Joshua Trees, Giant Sloths, and Climate Change

© Jimfeliciano, Stock Free Images

I’m a big fan of the Joshua tree.  Granted, it’s not much for climbing.  You wouldn't fasten a tire swing to it, or lean against its trunk to read a romance novel and sip sweet tea.  But just seeing a Joshua tree takes you somewhere... a place far off and long ago, when giant reptiles ruled the earth and our ancestors were just sniveling, naked fuzz balls hiding in dark holes.

I've enjoyed some wonderful trips to Joshua Tree National Park with my wife and kids, and every time we drive into the park I get this feeling that I’m entering some other world—some landscape from the distant past, maybe even the rocky surface of a far-off planet.  It’s a cool place, and the trees bring it to life.

But get this—there could come a time, within the next 60 to 90 years, when Joshua trees no longer grow in Joshua Tree National Park.  Already, the climate in this part of the Mojave Desert is becoming too hot and dry for the trees to flourish, and there is growing consensus that the Desert Southwest will experience some of the greatest future climate shifts, with temperatures increasing 6 to 7 degrees Fahrenheit this century. 

Research suggests that unless humans substantially reduce carbon emissions, Joshua trees could disappear from as much as 90 percent of their current range within my children’s lifetimes.  Already, drought conditions have caused the trees to stop reproducing within the park, and the area is only expected to get hotter and drier.  Someday, when my sons bring their own children to the park, the trees could be gone.

And here’s a kicker.  The trees may not be able to spread into areas with more suitable conditions as the climate shifts—because the animal that historically dispersed its seeds was hunted into extinction by humans some 13,000 years ago.  The Shasta ground sloth ate the seed pods of Joshua trees and dispersed them across large geographic areas.  Now, only a handful of small animals eat the seeds, and they do not travel long distances. 

In fact, during the Pleistocene, Joshua trees had a much larger range throughout the Mojave, Colorado and Sonoran Deserts.  But about 11,500 years ago, Earth’s temperatures rapidly increased as the Younger Dryas Period ended.  Of course, by this time humans had already stalked the Shasta ground sloth into oblivion, so Joshua trees were unable to disperse.  The trees died throughout their historic southern range, managing to hang on in pockets of what had been their northernmost limits.   

© Justinmetz, Stock Free Images

Now, thanks to human emissions of carbon and other greenhouse gasses, we are entering another period of rapid climate change—far more rapid than anything the planet has experience in at least half a billion years.  The Joshua tree, already reduced to a fraction of its potential range, will be further decimated.  Unless people take an active role in collecting the seeds and planting them in more suitable locations as the climate warms, the Joshua tree will only survive in small refuges at the northern edges of its current range—many miles from Joshua Tree National Park. 

J. S. Kapchinske is the author of Coyote Summer.