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The art and science of tree risk assessment continues to evolve as we
increase our knowledge base and with field experience. Trees do not fail at random.
Tree failures are predictable over a broad time range. And tree assessment is not
an exact science but a science nonetheless and one that is constantly advancing.
An arborist performing tree risk assessments must be well trained in
biology, tree structure, and tree mechanics. The arborist should possess a high level
of comfort and experience with the inspection process. It takes a trained eye to
recognize the subtle signs of impending mechanical failure. The possible result
of over-reading these signs is overreacting. Any existing signs of failure
must be thoroughly evaluated to determine cause. Because every tree is different,
performing tree risk assessments is a learning process. This article briefly
reviews structural defects and basic tree mechanics.
The process for evaluating the risk of tree failure begins with visual
inspection for defects (visual tree assessment, VTA), followed by sounding for
suspected decay and probing, if necessary, with a portable drill, increment borer,
or an advanced decay detection device. Formulating a decision involves considering
several factors, including multiple defects, species characteristics, location
and extent of decay, characteristics of decay organisms, crown size, crown ratio,
stem taper, exposure, target considerations, tree value, and owner attitude.
Components of Risk Assessment
The three components of tree risk assessment proposed by Matheny and Clark
(1994) are the tree's failure potential, an environment conducive to tree failure,
and a target.
Failure Potential
Although some trees without defects fail in major storms, the presence
of any defect will increase the chances of failure. Each species has its own profile
of defects. Some factors that must be considered include the species' growth habit,
tree condition, branch attachments, resistance to decay, condition of anchoring
roots, cultural or maintenance history, and previous damage. In addition, the
severity of any defects found should be considered. Other factors related to the
site such as intensity of use, soil condition, and prevailing winds must be
considered in conjunction with the defects present when assessing the potential
for failure. Any individual factor can directly impact tree safety (or, more
often, multiple factors impact the tree's failure potential). The size of the
tree or tree part that may fail is also important. Usually, the tallest, most
exposed tree and tree parts are of greatest concern.
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International Society of Arboriculture
