Shifts in woody plant defence syndromes during leaf development

Herbivores target young leaves in forests world-wide. How this strong and predictable selection pressure has shaped plant defence syndromes remains unclear. Specifically, whether young leaf herbivory has led to general global patterns of shifting leaf defences during leaf development (i.e. ontogenetic trajectories) remains unknown but likely. Using meta-analysis, we have synthesized developmental shifts in chemical, physical and indirect defence traits, as well as leaf nutrient content, during leaf expansion and maturation across 124 woody plant species. Leaf traits were compared for immature young leaves vs. mature leaves within studies, and these developmental shifts were then compared across studies to characterize general patterns. Traits shifted strongly during leaf maturation, giving rise to discrete defence syndromes, with young leaves having significantly greater nutrient and secondary chemistry concentrations but reduced toughness and indirect defence traits than mature leaves. These patterns corroborate a growing consensus on the importance of ontogeny in plant defences, illustrating ontogenetic trajectories at the scale of leaves. Trait developmental shifts were stronger in species with synchronous than asynchronous leaf flushing, but whether this reflects variation across biomes remains unclear. Future research on under-studied traits, such as alkaloids, cyanogenesis, leaf phosphorus content and indirect defence traits, and in a more biogeographic context will provide additional insights into the generality of shifts in defence syndromes during leaf development. A free Plain Language Summary can be found within the Supporting Information of this article.