Wood anatomical traits mediate life-history variations at the sapling, but not at the adult stage

Wood density (WD) is a central trait driving life-history variations among tree species. Density, however, is a property of wood that depends, in angiosperms, on three cell types, namely vessels, fibers and parenchyma. These wood cells are mainly involved in water transport, mechanical support and storage, respectively. Therefore, wood anatomical traits may provide a more mechanistic understanding of life-history variations than WD. Yet, studies formally comparing wood anatomical traits among ecological guilds or life stages of tropical trees have been scarce. This study examined the variation of wood functional traits between ecological guilds (i.e., light-demanding and shade-tolerant species) and life stages (i.e., sapling and adult wood), as well as the possible trade-offs between wood traits. Nineteen tree species with contrasting shade-tolerance were selected in a lowland tropical forest from eastern Amazonia. WD, fiber wall thickness, vessel lumen area, as well as fractions of fibers, vessels and parenchyma cells (i.e., axial, radial and total) were measured in sapling (up to 2.5 cm from the pith) and adult wood (> 5 cm to the bark). Overall, light-demanding and shade-tolerant species had different wood traits at the sapling, but not at the adult stage. Shifts in wood anatomical traits from sapling to adult wood were more common in light-demanding species than in shade-tolerants. Furthermore, at the sapling stage, wood allocation reflects a trade-off between growth and defense, with light-demanding species having traits that favor growth (i.e., low WD and fiber wall thickness, and wider conduits), while shade-tolerants had traits that maximize defense (i.e., high WD, fiber wall thickness, and parenchyma fractions). These findings represent valuable insights into wood allocation patterns among ecological guilds and life stages, and therefore, may expand our knowledge of trees life-history strategies.