Climate Change Threatens an Iconic Desert Tree

It’s not just the polar bear. Animals and plants in Earth’s other extreme environment—the desert—are endangered by rising temperatures.

Explorer: Bill Nye’s Global Meltdown boils down the facts on climate change, premiering Sunday, November 1, at 8/7c on the National Geographic Channel.

Close your eyes and imagine a species living in a harsh environment threatened by climate change. If you conjured up a polar bear, Cameron Barrows has a suggestion: Consider, instead, the Joshua tree—the gnarly icon of the Southwest’s Mojave Desert that looks like it sprang from a Dr. Seuss book.

“Animals living in the Arctic get a lot more attention than plants in arid lands, but desert plants like the Joshua tree are also threatened by a changing climate,” says Barrows, a research ecologist at the University of California, Riverside’s Center for Conservation Biology.

An environmental group recently petitioned the U.S. Fish and Wildlife Service to declare the Joshua tree, which can grow up to 40 feet tall and lives an average of 150 years, as “threatened” under the Endangered Species Act. So far, only one species, the polar bear, has been added to the national list based on the threats posed by climate.

Scientists say thousands of other species are at risk, too.  One study predicted under a scenario of changing climate that between 15 and 37 percent of Earth’s plants and animals will have populations so small by 2050 that extinction is virtually certain.

Other than the Arctic, deserts may have the most to lose as the planet warms because anything surviving there already lives on the edge.

Because plants and animals in ecosystems have complex relationships, it’s not just the Joshua tree at risk, says Kierán Suckling, executive director of the Center for Biological Diversity, based in Tucson, Arizona.

“The Desert Southwest and the Arctic are being ripped apart by climate change faster than anywhere else,” says Suckling, “because they are North America's most extreme ecosystems.”

Fewer Young Trees

Until the last ice age ended about 11,000 years ago, Joshua trees thrived throughout the U.S. Southwest and northern Mexico. As the Earth grew warmer and drier, their range contracted to California’s Joshua Tree National Park, where the Mojave and Sonoran deserts overlap, as well as small parts of Arizona, Nevada, and Utah.

Barrows, who recently completed the first year of a 20-year biological study of Joshua Tree National Park, found few or no young trees in roughly 30 percent of their range.

Based on climate models using a 3-degree Celsius (5.4-degree Fahrenheit) increase, the range of Yucca brevifolia could be reduced up to 90 percent by the end of this century, Barrows says. Under that scenario, it would exist only in isolated pockets, called refugia, scattered across the 800,000-acre national park.

Like most desert dwellers, Joshua trees have developed a suite of adaptations to survive in the desert. Because there is no groundwater to draw on, the succulent (not technically a tree) lacks the deep taproot of plants that grow in wetter climes. But when it rains in the desert, it pours. So Joshua trees have developed a shallow network of roots, each the size of a person’s little finger, that spreads five to six meters around each plant and sucks up rainwater like an industrial sump pump.

“The big guys just need a good rainstorm every other year,” Barrows says.

Seedlings, however, lack this extensive root network and depend on regular rain events to survive. With increasing frequency and duration of droughts, new Joshua trees are not replacing old ones. “In some areas we surveyed,” reports Barrows, “there are no baby Joshua trees at all.”

At one location in the Mojave Desert, Twentynine Palms, weather station records indicate that the mean temperature has increased 2 degrees Fahrenheit over 40 years. But the real change came in the nighttime lows, which are nearly 8 degrees Fahrenheit above average.

“What that means,” Barrows says, “is that even though precipitation is about the same, the evaporation rate is higher. Less water is available to the plants.”

Invasive Grasses and Nitrogen, Too

Joshua trees also face other threats, including the spread of red brome, a grass native to the Mediterranean. The invasive grass fuels intensely hot wildfires that can incinerate even the largest Joshua trees. Park records dating back to 1945 indicate that fires there are becoming larger, more frequent, and more destructive.

Helping drive the grass invasion is nitrogen, an air pollutant carried by winds from Southern California. It acts as a fertilizer of desert soil. “Fertilizing the ground doesn’t sound so bad if you’re a farmer,” Barrows says, “but if you’re a Joshua tree, that’s definitely not good news.”

Many other species, among the national park’s 700 plants, 40 mammals, and 40 reptiles, also are at risk from the changing climate. The piñon pine is a prime example. With a lifespan that can extend beyond 1,000 years, piñon are being squeezed out of the park even faster than Joshua trees. They are stressed by drought and attacked by beetles that had been held in check by lower winter temperatures.

And then there’s the fate of creatures that are inseparable from the Joshua tree – like the tiny yucca moth (Tegeticula spp.). In an extraordinary example of co-evolution, each species of yucca plant is pollinated by a single species of moth. Researchers don’t know if the moths are declining, but they say the possibility is troubling.

“If Joshua trees can hang out in refugia,” says Barrows, “the question becomes, ‘can moths survive there, too?’ If the answer is ‘no,’ they’ll both go extinct.”

Is the Joshua tree the “canary in the coal mine” of the arid Southwest? “The Joshua tree,” Barrows says, “is just one of a whole lot of canaries here.”

Researchers and activists agree that climate change poses a threat to Joshua trees, but they disagree on how best to protect them.

“We want to do everything we can to make sure these trees have the best chance at surviving,” says Taylor Jones, endangered species advocate for WildEarth Guardians, the New Mexico-based group that petitioned for the federal listing. “And that means using all available tools.”

But Barrows isn’t convinced that using the Endangered Species Act is the answer to what ails this symbol of the Mojave Desert.

“Climate change is an international issue,” he says. “The folks on the ground, the land managers at the park, don’t have the ability to fix that.”

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