“I strap a thumb-size ultrasonic sensor to a ponderosa twig and wire the electronic device to a computer. Then I wait, “listening” through the intermediary of a graph on the screen. Every time the twig releases an ultrasonic pop, the graph’s line jolts up by one step.” The sound sample below comes from a well-hydrated twig gradually drying. I converted ultrasonic data into audible “clicks.” As the twig dries, the rate of clicking increases. The 41 second sound sample is a compressed version of about 12 hours, a morning (when water is more plentiful) through afternoon (when dryness increases, then eases as the sun sets) on one day:

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