Tree Care - Expert Diagram on Hazard Tree Identification

In this article, Board Certified Master Arborist, Mark Webber addresses some of the relevant issues in tree-related litigation. He discusses physical signs that indicate a tree may be failing, industry specific vocabulary to assist attorneys in better understanding the science of trees, and other issues that may contribute to trees becoming hazardous.


Mark regularly applies his expertise to forensic casework involving failed trees that damage property or cause injury.

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Hazard Tree Identification

On a long enough timeline the survival rate of every tree drops to zero. For the majority of trees, this is a non-issue, but when trees fall near people, buildings, or infrastructure, they sometimes cause personal harm or property damage. In some instances, abnormal weather conditions can cause a structurally sufficient tree to experience an unforeseeable death; but in many situations, there are recognizable warning signs (defects) to indicate that a tree may be unhealthy or unstable.

The illustration below depicts a number of conditions that may notify a property owner or grounds keeper that a tree has become or may become a safety hazard.

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The presence or lack-of signs of failure may prove useful to better establish issues of notice and the appropriate standard of care in your case. The experts at Robson Forensic perform comprehensive analyses based on damage, signs of failure, growth, wound and decay rates, and many other variables.

Industry Vocabulary

  • Crown – refers to the totality of the tree that is above ground (branches, leaves, and reproductive structures extending from the trunk)
  • Diameter at Breast Height (DBH) - standard measurement of a tree’s diameter, usually taken at 4 1/2 feet above the ground
  • Felling - to cut a tree in such a way that it falls in the desired direction and results in the least damage to the tree
  • Girdling - method of killing trees by cutting through the stem, thus interrupting the flow of water and nutrients
  • Hazard Tree - a tree with a structural defect that predisposes it to failure and is located near a target (e.g. people, property, roadways, utility lines)
  • Reentrenchment (Overmature) - a quality exhibited by trees that have declined in growth rate because of old age and loss of vigor
  • Overstocked - a situation in which trees are so closely spaced that they compete for resources and do not reach full growth potential
  • Pruning - the act of sawing or cutting branches from a living tree
  • Snag - a dead tree that is still standing
  • Stand - a group of forest trees of sufficiently uniform species composition, age, and condition to be considered a homogeneous unit

Common Issues in Tree Care

  • Age – old trees are more likely to have severe structural defects than young trees.
  • Branch Spacing - branches should be well spaced, if several branches arise at one point, one or more of the branches are likely to fail.
  • Canker – the presence of a canker increases the likelihood of a failure near the canker.
  • Cracks – a deep split through the bark, extending into the wood of the tree indicates that a tree is already failing.
  • Dead Wood – dead wood is often dry and brittle and cannot bend in the wind like a living tree or branch. It is highly susceptible to falling.
  • Decay – the presence of decay may not indicate an immediate threat of failure, but advanced decay, including soft, punky, crumbly wood, or the presence of a cavity may indicate a greater hazard.
  • Epicormic Growth - new growth from buds, normally found at the base of a tree, on branches, or the main stem. Can be stimulated by pruning or other damage, infection, or insect infestation.
  • Failure History – trees that have experienced failures in the past are likely to again.
  • Lean – trees that develop with a natural lean do not have a high tendency for failure, unless the lean is aggravated by other factors (e.g. decay, end weight, snow, rain).
  • Response Growth - unique bulges and or swollen or sunken bark patterns. An indication of internal decay.
  • Shear Plane Cracks - cracks that result from wood tissue pulling apart, indicates an increased likelihood for failure.
  • Stand – trees that develop in closed forest stands typically develop narrower crowns and trunks with less taper than open-grown trees. When trees in a stand are exposed to open areas, during development or right-of-way clearance, they are more prone to failure.
  • Stem Girdling Roots - a dysfunctional root that grows against, or around, a tree’s stem (trunk) and inhibits the tree’s ability to absorb water and nutrients from the soil.
  • Stem Taper - the degree to which trunk or branch is wider or narrow over this distance and has a direct impact on potential failure.
  • Weak Branch Unions – there are several examples of weak branch unions, one such involves two or more similarly-sized, usually upright branches forming a “Y” or “V” formation.
  • Widow Maker - a tree branch that has failed or partially failed and is still lodged in the tree.
  • Wound Wood Formations - previous injuries or obstacles in growth can exhibit unique wood fiber patterns, providing information from a period well before a failure.

Forensic Tree Investigations

Our experts are frequently retained when tree failures are associated with catastrophic injury or property damage. Through the course of their investigations they are regularly tasked with determining how and why a tree failed, if it showed signs of failure leading up to an incident, when those signs first appeared, and what was the appropriate standard of care for the property owners/managers involved in a dispute.

For more information visit our Tree Forensics practice page.

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