Trees can make a lovely addition to your landscape, but they often come with their own set of risks. To ensure that your property is safe, regularly inspect your trees and mitigate risk factors, or hire a professional arborist to do so.

A formal tree risk assessment, performed by a trained arborist, is the best way to guarantee all of your bases are covered.During the assessment, the arborist will look at your trees, identify defects, then judge how likely that defect is to cause tree failure.

There are three levels of tree risk assessment:

Level 1: The arborist will view the tree in question, whether in person or through photographs.

Level 2: This requires a 360 degree ground level observation of the tree or trees in question. During this phase, the arborist will examine the roots, trunk and crown to identify obvious structural defects.

Level 3: Building on the information from Level 2, the arborist will then perform advanced diagnostic procedures, which may include extracting samples for lab analysis.

It must be recognised that there are limitations associated with Tree Risk Assessment which arise from uncertainties related to trees and the loads to which they are subjected. The scientific study of tree failure is relatively young and there is still a lot to learn.

At Treespan we now utilise the Quantified Tree Risk Assessment Method

The Quantified Tree Risk Assessment Method of 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

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