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 threemain trunks of a 32-year-old black locust tree,Robinia pseudoacacia(measuring 19.8 m at its highest point), were studied to determinewhether tree ontogeny can achieve a constant safety factor againstmechanical failure. Based on the properties of isolated woodsamples, the fresh density of sapwood decreased along radialtransects from bark to pith, while that of progressively olderheartwood samples increased, on average, towards the centreof 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 woodmoduli (averages of sap- and heartwood samples) increased, onaverage, towards the base of each of the three trunks of thetree. However, the mean fresh densities of wood samples increasedtowards the top and the bottom of each trunk and were lowestroughly near trunk mid-length. The mean density-specific stiffness(the quotient of Young's modulus and fresh density) of woodwas thus lower toward the top and the bottom of the trunks andhighest near trunk mid-length. Mean values of fresh wood density-specificstiffness were used to estimate the critical buckling heightsfor sections of the trunks differing in diameter and age. Theseestimates indicated that ontogenetic variation in the physicalproperties and relative amounts of sap- and heartwood in trunkscould maintain a constant factor of safety (approximately equalto 2) as a sapling grows in height and girth into a mature tree.This expectation was supported by data from 16 black locusttrees differing in height and diameter at breast height (DBH). Wood; elastic properties; tree height; biomechanics