Introduction

Tree risk assessments are an integral part of urban forestry. They ensure that business and land owners remain mindful of the risks posed to pre-existing trees on their site, in order to prevent further costs down the line should a tree be neglected and fall or decay. Trees can present risks to nearby people and buildings, and assessments help to mitigate this risk.

Quantitative vs Qualititatve Risk Assessment

Risk assessments used to contain numerical rankings in relation to a tree’s health and posed risk, but this has since been updated by the International Society of Arboriculture Best Management Practice method to replace numerical rankings with descriptive categories.

This was done to avoid confusion over accuracy of predictions, but numerical rankings can be and still are used in order to prioritise trees based on level of risk, with the most probably-at-risk trees being prioritised over the trees posing less risk based off of this numerical ranking.

Risk vs Hazard

A hazard is anything with the potential (no matter how small) to cause harm. Risk is the likelihood that a potential hazard will cause harm, and is situation dependent. For example, large trees with compromised structures are more likely to cause harm than small trees with the same probability of failure in remote areas.

There are three common evaluation methods in tree risk assessment, but which one should you choose?

Levels of risk assessment commonly follow the ANSI A300 Standards for Tree Care Operations, which can be viewed here.

ISA Tree Hazard Evaluation Form

The ISA Tree Hazard Evaluation Form, last updated in 1994, is widely used by commercial arborists as well as municipalities. Though the ISA have since created ISA Tree Risk Assessment BMP, this original evaluation method is still used widely in the industry.

Using this method, it will take approximately 20 minutes to complete a basic, 360-degree visual assessment. The time is likely to decrease as users become more familiar with the process behind this method.

Advantages

  • Level of detail is suitable for a basic visual assessment of an individual or small group of trees
  • Hazard rating can prioritize pruning, removals, and other mitigation options
  • Summarizes an industry accepted method for documenting risk & prioritizing hazard abatement

Disadvantages

  • Time required to complete form makes it less-than-idea for large groups of trees
  • Final numerical rating is a prominent aspect of the form, running the risk of being misused by arborists

This method is best used by commercial arborists, particularly those working with a smaller number of trees. Due to time requirements as well as labour costs, its usefulness in assessing large tree populations is limited.

USDA Community Tree Risk Evaluation Form

Developed in 2003, this evaluation form takes the user through a seven step process in order to identify defective trees, determine the severity of those defects, the potential consequences should the tree fail, and recommendations to reduce the tree risk.

They also apply numeric ratings which help to aid the prioritisation of remedial actions necessary on a population of trees.

Advantages

  • Simple to use & concise (ten trees to a printed page)
  • Fast to complete compared to other methods (10 minutes for each form)
  • Risk rating & tree defect codes makes paper data entry much quicker
  • Corrective action codes prescribe the necessary actions at the time of the assessment

Disadvantages

  • Lack of detail in site history & condition
  • Limited flexibility for user in describing unique problems (eg poor spacing)
  • Data consistency issues
  • Species-specific issues, for example live oaks (Quercus virginiana) being considered poor-tree architecture through the form, despite being considered one of the more resilient tree species around
  • Stem diameter is sole size measurement, height not included
  • Form’s layout doesn’t include sufficient space to list all the defect codes for trees with multiple issues

This form is best suited for commercial arborists working with a large population of trees.

ISA Tree Risk Assessment Best Management Practice Form

This form was developed simultaneously with ISA’s Tree Risk Assessment BMP Manual, drawing on risk analysis theory. It’s intended for trees receiving a basic (Level 2) risk assessment, and isn’t intended for any advanced assessing (level 3). It serves as a replacement for the older ISA Tree Hazard Evaluation Form.

This form takes around 20-25 minutes to complete for basic, 360-degree visual assessment. Time required is likely to decrease as the user becomes more familiar with the form.

Advantages

  • Lists multiple targets for a single tree, which innovated tree assessment methodology, providing a flexible, standardized means of dealing with multi-faceted assessment scenarios
  • Two pages, designed to guide user through a thorough visual assessment
  • Includes gridded spaces for users to map targets or draw major trunk defects
  • Discards numerical ratings, instead using decision matrices factoring in target, likelihood of failure, and consequence of failure to assess overall risk ratings

Disadvantages

  • Level of detail required increases time required
  • Replacing numerical ratings with four possible outcomes can limit the ability to prioritize mitigation efforts
  • Limited in assessing sizeable tree populations

This is best suited for commercial arborists/ urban foresters working with individual trees or smaller tree populations.

Conclusion

There are many different methods for performing tree risk assessments, with the above being the most common methods. It’s important that the arborist or urban forester you work with is licensed and a professional, as they will be able to perform the tree risk assessment and relay the information to you in order to determine the next steps necessary.

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