We have a fundamental and applied understanding of how differences in the wood chemistry of trees affects the durability of wood products. By comparison, relatively little is known about the ecological causes and consequences of species differences in wood chemistry; even less is known about how or why wood chemistry differs within species, across trees of different sizes. In a study published in AoB PLANTS, Martin et al. find strong and consistent differences in the wood chemistry of saplings and canopy trees, in several tropical hardwood species. These differences point to the importance of pathogens and tree biomechanics as evolutionary causes of size-dependent changes in wood chemistry.