Towering structures must be able to bend and sway when subjected to the forces of wind and ground movement, or they will topple, whether it's a building, a geological formation, tree—or even a cactus.
Especially if that cactus is a lofty saguaro, or Carnegiea gigantea, those iconic denizens of the American Southwest.
Jeff Moore , a University of Utah geologist specializing in geohazard assessment, grew up in the heart of saguaro country in Arizona. In his most recent research, he repurposed his geophysical toolbox for studying rock formations to analyze the structural properties of saguaro and how they respond to vibrations in their environment.
This detour into botany applied vibration analysis Moore developed for southern Utah's natural arches, bridges and towers in completely new ways that could help scientists better understand large, water-storing plants without harming them.
"Saguaros have always been in my life," Moore said. "These cacti have really strong cultural value and that helps motivate a scientific study". Saguaro are keystone species of the Sonoran Desert . They grow to up to 70 feet tall and hundreds of years old. "There's an ingrained culture of respect for these great cacti."
His study published last week in the American Journal of Botany introduces a new, noninvasive way to measure how living saguaro respond to transient disturbances, such as wind and ground movements, without defacing these beautiful giants. He put the techniques to work on 11 cacti of varying heights in the Tucson Mountains outside his eponymous hometown.
By analyzing these "ambient vibrations," Moore was able to determine each cactus's natural resonance frequencies, or the specific rates at which they sway, and how their flexibility and stiffness change with height, time of day and water content.
Saguaro's range extends from central Arizona to the Mexican state of Sonora. They grow slowly , with branches appearing after around 60 to 75 years. Their pleated trunks are covered in a thick, pliable skin armored with 2-inch spines.
"Saguaros are unique in that their morphology allows them to expand, to take up great quantities of water when it's available during monsoons and withstand periods of drought," Moore said. In time-lapse video, their stems can be seen swelling as they draw water, which would presumably alter their resonance frequencies.
Moore was visiting family in Arizona when it dawned on him he should apply his vibration-measuring methods to characterize the mechanical properties of the towering cacti. Tucson Mountain Park gave him permission to conduct research at Pima County's 20,000-acre preserve adjacent to Saguaro National Park. He spent a day rigging a light-weight seismometer to 10 cacti, selecting a representative sample in terms of height. All were single-column saguaro, known as "spears."
"It was really important to be able to compare them, these so-called spears as they're called when they don't have arms. The smallest was about two feet tall, and the tallest was nearly 25 feet." The seismometer was gently hung from each stem using a strap at about chest height.
He recorded just 15 minutes of seismic data on each cactus, which showed their resonance frequencies ranged from 0.55 to 3.7 Hz, with damping ratios between 1 and 2%. (Hertz, or Hz, is a standard unit of frequency, defined as the number of times an object completes an oscillation each second. The damping ratio indicates how quickly oscillations taper off following a disturbance.)
The study found each saguaros' resonance and stiffness varied widely across the height of its column. Generally, they were more stiff near the bottom and more flexible at the top.
"Saguaros vibrate much like a cantilever, but with some interesting differences," Moore wrote on Bluesky. "Stiffness varies between cacti (taller stems are stiffer), and for a single stem (taller stems have softer tops)."
Rather than water content driving short-term changes in vibration, as Moore had expected, the study identified daily cycles in resonance frequency, likely due to softening of cactus tissue as the day heated up and hardening as it cooled.
The study documented several modes of vibration, painting a complicated picture that reflects each cacti's internal architecture, supported by wooden ribs that are stronger in the stem's lower reaches.
"These cacti are vibrating every second of every day. When the wind picks up, they vibrate stronger, when the wind dies down, they vibrate less, but they are constantly in motion," Moore said. "Each saguaro is unique and moving at a combination of all its natural modes. It's swaying, but the pattern is more complex the closer you look."
Previously, people would have to cut up a cactus to gain such insights into the plant's flexibility.
"They would cut segments out and then bend them with weights to measure the flexural stiffness"—a measurement of a structure's deformability, Moore said. "You destroy the cactus this way, and you can't necessarily do that to pieces of a giant cactus's lower stem. It's too big and too stiff. The new approach is non-destructive, requires a short amount of data, and it provides that same information."
In other words, his noninvasive vibration technique provides a new framework for understanding the biomechanics and ecological resilience of desert plants. It could help scientists predict how saguaros and other large plants withstand strong winds and monitor their structural health.
