Jeff Moore is a professor of geology and geophysics at the University of Utah. He uses field observations and monitoring data to study the structural health of rock formations, including Utah’s iconic arches and rock towers.
“I'm from Tucson originally, and I was home visiting family, and around the time when we were working on the study of rock towers, I realized that I had all the skills necessary to sort of mount a similar study of saguaro cacti,” Moore said.
His study, published last month in the American Journal of Botany, analyzed resonance frequencies, or vibrations, of single-stem saguaro cacti in the Sonoran Desert.
Saguaro cacti, iconic symbols of the American West, can grow over 40 feet tall and live to be several hundred years old, Moore says. In Arizona, saguaro cacti are protected by state law.
“You can't just willy-nilly go strapping stuff to saguaros. You have to get permissions," Moore said. "So, I worked with the folks at Tucson Mountain Park to get permission to conduct the study, found the most lightest weight seismometer I could, and constructed everything from really lightweight materials in order that when I strap this seismometer to the cactus that it shouldn't cause any damage.”
Measuring how much a cactus bends under pressure helps scientists understand the stability of the cacti under conditions like high winds or earthquakes. In the past, this usually would require cutting a section of cactus and bending it with weights.
“Obviously, that's severely invasive and has its limitations," Moore said. "And so, my objective was to see what manner of structural characterization we could achieve from non-destructive testing."
Moore found that saguaros were generally stiffer near the bottom and more flexible at the top. He also documented daily cycles of vibrational frequencies, likely associated with softening of the cactus tissue as temperatures warm during the day.
“To my knowledge, nobody's done this on saguaro cacti previously," Moore said. "I hope that folks might find this useful and find at least the idea of resonance monitoring as something that could be applied in these columnar cacti."
