Determining Strength Loss from Decay

A formula for assessing strength loss due to decay was tested on hurricane broken trees and surviving trees exposed to hurricane force winds. When the formula was used with a 33% strength loss threshold, it would predict 50% of the failures in 90 mph winds while calling for "unnecessary" removal of 12% of surviving decayed trees. Large trees in the urban forest are declining at an alarming rate (3). At the same time, society is becoming more litigious. This combination of events demands that the detection, evaluation and management of hazardous trees must be the first priority for arborists and urban foresters. A tree is hazardous if it has both a structural defect that predisposes the tree to failure, and a target that would be struck if it were to fail. Healthy trees may be hazardous if they obstruct motorist's vision, raise sidewalks, interfere with utilities, or are particularly attractant to lightning. Many kinds of structural defects must be con- sidered when evaluating a tree. Some of these defects are illustrated in Figure 1 and more have been described in the literature previously (2, 4, 6). Evaluation of strength loss from decay in the trunk, limbs or roots has always been a problem for the arborist. It is well known that decay will structurally weaken the tree, but how much is too much? In the past the answer to this question was often based on qualitative factors as the experi- ence of the arborist, budget constraints, and the attitude of the property owner. In 1963 the United States Forest Service pub- lished a paper by Wagener (6) that documented tree failure in campgrounds. This was the first attempt to quantify a factor associated with tree failure. Using measurements of the diameter of the hollow crosssection of the trunk (d) and diam- eter of sound wood (D) the amount of strength loss (SL) was determined.