Ultrasound acoustic detection of cavitation events in water conducting elements of Norway spruce wood

Water in living conifer tree stems is transported in the sapwood area which consists of a network of small conducting elements, the tracheids. The liquid water in the conduits is under tension caused by evaporation from the needles, and thus in a metastable state which is maintained by cohesion between water molecules and adhesion of water to the conduit walls. The breakage of the liquid water columns inside the conduits by rapidly expanding gas bubbles is a common phenomenon defined as cavitation. Dehydration induced cavitation occurs when an air bubble is pulled into the lumen of a water-filled conducting element via the pores of the pit membranes from an adjacent conduit already embolised or from intercellular spaces filled with air. As a result, liquid water is replaced by water vapour and later by air. Since the continuity of the liquid water columns is interrupted, the hydraulic conductivity of wood is reduced leading to diminished water supply of the transpiring leaves. Cavitation events were detected by means of an ultrasound acoustic technique recording emissions in the high-frequency range of 100 kHz to 1 MHz. Emissions are due to elastic oscillations in the cell walls of the conducting element following the cavitation event, because the pressure rises rapidly in the conduit lumen as liquid water at negative pressure is replaced by water vapour very near vacuum pressure. The method is suitable to measure the onset and extent of cavitation in both intact and excised tree stems. Ultrasound acoustic detection combined with measurement of the hydraulic conductivity or the water loss can provide essential information on the resistance of species to stress-induced cavitation.