Impact of Tree Age and Size on Selected Properties of Black Locust (Robinia pseudoacacia L.) Wood

Black locust (Robinia pseudoacacia L.) is a non-native tree species that occupies a remarkable area in the forests of western Poland. It is mainly cultivated for the bee-keeping purposes as well as for its high quality wood. We investigated the impact of tree age and diameter on the selected structural, physical and mechanical attributes of wood of black locust that grows in conditions of mesic oligotrophic site. We analysed 200 samples originating from 18 trees that varied with age (38, 60 and 71 years old) and diameter (thin, medium and thick specimen selected according to Hartig's method). Individual wood properties were determined along with corresponding European or Polish standards. Structural and mechanical attributes were determined for moisture of 12%. We found significant impact of tree age on tree-ring width, latewood proportion, density, oven-dry density, basic density, share of woody substance, porosity, as well as radial, tangential, longitudinal and volumetric shrinkage, compression strength parallel to grain, static bending, coefficient of compression strength parallel to grain and coefficient of static bending. The older the trees, the higher values of individual attributes were observed. In turn, the effect of tree diameter was less profound and no significant impact of that feature was found for latewood proportion, anisotropy and almost all of the shrinkage parameters. Thin trees exhibited the lowest values of the analysed parameters, while medium ones-the highest. In general, the highest technical quality of the investigated wood can be found in the youngest trees, whose wood characterises with the properties significantly exceeding native Polish tree species such as oak or beech.