A wildlife protection group has asked the U.S. Fish and Wildlife Service to list Joshua trees as a Threatened species under the U.S. Endangered Species act. The group Wild Earth Guardians says that climate change is imperiling the survival of the iconic desert trees, which require cold winters to reproduce and thrive.
Under Federal law, USFWS must respond to the group's petition by either agreeing to study the issue further or declaring that the trees don't require protection. That response is called a "90-day finding," though the agency has been known to take a lot longer than 90 days to issue them.
In addition to climate change making desert winters too warm for the trees to reproduce easily, increased wildfires also pose a threat to the Joshua tree, as does summer drought. "It's already difficult to be a Joshua tree, since many factors need to come together in just the right way for them to reproduce," said Taylor Jones of Wild Earth Guardians. "We need to address climate change now if we want to see Joshua trees in the future."
That Joshua trees still exist at all is a testament to their tenacity. The droughty asparagus relatives, known to science as Yucca brevifolia, range throughout the Mojave Desert and neighboring lands, from Southern California's Antelope Valley to the western end of the Grand Canyon. (Some scientists suggest that eastern Joshua Trees ought to be considered a distinct species, Yucca jaegeriana.) In years past, conjecture many scientists, the trees were aided in getting their seeds from place to place by the Shasta ground sloth, a now-extinct cow-sized animal that would swallow the trees' golfball-sized fruit whole and deposit the seeds some miles away.
The ground sloth has been extinct for more than 10,000 years, and no other animal has stepped up to take over its suggested role as Joshua tree seed disperser. That means the trees have a handicap in moving their seeds to places with more suitable climates. Modern-day Joshua trees mainly have their seeds dispersed by local rodents such as desert woodrats, who don't range over wide areas of the countryside.
Since the extinction of the ground sloth, in fact, Joshua trees tend to "migrate" through seed dispersal at rates of around seven feet per year. Habitat that's expected to be suitable for Joshua trees in the year 2100 under current models of climate change is well north of much of the trees' current range. The current northern limit of wild Joshua trees is south of Tonopah, Nevada; likely end-of-century habitat for the trees near Fallon, Nevada is about 120 miles north. At seven feet per year, 750 years per mile, it would take Joshua trees in excess of 75,000 years to make that trip.
And that all assumes that the trees' seeds would find places suitable for germination and survival to an age sufficient to start producing seeds, generally around 50 years according to current thinking. For best germination and survival, Joshua tree seeds need cold, wet winters and cool summers for several tears in a row. They also prefer to germinate in the cover of native desert shrubs, especially blackbrush (Coleogyne ramossissima), which provides shelter from both sun and drying wind as well as foraging herbivores. Coleogyne is another relict of cooler, wetter times in the Mojave, and it isn't doing all that great under current climate conditions either.
Fire is another major threat to the trees; they can recover reasonably quickly from a single low-intensity fire, but repeated fires fueled by invasive grass and herb species are often too much for the trees to handle. What's more, even a single fire can deprive future trees of their blackbrush germination cover: Coleogyne can take millennia to come back after a fire.
Summer drought poses another hazard: while the trees are quite capable of surviving a dry summer, animals such as rabbits and rodents often turn to the trees as sources of summer moisture in dry years by chewing through their bark to reach green tissue beneath. That can girdle and kill the trees, as noted by USGS researchers in Joshua Tree National Park during the last decade.
The climate change threat to Joshua trees is dire enough that some scientists have predicted the trees will be extinct in their eponymous National Park by the end of this century. Other scientists, including Cameron Barrows of the University of California at Riverside, suggest that higher elevations of the park may still hold populations of Joshua trees well into the next century, as long as fires don't burn them out.
This isn't the first Endangered Species Act petition to focus on climate change as the leading threat to a species. In 2008, USFWS agreed to list the polar bear as Threatened due to widespread and mounting melting of its sea ice hunting habitat. In 2010, USFWS declined to list the American pika under ESA; the pika, a rabbit relative that lives in alpine habitat at and above treeline, is likely to see most of its habitat become too warm in the next century or two.
Like the pika and the polar bear, Joshua trees are likely to see as much of 90 percent of their habitat become inhospitable to the species by 2100, prompting Wild Earth Guardians' petition.
"Joshua trees are an irreplaceable part of the Southwest and we must protect them," said Taylor Jones. "Because Joshua trees grow so slowly, they cannot quickly adjust to our changing climate and will need safeguards to ensure they are here for future generations."