“One must summer and winter with the land and wait its occasions,” Mary Austin wrote in her 1903 desert classic, “The Land of Little Rain.” It’s true – the desert can cast its spell in an instant, but understanding its life is the work of decades.
Scientists have come to appreciate this long-term perspective. Take a familiar desert life form – the ocotillo. Researchers have studied the iconic plant extensively. Yet they didn’t know the answer to a basic question: how long do ocotillos live? Biologist Peter Scott sought the answer in Big Bend National Park.
“I wasn't thinking much about a long-term study of survival,” Scott said, “but I had other reasons to want to have them individually marked. But then as time went by, I started thinking I really should go back and see how many ocotillos were still alive.”
Now an Indiana State University emeritus professor, Scott began studying Big Bend ocotillos in 1986 as a PhD student. Pollination was his focus. But he needed to mark his study plants, and a colleague recommended aluminum tags affixed with plastic ties and wire. When Scott returned in 2018, he found that advice had been sound.
“And sure enough, they barely lasted 30-some years,” he said. “Often there was just one strand left, but it was attached to the base of the plant, and the inscribed number in the aluminum was still barely legible.”
Scott didn’t have GPS during his initial work, and locating the plants took some doing. But he was able to find 95 of the 140 ocotillos he’d marked, at eight sites in the park.
Biologists had assumed ocotillos were “long-lived” – but what that meant was imprecise. Some had estimated an average lifespan of less than 60 years, while others projected a maximum age of 200. A desert tree, like, say, a mesquite, can be cored, to count its annual rings. But the rings of an ocotillo’s trunk can only be accessed by uprooting and killing the plant. In New Mexico, a scientist had counted the rings on dead ocotillos. One was 107 years old, showing that ocotillos can live at least a century.
64% of Scott’s ocotillos had survived the 33-year span. The plants had all been mature when he’d marked them. It likely takes 20 to 25 years for an ocotillo to reach maturity and begin flowering. With statistical analysis, Scott concluded that, on average, a mature ocotillo lives another 51 years. Ocotillos, then, are granted the biblical three score and 10 – an average lifespan of 70 to 75 years. Some live quite a bit longer.
Most of the ocotillos were producing more flowers than they’d been three decades earlier. These plants are producing 5 to 10,000 seeds a year, Scott said, or up to half a million during a lifetime. It’s a stunning figure, but it’s adaptive. Pocket mice and other rodents feast on the seeds as they fall. And ocotillo saplings are vulnerable to jackrabbits until they become tough and spiny, a decade into their lives.
And even for the seeds to germinate and take root requires a “jackpot” of rainy conditions, Scott said.
“I'm sure it needs more rain than an adult would, proportionately for its size,” he said. “And we don't know yet what the recipe for that is with ocotillos, but it probably calls for a few years of better than average rain, even though it's a desert plant adapted to survive dry periods.”
As Scott’s study shows, patience and perseverance are the watchwords for desert plants, and for desert science.
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