Giant cactus plants ‘loosen up’ to bend with the desert wind

The saguaro cactus is engineered for endurance in one of North America’s toughest environments.

Rising up to 70 feet tall, with arms reaching skyward like desert sculptures, the cactus endures searing heat, fierce monsoon winds, and shifting sands.

But how does the saguaro manage to stay upright? A new study reveals that this giant cactus constantly flexes with subtle vibrations invisible to the naked eye.

The silent sway of desert giants

Every second of every day, these massive cacti are moving. When the wind kicks up, they sway more. When it’s calm, they sway less. They don’t creak like trees or wobble like jelly, but they’re always in motion.

A new study found that each saguaro moves in its own unique pattern, depending on its height, stiffness, and internal structure.

But here’s the catch – until recently, the only way scientists could figure out how a cactus moves was to cut it apart and test its pieces with weights.

Obviously, this meant destroying the cactus, which isn’t exactly ideal when the plant you’re studying takes 60 to 75 years just to grow its first arm.

Studying the desert cactus

That’s where Jeff Moore, a University of Utah geologist, stepped in with a fresh idea. He had been using vibration analysis to study rock formations like natural arches and towers in Utah.

On a family trip back to Arizona, he realized that the same technique might work on the giant cacti he grew up around – without cutting them down.

Moore reached out to Tucson Mountain Park for permission to conduct a small experiment. He selected 10 single-column saguaros – called “spears” because they don’t yet have arms.

The plants ranged in size from just 2 feet tall to nearly 25 feet. Using a lightweight seismometer strapped gently around each cactus at chest height, he recorded just 15 minutes of vibration data for each plant.

What the cactus vibrations revealed

That short recording revealed a lot. Each cactus had its own set of natural resonance frequencies – the speeds at which it tends to sway.

The frequencies ranged from 0.55 to 3.7 Hz. He also measured damping ratios – how quickly the cactus stopped moving after a disturbance – and found them between one and two percent. The cactus was stiffest at its base and more flexible at the top.

“Saguaros vibrate much like a cantilever, but with some interesting differences,” noted Moore. “Stiffness varies between cacti (taller stems are stiffer), and for a single stem (taller stems have softer tops).”

Moore originally thought the cactus’s water content – which changes during monsoon seasons – would have the biggest impact on its flexibility. But the data told a different story.

What actually caused short-term changes in vibration was the heat of the day. As temperatures rose, cactus tissue softened slightly, making them more flexible. As the desert cooled down, they stiffened again.

Saguaros are built to bend

Saguaros grow slowly and live for centuries. Their tall, pleated trunks can expand to store huge amounts of water.

These pleats are covered in tough, stretchy skin and protected by two-inch-long spines. Inside, they’re supported by wooden ribs, which are much stronger near the bottom and more flexible toward the top.

The new research shows how all these features come together to let a saguaro bend and sway without breaking. It also offers scientists a noninvasive way to study large desert plants in the wild, without harming them or interrupting their natural growth.

“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.”

A keystone species in the desert

Saguaros aren’t just interesting plants. They’re a keystone species in the Sonoran Desert, providing food and shelter for birds, insects, and other animals.

Their presence is a sign of a healthy desert ecosystem. Saguaros also hold cultural value for many people living in the region.

“Saguaros have always been in my life,” Moore said. “These cacti have really strong cultural value and that helps motivate a scientific study. There’s an ingrained culture of respect for these great cacti.”

Moore’s new method offers a way to learn more about these plants while protecting them.

The technique could come in handy as scientists look for better ways to track plant health, understand how different species handle climate stress, and even design buildings that learn from nature’s designs.

The study shows how even the quietest, slowest-growing organisms can reveal something new – as long as we’re listening.

The full study was published in the journal American Journal of Botany.

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