Tree Risk Assessment and Tree Risk Management

The benefits that urban trees provide to our cities are not without their share of risks. In a multitude of situations, trees can pose risk when they are adjacent to human activities, infrastructure and services. The degree of risk relates to three important variables: the likelihood of a tree-related conflict or tree-related failure actually occurring, the likelihood that a target could be affected or stuck as a result of this conflict/failure and to what degree of severity the target could be impacted by this event.

Once risks have been categorized, urban foresters must attend to reducing these risks and maintaining them a level that is tolerable and reasonable for community safety, all while fostering urban forest health and function.

Tree risk management is a complex process intended to balance the benefits that trees provide against the harm that can arise from their proximity to people and property (Barrell, 2021).

This process takes into account the condition of trees analyzed within their greater interaction with humans and the sites in which their are located. Tree risk assessment must account for the specificity of each tree, and site encountered, along with possible targets in the landscape.

What is critical in this practice, is properly assessing the degree of risk and making sure subsequent interventions are proportional to the level of risk.

Tree risk management has an important place in urban forest strategic planning, arboricultural operations and even for tree conservation in general. While tree risk assessment helps urban foresters establish priorities and guide operations, different methodologies have different advantages and limitations. Understanding the distinct approaches that exist, along with the assessor's inherent biases, can assist practitioners in making reasonable decisions about trees. 

This week, we will cover the fundamental steps, guiding principles and even a few methodologies and resources pertaining to tree risk assessment, in a wider context of tree risk management. We will also explore the topic of risk mitigation as it relates to tree care operations.

This section's readings focus on defining the guiding principles, terminology and approaches to tree risk assessment.

Some optional readings will also offer comparative studies of different approaches and methodologies internationally, drawing from studies from Uruguay and the United Kingdom. In our next course, we will discover how one Canadian consultant, is using technology to elevate tree risk assessment methods and gain more precise data on tree condition.

Learning Objectives

By the end of this topic, you should be able to:

• •  Distinguish between a target, tree hazard, tree risk, tree risk assessment and tree risk management
  •  Explain the three variables that make up tree risk 
  •  Recognize the different roles between tree risk managers, tree risk assessors and tree care providers/tree workers
  •  Describe three different levels of risk assessment and which tasks can be covered in each of these three levels
  •  Identify different tree risk assessment methodologies that exist
  •  Be familiar with different interventions and approaches to mitigate tree risk

Topic Readings

This week is all about context. Where you work and where trees are located will influence tree risk management decisions and assessment techniques. In some regions, the strategies and methodologies used, will be required by local laws and regulations. Risk categorization and risk mitigation approaches are very site specific. It is important to be aware of the frameworks, constraints, and land-use of the area under review when conducting risk assessment work.

The following readings have been curated to help you understand the importance of context, and tree risk management applications in different parts of the world.

Required Reading (will reinforce concepts covered in class)

Smiley, E.T., Matheny, N., and Lilly, S. (2017). Tree Risk Assessment. Routledge Handbook of Urban Forestry. Routledge, London, (pp. 478–488)

Optional Reading

Looking to get up to speed? Consider this optional reading to introduce you to the topic as well as a useful methodology for Assessing Tree Risk. This source covers the basic elements of ISA's Tree Risk Assessment Qualification (TRAQ) system. However, reading and understanding this source is not a substitute for tree risk assessment training and formal credentialing.

• Smiley, E.T., Matheny, N., and Lilly, S. (2017). ISA Best Management Practices BMP: Tree Risk Assessment Second Edition. International Society of Arboriculture, Champaign IL.

Optional Comparative Research

• Coelho-Duarte, A.P., Daniluk-Mosquera, G., Gravina, V., Vallejos-Barra, O. and Ponce-Donoso,M. (2021). Tree Risk Assessment: Component analysis of six visual methods applied in an urban park, Montevideo, Uruguay. Urban Forestry & Urban Greening., 59.
• 6 Tree Risk Assessment Methods Uruguay 2021.pdf Download 6 Tree Risk Assessment Methods Uruguay 2021.pdf 
• Barrell, J. (2021). The implications of recent English legal judgements, inquest verdicts, and ash dieback disease for the defensibility of tree risk management regimes. Arboricultural Journal, 43:1, 3-18.
• Tree Risk UK Barrell 2021.pdf Download Tree Risk UK Barrell 2021.pdf 

PRACTICAL RESOURCE

The following document was developed by the United Kingdom's National Tree Safety Group in 2011. This text provides guidance on trees and public safety in the UK for owners, managers and advisers. It has also been useful in the development of North American guidance documents such as ISA's Tree Risk Assessment Best Management Practices. and is definitely worth a read if you're looking to know more about tree risk management in urban forestry.

Common sense risk management of trees UK NTSG 2011.pdf Download Common sense risk management of trees UK NTSG 2011.pdf 

Fundamental Concepts

Lecture Video in Zoom archive & Powerpoint presentation notes HERE Download HERE

Topic Self-review

1- How is tree risk managed in your community? Is there a process or methodology that must be followed when reporting on tree risk? What qualifications are required, in your region, to assess trees for risk?

2- What mitigation measures can be integrated into urban forestry operations to manage tree risk at tolerable thresholds all while promoting the health and function of your local urban forest?

3- How can tree risk assessment be subjective? What can tree risk assessors do about it?

4- Why is assessing site important when conducting tree risk assessments?