What Is Tree Risk?

Tree risk is commonly defined as the likelihood of a tree or tree part failing and causing harm to a target. Risk is not determined by tree condition alone. Instead, it reflects the interaction of three factors:

Tree risk increases as the likelihood of failure, likelihood of impact, and severity of consequences increase. For example, a decayed tree in a remote woodland may present little risk because targets are rarely present. The same tree next to a playground, roadway, or residence may represent a high-risk condition. Risk is therefore not absolute. It is a combination of tree biology, structural condition, site occupancy, and potential consequences.

How Do Arborists Evaluate Risk?

Professional arborists often use standardized assessment methods developed by the International Society of Arboriculture (ISA). One of the most widely recognized credentials is the Tree Risk Assessment Qualification (TRAQ), which provides specialized training in evaluating tree defects, site conditions, targets, and risk mitigation options.

A TRAQ assessment does not predict exactly when a tree will fail. Instead, it evaluates the probability that a tree or tree part could fail and cause harm within a specified time frame. The goal is to support informed management decisions based on the best available evidence.

TRAQ-qualified arborists combine the factors related to the likelihood of failure, likelihood of impact, and severity of consequences to assign an overall risk rating and develop management recommendations. Possible recommendations may include continued monitoring, structural pruning, cabling or bracing, or even tree removal.

The purpose of a risk assessment is not to identify trees that must be removed, but to provide information that supports appropriate and proportionate management decisions. The ISA also outlines three distinct levels of inspection. An arborist will select the appropriate level based on the client's goals, the budget, and the level of detail required.

Hidden Defects and Decay

Unlike forest trees that grow closely together and shield one another from the elements, isolated urban and suburban trees face wind from all directions. They must also cope with restricted root space, soil compaction, and crown imbalances that alter how they naturally distribute physical stress. While healthy trees adapt using wide trunk tapers and strong branch attachments, urban and suburban environments often cause structural weaknesses like V-shaped branch unions or heavy lever-like limbs.

However, not all structural defects, whether on urban, suburban or rural trees, are visible from the outside. Internal decay can develop for years before showing clear warning signs. Property owners should look out for hidden dangers indicated by mushrooms or fungal conks on the trunk, hollow sounds when tapped, cavities, cracks, a sudden change in lean, or soil shifting around the tree's root plate due to a compromised root system.

Signs that may indicate internal defects include:

• Mushrooms or fungal conks on the trunk or at the base
• Hollow sounds when tapped
• Cavities
• Cracks
• Sudden changes in lean
• Soil shifting around the root plate

For tips to recognize defects and decay on trees, follow this Arbor Day Foundation Tree Risk Guide and this Alabama Extension Common Tree Defects List.

Know Who Is Responsible

Tree responsibility depends on ownership and jurisdiction, not proximity.

Before and After the Storm

Step 1: Perform a Pre-Storm Assessment

Tree risk management should begin before severe weather occurs. Property owners and municipalities should regularly inspect trees located near homes, buildings, roads, sidewalks, playgrounds, and utility infrastructure.

Particular attention should be paid to:

• Deadwood
• Cracked limbs
• Included bark
• Leaning trees
• Root damage
• Signs of decay

Step 2: Engage in Preventive Maintenance

Risk can often be reduced through:

• Structural pruning of young trees
• Removal of dead or broken branches
• Subordination of codominant stems
• Root zone protection
• Diversified species selection
• Proper planting and maintenance

Trees that develop strong structure early in life generally experience fewer failures later.

Step 3: Practice Utility Safety

Never approach trees touching power lines, limbs entangled in electrical conductors, or uprooted trees contacting utility infrastructure. Report electrical hazards immediately to the utility provider.

Before digging or performing root-zone work near utilities, contact Call Before You Dig (811). Only qualified line-clearance professionals should work near energized electrical systems.

Step 4: During and After Storm Events Make Sure You...

• Assess safety first.
• Stay clear of downed utility lines.
• Photograph damage for documentation and insurance purposes.
• Restrict access to hazardous areas.
• Contact qualified professionals when needed.

Step 5: Assess, Don’t React Emotionally

Not all damaged trees require removal. Trees compartmentalize damage. The Compartmentalization of Decay in Trees (CODIT) biological model can explain this. CODIT tells us that trees isolate damage by creating physical and chemical boundaries to prevent the spread of pathogens. Rather than “healing” wounded tissue like mammals, trees use these internal “walls” to seal off the affected area, effectively sacrificing the damaged wood to protect the health of the remaining structure.

In some cases, corrective pruning or structural support systems may restore acceptable levels of safety and function. Professional assessment is particularly important for mature trees with significant landscape, ecological, or historical value.

Step 6: Observe for Structural Red Flags

Seek professional evaluation if you observe soil cracking, root plate movement, or cracks extending through the trunk. Remember: trees often fail at the roots, not the trunk. Beyond uprooted root plates, some high-risk scenarios for storm-damaged trees in neighborhoods often involve split stems under tension, trees resting on houses, or limbs entangled in wires. Therefore, the safest action is to secure the area, if possible, and call a professional.

Step 7: Hire a Professional, if Needed

If you are considering hiring a licensed arborist, you can use the CT Tree Protective Care Association Directory to find one. For formal tree risk assessments, look for professionals who hold an International Society of Arboriculture (ISA) TRAQ (Tree Risk Assessment Qualification) credential. You can use the ISA directory, filtering by the TRAQ qualification, to cross-reference with the CT licensed arborists list. Under Connecticut state law, only individuals licensed by the state may advertise, solicit, or contract to perform tree care services (arboriculture).

Avoid companies that solicit door-to-door or recommend topping, which increases long-term risk. Also consider that cabling and bracing systems can sometimes reduce tree risk, but they must be properly designed and installed by trained professionals who understand load distribution and dynamic forces.

Balancing Risk and Benefit

Trees provide substantial benefits, including shade and cooling, air-quality improvement, stormwater interception, carbon storage, wildlife habitat, and psychological and physical health benefits. Risk management should seek to preserve these benefits whenever possible.

Removing trees solely because they possess minor defects can reduce canopy cover, increase heat exposure, diminish environmental benefits, and create long-term ecological costs. Thus, the goal of risk management is not to eliminate all risk (an impossible task), but to make informed decisions based on science, probability, site use, and community values.

Risk tolerance also varies by site. A defect that may be considered acceptable in a natural area with limited public access may require mitigation in a schoolyard, playground, downtown streetscape, parking lot, or other high-occupancy location. Modern arboriculture therefore focuses on reducing risk to an acceptable level rather than attempting to eliminate all risk, which is rarely achievable in living systems.