Figure 6 Aspen that has failed at Ceratocystis fimbriata canker. If the amount of sound wood exceeds that established by the line on the graph, the tree can be considered relatively safe from failure. Figure 5 Lodgepole pine along a well-used trail shows effects of pedestrian traffic on root system . The trunk must support the weight of the entire crown and any structural injury increases the chance for failure. Trunk wounds provide entry for wood-rotting fungi that reduce the volume of sound wood and increase the probability of stem breakage.
Forked trunks are structurally weaker than single stems.
The thickness of sound wood in the outer shell determines structural strength in trees with rot defect. The minimum standards for safety based on bole diameter are presented in Figure 7. Rots must be considered together with other defects. For example, a leaning tree can tolerate less rot defect than an upright tree. Figure 8 Damaged Engelmann spruce root is checked for decay with an increment borer.