Development of an acoustic method for wood disease assessment

Abstract

Over the past years, the Lecanosticta acicola fungus or brown band is seriously damaging the pine forests of northern Spain. This fungus happens to dry the leaves and branches of trees. The present article describes the equipment and methodology for assessing the severity and spread of diseases in trees quickly, easily and cheaply. The concept is based on a basic assumption: a sick tree does not sound the same when it is hit as a healthy one does. The breakthrough character of the present work comes from a basic mechanical principle: if a tree loses water its density is reduced, so the sound that it makes when it is hit, also known as its acoustic response, is higher. If the tree is healthy it will grow, so the sound will be the same or deeper. The human ear cannot distinguish these variations or quantify them, but a microphone with an acquisition system can. After initial proofs of concept on real trunks, the function that relates the frequency increase to the degree of dehydration of the tree, and therefore to the severity of the disease, has been identified. Then, the method has been tested in a real forest with sick pines and compared to visual inspection of an expert. After several months of tests, it has been concluded that the trend of the acoustic method coincides with the visual assessment, with the difference that it is objective, while visual inspection is not. It has even demonstrated to be able to foresee the healing of trees before new leaves start to sprout.