Analysis of Ultrasound emissions from dehydrating Sapwood

Acoustic emission (AE) testing has an enormous information potential on physiological events during wood dehydration. The increase in the sensitivity of new detection equipment and powerful software tools developed during the past decade, which allow detailed analysis of millions of waveforms, open new perspectives to utilize this information potential. Secondary xylem (wood) emits AEs over a broad frequency range during dehydration. Signals emitted in the high-frequency range (> 15 kHz) are used to detect mechanical failure induced by lumber drying as well as cavitations (breakage of the water column inside wood elements) in drought stressed plants parts, because the background noise can be selectively filtered out. Whereas the analysis of AE waveform patterns is nowadays successfully used to optimize kiln conditions in order to avoid checking, hydraulic vulnerability assessment of sapwood is still focused mainly on counting all AEs surpassing a defined detection threshold. AEs from dehydrating sapwood remain therefore somehow a black box, although it is well known that most of the AEs during wood dehydration come from cavitations. The proposed project will focus on the physiological information potential of AEs from dehydration stressed sapwood concerning cavitations and internal checking in living trees. The hydraulic properties (conductance and vulnerability) of conifer and angiosperm sapwood will be analyzed by the extraction of AE waveform features, such as the peak amplitude, the AE duration or the AE energy. AE feature extraction, combined with physiological and anatomical investigations, will be used as a tool to separate harmful cavitations (impairing hydraulic conductance) from less harmful ones. The establishment of a reliable, readily automated method to detect harmful cavitations would be a break-through in the assessment of the hydraulic behavior of sapwood. AE feature extraction should as well allow a clear distinction between AEs produced by cavitation events and those induced by mechanical failure during dehydration stress. The investigations will be done on tree species which are economically important in Austria, such as Norway spruce (Picea abies), where checking in living trees is a severe problem, and drought-sensitive hybrid poplars (Populus x euramericana, Populus x canescens). The testing methods developed will help to select trees with high hydraulic safety and lowered susceptibility to checking, which is of importance for short rotation forests in a changing environment.