Saguaro Cacti Thrive When the World's Massive Volcanoes Erupt, Study Finds

The majestic saguaro cactus only grows in the Sonoran Desert. John Hunnicut II/Flickr

Look at a world map and remind yourself that all those lines demarcating national borders are completely imaginary, human constructs. Our planet and its various ecosystems don't recognize lines on a map, and what happens in one part of the globe can lead to surprising results in another.

That's precisely the case with a new finding that volcanic eruptions on the other side of the planet can measurably affect the Sonoran Desert's iconic saguaro cactus. According to new research, massive volcanic eruptions affect the global climate enough to influence plant growth patterns on the other side of the world.

Biogeographer Taly Drezner studied 30 different saguaro (Carnegiea gigantea) populations across 19,300 square miles (50,000 square kilometers) of desert, and will present her research at the Ecological Society of America annual meeting on Aug. 9 in Fort Lauderdale, Florida.

"I started noticing that these saguaro age cohorts followed notable volcanic eruptions," says Drezner in a press release. "I knew that volcanoes drive milder summers and winters, and typically more rainfall for an extended period--two to three years after the event, which is a perfect window of time for the saguaro to get established and have a chance to survive."

After estimating the ages of the cacti, she compared the data to global volcanic dust records. The Sonoran is the hottest desert in North America and spans two Southwestern U.S. and three northwestern Mexican states. Temperatures regularly exceed 104 degrees F (40 degrees C) for weeks straight in the summer months, which is when saguaro seedlings germinate. Volcanic eruptions helped tame some of the summer heat.

Take the gigantic Krakatoa eruption of 1883, for instance. That cataclysmic event threw so much ash, dust and sulfur dioxide into the atmosphere that the world experienced a haze that reflected sunshine and dropped global temperatures. A bevy of other significant eruptions around the same time — Mt. Pelée, Soufriere, Santa Maria (1902), Ksudach (1907) and Katmai (1912), for instance — contributed to environmental conditions that helped saguaros thrive in higher numbers than usual. In what could be called a saguaro baby boom, the cacti that started their lives during that period represent a disproportionately large percentage of all saguaros living today.

The volcano Anak Krakatau (Child of Krakatoa) has grown in place of the much larger Krakatoa, which cataclysmically erupted in 1883.
DeAgostini/Getty Images

"The saguaro is key to the survival of many species," said Drezner. "Almost every animal in the Sonoran uses them in some way, as a nest site, or food, or a cool refuge."

Though once they start growing the long-lived saguaro can live up to 150 years, it can be incredibly difficult for young saguaro seedlings to make it past their first summer — there's a nearly 100 percent mortality rate for the cactus in the first few years of its life, when it measures only a few millimeters. All the young saguaro that crop up in a given year will die unless a summer is just cool enough and a winter just wet enough.

"It is likely that other species in other ecosystems are also affected by global-scale, large geological events, but have not yet been identified," the paper says.