BMP - Tree Risk Assessment - 29

In some locations where strong winds are rare or infrequent, the wind
speed at which failures of structurally sound trees occur may be lower. For
example, in some tropical areas where gale force winds are rare, trees may
fail at wind speeds that would not damage most adapted temperate-zone trees.
Trees adapt to their locations and to the wind speeds that commonly occur in
an area. Most trees have significant additional strength to resist occasionally
higher wind speeds.
When evaluating wind loads during tree risk assessment, the assessor
should consider the normal range of wind and weather conditions for the
region. For example, strong winds may be within the range of normal conditions, but a tornado, hurricane/typhoon, or microburst would be outside
the realm of consideration. If a region is prone to strong storms or heavy
snowfall and such events are likely to occur in the established time frame,
the assessor should consider the likelihood for failure during such events.

Tree Failures
Trees fail when the load on them exceeds their strength. Some tree failures can be predicted because certain structural defects or conditions
often lead to structural weaknesses. Arborists should recognize and assess the common conditions that can predispose trees to failure in order
to categorize the likelihood of failure (see Appendix 3). Yet, through
response growth (see Appendix 4), trees can strengthen weak areas and
support loads, thereby reducing the likelihood of failure. A visual assessment includes looking for and considering the significance of structural
conditions, individually and in combination, that may increase or decrease
the likelihood for failure (Figure 6). Not all conditions and defects have
a significant impact on tree structure.
When more than one defect or condition is present in a tree, the possible combined effects on likelihood for failure should be considered. For
example, a trunk lean of 10 degrees may not be of great concern on many
trees, but if there is a large, decayed root on the side opposite the lean, then
the likelihood of failure increases if significant loads are likely to occur and
the tree has not compensated for the defect with response growth.
Tree failures usually occur when there is a critical combination of tree
defect(s), conditions, and contributing environmental factors such as wind,
rain, freezing rain, or snow. With the exception of sudden branch drop, calmday tree failures are very rare and usually result from extreme defects.

BMP - Tree Risk Assessment

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