Abiotic stress rather than biotic interactions drives contrasting trends in chemical richness and variation in alpine willows

Plants produce an astonishing diversity of specialized metabolites as defences against herbivores, pathogens or detrimental abiotic conditions. Plants growing at different elevations are exposed to different biotic and abiotic conditions and typically show pronounced differences in their chemistry. Understanding how these differences arise through changes in various measures of chemical diversity can inform us concerning factors that contribute to the variety of metabolites found among plants. We focused on elevational changes in concentration, richness and intra- and interspecific variation in specialized chemistry in willows (Salix, Salicaceae) and compare them among metabolite classes with different functions. We aim to show how these various measures of chemical diversity change with elevation to reveal trends contributing to changes in plant chemistry along major ecological gradients. We studied chemistry, herbivory and fungal pathogen damage in an assemblage of seven willow species along an elevational gradient in the Alps (800-2600 m a.s.l.). We examined trends in chemical diversity using untargeted metabolomics, and further quantified trends in three specific classes: proanthocyanidins and salicinoids involved in biotic interactions, and flavonoids involved mainly in abiotic protection. We use measures of willow chemistry that take structural relatedness of metabolites into account to show if the roles of structurally distinct metabolites change with elevation. Willows from low elevations exhibited greater proanthocyanidin concentration and structural richness of flavonoids. In contrast, willows from high elevations showed greater structural richness of salicinoids and greater variation in total metabolite composition at both the intra- and interspecific levels. The trends in salicinoid richness and proanthocyanidin concentration were explained by elevational changes in temperature. Our results show how elevational differences in plant chemistry arise through trends in various aspects of their chemical diversity. Willows at high elevations showed reduced structural richness of metabolites involved in abiotic protection. This may reflect focused investment in metabolites with the highest ecological benefit relative to their concentration in high-elevation willows. At the same time, they possessed greater richness of metabolites involved in biotic interactions, while variation in microhabitat preferences among high-elevation species likely contributed to the high variation in their total metabolite pool. Read the free Plain Language Summary for this article on the Journal blog.