Size- and Age-dependent Variation in the Properties of Sap- and Heartwood in Black Locust (Robinia pseudoacaciaL.)

Dissection and mechanical bending experiments showed that the cross-sectional area and elastic moduli of sap- and heartwood varied within the trunk and branches as a function of the distance from the top of a 43-year-old black locust tree (Robinia pseudoacaciaL.). Wood in branches less than 1 m from the top of the tree consisted entirely of sapwood; the majority of the wood from more basipetal (and older) parts of the tree was heartwood. The Young's elastic moduli of sap- and heartwood increased towards the base of the trunk, and, on average, the modulus of the sapwood was 35%less than that of the heartwood. Younger, more distal tree limbs, therefore, were more flexible than older portions of the same tree. Simple bending experiments showed that the flexural rigidity of young limbs was governed by the location, physical properties, and the relative quantities of the two types of wood. The rigidity of limbs increased toward the base of the tree, and was dominated by sapwood in young limbs and by heartwood in the oldest parts of the tree. These trends predict that the younger, distal limbs of this tree can more easily deflect and bend in the wind, thereby reducing drag and the total bending moment on the tree trunk, while older limbs and the trunk are sufficiently rigid to support static self-loadings. Further study, however, is required to determine whether the trends reported here apply to all trees of this species and to trees of different species.