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Study Predicts Near Extinction Of Joshua Trees

By Nicholas Gerbis
Published: Thursday, July 25, 2019 - 3:29pm
Updated: Tuesday, August 6, 2019 - 1:44pm

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Science and innovation coverage is supported in part by Palo Verde Generating Station

Joshua trees
Joshua Tree National Park - Flickr

The iconic Joshua trees of the Sonoran and Mojave Deserts could face near extinction by century's end due to climate change and human impacts according to research in the journal Ecosphere.

Researchers used climate models, plant distribution data and citizen scientists to predict where Joshua trees in Joshua Tree National Park might best survive harsher climate conditions.

They found that medium to strong efforts to limit climate change would reduce such "refugia" to less than 20% of the plants' habitat after 2070, and that business-as-usual warming scenarios would virtually wipe out the species in the park.

Lead author Lynn Sweet of University of California, Riverside said the treelike yuccas would hang on longer in certain areas.

"The best place to be a Joshua tree — the place where new Joshua trees are occurring, the place where old Joshua trees aren't dying — will be likely these higher and upper elevations," she said.

Large and long-lived, adult Joshua trees can weather short-term droughts and other stressors, but young plants are highly vulnerable to low levels of precipitation.

Three scientists look at a dead Joshua tree.
Nicholas Graver / NPS.
Study lead Lynn Sweet, back center, pictured here with two volunteers measuring a dead Joshua Tree.

The study was limited to Joshua trees (Yucca brevifolia) within the park, but the same principles are likely to hold true for Joshua trees outside the park, as well as for an eastern variant known as Jaeger's Joshua tree (Yucca brevifolia var. jaegeriana).

"Our finding of potentially very little habitat being left, if any, within the park is certainly troubling for its persistence throughout the range," said Sweet.

Joshua trees also face threats from invasive species, which grow in the park when smog from the Los Angeles Basin fertilizes the ground. These plants then die and dry up, increasing fire risk.

Other hazards include the direct impacts of visitors and off-road vehicles.