Managing Tree Risk
By Colleen Del Vecchio and Davinna Ohlson
With winter upon us, it’s critical to prepare the spaces that we live and work in for cold temperatures and inclement weather. Whether you are a property owner or a park manager, these preparations should include a quick visual inspection of the trees that you are responsible for. These can include trees in your home and neighborhood, public parks, and school campuses—any place where tree failures can put people or property at risk.
Arborists are qualified tree care professionals who are trained in the management, cultivation, and care of trees. One important responsibility of an arborist is assessing and managing tree risk. You can hire an arborist for a professional assessment of your trees, but we offer some guidelines here for doing a preliminary visual inspection yourself.
What is Tree Risk?
Tree risk is the likelihood of a tree failing and impacting a resource of concern, known in the arborist world as a “target.” Tree risk considers not only the probability of a failure event itself – think breaking branches or toppling trees – but the severity of the potential consequences associated with such an event. In general, tree risk is considered in a human context. Targets are defined as people, property (e.g., buildings or cars), or activities providing a human benefit (e.g., a botanical garden or hiking trail) that could be injured, damaged, or disrupted by tree failure (Dunster 2017). Hence, if there are no targets, a tree’s risk is likely considered negligible, even if there is failure.
What is a Tree Risk Assessment?
A tree risk assessment is a systematic process for evaluating structural defects, conducted by arborists specifically trained in this practice. Tree risk assessments promote tree longevity while providing a beneficial service to people and communities. When conducting a tree risk assessment, the arborist inspects and documents:
1) the tree’s structural and health condition, 2) site and environmental factors that may influence the likelihood of failure, 3) potential targets, and 4) species-specific failure patterns to ultimately determine the severity of the consequences, should failure occur. A tree can be healthy, but if it has weak attachments, such as long, heavy, horizontal branching, its structure of over-extended branches is a load defect and might contribute to conditions of concern depending on other factors assessed in the inspection.
As a management practice, tree risk assessments are particularly helpful in public land settings such as city parks. Imagine you are walking along a path through your favorite park on a blustery autumn afternoon.
Large oak trees are scattered throughout, and a picnic area is nestled in a grove of trees. You decide to take a break and relax at one of the picnic tables. While you sit there enjoying the crisp air, you see other people stopping to rest at the tables next to you and children playing tag amongst the trees.
As the city who owns this park, there is a lot to manage between the visitors, facilities, and landscaping. You observe the park manager passing through the picnic area on his way to the pavilion. He stops to look at a large crack at the base of one of the oaks next to your picnic table and comments that he doesn’t think he’s seen it before. Come to think of it, neither have you.
In keeping with the city’s park management plan, the park manager contacts the city arborist to conduct a tree risk assessment. The next day, the city arborist comes out to the park to complete her assessment. She first identifies the species and gathers height, trunk diameter, and canopy data. She then completes a visual inspection of the tree, documenting its health and species profile, load factors, tree condition, and defects. She identifies a large amount of decay in the main trunk, which caused the visible crack. She then inspects the crack further to determine its length and depth, if the tree has any growth response to the crack, and potential causes. Due to the crack’s location, the entire tree would be the expected failure point. She also records site factors, including topography, soil conditions, and common weather and wind patterns. Next, she assesses what the entire tree may impact. Remember those kids running around that tree? Don’t forget about yourself taking a break at the picnic table.) She identifies multiple targets, including people utilizing the picnic area, the children running around the trees, picnic tables under the tree, and even other trees near the oak. Lastly, the arborist categorizes the likelihood of failure, impacts to people or property, and the severity of the consequences should the tree fail and hit the targets.
Ultimately, the arborist determines that the likelihood of failure for this tree is imminent. The crack is an indication of the tree actively failing due to extensive decay in the trunk. She determines that because of a recent storm, which had high winds, the load the wind placed on the trunk caused the vertical compression crack.
Therefore, she concludes that tree failure and impacts to targets are very likely and that the consequences of failure could be significant. After considering these factors together, she determines the risk rating of the oak to be extreme.
The arborist identifies possible options for mitigating the risk:
Weight reduction pruning. Although the tree in question is actively failing, risk can be somewhat mitigated by pruning to reduce the overall weight of the tree, thereby reducing the stress on the main trunk, and reducing the wind load in the tree’s canopy during storm events.
