow species showed weak genetic differentiation across elevation, suggesting limited population divergence. Together, these results indicate that elevational shifts in willow chemistry involve broad but uneven reorganization across the metabolome, with different metabolite groups and measures of diversity plastically responding to different drivers.