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October 15, 2019

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Study predicts sharp decline in Joshua trees, a key Mojave Desert species

Joshua Trees

A Joshua tree sits in the foreground of this image taken of Lovell Canyon Tuesday, July 2, 2019.

Joshua Trees

A view of Joshua Trees near Calvin's Rock in Gold Butte National Monument Thursday, Feb. 23, 2017. Launch slideshow »

The iconic trees that give Joshua Tree National Park its name could be virtually extinct in the park by 2070 if no action is taken to reduce global greenhouse gas emissions, a new study from UC Riverside has found.

The study focused on western Joshua trees within the national park in California’s San Bernardino and Riverside Counties, about 180 miles south of Las Vegas. Tree counts have been declining in the park since the mid-20th century, particularly in the warmest, lowest-elevation areas, the study notes.

Anticipated increases in temperature due to climate change will only worsen the trend, according to the study published in the journal Ecosphere.

“Our results provide a striking estimate of what may happen if business-as-usual policies that affect carbon emissions are to continue,” the study warns.

If greenhouse gas emissions continue on their current projection and exacerbate warming trends, Joshua trees will be reduced to .02% of their historic range in the national park by 2070, the study predicts. If emissions are moderately mitigated, Joshua trees’ range will be limited to 13.9% of their historic range.

Under a best-case scenario in which global emissions are sharply curtailed, the trees could maintain 18.6% of their historic range in the park.

“We can do something about climate change, and we do something about how these trees persist in the landscape,” emphasized Lynn Sweet, a UC Riverside plant ecologist who led the study.

Although the study focused on Joshua Tree National Park, it could have implications for a subspecies of Joshua trees found in Southern Nevada known as the eastern Joshua tree, Sweet said.

“If there are predicted to be hotter, dryer temps in (Clark County) as well, we’d expect to see probably a decrease in new trees establishing in the landscape, and probably some mortality in adult trees and a shifting in where those trees (are),” she said.

Driving the plant’s vulnerability to climate change is its long seedling phase, which lasts for the first approximately 30 years of the plants’ lives; Joshua trees live an average of 150 years. The young plants have less water storage capabilities than adult Joshua trees and a smaller root system, making them less resilient to adverse conditions, Sweet said.

“Adult trees have all the physical resources to survive and persist through droughts, whereas small trees just don’t,” she said.

Another climate-change fueled threat that could hasten the decline of the trees is wildfires. A prior study cited in this one found that about 10% of Joshua trees could survive in the event of a wildfire. Their slow growth also means that recovery after a mass die-off due to a fire could take years or decades.

Throughout the Mojave Desert, including in Southern Nevada, Joshua trees are considered keystone species, crucial to the health of the overall ecosystem. In Clark County, many invertebrates feed on their flowers, roots and stems, and they are also used as hunting perches for some species of birds and owls, notes a 2018 study from Southwest Ecology, LLC submitted to the Clark County Desert Conservation Program.

The ecological significance of the trees coupled with their decline in areas farther south highlights the need to protect them in Southern Nevada, said Patrick Donnelly, the Nevada state director for the Center for Biological Diversity.

“We’re looking at a real possibility where Joshua trees are extinct across the southern portion of their range. That would mean Nevada is all they have left,” Donnelly said. “That means we need to do everything we can to protect Nevada’s Joshua trees and bring a high level of scrutiny to any projects that could threaten them.”