The evolution of bark mechanics and storage across habitats in a clade of tropical trees

Premise of the study: Bark functional strategies vary conspicuously within communities. As a result, predicting most community level bark traits based on environment often reveals little association. To complement this community-based view, we took a clade-based approach to study potentially adaptive differences in bark water storage and biomechanics across habitats and examined ontogenetic mechanisms that lead to these differences. Methods: We studied the branches of nine species in the simaruba clade of Bursera in dry to wet, fire-free neotropical forests. We measured mechanical properties from branch tips to bases, as well as the relative area and water content of bark. Using raw data and phylogenetically independent contrasts, we then tested predictions regarding trait associations with environment and mapped branch tip-to-base ontogenetic changes. Key results: Across our wet-dry gradient, bark water storage was greater in drier habitats, whereas bark tissue mechanical rigidity was greater in the taller species of moister forests. Bark was the principal mechanical tissue in branch tips and an important contributor even in branches 3 m long. Within species, bark contributions to mechanical support and water storage came mostly through a tip-to-base increase in bark quantity rather than alterations in tissue properties. Quantitative developmental alterations in proportions of bark to wood led to habit differences. Conclusions: Our clade-based approach shows that, in marked contrast to most community-based results, environment can strongly predict bark functional traits across species in ways that seem plausibly adaptive.