The desert is getting too hot for Joshua trees. Can we just … move them?
To protect the southwest’s iconic Joshua trees, scientists are tinkering with their genetics and searching for climate havens in—and sometimes outside of—their existing habitat.
Six years ago, Jeremiah Joseph was driving through a thick stand of Joshua trees during the early morning hours. The arms of the trees stretched out of the darkness and into the light cast by his car’s headlights, and for a split second, he thought they were people trying to flag him down.
Joseph, a member of the Lone Pine Paiute-Shoshone tribe in California, had spent his whole life surrounded by Joshua trees, but he says that morning was the first time he took a serious interest in them. That same year, the tribe had received 38 Joshua trees that were dug up to make room for a highway expansion project. Joseph was part of the group tasked with replanting them, and he began to learn more about the dire fate facing this species: Without human intervention, both the eastern and western subspecies of Joshua trees face a perilous future.
Joshua trees—the symbol of the Mojave Desert, of a specific kind of California dream—are projected to lose as much as 80 percent of their habitat by the end of this century, according to a study published last summer.
This dire prediction has left scientists, lawmakers, tribes, and communities scrambling to figure out how this iconic species will survive. Also last summer, California released a conservation plan to protect western Joshua trees by preserving land, restricting tree removal, and reducing wildfire risk. The conservation plan is unique; instead of conserving a species currently showing signs of decline, it protects future trees from future threats.
Conservationists working with Joshua trees see two options for a viable future habitat. The first is in climate “refugia,” patches of land within or near Joshua trees’ natural range where they would be relatively shielded from climate change. The second is more dramatic and more complicated: inching them toward more hospitable environments outside of their natural range, a strategy known as assisted migration.
On the Lone Pine Paiute-Shoshone reservation, where tribal leaders are working with the state of California on conservation plans, moving trees outside of their current habitat has already shown promise. The 38 trees uprooted by highway construction were replanted on the reservation 25 miles north, outside of the trees’ typical range. Six years later, they are still successfully growing. Even mature trees that weren’t expected to make it have put up offshoots in their new homes.
“We know, just by the science and just by how rapidly things have been changing, especially for the environment around the Joshua tree, that it's vital that we kind of get it right and that we make the right decisions moving forward, especially for the ones that are barely hanging on,” Joseph says.
Why Joshua trees face an uncertain future
Joshua trees are ancient; they began growing in the southwest around 12,000 years ago, when Ice Age glaciers receded. And while their average lifespan spans about 150 years, some are estimated to have been alive for 500 years or more.
Despite their names, Joshua trees are a type of succulent, a member of the agave family. And saving them means saving entire desert ecosystems. They are essential habitats for dozens of birds like the western screech owl and Scott’s orioles, as well as native reptiles and insects.
Currently, they are so ubiquitous it might be hard to believe they are at risk.
A 2022 review estimated there could be anywhere from 4.3 million and 9.8 million western Joshua trees in California. This relative abundance led to grievances from homeowners and developers who, as a result of California’s conservation plan, must pay fees ranging from around over a hundred to several thousand dollars to remove any Joshua trees from their land.
But it’s the climate of the future, rather than current conditions, that has scientists and officials worried. Climate change will supercharge the desert’s already intensely hot and dry climate, says Jeremy Yoder, a biologist at California State University, Northridge.
“The Mojave includes some of the hottest, driest conditions in the continental U.S., and all of the species that live there already are adapted to those conditions,” Yoder says. “Of course, if you push them to even more extreme conditions, they're going to be in trouble.”
In recent years, wildfires fueled by invasive grasses have added to those extreme conditions, burning millions of trees in the last five years alone. Add in the Mojave’s rapid urbanization and energy development, and their viable habitat shrinks even more.
One of the challenges to maintaining a healthy population of Joshua trees is ensuring enough juvenile trees replace their dying elders. For Joshua trees, the conditions are specific: trees only flower under certain conditions, usually every few years, and the two sub species of Joshua tree can each only be pollinated by one species of moth.
