BMP - Tree Risk Assessment - 47
Over-mature trees in natural settings may reconfigure as they age and
deteriorate, a process sometimes called " natural retrenchment. " They may
continue to grow trunk diameter while branches die and fail-reducing overall
height of the tree and increasing stability. Where tree risk is a concern, tree
risk assessors can imitate this process by recommending crown reduction.
Eventually, however, even tall stumps without branches will fail when the
roots decay. For this reason, trees near targets that are retained for wildlife
habitats should be monitored and considered for removal or mitigation if the
risk exceeds allowable thresholds.
Mitigation may involve more than one action. The tree risk rating based
on the highest risk factor assigns an overall tree risk, but typically there are
other factors that also should be considered, which may require additional
actions if the tree is to be retained. Once the highest risk factor has been
mitigated, the tree risk rating goes to the next highest risk factor.
With some extreme-risk trees, mitigation may involve the immediate restriction of use within the target zone before removing the tree. Tree removal
may require using a crane or an aerial lift because additional loads on the
tree-from the weight of a climber or from lowering branches-may cause
the tree to fail. However, on some occasions, tree removal is not necessary,
even for high-risk trees. Removing the target, pruning, or installing a support
system may significantly reduce the risk posed by the tree to a level that is
acceptable to the client.
Mitigation
Guidelines for tree risk mitigation:
* Extreme-risk trees should be mitigated as soon as possible. Immediate
action may be required to restrict access to the target zone.
* High-risk trees should be mitigated as soon as it is practical.
* Moderate-risk situations may not require mitigation but, if deemed
necessary, could be mitigated when budget, work schedule, or pruning
cycle allows. If the risk is acceptable to the client, the tree(s) could be
retained and monitored.
* Low-risk trees should be retained and monitored (if appropriate) or
mitigated, if desired, when the budget, work schedule, or pruning cycle
allows.
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BMP - Tree Risk Assessment
Table of Contents for the Digital Edition of BMP - Tree Risk Assessment
Table of Contents
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