Trees under water stress produce sounds when their water vessels crack. Ultrasonic microphones can capture these sounds and thus detect whether trees are suffering permanent damage due to drought. Swedish researchers have succeeded in implementing this method in the field for the first time. The idea is to detect climate stress in forests. Drought-related decline is most likely linked to the phenomenon of embolism or ‘air bubbles’ in the xylem. Microphones can detect when trees are affected by embolism. The study was conducted on young birch and pine trees. Some of them were watered with normal water and others with water that had been treated to break down its molecules and form air bubbles in the xylem. Trees in which embolism was induced emitted more ultrasound than the others and lost their ability to absorb water. In pine trees, embolism occurred in both the trunk and the crown, while in birch trees it was detected only in the trunk. The research team plans to work on other species and also on more mature trees, with the challenge of thicker bark to overcome.
