Size-dependent changes in wood chemical traits: a comparison of neotropical saplings and large trees

Wood anatomical traits are important correlates of life-history strategies among tree species, yet little is known about wood chemical traits. Additionally, size-dependent changes in wood chemical traits have been rarely examined, although these changes may represent an important aspect of tree ontogeny. Owing to selection for pathogen resistance and biomechanical stability, we predicted that saplings would show higher lignin (L) and wood carbon (C(conv)), and lower holocellulose (H) concentrations, compared with conspecific large trees. To test these expectations, we quantified H, L and C(conv) in co-occurring Panamanian tree species at the large tree vs. sapling size classes. We also examined inter- and intraspecific patterns using multivariate and phylogenetic analyses. In 15 of 16 species, sapling L concentration was higher than that in conspecific large trees, and in all 16 species, sapling H was lower than that in conspecific large trees. In 16 of 24 species, C(conv) was higher in saplings than conspecific large trees. All large-tree traits were unrelated to sapling values and were unrelated to four life-history variables. Wood chemical traits did not show a phylogenetic signal in saplings, instead showing similar values across distantly related taxa; in large trees, only H showed a significant phylogenetic signal. Size-dependent changes in wood chemistry show consistent and predictable patterns, suggesting that ontogenetic changes in wood chemical traits are an important aspect of tree functional biology. Our results are consistent with the hypothesis that at early ontogenetic stages, trees are selected for greater L to defend against cellulose-decaying pathogens, or possibly to confer biomechanical stability.