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Tree_Risk_Assessment

What is Risk?

  • The ISA definition of Risk, is the combination of tree failure with its consequences. Such consequences include: injuries or fatalities, property damage, service disruption, and environmental damage. If a tree falls but there is no target surrounding it, the tree is not a risk.
  • Using the ISA's BMP, certified arborists are tasked to do the following:
    1. Predict the likelihood of failure
    2. Predict the possibility of impacting a target
      • Unlikely/Very low -- very slim chance of a failed tree part hitting a target
      • Somewhat likely/Low -- not likely that failed part will impact the target
      • Likely/Medium -- failed part could hit the target
      • Very Likely/High -- Hitting the target is very likely
    3. Understand the consequences of impacting a target
      • Negligible -- consequence is almost nothing -- impacts to property and injury is minimal
      • Minor -- low to moderate property damage, break a fence or two.
      • Significant -- damage to moderate to high-value property, considerable disruption or personal injury
      • Severe -- serious damage (to high-value property and important services), injury or death is likely
  • "Hazard trees are trees that have been assessed and found to be likely to fail and result in an unacceptable consequence." Places like UBC are target-rich environments.

Tree risk management

There are many reasons why we should do tree risk management. These include:

  • Reduce insurance costs. Damage from tree failure can cost millions of dollars in compensation!
  • Safe environments for citizens
  • asset protection
  • conflict management and mitigation
  • Improved public image and customer services
  • Enhanced forest ecology
  • Improved tree health

Who takes responsibility for tree risk management? Well, it goes in a hierarchy...

  • Those who have legal responsibility for managing the property (owner)
  • Those who are doing the assessment (tree risk assessors), considers training, for knowing the limits of their expertise
  • Tree manager
  • The arborist who works on the tree

5 key Aspects/steps to Tree Risk Management

  • Firstly, identify the risk -- what is it?. What is it associated with?
  • Evaluate the risk -- When is this risk imminent? How serious is the risk?
  • Mitigate the risk
  • Monitor the same risk -- come back and double-check to see that mitigation measures worked as planned.
  • Communicate about risk -- this is an important, often overlooked step: everything we do in urban forestry is communication (e.g. warning signs, notifications for weather, etc.)

 

Rating Tree Failure

Tree Failure can be rated using a Risk matrix with the key factors described.

  • Extreme risk
    • FAILURE IS IMMINENT, high likelihood of impact a target, consequences are severe
  • High risk 
    • Very likely and consequences are significant, or not likely but consequences are significant
  • Moderate risk
    • Failure is likely or very likely & consequences are Minor, or failure is somewhat is likely and consequences are significant or severe.
  • Low risk
    • Failure is unlikely & consequences are Minor, or failure is somewhat likely and consequences are negligible
    • We don't do much with this risk, mitigation measures are not normally recommended, might opt toward monitoring to observe changes in condition

Taking all this together, we can create a risk matrix that looks something like this:

  Negligible Minor  Significant Severe Very likely Low Moderate High Extreme Likely Low  Moderate High High Somewhat likely Low  Low Moderate Moderate unlikely Low  Low  Low  Low 

 

Tree Risk Tolerance

  • Tree risk tolerance is how much risk is acceptable to a tree owner. Acceptable risk is dependent on the individual: different people perceive risk differently.
  • Some factors that inform a person's definition/parameters of Acceptable risk include:
    • Historical and heritage value
    • Environmental significance
    • Aesthetics
    • Existing risk policy
  • Very important is the fact that risk tolerance is often driven by... the size of our budget!

How Should We Mitigate Risk?

Here are a few considerations when trying to mitigate the risk of a tree:

  • Target management -- instead of changing the tree, change its surroundings. Move the people, move the buildings, move the pathway
    • Can we direct pedestrians away from hazard trees or away from tripping hazards that trees can create?
    • Can signs be used to mitigate risk by redirecting pedestrian/vehicle traffic?
  • Sometimes, trees will have to be cut. Sometimes, restarting from scratch is the best way to mitigate risk (Vancouver has been logged before, lol).
  • We could use tools and materials to support a tree with risk.
    • Dynamic systems using cables, bolts, props, etc., to support defected, likely-to-fail tree limbs.
    • This may become an optimal or widely used methodology in the future, but more time and tests have to pass before this can be confirmed. We will see!

Residual Risk 

  • Residual risk is the risk that remains after mitigation efforts. Residual risk can come back to haunt a tree (and arborists I guess) if unsustainable management practices are employed. So the level of residual risk to be tolerated must be determined and decided upon by the tree owner and manager.
    • For example, topping a tree significantly decreases if not totally eliminates the likelihood of entire tree failure in the short term, but increases the likelihood of (branch) failure in the long term. This is because new growth will occur from the stubs (often as buried epicormic buds in the vascular cambium), and so create structurally weak areas. Failure potential will rise dramatically as these epicormic shoots enlarge.

Notes

  • It is impossible to eliminate all risks associated with a tree? YES! (cut them down lol).
  • We can learn a lot about future tree risk by looking at cases today.
  • Tree risk is much lower than one would initially assume!