Tracking Saguaros—75 Years of Change in the Cactus Forest

The cactus forest at Saguaro National Park is changing. In 1941, concerned park researchers established 6 plots containing saguaro (Carnegiea gigantea) in the Rincon Mountain District to track park populations of the iconic giant cactus over time. I can imagine the mounting concern that park staff shared in the 1940s as they witnessed an increasing number of large saguaro deaths and saw growing patches of bare ground in the cactus forest. A result of wood cutting, dense rodent populations, and overgrazing in what was then Saguaro National Monument. The enterprising park staff worked to exclude cattle and wood cutting from parkland and turned to science to learn more about how to care for saguaro.

Saguaro census volunteers document the height of cacti. Photo courtesy of Saguaro National Park.

Every year since 1942 (except 1955), the height of each saguaro within the plot was carefully noted along with deaths and germination of new cacti. After 75 years, the results of this remarkable study were published by Thomas Orum et al. (2016)—revealing a staggering decline in the oldest saguaro over the decades and a flourish of young saguaro emerging in the 1980s.

Saguaro are a long-lived species and studying their dynamics continually for decades is critical. We need to understand how their population responds to prolonged drought, deep freezes, changes in neighboring vegetation, and years of ample rain. What we know so far is that older saguaro are most vulnerable to extreme winter cold. And that young saguaro germinate best in the wettest years. Young saguaros have the highest survival when sheltered by native nurse plants such as paloverde and mesquite.

In 1990, Saguaro National Park greatly expanded the scope of this historic saguaro study by establishing the Saguaro Census. The census documents saguaro in a network of 45 plots across both districts of the park every 10 years. Powered by volunteers, this citizen science project gives more than 20,000 saguaro a health check up every decade, and it’s time to do it again!

Please join Sky Island Alliance in the field for the Saguaro Census this winter. Sign up to save your spot today: