Variations of the mechanical properties of Acer saccharum roots

Abstract

Longitudinal and transverse variations in the compressive strength σ c and stiffness measured in bending and torsion (E and G, respectively) were determined for wood samples removed from three old lateral roots of a large sugar maple Acer saccharum L. whose canopy had been heavily pruned on one side such that the three roots had loading conditions assumed to be dominated by different stresses: tensile stresses along the axis of root I, compressive stresses along root III, and torsional shear stresses in root II. Values of σ c , E, and G tended to decrease from the base toward the tip of each root, but maximized at approximately 1 m from the base (perhaps as a consequence of fibre fatigue near the base of the root system). Comparisons between the mechanical properties of wood sampled from the upper and lower root surfaces showed that the wood along the lower surface of roots I and II was stiffer and stronger; the reverse was found for root III. Longitudinal variations in the geometry and size of root transections resulted in a curvilinear pattern of root taper; maximum taper occurred at 1 m from each root base. These variations, which resulted in longitudinal changes in the second moment of area I and the polar second moment of area J, were sufficient to mask the effects of longitudinal variations in E and G on the flexural and torsional rigidity of roots (El and GJ, respectively) both of which decreased exponentially away from the base of the tree. Based on the correlations observed between the stresses presumed to dominate the loading conditions of each of the three roots and the mechanical properties of wood samples, it appears that the 'biomechanical plasticity' of woody roots holds the potential to maximize the individual ability of roots to cope with the dominant stresses attending their growth and development.