It’s these niche adaptations that make them particularly vulnerable to changing conditions. Research has shown they grow slower and reproduce less often than other plants, and there is evidence that hotter and drier conditions reduce the number of juveniles.
More resilient trees growing in more resilient habitat
Scientists and conservationists say Joshua trees’ best chance of survival lies in their climate refugia. These areas typically sit at higher elevations, though certain weather patterns and water availability are also factors for certain trees. Scientists like Yoder who study these arid landscapes have been working to identify refugia for years. Pinpointing these habitats requires a complex combination of scientific models, on-the-ground observations, and, more recently, satellite data. Last summer’s study modeling their habitat loss predicted promising news: more areas might qualify as a refuge than previously thought.
However, these regions aren’t necessarily on protected land, which might leave future habitat vulnerable to development.
And wildfires will still be a risk even on conserved land. In 2020, over 40,000 acres in one potential refugia in the Mojave National Preserve in California were burned by a devastating wildfire that killed 1.3 million Joshua trees.
To make sure trees planted in these climate refuges have the best chance at survival, scientists are also looking for trees with the hardiest and most adaptable genes.
While sequencing these genes is still underway, scientists are already learning more about how these plants survive in such harsh conditions. Last September, for instance, a paper led by Karolina Heyduk, assistant professor at the University of Connecticut, showed that Joshua trees have a unique form of photosynthesis in which they take up carbon dioxide at night to reduce water loss.
“I'm hopeful it means that they, especially the older plants, are able to handle some of this heat and drought,” Heyduk said. “What it means for future generations is still a little less clear.”
Both Heyduk and Yoder are also part of an effort to sequence the genome of Joshua trees so that they can identify the specific genes associated with surviving the harshest desert conditions. That genetic map would allow them to select the hardiest seeds for storage in seed banks, and areas burned by fire could eventually be planted with these reserves.
Finding a 22nd century future home
Given Joshua trees’ vulnerability and the slow and costly pace of studying their genetics, some conservationists involved in California’s conservation plan are also wondering if they could help Joshua trees migrate to a less extreme habitat at more northern latitudes or higher elevations.
The study modeling Joshua trees’ future refugia found that some new habitats for western Joshua trees may emerge north of their current range, and new habitats for eastern Joshua trees may emerge at higher elevations in Arizona’s Hualapai Mountains.
Helping plants migrate to new ranges is a strategy scientists are studying for other species, such as Douglas firs in Oregon and deciduous trees in Minnesota’s Northwoods. For Joshua trees, that migration would likely happen at a shorter distance, helping move the trees to spots where they would migrate naturally a little sooner than they would do on their own, says Drew Kaiser, a senior environmental scientist at the California Department of Fish and Wildlife. But Kaiser also says there are also early discussions of long-distance assisted migration experiments managed by tribes, land managers, and researchers.
Assisted migration is controversial in the science community because it is both complex and uncertain. To sustain itself, the ecosystem in which Joshua trees exist—including the yucca moths that pollinate them—would also need to migrate. It would also be expensive, and those resources might be better used to breed more resilient trees and reduce wildfire risk where the trees already grow.
Migrating individual genes might be a more viable option; if Yoder, Heyduk, and the other scientists involved in the genome project can pinpoint which genes help trees best weather drought and extreme heat, they might be able to migrate those genes into more vulnerable populations of Joshua trees.
“There's a lot of people doing their best to take advantage of the fact that we do have time to act,” Yoder says. “We can act now to prevent having to do more drastic things later.”
In Lone Pine, the tribe has continued to transplant trees removed for infrastructure projects and will soon begin growing juveniles from seed, too.
In addition to serving as an essential part of the ecosystem, Joshua trees here are culturally significant. A Joshua tree’s flowers, seeds, leaves, and roots can be eaten, and the leaves and roots are used to make cordage and dyes for weaving baskets, nets, sandals, and mats.
“As we've worked with the tree, we've noticed it's actually helping us preserve our history and our identity,” Joseph said. “In our efforts to protect our cultural resources, the Joshua Tree and our history have developed a symbiotic relationship."
