Ultrasonic acoustic emissions as indicators of tree drought stress in outdoor forest settings: Testing the concept using cut saplings

Drought-induced air embolism (air blockage of xylem sap) is an important mechanism behind observed forest declines and serves as a physiological link between climate and reduced tree growth. It has been known since the 1980 s that vapour bubbles generated inside drying plant tissue emit ultrasonic acoustic emissions (UAEs), but it remains uncertain whether acoustic studies can be applied inside complex soundscapes of forests to monitor xylem embolism in real-time. In this study, we show that microphones mounted on, or adjacent to, stems of young birch (Betula pendula) and Scots pine (Pinus sylvestris), can indeed be used in outdoor environments to detect embolism in the xylem sap. Vapour formation via cavitation in the xylem was artificially generated inside cut saplings by a surfactant that reduced the cohesive forces of water and forced the water column inside the xylem to break. Birch trees with induced xylem embolism experienced reduction (-60 +/- 11 %; mean +/- sterr) in water uptake in similar to pine (-49 % +/- 5). Both control trees and trees with induced embolism generated UAEs, but trees with hydraulic failure due to xylem embolism emitted UAEs more frequently than control trees. The vertical distribution of UAEs from the stems differed between species where birch only emitted more UAEs than the control from basal part of the tree trunk (+58 % +/- 28), while these pulses where emitted more often from both basal sections (+35 % +/- 2) and upper sections (+53 % +/- 4) of the pine trunk from our treatment. We conclude that both substrate-borne and airborne UAEs can be used to indicate hydraulic failures inside forests and that bioacoustics monitoring reveal a previously unknown species-specific difference in how embolism appear inside the trunk of birch and pine.