Anatomical underpinnings of wood density better associated with growth and mortality of tropical trees

Background/Question/Methods: Wood density is a key functional trait related to several major aspects of a plant’s ecology. Prevailing understanding considers similar wood densities of species as indicative of analogous ecological strategies. However, same wood density value can be achieved through different combinations of underlying anatomical components, suggesting that the composite nature of wood density masks greater variation in ecological strategies among coexisting species. Here, we decompose wood density into underlying components, identify major anatomical drivers of wood density variation, and examine relationships between these components and whole-plant function.  We tested two predictions: 1) wood density variation is largely influenced by fiber and parenchyma traits variation; 2) anatomical traits will be more strongly associated with species performance compared to wood density. In Barro Colorado Island, Panama, juvenile wood was sampled from three individuals (≥ 20 cm diameter) each of twenty tropical forest tree species varying along the tradeoff axis between growth and mortality. We quantified wood density and two categories of anatomical traits; 1) cell morphological traits (vessel diameter, vessel density, fiber wall thickness, and fiber lumen area), and 2) tissue fractions (vessel, parenchyma, fiber wall, and fiber lumen fractions). Species performance was measured as sapling relative growth and mortality rates.

Results/Conclusions: In variation partitioning analysis, juvenile wood density was best predicted by fiber lumen size for cell morphological traits and fiber wall fraction for tissue fractions. Relative growth rates showed moderate associations with fiber wall fraction (R = -0.52), fiber lumen fraction (R = 0.45), and fiber lumen size (R = 0.45). Mortality rates were moderately associated with fiber wall fraction (R = - 0.54), vessel diameter (R = 0.62), and vessel density (R = - 0.46). Prominence of fiber traits, particularly partitioning of fiber area between lumen and wall, confirms a major role for biomechanical demands in driving wood density variation among coexisting species.  Nevertheless, wood density was independent of interspecific variation in parenchyma fractions. Anatomical trait variation had consequences for whole-plant function. Associations between relative growth rates and fiber traits demonstrate carbon investment costs of making thicker fiber walls of constant density. Such investment translates to higher survival shown by associations between fiber traits and mortality rates. Safety rather than efficiency of hydraulic transport appears to be of greater priority for survival among coexisting species. Relatively stronger associations between species performance and anatomical traits in our results corroborate the notion that wood density masks variation in ecological strategies.