A non-invasive method for evaluating trees

Abstract

Recently high winds raged across central Europe felling trees with the inevitably consequence that some of the trees damaged property and in a number of cases caused personal injury or the tragic loss of life. The risk posed to property and life by wind-blow damage to a tree is assessed by a number of factors with the principal determinants being the location of the tree and its apparent health. Tree owners governed by English law have a legal responsibility to act in a prudent manner to ensure that their trees are not a liability to persons or property and should, if necessary, inspect their trees at regular intervals. Typically an arboricultural consultant is called upon to assess the health of a tree, as an expert visual appraisal is critical in any tree inspection. Although the extent of a defect within a tree cannot normally be assessed by a visual assessment alone and various proprietary mechanical and sound wave instruments may be used to gauge the risk presented by the tree. Decay in mature trees generally manifests itself as a central column of rot that tapers from the roots within the trunk of the tree. The aim of an internal inspection is to ensure that the trunk has a minimum of 30% of sound wood surrounding the rot. Whereby a tree can heal itself by surrounding the rot with a defensive layer that isolates the decay and allows the tree to regenerate. Moreover, a hollow tree that had decay may present little risk and withstand greater wind force than a solid tree of similar size. Unfortunately the mechanical removal of a core sample for inspection or probing a tree with a rot detecting drill can breach the natural defence mechanisms of a tree and cause an infection to spread into the surrounding sound wood. This paper describes the progress of a joint research programme that is developing a practical method to non-invasively detect decay in trees by propagating stress-waves across the trees and analysing the resultant time-frequency components in the waves.