How Tree Selection and Location Influences Tree Risk Assessment in the Urban Area

How Tree Selection and Location Affect Tree Risk Assessment in the Urban Area

Due to the close proximity of trees in the urban environment, a tree part or whole tree failure can result in property damage and personal injury to workers, residents or the general public. Since we value the urban forest we live and work in, our understanding of tree selection and location play a pivotal role in whether a tree will provide the specific benefits intended.

Proper tree selection analyzes the cultural growth requirements of a tree with the specific site location. Since trees are long-lived organisms, the growth characteristics that make a tree unsuitable for a location do not manifest for ten or twenty years.

The layperson may not fully understand the growth characteristics of trees, or fail to consider the size of the tree at maturity. A small pine tree planted near the neighbors property line doesn’t become a problem until two decades later when the tree roots encroach onto the neighboring residence and the tree crown extends over the property line. These conditions can create unintended hazards that may increase risk and potential liability to property owners.





What makes a tree “good or bad”? Trees possess growth characteristics that may or may not be compatible for a given site, location or environment. A normally “good” tree can become a “bad” tree in the wrong location. Just like a tree with a “bad” reputation might become a “good “ tree when planted in a suitable location.

Ficus trees are abundant throughout Southern California. There are many admirable species used in a variety of settings such as street trees, accent, front yards and park sites. However, it is easy to confuse the different species and end up with an inappropriate large growing species planted in a confined space. When that happens, little can be done to correct the problem.



This Ficus, although a magnificent specimen, is in the process of causing a retaining wall failure and creating a tripping hazard by raising the concrete sidewalk. There are no easy solutions for incorrect tree selection. However, a property Owner has a duty of care to protect the public from foreseeable accidents that could arise from incorrect tree location, selection, maintenance practices, or lack thereof.

Good Tree or Bad Tree?

The Eucalyptus tree has earned a reputation as a “bad” tree because certain species are known to drop limbs or unexpectedly fail. However, when located and used properly in large open spaces and parks, they can provide excellent use for background, screening or massing. However, when used adjacent to homes, their use becomes questionable.



The picture on the left shows Eucalyptus trees used in a city open space, the picture on the right shows a Eucalyptus tree in the same stand of trees after it unexpectedly uprooted and fell onto an adjacent residence. Does this make Eucalyptus a bad tree or do we question the wisdom of the selection, location and maintenance practices that played a role in the tree failure?

Incorrect tree selection is often times compounded by poor arboriculture maintenance practices, especially in attempts to alter the normal growth characteristics of the tree that is causing the problem.

The most egregious of pruning practices involves topping the tree in an attempt to reign in unwanted tree height. Older, mature trees may not have the ability to compartmentalize large topping cuts, resulting in decay of the main stems. As the decay worsens, it spreads down the main stems, slowly killing the tree, and in the process creating increased risk for limb or whole tree failure.

Trees create risk only if there is a target nearby. Targets include overhead utility lines, buildings, structures, sidewalks, paths, pedestrians, streets or highways with automobiles. If there is a target, all trees create a certain level of risk. Trees acquire injuries and hazards that may increase over time. As the hazard worsens, the risk level posed by the tree may also increase. The property owner may not be aware of the potential risk and liability until an accident occurs.


The picture on the left show a topped Cedar, the picture on the right, a topped Aleppo pine dying from decaying stem from the topping cuts. The Aleppo is at a street corner adjacent to pedestrian sidewalks.

It failed due to an “Act of God”

The “Act of God” defense implies some kind of natural, unavoidable, or unforeseeable catastrophe that interrupts the expected course of events.

Was the failure really an unforeseeable “Act of God”, or did the owner fail to meet the “duty of care” requiring an Owner to make a reasonable effort to identify foreseeable problems cause harm to people or property? Assuming the Owner is a layperson, they would not be qualified to inspect and assess the risk posed by their tree. Is it their “duty of care” to call an arborist for a tree risk assessment?

There is no doubt that hurricane or tornado force winds may cause substantial tree loss through various kinds of failures. When wind velocities exceed the calculated failure threshold of a tree, even the healthiest tree will fail. Catastrophic events of this nature represent an “Act of God”.

