BMP - Tree Risk Assessment - 75
Appendix 5
Description of Selected Types of Advanced Tree Risk
Assessments
Advanced assessments are performed to provide detailed information about
specific tree parts, defects, targets, or site conditions. Many techniques can
be considered for advanced risk assessment. A few commonly used assessment methods are described here.
Aerial Inspection
Aerial inspection (crown inspection) is the inspection of the aboveground
parts of a tree not visible from a ground-based inspection, including the upper
trunk and the upper surfaces of stems and branches. Aerial inspections usually include a visual assessment for defects, conditions, and response growth.
Conditions of particular importance include inspection of significant branch
unions, cracks in branches, sunscald on the tops of branches, and bark damage from bird or animal feeding. In addition, aerial inspections may include
evaluation of internal decay.
Aerial inspection can be done from an aerial lift, adjacent building, ladder, unmanned aerial vehicle, or by climbing the tree; however, the tree risk
assessor should determine that the tree is safe to climb before entering the
tree. Visual inspection from the ground using binoculars is not considered
an advanced assessment, but it may be part of a basic assessment.
Assessment of Internal Decay
It is difficult to estimate or quantify the location and extent of internal wood
decay during most basic assessments. When necessary to more accurately determine the location and extent of decay, it can be estimated with one of several
decay-detecting techniques, including drilling and the use of sonic devices.
After estimates are made of the location and amount of solid wood present
around a column of internal decay, several methods are available to evaluate the significance of the decay. Some methods are based upon engineering
models of pipe strength and recommended thresholds for minimum solid
wall thickness. Modifications for species, location and amount of decay, dimensions of the tree, additional defects, and site conditions should be made
by adapting the thresholds, but there is little guidance for such adaptations.
Other methods adapt mechanical principles or engineering models in order
to compare expected wind loads with the estimated load-bearing capacity
of the tree.
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