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What is QTRA?

The Quantified Tree Risk Assessment Method

Tree safety management is a matter of limiting the risk of harm from tree failure while maintaining the benefits conferred by trees.  Although it may seem counterintuitive, the condition of trees should not be the first consideration.  Instead, tree managers should consider first the usage of the land on which the trees stand, and in turn this will inform the process of assessing the trees.

The Quantified Tree Risk Assessment (QTRA) system, developed by Mike Ellison at Cheshire Woodlands, applies established and accepted risk management principles to tree safety management.  The system moves the management of tree safety away from labelling trees as either ‘safe’ or ‘unsafe’ and thereby away from requiring definitive judgements from either tree assessors or tree managers.  Instead, QTRA quantifies the risk of significant harm from tree failure in a way that enables tree managers to balance safety with tree values and operate to predetermined limits of tolerable or acceptable risk.

By quantifying the risk from tree failure as a probability, QTRA enables a tree owner or manager to manage the risk in accordance with widely applied and internationally recognised levels of risk tolerance.  QTRA further provides a decision-making framework which considers the balance between the benefits provided by trees, levels of risk they pose, and costs of risk management.

QTRA decision making framework

Using QTRA, the land-use (Target) upon which trees could fail is assessed first.  By valuing the Target first, the tree owner and the risk assessor are able to determine whether or not, and to what degree of rigour, a survey or inspection of the trees is required.  Where necessary, the tree is then evaluated in terms of its size and probability of failure.  Ranges of value for Target, Size, and Probability of Failure are entered into a QTRA calculator which generates a traffic light colour-coded risk of harm.  The tree owner can then compared the risk to advisory levels for risk tolerance.
By taking a QTRA approach, tree owners commonly find they spend substantially less resources on assessing and managing tree risk than they did previously, whilst maximising the many benefits their trees provide.  Moreover, in the event of a 'tolerable' or 'acceptable'  risk being realised, they are in a position to demonstrate the risk has been managed reasonably and proportionately.

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