Mechanical reinforcement of tracheids compromises the hydraulic efficiency of conifer xylem

Wood structure and function of juvenile wood from 18 conifer species from four conifer families (Araucariaceae, Cupressaceae, Pinaceae and Podocarpaceae) were examined for a trade-off between wood reinforcement and hydraulic efficiency. Wood density and tracheid 'thickness-to-span' ratio were used as anatomical proxies for mechanical properties. The thickness:span represented the ratio of tracheid double wall thickness to lumen diameter. Hydraulic resistivity (R) of tracheids on a cross-sectional area basis (RCA) increased over 50-fold with increasing density and thickness:span, implying a strength versus efficiency conflict. The conflict arose because density and thickness:span were increased by narrowing tracheid diameter rather than by thickening walls, which may be developmentally difficult. In the Pinaceae and Cupressaceae species, density and thickness:span correlated strongly with protection from drought-induced embolism, suggesting that mechanical strength was required in part to withstand tracheid collapse by negative sap pressure. These species showed a corresponding trade-off between increasing RCA and embolism protection. In contrast, species of Podocarpaceae and Araucariaceae were overbuilt for their embolism protection and were hydraulically inefficient, having greater density, thickness:span and RCA, none of which were correlated with vulnerability to embolism.