The Iconic, Vanishing Joshua Tree: Species Collapse

Evolution, climate change and sprawl threaten to wipe out Joshua trees across North America
Staff Writer

Editor's Note: This is Part 1 of a three-part series about the precarious survival of the Southwest's iconic Joshua trees, which are threatened by climate change and urban sprawl. Click here for Part 2, and here for Part 3.

Chris Smith has a recurring nightmare. As he pulls up to his field site in the Nevada desert, something is amiss. Instead of a forest of twisted, charismatic Joshua trees, condominiums dot the landscape.  Smith, an evolutionary ecologist from Willamette University, is among a group of researchers who have devoted their careers to this fascinating plant. But these scientists are now seriously worried. A combination of climate change and urban sprawl is challenging the survival of this idiosyncratic species.

To preserve the Joshuas, people must make a decision about the kind of planet they want to experience, Smith said.

“Do we prefer a world in which there is a diversity of organisms, such as weird plants like Joshua trees?” he asked. “Or are we content to live in a world in which the hardiest human-adapted organisms like crabgrass and Canadian geese are the only things around us?”

To find Joshua trees, you must travel to the Southwestern deserts of the United States. A popular place to see them is in Joshua Tree National Park. This protected area, located about 120 miles east of Los Angeles in the Southern California desert, attracted one million visitors in 2011.

Within the park, Joshua trees dot the desert to infinity. Their green, spiky heads bring a splash of life to the scene. One popular legend describes how early Mormon pioneers decided the plant resembled the prophet Joshua reaching up to the sky in prayer. They gave this yucca its common name in the mid-19th century and it stuck.  

The “tree” in the Joshua’s title is actually a misnomer. As yucca plants, Joshuas are related to agaves, orchids, grasses and palm trees, Smith said.  They hold water in their spongy interior, like cacti, and the resulting lack of tree rings makes it impossible to know the exact age of each plant. People often call the Joshuas “Dr. Seuss trees” because of their funny, twisted shape. On average, full grown plants stand between 20 and 40 feet tall, towering over other desert vegetation.

As the Joshua trees grow, their long, narrow leaves die back and hang on as dry remnants. At the tips of the branches live bulbous clusters of stiff living leaves, resembling spiky pompoms. In the spring, thick bunches of creamy white flowers the size of ostrich eggs appear, preceding the plant’s small, greenish-brown fruit.

For the animals in the desert, each part of the plant serves a valuable purpose, said Joe Zarki, public information officer for Joshua Tree National Park. Red-tailed hawks and American kestrels sit on Joshua trees’ high branches, looking out over the desert for prey, while Woodpeckers nest in hollowed-out holes in the plant. The handsome loggerhead shrike, the only predatory songbird, impales its victims on Joshua spines to attract mates.  And, when the desert endures extended droughts, animals such as jackrabbits and ground squirrels chew through the plant’s hard outer layers to get moisture from the spongy interior. 

“We wondered if it represented a last canteen in the desert,” Zarki said.

The plants grow in different formations, including loose circles that hint at one of their many adaptations to harsh desert life: they can clone themselves.

If a plant is damaged, Zarki explained, it creates stolons—offshoots of the root of the parent plant, which can grow into a healthy adult Joshua tree. For plants that can endure freezes, scalding temperatures, droughts and wildfires in the same year, this backup plan is crucial.

But the Joshuas can’t rely solely on cloning. They need pollination to maintain healthy populations. And when it comes to the way they are pollinated, these desert kings are picky.

“All yuccas are exclusively pollinated by different species of yucca moths,” said Olle Pellmyr, professor of biology at the University of Idaho. “They are quite unusual in that regard.”

Both subspecies of Joshua tree depend solely on one type of moth in a mutually beneficial relationship so unique that Darwin called the yucca-yucca moth interaction "the most remarkable fertilization system ever described." The two are now perfectly suited to each other, like lead dancers in their last performance for The Royal Ballet.

Click here for Part 2.


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