Hazard Tree Identification Expert Overview

In this article, forensic arborist Christopher Larson 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.

Hazard Tree Identification Expert Witness

Forensic Tree Investigations: 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 groundskeeper that a tree has become or may become a safety hazard.

Hazard Tree Identification Expert Diagram

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.

Tree Industry Vocabulary

  • Crown – refers to the totality of the tree that is above the lowest branch, includes all branches and foliage. 
  • Diameter at Breast Height (DBH) – standard measurement of a tree’s diameter, measured at 4 1/2 feet above the ground in the United States.
  • 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 – restriction, compression, or destruction of the root, stem, or branch tissues that inhibits the flow of water and sugars; can be a method employed to intentionally kill a tree by cutting through tissues around the stem.  [see also Stem Girdling Root]
  • Hazard Tree – a tree that is likely to fail and impact a target (e.g. people, property, roadways, utility lines).
  • Retrenchment – a natural process in which an overly mature tree reduces its crown and increases trunk girth to consolidate resources and increase longevity. Retrenchment Pruning is a deliberate attempt to mimic the retrenchment process by pruning to reduce overall tree size and height.
  • Overstocked – a situation in which trees are so closely spaced that they compete for resources and do not reach full growth potential.
  • Pruning – removing branches or roots from a tree or other plant by the act of sawing or cutting.
  • 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.
  • Structural Root Plate – woody roots, close to the trunk, that function in anchorage and stability. Compromising the integrity of the structural root plate can destabilize the tree.

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 – separation in wood fibers; severe cracks, through the bark, extending into the wood, indicates that the tree or tree part 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 – undergoing decomposition; 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 generally do not have a high tendency for failure, unless the lean is aggravated by other factors (e.g. defects, decay, end weight, snow, rain).
  • Response Growth – a form of adaptation in which new wood is produced to compensate for higher strain.  This new wood often takes on the form of unique bulges and or swollen or sunken bark patterns. Can be an indication of internal decay.
  • Root Rot – decay located in the roots of a tree.
  • 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), squeezing, compressing, and inhibiting the tree’s ability to absorb water and nutrients from the soil.  The tree stem can be weakened by compression or “girdling” and be more prone to snapping off at this point.
  • Stem Taper – the degree to which trunk or branch diameter changes in width over its length. This relationship 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 narrow angled “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, submit an inquiry or call us at 800.813.6736.

Featured Expert

Christopher Larson, Board Certified Master Arborist & Tree Expert

Christopher Larson, BCMA, TRAQ

Board Certified Master Arborist & Tree Expert
Chris Larson is a Board Certified Master Arborist with over 20 years of experience in urban forestry, arboriculture, and landscape design/maintenance. He applies his expertise to a broad range of… read more.


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