The evolution of Quantified Tree Risk Assessment. Most arborists have been asked a question, something along the lines of ‘Is my tree safe?’
And the response is likely to be carefully measured to avoid liability. After all, if you are the last person to assess the tree before it falls and injures someone or causes damage to someone else’s property, you might well expect your judgment to be questioned.
Historically, the management of tree safety has often been passed to arborists and other specialists who have inspected trees and provided management decisions based on their own tolerance of risk
The most obvious problem with this approach is that it places responsibility for the safety of trees onto the arborists. This often forces them into a risk-averse position where they are likely to end up managing not the primary risk from falling trees but the secondary risk of being found culpable, which in turn could put their livelihoods at risk. Placing the responsibility for management decisions on the tree inspector without any meaningful guidance can be costly at best, but in some cases the negative impact on the tree asset can be catastrophic. It is not unknown for urban populations of urban trees to be seriously degraded by highly risk-averse management decisions that are all too often driven by the irrational fear of a few high profile experts.
Developed in the mid-1990s, Quantified Tree Risk Assessment was at that time a revolutionary approach to assessing the risks from falling trees and branches. Over more than two decades, the method has evolved to provide tree owners with an alternative to the usual game of risk table tennis where they pass the buck to the tree inspector, who is well-practiced in passing it straight back with recommendations to remove and prune trees along with all of the associated costs. And those costs can be complex, usually being far greater than the simple financial expense of commissioning the work. These extend to all of the values that were provided by the trees, from the visual amenity and landscape value to habitats for flora and fauna, the loss of which can have wide-ranging knock-on effects for the environment and local stakeholders. What is the tree owner to do when faced with a professional report that recommends the removal and pruning of trees? Should they implement the recommendations, often at great financial cost and perhaps contrary to their broader objectives, or should they take a chance on the trees rather than divert their finite resources into reducing risks from trees that have never before given them any particular cause for concern. Even though they might sometimes question the validity of expert reports, land owners will often foot the bill rather than be exposed to the risk of litigation.
By taking a risk-based approach that separates management decisions from the assessment of risk, the QTRA method has allowed arborists to carry out objective assessments of trees and provide a measure of the risks along with management guidance based on the tree owner’s priorities. By quantifying, we can not only compare the risk from one tree with the risk from another but we can also manage risks to predetermined thresholds of tolerability and acceptability. This approach works well in large organisations too. Councils can develop a tree risk management policy based on their specific strategic management objectives and their tree inspectors can then manage tree risk on the ground based on the methodology and risk thresholds that have been agreed and documented by the organisation.
Quantified Tree Risk Assessment has evolved to a method that uses quantification to establish the level of risk from a tree but simplifies the communication of risk using a simple traffic light system of colour-coded outcomes. The QTRA assessment process follows that set out in the International Standard AS/NZS ISO 31000 – Risk Management (2018). Risk management decisions for very high and very low risks are usually clear-cut where we control red risks and know that we don’t need to do anything about green, but in the middle the tolerable risks require a little more thought if management decisions are going to be reasonable and proportionate and consider the client’s overall management objectives. QTRA is a method for arborists who want to do the best for their clients and are prepared to put in a little extra thought where risks are approaching key thresholds and the tree owner needs to be able to take an objective view of the balance between risks and benefits.