However, not every tree part or whole tree failure occurs due to an “Act of God”. Trees incur defects throughout their lifespan. As they age, the defect grows and becomes increasingly hazardous. Development and construction brings targets into closer proximity, increasing the risk of existing, older trees. Renovation projects with grade changes can disturb tree roots. Deferred maintenance and incorrect pruning practices, and environmental factors may increase the stress on an already declining tree, compounding existing hazards


What is a Tree Risk Assessment?

A tree risk assessment is a systematic process that identifies tree hazards, and reviews and ranks the risk factors into risk categories. The highest risk category represents a hazardous tree. Not all tree hazards are serious and most trees in the urban landscape do not represent imminent threat of failure. The risk assessment process examines the whole tree in its site and environmental setting; it then focuses in on individual tree parts and components.

Why should a tree risk assessment be undertaken? Anticipating tree problems, identifying them and making recommendation to prevent future damage before it is too late is a primary reason for undertaking a tree risk assessment. A property owner concerned about their trees recognizes the potential damage or injury a tree failure could cause, as a matter of due diligence, they hire a certified risk assessor to perform a tree risk assessment. In so doing, they have taken a pro-active step to reduce the risk associated with a hazardous tree. This has positive health benefits for the tree and forms the basis of a legal defense should the tree fail. The owner has made a reasonable effort to identify and mitigate a tree hazard that could cause injury or property damage.

A tree risk assessment should only be undertaken when there is a potential target that might be injured or damaged should the tree fail. If there is no target, then there is no risk! We are not concerned about a tree failure if it does not threaten people or property. A target is defined as people or property that are within striking distance of the tree or component part.

What does an assessment determine?

When a target exists, ALL trees pose some degree of risk all of the time. A tree risk assessment provides the means to evaluate:

• The probability of component part or whole tree failure
• The size of the defective part
• The target area, including frequency and use, as well as any other risk factors that require consideration

Each factor is assigned a value, which are summed up to derive an overall risk rating. The rating is then compared to a predetermined risk level, and if the rating number exceeds a certain level, the tree or component part is rated as hazardous and some form of mitigation is determined.

If you are a property owner, property manager, builder, developer, attorney or insurer, you most likely are involved with trees in some manner. For property owners, homeowner associations, and property management companies, while trees are assets that can increase the property and rental value, they also create risk and potential liability to the owner.

For construction and development companies, encroachment into the wild land urban interface increases the likelihood of tree damage and failure to retained trees caused by construction practices. Attempts to preserve remnant forests or wild land specimen trees in the developed landscape adjacent to new home or commercial development can prove very difficult and create a high level of risk.

Attorneys and insurers must determine the factors involved in tree accidents and resultant litigation.

The current standard of care to determine tree hazards and their related risk is by having a tree risk assessment performed. The Pacific Northwest Chapter of the International Society of Arboriculture has designed and is administering training in Tree Risk Assessment in Urban Areas and the Urban/Rural Interface. The training is only available to certified arborists and is highly technical in nature. Those that pass the test receive certification as a Certified Tree Risk Assessor.

Jeremy Rappoport is an ISA certified arborist, WE-9083A and certified tree risk assessor, #CTRA 1220. He is a C-27 California Landscape Contractor, #436000 and is a professional Horticulturist with a Bachelor of Science degree in Ornamental Horticulture from California State Polytechnic University, Pomona.


Acknowledgements:
Dunster, J. 2009. Tree Risk Assessment in Urban Areas and Urban/Rural Interface: Course Manual. Silverton, Oregon: Pacific Northwest Chapter, International Society of Arboriculture

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ABOUT THE AUTHOR: Jeremy Rappoport, President of Rappoport Development Consulting Services LLC
Mr. Rappoport is an ISA (International Society of Arboriculture) certified arborist, #WE-9083A, and a certified tree risk assessor, #CTRA 1220. Additionally, Mr. Rappoport is a C-27 California Landscape Contractor, #436000 and professional Horticulturist, with a Bachelor of Science degree in Ornamental Horticulture from California State Polytechnic University, Pomona.

Copyright Rappoport Development Consulting Services LLC

Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.For specific technical or legal advice on the information provided and related topics, please contact the author.