BMP - Tree Risk Assessment - 76

These models have limitations because they are based upon certain assumptions that may not coincide with actual tree conditions. For example,
trees may differ from common models for strength loss due to decay when
tree trunks are not circular in cross section, or when they have included
bark, nonlinear fibers, or off-center decay. Wood in trees also does not have
uniform strength throughout; some areas may be stronger and some weaker.
While high precision can be achieved during measurements, assumptions
required to complete any evaluation carry an inherent possibility of error,
which could be cumulative in the calculations. Nevertheless, mathematical
models and calculations can be useful in some advanced tree risk assessments.
Several decay-detection devices are on the market, but not all have been
demonstrated (through independent research) to be effective tools, and
there may be differences in precision, resolution, and reliability. Two wellestablished technologies are described.
Drilling. Two types of drilling tools can be used to evaluate the extent
of decay: a handheld electric drill and a resistance-recording drill. Both distinguish between solid and decayed wood by the resistance to penetration as
the drill moves through the wood.
It is important to carefully select testing locations so that the size and
configuration of the decay column can be estimated. Before testing, sounding
or visual assessment should be used to determine the best locations to test.
The tree risk assessor should conduct sufficient testing to visualize the approximate extent of the decayed area. However, the tree risk assessor should
take care to avoid unnecessary or excessive wounding. Tree risk assessors
should also consider that drilling into decay can breach CODIT walls (especially wall 4), which may allow decay to spread. The number of drillings
should be as few as possible but as many as needed.
One type of drilling device is a handheld electric drill fitted with a long
(8 to 18 inches, 20 to 45 cm), small-diameter (1/8 inch, 3 mm), full-fluted
drill bit. Evaluation is primarily limited to the advanced stages of decay.
Accuracy relies in large part on the experience and expertise of the operator.
A resistance-recording drill drives a small-diameter (1/8 inch, 3 mm) flattipped spade bit into the tree. As the bit penetrates the wood, the resistance
to penetration is recorded. With training and experience, an inspector can
distinguish solid wood from voids and decay. Incipient decay, effectiveness
of compartmentalization, and response growth rates may be estimated from
profiles created by some high-resolution resistance drills.
Sonic Assessment. Sonic wood assessment instruments send a sound
(stress) wave through the wood and measure the time for the wave to travel
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BMP - Tree Risk Assessment

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