A hazard tree is a tree with structural defects, such as compromised branches or stems, that pose a risk to nearby people, objects, or infrastructure. Evaluating hazard trees involves assessing their potential for failure, environmental conditions, and the presence of a potential target.

Figure 1. A tree with dead and broken branches. These branches are likely to impact nearby potential targets.

Common structural defects include dead branches, weak branch unions, cracks, root issues, poor tree form, and wood decay, which increase the risk of failure and potential consequences, according to the International Society of Arboriculture (ISA). Risk assessments are categorized into three levels: level 1 involves a limited visual inspection to identify obvious hazards; level 2 includes a detailed evaluation of the condition of a tree and the site where the tree grows; and level 3 involves advanced techniques and tools for precise analysis. These assessments guide appropriate interventions, balancing tree health, safety, and ecological considerations. This Extension publication is part of a series that discusses hazard trees and risk assessment programs. Specifically, it focuses on common structural defects of trees and risk assessment levels. It is a valuable resource for decision-makers and policymakers in cities and municipalities, urban foresters, certified arborists, and the general public.

What Is a Hazard Tree?

A hazard tree is one with compromised structural stability or weakened parts, such as branches and roots, caused by biological or nonbiological factors. Determining whether a tree is hazardous involves evaluating three key aspects.

• Potential for failure. The likelihood of the tree or its parts, such as branches, breaking or falling.
• Environmental factors. Conditions in the local growing environment that could contribute to failure.
• Target presence. A person or object that could be harmed or damaged. Without a potential target, a tree cannot qualify as a hazard.

Common Structural Defects of Trees

Inspecting tree structural defects is essential in tree risk assessment. According to the ISA, the most frequently encountered defects include the following:

• Dead or dying stems and branches. These typically indicate poor tree health and can be hazardous if located near targets. However, some species may retain dead branches for long periods while the trees are still healthy. Examples are honey locust (Gleditsia triacanthos); black locust (Robinia pseudoacacia). The risk level helps determine whether removal is necessary, especially for branches threatening people or structures, such as houses. If a dead tree poses no danger, it can be left standing in place to become a tree for wildlife, sometimes called a snag, which can provide shelter and food for animals.
• Broken branches. Branches can snap under stress from wind, rain, or other forces and may remain suspended in the canopy, posing risks. Factors determining how hazardous a hanger branch is include the branch size, how well it is still attached, and the tree species.

• Weak branch unions and codominant stems.
  • Branch unions can be categorized as U-shaped or V-shaped based on their angle of attachment. In general, U-shaped unions are stronger, whereas V-shaped unions indicate narrow angles of attachment. They are inherently weaker and may contain what is called included bark, a condition where bark grows within the union, resembling a crack.
  • Codominant stems occur when two branches or stems of similar base diameter emerge from the same point. These formations often have weak attachments and may have included bark, reducing their mechanical stability against forces such as wind.

• Cracks (traverse, radial, and longitudinal). These can be caused by freezing atmospheric temperatures (frost cracks), fire (fire cracks), lightning, or other factors (heavy snow loads, strong wind loads).
  • Traverse cracks that span across the short axis of the stem or branch are particularly dangerous as they weaken wood’s structural integrity.
  • Longitudinal cracks along the stem or branch, while less critical, can allow fungi and pathogens to invade, leading to wood decay.
• Root defects. Problems such as diseased or broken roots, restricted root growth, and stem girdling roots (roots encircling the tree base) compromise a tree’s stability and overall health. These defects often contribute to tree decline and increase the likelihood of failure.
• Poor tree architecture. Tree architecture refers to the structural organization and growth patterns of a tree, including its branching, canopy shape, and overall form, as influenced by genetics and environmental factors. Some common problems indicating poor tree architecture are as follows:
  • Trees with uneven crowns struggle to distribute mechanical stresses from snow or wind evenly, increasing failure risks.
  • A pronounced lean can reduce stability, though some leaning trees remain stable for extended periods. Rapidly worsening lean, however, is a warning sign of impending failure.

Figure 8. Visible cavities and sap leaking through the bark are common indicators of wood decay.

• Wood decay. Fungi causing wood rot to weaken the structural integrity of trees. Indicators include fruiting bodies, such as conks and mushrooms, though not all decaying trees show these indicators. Other indicators include visible cavities, carpenter ants, cavity-nesting wildlife, cracks, past pruning wounds, and sap leaking through the bark. These indicators highlight internal decay that may not always be externally obvious.

Levels of Risk Assessment

Risk assessment is a systematic process focusing on estimating the likelihood of structural failure and the severity of its consequences, such as property damage, injuries, fatalities, road blockages, or disruptions to power systems. The probability of failure is calculated within a given time frame and considers the chance of the tree affecting a specific target. Certified arborists are professionals who can identify potential hazard trees and perform tree risk assessments. Various systems exist for assessing risk, each with unique approaches and assumptions.

The ISA identifies three levels of tree risk assessment, which are standard for certified arborists.

• Level 1: Limited visual inspection
• Level 2: Basic inspection
• Level 3: Advanced analysis

The level chosen depends on the tree’s condition, site characteristics, and the client’s needs. Municipal trees, such as those on streets, often undergo level 1 assessments for preliminary screening. Homeowners who notice tree defects generally hire certified arborists to perform more detailed level 2 or level 3 assessments.

Level 1: Limited Visual Assessment

This is a quick evaluation performed by walking, driving, or using drones to visually inspect trees for potential hazards. The inspection is conducted from a fixed perspective (i.e., only views one side of a tree), to spot obvious structural defects.

Level 2: Basic Assessment

In this comprehensive inspection, a certified arborist examines the tree, its surroundings, and nearby potential targets. The process involves inspecting the trunk, its surrounding site, roots, branches, and crown from multiple angles. Basic tools, such as a clinometer, measuring tape, binoculars, mallet, and decay probe, may be used to assess structural defects and record measurements.

Level 3: Advanced Assessment

This detailed evaluation is performed when a basic assessment identifies issues requiring further investigation. It is more thorough, costly, and time intensive, requiring client approval before proceeding. This level of assessment is commonly used for high-value trees and most likely performed by private consultants. Advanced assessments may include the following:

• In-depth descriptions of defects or tree sections.
• Sophisticated tools, such as sonic tomography and resistance-recording drills, for decay detection.
• Load and stability analysis.
• Laboratory testing of tree and soil samples.
• Root excavation.
• Target impact evaluation.
• Aerial inspections using tree climbing or cranes.

This level provides a precise understanding of risks, helping to determine necessary actions.

Conclusions

Identifying and addressing hazard trees is essential for ensuring safety. Property owners and arborists can make informed decisions about tree care by recognizing common structural defects and assessing risk levels. Property owners can search for certified arborists in their area at Trees Are Good, the International Society of Arboriculture website.

Suggested Literature

• A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas, 2nd Edition by Nelda P. Matheny and James R. Clark.
• Arborists’ Certification Study Guide, 4th Edition. Copyright © 2022 by International Society of Arboriculture.

Georgios Arseniou, Extension Specialist, Assistant Professor, Forestry, Wildlife, and Natural Resources, Auburn University

New April 2025, Spotting Danger: How to Identify & Assess Hazard Trees, FOR-2175