Scientists “Listen” to Saguaros — Seismology Reveals Cactus Health Without Cutting

Listening to the Desert Giants

Arizona’s iconic saguaro cacti, which appear as immobile, spiny sentinels across the Sonoran Desert, are in fact constantly vibrating. A University of Utah geologist has adapted seismic instruments—tools normally used to measure rock vibrations and monitor earthquakes—to record those natural frequencies and probe a cactus’s internal structure without harming it.

Non-invasive method and fieldwork

Published last month in the American Journal of Botany, the study repurposes ambient vibration seismology to study plant biomechanics. After securing permission from Pima County, the researcher carried out fieldwork in Tucson Mountain Park, a 20,000-acre preserve adjacent to Saguaro National Park. He located single-column "spear" saguaros and strapped lightweight seismometers to each plant.

Data collection was brief: each cactus was recorded for roughly 15 minutes. The team sampled 11 saguaros of varying heights to capture a range of natural vibration behaviors.

What the vibrations reveal

Although saguaros look rigid, they respond to wind, environmental forces and seismic energy with measurable motion. The study found that each cactus’ resonance frequencies shifted between day and night, suggesting that the outer cortex and skin become alternately more and less rigid on a daily cycle. Researchers had expected short-term changes to be driven primarily by water content; the results indicate the picture is more complex.

“One of my main tools is using ambient vibration seismology to study the properties and monitor the properties of rock structures, like arches and towers,” Moore said. “I realized that all my same measurement techniques and analysis techniques could be applied in this area of biomechanics and for the same reasons.”

Why this matters

The saguaro is a keystone species in the Sonoran Desert, providing shelter and food for birds, bats, insects and other wildlife. Unlike trees, mature saguaros are supported internally by a wood-like rib framework beneath their water-rich flesh and long spines. Because saguaros are protected by Arizona law, invasive study methods that slice into plants are undesirable and often illegal without permits.

Non-invasive vibration monitoring offers a way to assess structural health and stiffness without damaging these long-lived plants. That capability is increasingly important: saguaros have declined in recent years due to rising temperatures, prolonged drought, wildfires and pressures from invasive species. A 2025 study at the Desert Botanical Garden in Phoenix found saguaro mortality rose from roughly 2% (2015–2020) to about 7% in 2023 before falling to just over 3% in 2024. Nonnative burros have also been identified as a threat because they eat the vegetation that shades young cacti.

Next steps

Moore hopes other researchers will adopt this seismology-based technique as a monitoring tool to track changes in cactus structure and health over time. Because the equipment is sensitive, portable and non-destructive, it could help land managers and scientists detect early signs of structural decline and better protect these culturally and ecologically valued desert sentinels.

Reporting note: This research was reported by John Leos for The Arizona Republic and appears on azcentral; environmental coverage received support from a Nina Mason Pulliam Charitable Trust grant.