Arborists use formulas to estimate the risk of failure in trees with decay. The study investigates the appropriateness of using formulas to determine the risk of tree failure. Two variables were examined, whether trees improve wood toughness in the vicinity of mechanical stress concentrations, and how well the formulas estimate the loss in moment of inertia due to decay. Red maple (Acer rubrum) callus wood toughness was consistently greater than for normal wood, but, in some cases, the formulas significantly underestimated the actual loss in moment of inertia. The improvement of callus wood toughness is not related to increases in mechanical stress on the tree, but instead seems to be related to callus cell anatomy.