BMP - Tree Risk Assessment - 25

with a radius equal to about 1 to 1.5 times the height of the tree. Trees with
asymmetric crowns or leans may have irregularly shaped target zones. For
a branch failure, the target zone is the area in which the branch could strike.
The target zone for dead trees or trees with dead or brittle branches is
generally larger than those with live, flexible branches, because dead and
brittle branches are more likely to shatter upon hitting the ground and may
spread debris some distance beyond the tree (Figure 4). In some situations,
the target zone also may be larger when one tree could cause the failure of
others that it could fall into. Conversely, the target zone may be reduced in
size by adjacent larger trees, strong lower branches, or other factors that will
not allow the tree or branch to fall in a given direction.
Trees and branches sometimes fall in unusual ways, striking outside of what
would normally be considered the target zone. The direction of tree failure is
often more closely related to wind direction than to the location of the defect
on the tree. The direction that wind will come from during a storm is unpredictable and should not be confused with the direction of the prevailing wind.

Occupancy Rate
Targets can be categorized by the amount of time that they are within the
target zone-their occupancy rate. Occupancy rates can be classified as constant, frequent, occasional, or rare. Constant occupancy means that the target

Figure 4. The target zone is the area where a tree or tree part is likely to land if it were
to fail. Due to the brittle dead branches at the top of this tree, the target zone radius
is equal to 1.5 times the tree height.

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BMP - Tree Risk Assessment

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

Table of Contents
BMP - Tree Risk Assessment - Cover1
BMP - Tree Risk Assessment - Cover2
BMP - Tree Risk Assessment - i
BMP - Tree Risk Assessment - ii
BMP - Tree Risk Assessment - Table of Contents
BMP - Tree Risk Assessment - iv
BMP - Tree Risk Assessment - 1
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