BMP - Tree Risk Assessment - 26

is always present or may be present nearly all day, most days of the week.
Frequent occupancy is when the target is within the target zone a large portion
of the day. Occasional occupancy refers to infrequent or irregular occupancy.
Rare occupancy means that the area is not commonly used by people or other
mobile targets.
Not all targets are present in the target zone at all times. Static targets are
constant. Movable and mobile targets can be in any of the four classifications.
Occupancy rate is a primary component in an assessment of the likelihood of impacting a target. The tree risk assessor can consult with the client
to determine targets and their occupancy rates. For example, a field with large
trees may be used as a parking lot (car park) during football games, but it
might be unoccupied most of the time. The client or tree manager may also
define whether the risk assessment is to consider low- or high-use times, or
both. Continuing the example, if the risk assessor is not told about the high-use
times, then the target rating would be much lower than if the high-use times
are considered exclusively.

Site Factors
Site factors have significant influence on both the likelihood and consequences
of tree failure. Site factors that can be used to evaluate the likelihood of tree
failure impacting the target include the history of previous failures, including
branch, trunk, root, and soil failure; vegetative cover type; land use; and wind
exposure (Figure 5). It is important to consider the significance of site changes
such as forest clearing, trenching, earth excavation or filling, groundwater
lowering or raising, infrastructure repair, or other construction. For example,
a fully exposed lone tree, retained after removing adjacent trees, may be more
likely to fail than the same tree within a stand protected from the wind. Site
use can be assessed from features such as sidewalks, pavements, paths, play
areas, picnic tables, and other objects.
Site examinations can provide information on root damage and/or disease
and susceptibility to root failure. Soil factors such as frequency of saturation;
compaction; erosion; textural gradients; restrictions to root growth from
shallow, impermeable layers; and restrictions by roads, rock, or building
foundations may also be considered. Trees are more susceptible to windthrow
during storms when the soil is saturated. Site topography, including slope
and aspect, should also be considered.
Because trees grow and develop, constantly responding to the forces and
conditions that they are exposed to, they adapt over time. Assessors should
take note of site changes that any trees present may not have had sufficient
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BMP - Tree Risk Assessment

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