Installing a fence around the tree. Risk to targets can be mitigated if the picnic tables under the tree are relocated to other parts of the picnic area, and a fence or other suitable barrier is installed around the tree to prevent people from entering within the failure zone. This approach may be appropriate if the tree has cultural or historical significance and if its removal would be considered a significant loss to the community.
Tree removal. This is the most extreme of the three options but may be the most appropriate as whole tree failure appears imminent.
After the City received the arborist's risk assessment report, the action item they chose was to remove the tree followed by a tree replacement. The City’s tree management plan states that extreme risk is beyond their tolerance and puts too much risk on the community.
Should a Tree Risk Assessment be Conducted for My Home?
Mature trees offer a wide range of benefits for people and the environment. They beautify communities, contribute to social and emotional well-being, and provide habitat for wildlife. Maintaining your trees can prolong their life and raise your home’s market value. As a property owner, hiring an arborist to conduct a tree risk assessment is a proactive step that can help identify high-risk situations before accidents happen. The arborist will provide tree care recommendations so that damage to property or people can be avoided. Preventative tree care measures can result in cost savings in the long run, provide a higher level of safety to your home, and bring you added peace of mind.
Tree risk assessments have specified timeframes since trees are constantly growing and the environment is continuously changing. The tree’s conditions and level of risk can change over a short period of time. A tree assessment report typically has a timeframe of 6 or 12 months, which projects how long a tree’s condition is anticipated not to change. When weighing out whether to have a tree risk assessment conducted for your property, keep in mind that the assessment will reflect a snapshot in time. Tree care recommendations should be implemented as soon as possible after the assessment to maximize the value of the assessment, minimize risk, and effectively conserve your tree resources.
Of course, not all trees require management. Tree risk assessments are intended for trees that can cause damage or injury if they fail. Typically, trees that require maintenance for safety are surrounding structures, cars, sidewalks, or locations that people frequent. As a homeowner, pay close attention to the trees where your children play as well as trees that overhang your house, driveway, and sidewalk. For more natural settings where there are no targets, trees require little to no management or human intervention.
Large trees on your property, and especially those next to your home, can be intimidating, both in size and responsibility, but they can be managed with thoughtful effort. It’s likely that you already manage your trees in beneficial ways, such as removing damaged or dead wood and branches. We would like to “leaf” you with some additional recommendations and potential next steps to help ease your mind and keep you, your family, and your neighbors safe.
Get to know your trees. Pay attention to your trees through each season. Know what they look like when dealing with heat, wind, rain, or any other seasonal condition in your area.
Make a mental checklist and do a basic visual inspection. Start your visual inspection at the base of the tree and work upwards, looking at the trunk and then at the canopy (you might need a pair of binoculars). Take note of things that appear out of the ordinary, such as cracks in the bark or at branch connections, signs of decay (e.g., fungus or cavities), sap oozing out of the bark, discolored bark, or leaves dying on a branch in the upper canopy. If you see anything out of the ordinary or sudden visual changes to your tree, it may be time to contact an arborist for professional advice and care.
Getting to know your trees and when you should call a local arborist is a worthwhile management plan for any homeowner. Any signs of change from your observations could be lifesaving to yourself, your family and neighbors, and the tree. You can even call a local arborist to walk your trees with you for the first time, helping you understand what to put on your basic inspection checklist. There’s no set time frame on how frequently trees need to be professionally inspected or trimmed. Each tree is an individual growing in a unique location with unique conditions. Although these factors make it nearly impossible to have a single risk management plan for all trees, the recommendations above are a useful first step for any tree owner and highlight the importance of caring for trees, especially as the seasons change.
Fun Fact: Similar to deciduous trees dropping their leaves in the fall, trees naturally shed their branches, or self-prune, through a process called cladoptosis. This is a tree’s own way of managing itself when at risk of failure. Trees may self-prune as part of their normal physiology, or they may shed branches in response to environmental and physiological stressors, such as drought, disease, age, or other unique site factors.
References
Dunster, Julian A. 2017. Tree Risk Assessment Manual, 2nd Edition. International Society of Arboriculture (ISA).
Live Oak Associates provides a comprehensive range of biological consulting services to organizations throughout California. Partner with us to discover how our team can help you with your next project.