Mechanical properties of black locust (Robinia pseudoacacia L) wood. Size and age-dependent variations in sap- and heartwood

Variations in the density and stiffness (Young's elastic modulus) of fresh wood samples drawn from different parts of the three main trunks of a 32-year-old black locust tree, Robinia pseudoacacia (measuring 19.8 m at its highest point), were studied to determine whether tree ontogeny can achieve a constant safety factor against mechanical failure. Based on the properties of isolated wood samples, the fresh density of sapwood decreased along radial transects from bark to pith, while that of progressively older heartwood samples increased, on average, towards the centre of each of the three trunks. Along the same radial transects, the Young's elastic modulus of sap- and heartwood increased. In terms of longitudinal changes in wood properties, mean wood moduli (averages of sap- and heartwood samples) increased, on average, towards the base of each of the three trunks of the tree. However, the mean fresh densities of wood samples increased towards the top and the bottom of each trunk and were lowest roughly near trunk mid-length. The mean density-specific stiffness (the quotient of Young's modulus and fresh density) of wood was thus lower toward the top and the bottom of the trunks and highest near trunk mid-length. Mean values of fresh wood density-specific stiffness were used to estimate the critical buckling heights for sections of the trunks differing in diameter and age. These estimates indicated that ontogenetic variation in the physical properties and relative amounts of sap- and heartwood in trunks could maintain a constant factor of safety (approximately equal to 2) as a sapling grows in height and girth into a mature tree. This expectation was supported by data from 16 black locust trees differing in height and diameter at breast height (DBH). (C) 1997 Annals of Botany Company.