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Southern California Without Joshua Trees?

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Sheep Pass in Joshua Tree National Park | Photo by Chris Clarke

Will there be any Joshua trees left in Joshua Tree National Park in 2111? That depends on who you ask.

In late March, a study released by USGS scientist Ken Cole made the alarming statement that assuming a global temperature increase of 3.5-4 degrees Celsius in the next 60 to 90 years, Joshua trees are likely to be eradicated in much of southern California. Cole and his team modeled changes in both temperature and precipitation that are expected toward the end of the 21st century, and concluded that the vast majority of California's Joshua tree woodlands will be unable to cope with late-century heat and drought.

The few remaining places in the state where Joshua trees will be likely to survive are in the Northern Mojave, according to Cole. A few remnants of what is now the world's largest Joshua tree forest may hang on in the Mojave National Preserve, and scattered groves may persist in the Inyo Mountains above Owens Valley. But in the rest of California, including the trees' namesake national park, Joshua trees will persist only in memory.

A map published in the study accentuates the peril with which the trees are confronted.

“Past and ongoing shifts in Joshua tree distribution support future modeled range contraction,” Kenneth L. Cole et al

Red areas, including the one closest to the bottom of the map that includes Joshua Tree National Park, show current populations of the tree that are unlikely to survive. The far smaller orange and yellow patches denote places where the tree might hang on, and nearby spots they could migrate into. The bright green areas are places where Cole expects the trees might do well if people decide to plant them there, which we don't quite know how to do yet.

Joshua trees have faced global warming in the past. Before the end of the Pleistocene about 11,000 years ago, when the deserts were cooler and wetter, they lived as far south as Mexico and much closer to sea level. When the Ice Age ended and things started heating up, the trees faced much the same dilemma as they do now: unless their seeds somehow got planted in cooler places, their species ran the risk of dying out.

They survived then, so why not now? The difference is that the trees had help dispersing their seeds back then from a large animal long since extinct: the Shasta ground sloth. The sloths, which were about the size of a large cow, enjoyed a largely Joshua-tree-based diet; the trees' leaves, flowers and fruit abound in the deserts' millennia-old deposits of mummified sloth dung. A sloth could take a big mouthful of Joshua tree fruit, swallow, then wander a few dozen miles before planting the conveniently fertilized seeds.

Now that the sloths are gone, Joshua tree seeds are dispersed by much less efficient means: carried off by packrats or squirrels, washed downslope by floods, gobbled up by the occasional starving coyote. As a result, the trees are much less likely to disperse their seeds into new, less hostile settings.

Still, there's a chance that Southern California's Joshua trees may not face quite as bleak a future as all that. At an April 2 conference in Palm Desert entitled "Climate Change In The California Desert, sponsored by the National Parks Conservation Association, desert biologist Cameron Barrows offered a bit of hope that the trees - if we play our cards right - might yet persist in the park that bears their name. Presenting data now being reviewed by National Park Service scientists that will be published later this year, Barrows, a researcher at UC Riverside's Center for Conservation Biology in Palm Desert, pointed out that Cole's study covered the entire range of the tree, and thus lost some of the fine detail. "We focused on just the Joshua Tree National Park area, so we were able to look at smaller units of the landscape and catch some of that detail," said Barrows.

Based on that finer-grained detail, Barrows and his team suggest that a reduced population of the trees may well endure in the well-trafficked northwestern section of the park between Queen and Quail mountains. "There's one problem," cautioned Barrows. "Most of the really destructive wildfires in the park in the last decade have been right in that same area. That's a problem the National Park Service is going to have to address."

"We shouldn't look at this and breathe a sigh of relief," Cameron added. "This is still a real threat."

Whether or not Joshua Tree National Park will be faced with the necessity -- along with Glacier -- of changing its name, Southern California and its deserts will take a major hit from a warming world, with serious consequences to biodiversity. A study by Purdue University's Noah Diffenbaugh, published in Geophysical Research Letters in 2008, included a map of areas in North America most vulnerable to extreme changes from a two degree Celsius rise in average global temperature:

Diffenbaugh et al., Geophysical Research Letters, August 2008

It's clear from that distressingly red patch stretching from Long Beach to Vegas that we're in for trouble here in Southern California. And not just us, but all our neighboring species as well. Another map much in evidence at the Climate Change in The California Desert conference, this one put out by the Nature Conservancy a decade ago, points out that Southern California is a biodiversity hotspot:

Biodiversity Hotspots; The Nature Conservancy

In other words, even if the Joshua tree survives, the extremely biodiverse country it lives in is very vulnerable to climate change, and we have a lot to lose. All the more reason to ride that bike to the supermarket instead of firing up the SUV.

Chris Clarke is a Palm Springs-based natural history writer and environmental journalist currently at work on a book about the Joshua tree. He comments on issues surrounding California's desert every Wednesday on KCET's blog, SoCal Focus.

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