The evolution of mechanical properties of conifer and angiosperm woods

The material properties of the cells and tissues of an organism dictate, to a very large degree, the ability of the organism to cope with mechanical stress induced by externally applied forces. It is, therefore, critical to understand how these properties differ across diverse species and how they have evolved. Herein, a large data base (N = 84 species) for the mechanical properties of wood samples measured at biologically natural moisture contents (i.e., "green wood") was analyzed to determine the extent to which these properties are correlated across phylogenetically diverse tree species, to determine if a phylogenetic pattern of trait values exists, and, if so, to assess whether the rate of trait evolution varies across the phylogeny. The phylogenetic comparative analyses presented here confirm previous results that critical material properties are significantly correlated with one another and with wood density. Although the rates of trait evolution of angiosperms and gymnosperms (i.e., conifers) are similar, the material properties of both clades evolved in distinct selective regimes that are phenotypically manifested in lower values across all material properties in gymnosperms. This observation may be related to the structural differences between gymnosperm and angiosperm wood such as the presence of vessels in angiosperms. Explorations of rate heterogeneity indicate high rates of trait evolution in wood density in clades within both conifers and angiosperms (e.g., Pinus and Shorea). Future analyses are warranted using additional data given these preliminary results, especially because there is ample evidence of convergent evolution in the material properties of conifers and angiosperm wood that appear to experience similar ecological conditions.