BMP - Tree Risk Assessment - 41

municate the level of risk and to assist in making recommendations to the
owner or risk manager for mitigation and inspection frequency. The priority
for action depends upon the risk rating and the risk tolerance of the owner
or manager.
Extreme-The extreme-risk category applies in situations in which failure
is imminent, there is a high likelihood of impacting the target, and the consequences of the failure are severe. The tree risk assessor should recommend
that mitigation measures be taken as soon as possible. In some cases, this
may mean restricting access to the target zone area to avoid injury to people.
High-High-risk situations are those for which consequences are significant and likelihood is very likely or likely, or consequences are severe and
likelihood is likely. This combination of likelihood and consequences indicates that the tree risk assessor should recommend that mitigation measures
be undertaken. The decision for mitigation and timing of treatment depends
upon the risk tolerance of the tree owner or risk manager. In populations
of trees, the priority of high-risk trees is second only to extreme-risk trees.
Moderate-Moderate-risk situations are those for which consequences
are minor and likelihood is very likely or likely; or likelihood is somewhat
likely and consequences are significant or severe. The tree risk assessor should
recommend mitigation. The decision for mitigation and timing of treatment
depends upon the risk tolerance of the tree owner or manager. In populations of trees, moderate-risk trees represent a lower priority than high- or
extreme-risk trees.
Low-The low-risk category applies when consequences are negligible
and likelihood is unlikely; or consequences are minor and likelihood is
somewhat likely. Some trees with this level of risk may benefit from mitigation or maintenance measures, but immediate action is not usually required.
Tree risk assessors may recommend retaining and monitoring these trees or
mitigating the risk. Although the risk is already in the lowest category, the
actual risk may still be reduced, if desired.
Most trees have more than one failure mode (location or manner in which
failure is likely to occur) and may have multiple potential targets. For example, a tree with excessive root decay may also have several dead branches;
the whole tree could fail from root decay, and dead branches may fail. Similarly, the whole tree may fall on a house, while the dead branches would fall
only on the driveway. When evaluating individual trees, it is appropriate to
evaluate each failure mode/target combination as an independent event and
to report its risk, recommended mitigation options, and residual risks. The
assessment report should state what specific risk each rating applies to.
41

BMP - Tree Risk Assessment

Table of Contents for the Digital Edition of BMP - Tree Risk Assessment