Climate change threatens to wipe out one of the parks' iconic plants.
By Seth Shteir
"Repulsive" isn’t the kind of word you associate with a national park. But that’s exactly how explorer John Fremont described the Joshua tree when he traversed the Mojave Desert in the mid-1800s. Those who followed in his footsteps, however, found inspiration in the landscape: Mormon settlers likened the tree’s bizarre, outstretched limbs to a prophet named Joshua pointing to the Promised Land and called it “the praying plant.” And one has to wonder if Dr. Seuss, author of The Lorax, didn’t fashion his Truffula trees after the quirky plants, the largest of all American yucca species.
Ken Cole, a scientist with the U.S. Geological Survey, has been drawn to the Joshua tree for decades, but it’s the species’ future that concerns him. Cole’s research predicts that by 2100, climate change will reduce the Joshua tree’s range by 90 percent. The hotter, drier temperatures associated with a shifting climate will suck up precious water and cause thirsty wildlife to eat younger plants—and if the next generation of trees can’t establish itself, Yucca brevifolia brevifolia will eventually disappear from its southern range in Joshua Tree National Park.
Plants generally deal with long-term changes in climate by shifting south as temperatures drop or north as temperatures climb. To do this, they depend on animals, wind, water, and even humans to disperse their seeds. Although some scientists believe that Joshua tree seeds are still being transported by rodents, wind, and water, Cole thinks that the extinct Shasta ground sloth once acted as its Johnny Appleseed.
Roughly the size of a Volkswagen Beetle, the prehistoric Shasta ground sloth ate the leaves, seeds, and fruits of the plant, traveled great distances, and, along the way, passed the seeds through its digestive system, fertilizing them with its manure. But when the sloth went extinct about 12,500 years ago, so did the Joshua tree’s ability to hitchhike to suitable habitat.
That inability to spread its seeds long distances in response to a changing climate certainly presents a problem, but the park’s frequent wildfires pose a more immediate danger. Catastrophic fires are a relatively new phenomenon, caused by nitrogen pollution blown eastward from the Los Angeles area. The nitrogen seeps into the rocky desert soil and fertilizes invasive grasses, causing them to fill in the open spaces between the trees. When summer lightning strikes, the grasses fuel huge fires that can engulf whole Joshua tree forests. Some species, like lodgepole pines, thrive with periodic fires, but Joshua trees aren’t well adapted to this new threat.
And that spells trouble for the dozens of species that rely on them. Joshua trees are like living hotels in the middle of the desert, providing food, shelter, and water for wildlife. Rodents like the antelope ground squirrel, black-tailed jackrabbit, and wood rat chew through the bark to suck moisture out of the trees during dry spells. Red-tailed hawks perch on spiky branches to scan the rocky landscape for prey. Ladder-backed woodpeckers nest in the cavities of dead trees and drill holes in the limbs to search for juicy insects. Colorful yellow and black Scott’s orioles hang their pendulous nests on the underside of the lance-shaped leaves.
Without Joshua trees, many of these animals would perish, along with the species that prey on them—such as coyotes, hawks, and owls. “As Joshua tree forests disappear,” says Joe Zarki, the park’s chief of interpretation, “you begin to worry about the quality of the habitat for all the creatures that fit in that niche.”
Complicating all that is the fact that some scientists now believe that Yucca brevifolia jaegariana and Yucca brevifolia brevifolia are two distinct species of Joshua tree—not two subspecies. The former is a smaller, more compact tree found in the northern Mojave; the latter, with its longer trunk and broader canopy, grows in the southern parts of the Mojave Desert, including Joshua Tree National Park. If Yucca brevifolia brevifolia is indeed a separate species, the park itself represents a large portion of its range. Its disappearance would not only be a blow to the park ecosystem but could spell disaster for the overall health of the species. And if there are two distinct species of Joshua trees, it means the Park Service and other land-management agencies aren’t dealing with one species with a moderate threat but two species even closer to the brink.
Scientists, land managers, and environmentalists don’t always agree on how to save the Joshua tree. Some suggest “assisted migration,” a process where humans relocate the plants to more suitable habitats. Critics argue that Joshua trees are difficult to grow under nursery conditions let alone to propagate as forests on an open landscape.
This may be just the first of many dilemmas facing the Park Service, as its land managers try to address the impacts of climate change on countless species. “National parks are one place where we can actually measure the effects of climate change,” Zarki says. “They are also symbols of our best aspirations of the natural world. If we can successfully grapple with climate change in the national parks, we can in other areas as well.”