The decay of wood in a standing tree is often the cause of tree failure. However, the accurate measurement of decay and the non-invasive sensing is still in its infancy. A review of the current sensing methods commonly used for decay detection in standing trees is presented. Methods are compared in terms of the fundamental of measurements, hardware implementation, damage caused to tree and the ease of use. Invasive technique with decay detecting devices, such as increment borer, borescope, decay detecting drill, shigometer, fractometer and radiographic meter, and non-invasive technique involving electrical resistance, microwave, nuclear magnetic resonance and acoustic methods are discussed. This review paper aims to help researchers in this field to identify the remaining issues.
