Fibre wall and lumen fractions drive wood density variation across 24 Australian angiosperms

Wood density is considered a key plant trait, affecting mechanical and physiological performance, yet its biological meaning is still rather unclear. Accordingly we investigated the anatomical underpinnings of wood density in trees and shrubs. We measured wood density and anatomical traits in distal stems 4-10 mm diameter under bark in 24 Australian species. Proportions of wood components that are functionally distinct were analysed, including fibre wall and lumen, vessel wall and lumen, and axial and ray parenchyma. Wood density was mainly driven by the density of wood outside vessel lumens (density NV) rather than by vessel lumen fraction. In turn, densityNV variation was chiefly affected by fibre wall and lumen fractions. Considerable anatomical variation was observed at a given density NV, especially among medium-density NV species (0.60-0.85 g cm23); this range of medium density NV roughly translates to 0.50-0.75 g cm23 of overall density. The anatomy of these species formed a continuum from low fibre lumen and medium parenchyma fractions to medium fibre lumen and low parenchyma fractions. Our data suggest that wood density is an emergent property influenced by a complex anatomy rather than an unambiguous functional trait, particularly in medium-density species. With much anatomical variation, they likely represent a wide range of ecological strategies.