Joshua trees may be migrating north in response to climate change

 

Last spring, Joshua trees put on a magnificent show in the Mojave Desert. Nearly all at once nearly all of them bloomed, sprouting dense bouquets of waxy, creamy-green flowers from their Seussian tufts of spiky leaves. The bloom was so sweeping and abundant -- and such a contrast to the typical pattern, where only a small number of trees bloom in any given year -- that it was called "a once-in-a-lifetime phenomenon."

This spring was far less flowery, and yet standing among the giant yuccas in late March, in the Tikaboo Valley north of Las Vegas, U.S. Geological Survey ecologist Todd Esque still had the sense he was witnessing something historic. This, he suspects, is the leading edge of the entire species -- "leading" because the trees appear to be marching in the same direction the climate that suits them is expected to shift, with an old, established population of Joshua trees flinging out new recruits in a distinctly northern pattern. That's exactly what scientists believe the shaggy beasts need to do -- and quickly -- to survive in a warming world.

The pattern is obvious to the naked eye here because it's occurring where the Mojave Desert gives way to the Great Basin, where stands of Joshua trees fade to sagebrush. "I get chills when I look at that population," says Esque.  "We know from the paleo record that plants and animals have moved north and south by hundreds of miles -- if not more -- in response to climate change. To see it in our lifetime, at a time when it really matters if they can move or not, it's neat."

The news about Joshua trees of late has mostly been gloomy, so much so that some people have begun to imagine a future in which Joshua Tree National Park is without Joshua trees. Fires carried by non-native grasses have been picking off the plants. There is evidence that in the hottest, driest spots it occupies, the trees are already plodding down the road to extinction by failing to reproduce. One study projected that 90 percent of their current habitat could be inhospitable by century's end.

And so Joshua trees face the modern mandate familiar to so many species: move or die. The same study projecting a 90 percent reduction in habitat also cast doubt on Joshua trees' ability to migrate far enough quickly enough to keep them on the map in significant numbers. It found evidence that the Shasta ground sloth was once one of the plant's major seed dispersers. The sloth, of course, is extinct, and the trees now mostly depend on smaller creatures -- squirrels and kangaroo rats -- to spread their seed. The sloths, large mammals that they were, are assumed to have dispersed the seeds over greater distances than the rodents now do, meaning Joshua trees might be able to make small steps to new territory, but not the great leaps that may be necessary.

But really, says Esque, we don't know how quickly Joshua trees are capable of moving, or even if they can move at all. It's possible the new trees in the Tikaboo Valley represent a "static front," he explains, "where they keep casting out young trees, but every 30 years there’s a drought that might kill them, so the population can never really move." Nor do we know for certain that sloths dispersed seed across great distances, because we don't know how widely the animals actually ranged. "There are a lot of questions, probably way more than answers," he says. Which is why it's so exciting that he and his colleague Chris Smith, an evolutionary biologist, may have discovered the trees' forward march. If they can confirm that it is the species' leading edge, they can begin to gain greater insight into its potential mobility, and with that its prospects for the future.

In March, Esque, Smith and a group of citizen scientists spent four days collecting data to do just that, by mapping the distribution of old and young Joshua trees in the Tikaboo Valley. As it happens, the Tikaboo is the only place scientists know of where the distinct eastern and western populations of Joshua trees meet and mingle. So they took tissue samples from the burgeoning population, too, in hopes of identifying whether either the eastern or western trees, or their hybrids, were winning the "race north."

"As you move northward (in the Tikaboo), the big Joshua trees thin out, they get shorter and shorter, younger and younger, then you get to a point where there aren’t any anymore," Esque explains. The youngest, he believes, are less than a decade old. "That’s the edge of Joshua trees as we know them. The potential is right there for the species' migration."

Cally Carswell is a contributing editor for High Country News, and is based in Santa Fe, New Mexico. She tweets @callycarswell.

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