Defence traits vary across leaf ages in the thistles (Cardueae)

Abstract Plants produce a variety of structural and chemical defences to deter herbivores, which can covary across every scale of biological organization. Along these lines, it has been suggested that young leaves differ from old leaves in their defence strategies, but studies have largely focused on woody plants and/or chemical traits. Thus, we lack an understanding of how structural and chemical defences ontogenetically covary across a large portion of plant diversity. We investigate the range of structural and chemical defences of thistles (tribe Cardueae), which comprise herbaceous species with a wide spectrum of defensive traits. We collected structural and chemical trait data across species and leaf ages from field collections, herbarium specimens and the literature. Species' young leaves tend to produce similar suites of traits, with greater interspecific variation in older leaves. Young leaves are dominated by chemical defences, while older leaves produce a broader range of both chemical and physical defence traits. Leaf spininess is associated with a distinct chemical profile that differs from that of non‐spiny species. Our study demonstrates how defence strategies can vary across the leaves of herbaceous species and their ages. Expanding such efforts will be critical to further our macroevolutionary understanding of variation in plant defences. Read the free Plain Language Summary for this article on the Journal blog.