Environment Conducive to Failure

A weakened tree that is exposed to additional loads from wind, ice, or other factors obviously will have an increased likelihood of failure, especially if the load is unusual in direction or magnitude. Most tree failures occur during or as the result of storms, and exposure to rain, snow and ice loading, and lightning increases risk of branch and tree failures. Many site factors and past history can influence tree condition and the types and severity of the defects present.

Some examples of stress factors and the injuries or defects they can cause are listed in Table 1.

Table 1. Stress factors and the injuries or defects they cause.

Target

Theoretically, without a target there is no hazard. However, in urban settings, we rarely can completely discount target potential. Targets include facilities, people, and personal property. In various tree-rating approaches, targetsmay be prioritized by intensity of use or exposure to people.

Inspections and Documentation

Inspecting trees for defects must be a careful and systematic process. The entire tree must be inspected. Inspections should be done once a year and following storm events. Inspections are best conducted during the leaf-off season for temperate climate hardwood species to facilitate observation and inspection. Always document the evaluations, recommended actions, and actions taken. Keep permanent records.

Structural Defects

Identification and correction of structural defects such as weak branch attachments, leaning, cracks, wounds, deadwood, and decay may reduce the failure potential (and, therefore, reduce risk to property and injury to people).

Branch Attachments and Branch Failures

Weak branch attachments include unions with included bark and branches formed from epicormic buds. A weak union with included bark has bark present inside the branch union. There is little wood tissue attachment between the codominant stems or branch union with included bark. Weak unions are common in species with an opposite bud set. Examples include maple (Acer) and ash (Fraxinus) species. Weak unions are often easy to evaluate. Weak unions with open cracks or decay are obvious hazards. The propagating rib on opposite sides of the weak union can be an indicator of an internal crack and, in some cases, decay.

When trees are topped, overpruned, or stressed, they produce epicormic buds. Branches from epicormic buds are weakly attached, especially if there is internal decay below the attachment, which is often the case below old topping cuts. Weak unions are one of the most common causes of branch and stem failures during storm events and one of the easiest defects to prevent. Guidelines call for subordination or removal of all competing leaders beginning the second year after planting. This practice should continue over several years to produce one central leader.