It's Still Possible to Stop the Extinction of America's Joshua Trees by 2070

"This outlook for US desert national parks is dire."

If you took the Truffula tree imagined by Dr. Seuss, added more gnarled arms, and plunked it down in the Mojave Desert, you’d get the Joshua tree. Technically, the plant is known as the Yucca brevifolia — it picked up the name Joshua after passing Mormon immigrants were reminded of the biblical figure, his arms raised in supplication.

Today, it is scientists and conservationists who are making a plea because of a problem of biblical proportions: Climate change is driving the death of the Joshua tree. According to a study released in the June edition of Ecosphere, it is inevitable that their numbers will soon decline. However, the extent of that decline depends on what humans choose to do to next.

This study, first-authored by University of California Riverside plant ecologist Lynn Sweet, Ph.D., is based on a combination of statical models and robust field data. The models revealed a somber forecast: If there are no future reductions in carbon emissions, then Joshua Tree National Park will retain just 0.02 percent of its Joshua trees by 2070.

In the very best case scenario — a world where there’s been a major effort to reduce heat-trapping gasses in the atmosphere — about 19 percent of Joshua trees will be alive in the year 2070. 

The researchers collected field data to see whether climate change is already affecting Joshua trees. A large group of volunteers helped the scientists gather data on more than 4,000 Joshua trees. One of the goals of this study, the team tells Inverse, is that people will hear about it and volunteer to take part in studies like these in the future.

Joshua tree
The Joshua tree is not actually a tree but a type of perennial shrub.  

Joshua trees are able to survive in the desert because they store large reserves of water. However, younger Joshua trees in the hotter, drier parts of Joshua Tree National Park are either struggling or dying. The researchers suspect this is happening because they lack this reserve, making them more susceptible to the 376-week drought that plagued California until March. Possibly in response to these pressures, Joshua trees are migrating to parts of the park with higher elevations, more moisture, and cooler weather.

It was also apparent that fires pose a significant threat to Joshua trees. Statistically, fewer than 10 percent of Joshua trees survive wildfires, which have been exacerbated in recent years by car smog and industrial exhaust.

However, the scientists write that it does not have to be a hopeless situation:

This outlook for US desert national parks is dire; however, while it is impossible for park managers to tackle the primary sources of climate change directly, if managers can identify, verify, and protect features that buffer against the effects of a warming and drier climate, there might be a somewhat more optimistic future.

Joshua rees
Joshua trees are confined mostly to the Mojave Desert.

What really, truly, absolutely needs to happen is for societies to take large-scale action and reduce heat-trapping gasses in the atmosphere. And in the interim, preventative measures can be taken. The US Park Service, which was a partner on this project, now plans on removing invasive plants as a result of what was uncovered here. The on-the-ground research demonstrated that smog deposits nitrogen on the ground, which then feeds non-native grasses. These become kindling for wildfires.

The goal of this study, Sweet tells Inverse, was to create “aim-actionable specific advice for conserving the Joshua tree.” That’s why the scientists wanted to collaborate specifically with park staff on the development and execution of the project.

If the Joshua tree disappears, we won’t just lose a lovely quirk of biodiversity. Desert night lizards live under the plant’s fallen branches, while the Scott’s oriole and the cactus wren nest in its branches. The primary pollinator of the plant is the Yucca moth — and it only pollinates Joshua trees. These are connected entities within an ecosystem. When one factor is eliminated, it can all be destabilized.

Abstract:

U.S. national parks protect a natural heritage of global significance; those parks, especially those in the arid southwest, are threatened by climate change. Identifying climate refugia within our national parks using not only statistical models, but also validating predictions using robust field data should provide focus for managers in their stewardship of parks’ biological resources. In the region surrounding Joshua Tree National Park (JTNP), which straddles the Colorado and Mojave deserts in southern California, previous research has predicted the widespread demise of its namesake iconic species, the Joshua tree (Yucca brevifolia) due to climate change. In order to assess whether climate refugia exist for Joshua trees in the future at JTNP, we employed both field measurements and statistical models. We used current distribution point data together with historic climate data, to match conditions when the existing Joshua trees established, in order to predict the distribution of continuously suitable conditions (refugia) at the end‐of‐century. While the high and moderate mitigation could result in refugia for approximately 19% and 14% of the original area within JTNP, respectively, the business‐as‐usual scenario indicated an almost complete elimination of Joshua trees from the park. In order to validate model predictions, using teams of community scientists, we measured the demographic patterns of Joshua tree stands from low to upper elevations within JTNP. Recruitment within stands shows a strong concordance with modeled climate refugia; high‐recruiting stands were within or closer to modeled refugia and in areas with lower climatic water deficit, higher precipitation, and lower maximum temperature than low‐recruiting stands. These findings most importantly indicate the importance of regional to global mitigation strategies for carbon emissions, as reflected in the difference between maintenance of refugia vs. an almost complete elimination of the species from the park by the end‐of‐century. This also underscores the need to protect areas predicted to support refugia from multiple management threats. Rather than an ominous prediction of extinction, climate refugia provide land stewards with targets for focusing protective management, giving desert biodiversity places to weather the future.