Does included bark reduce the strength of codominant stems?

Abstract

One of the most common locations for the aboveground portion of a tree to fail is at the junction of two or more codominant stems. Due to the frequency of failures at this point, a study was undertaken to get a better understanding of the mechanical strength of this point and to determine if included bark reduces the strength of the union. Eighty-four codominant stems were removed from 26 felled maple trees. These crotches were securely anchored and split apart using measured force. Breaking force varied from 64 to 2,363 kg. The regression line produced from the comparison of stem diameter and force required for breaking the union when there was no included bark was Force = Diameter * 613 - 1388, r2 = 0.92. When only those unions with included bark were analyzed, the regression line was Force = Diameter * 537 - 1285, r2 = 0.76. There was a significant difference between the regression lines (p < 0.05). Codominant stems that have bark trapped in the union are significantly weaker than those that do not have bark included. The differences appear to be greater with smaller-diameter stems than with larger stems.