Watch a saguaro "dance" as part of a seismic new study of the cactus

Saguaros never stop swaying to the rhythm of the Sonoran Desert.

A new study by a researcher at the University of Utah shows the cactuses are in constant motion, whether the wind is blowing or not, and their subtle dance offers clues about their internal architecture.

Geologist Jeff Moore strapped a seismometer to 11 saguaros of varying heights at Tucson Mountain Park to produce what he called a "structural characterization" of each cactus by measuring its ambient vibrations. In the process, he may have pioneered a new way to study the biomechanical properties of the plants without damaging them.

"These cacti are vibrating every second of every day. Whether we can see it or not, they're always moving," he said. "They don't have to move a lot. These seismic sensors are incredibly sensitive."

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For the past decade or so, Moore has been using similar seismic techniques to assess the structural integrity of Utah's famed rock bridges, arches and towers. He said he got the idea to try it out on saguaros while visiting family in Tucson.

Moore figured the cactuses would behave like any other cantilever, the architectural term for a structural element that is only fixed in place at one end.

"I was expecting a rather boring data set that adhered closely to analytical predictions," he said.

Instead, the vibrations he measured revealed surprising frequency fluctuations stemming from the structural properties of the saguaro and other factors.

"With 15 minutes of data, I can describe the change in flexural properties of the cactus from top to bottom and amongst populations," Moore said. "That's what I think is really cool. The data generated to understand the structural properties of the cactus come from a very short time period."

His findings were published on Oct. 21 in the American Journal of Botany.

Under their sway

In the past, he said, the only way to gauge the stiffness and flexibility of a saguaro was to cut off sections of it and physically bend them using weights to measure those properties. "Obviously, that's something that can only be done very rarely," Moore said. "If we wanted to do something more broadly and without injuring the cactus, then this is where I think these ambient vibration measurements can come in."

The study represents a homecoming of sorts for the researcher.

Moore grew up in Tucson, where he spent his childhood picking through the ribs of fallen saguaros and studying the giant cactuses while waiting for the school bus. "They've always been in my life and always been around in various ways," he said. "I've grown up around them."

Moore was part of the first graduating class at Catalina Foothills High School in 1996, then earned his bachelor's degree in geological engineering from the University of Arizona in 2000 before leaving to attend graduate school at the University of California, Berkeley. He is in his 13th year at the University of Utah, where he specializes in geohazard assessment.

Moore said he conducted his saguaro study with permission from Pima County in a part of Tucson Mountain Park not far from the Arizona-Sonora Desert Museum.

He said he chose an area away from any roads or hiking trails to limit outside vibrations and unwanted attention. He only collected data from single-stemmed saguaros because the presence of arms would have complicated the measurements.

Seismometer technology has advanced greatly in recent years, with small, high-quality instruments now readily available and affordable for professionals and amateur scientists alike. Moore said the device he used came from a Panama-based company called Raspberry Shake, which makes a seismometer light enough that it can be "attached on the outside of a saguaro stem without causing any damage."

He would mount it directly to the cactus using a nylon strap and a special 3D-printed bracket, then come back after 15 minutes to collect the instrument.

His test subjects ranged in size from about two feet to almost 25 feet tall. It didn't matter where on the stem the device was placed, so he would attach it where it was easiest to reach.

The specific rates at which the cactuses swayed -- also known as their natural resonance frequencies -- varied based on their relative height, stiffness, flexibility and other factors, such as time of day and water content.

"Overnight it sort of sways at a faster frequency, and as the day warms, the frequency drops," Moore said.

Some of the results were confounding at first, until he remembered his childhood lessons in saguaro anatomy. Shorter cactuses tend to have ribs that are more uniform in thickness, while the "bones" of taller specimens get thinner in diameter near the tips of the stems. "This manifested in the data," Moore said.

Growth potential

With additional study, he believes ambient vibration measurements could be used to assess the structural health of saguaros and study their tolerance to everything from extended drought to major wind events -- all without harming the cactus.

He also thinks the technique might provide a way to track how much water the plants take up after a rain event, though his own experiments in that regard proved inconclusive.

Tucson ecologist Bill Peachey has spent decades studying saguaros, including the roughly 130 of them he's been keeping tabs on near Colossal Cave since 1997. He said he's probably poked around in more cactus guts than just about any researcher on Earth, so he applauds Moore's effort to find new, noninvasive ways to study the plants.

"It's very admirable," Peachey said. "This guy really put a lot of thought into this. What it shows is you can make really sensitive measurements."

Even so, Peachey questions how useful such seismic data can really be, even when it comes to some of the things Moore has suggested.

But just because he can't think of a practical application for the measurements doesn't mean there isn't one. Simply trying something new like this can lead to more discoveries down the line, Peachey said. "If nothing else, this could give people the idea that maybe they should try something new, too."

Moore said he hopes to see other researchers build on his work.

"It's my hope that it sort of unlocks a new measurement for the botanist's toolbox," he said. "I'm hoping that other folks will run with it a little bit and see where it goes."

Contact reporter Henry Brean at [email protected]. On Twitter: @RefriedBrean

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