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MICHELE NORRIS, Host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

ROBERT SIEGEL, Host:

And I'm Robert Siegel.

Today, we have another story in our series Climate Connections with National Geographic. If you go to the Mohave Desert, you'll find trees that look like something out of Dr. Suisse. They're short and narrow with dagger-like spines. They're called Joshua trees. It's said that Mormon pioneers thought their limbs looked like the outstretched arms of Joshua guiding them to the promise land.

Well, Joshua trees even have a national park named after them in Southeastern California. But, as NPR's Elizabeth Shogren reports, some scientists are worried that climate change could push the tree to extinction.

ELIZABETH SHOGREN: To understand the Joshua tree and the challenges it faces from climate change, you have to take a trip underground and 13,000 years into the past.

KEN COLE: You might want a helmet because it's really easy just to bang your head on a rock.

SHOGREN: I'm about 15 miles from Las Vegas with U.S. Geological Survey scientist, Ken Cole. This is some of the most parched land in the country. During the Ice Age, Joshua trees thrived here and throughout much of the southwest. Now, the closest one is dozens of miles away at higher altitude.

Why did the trees thrive in ancient days but not now? Cole says it's because a large, now extinct, plant-eating animal called the giant ground sloth roamed this area and helped Joshua trees. He says the evidence is in Gypsum Cave.

COLE: What we're going to have to do is walk down this debris slope here. And then you see a black hole? I think it's right about there; you're going to have to lower yourself down about 5 feet.

SHOGREN: Cole takes the lead. His colleague, Sandy Swift, brings up the rear. We make it through the narrow entrance and the cave opens up into a big room.

What does the ground sloth look like?

COLE: To me, the ground sloth looks like a fuzzy Volkswagen Beetle.

SHOGREN: Scientists who excavated here more than 70 years ago found parts of skeletons, hides and hair from ground sloths and layers and layers of dung.

COLE: The reports said there were so much Joshua tree leaves and seeds and fruits in these deposits that Joshua tree must have been the favorite food of the ground sloth.

SHOGREN: And Cole Says that was great for Joshua trees. After eating the fruits, the sloth could travel 10 miles or more. Then they left the seeds behind in piles of manure that were great fertilizer for new trees. Most of the sloth dung was removed from the cave decades ago. But Cole says they're still some here in a small alcove off the main cave.

Sandy Swift crawls right in.

SANDY SWIFT: You can (unintelligible) of the dung down here.

SHOGREN: It's unbelievable that something put here 13,000 years ago still has an odor, but it does. The discovery clearly delights her.

SWIFT: It's not everybody that can say they know what sloth's (bleep) looks like.

SHOGREN: The ground sloth didn't survive the big warm-up after the Ice Age, so it couldn't play Johnny Appleseed for Joshua trees anymore. Since then, the trees' range has shrunk to one-tenth of what it was. Cole says no modern animal is capable of helping the Joshua tree migrate long distances. So he predicts global warming will shrink the tree's range still more. That could have a huge impact at a national park 200 miles away. Cole predicts Joshua trees will go extinct in 50 to 100 years in Joshua Tree National Park.

So I went to the park to ask rangers like Joe Zarki what they make of that prediction.

JOE ZARKI: One of the questions I guess we talk about, at least internally on an informal basis, is what do you have here as a national park without Joshua trees?

SHOGREN: Joshua trees are a type of yucca that grow 30 feet or taller. The ends of their short branches are covered with clusters of green leaves with spiny tips. An early explorer named John Freemont called it the most repulsive plant in the vegetable kingdom.

ZARKI: People nowadays, I think, look at it and see this really distinctive, you know, sort of whimsical-looking plant.

SHOGREN: Do you like them?

ZARKI: I like them, yeah. I mean, it's a plant with character. It's got a lot of character.

SHOGREN: Zarki and I are touring the park with Jim Cornett, an independent ecologist who has been studying Joshua trees for almost 20 years. Cornett doesn't buy Ken Cole's grim prediction about the trees disappearing from the park.

JIM CORNETT: There's no question that a warming of the climate would reduce the suitable habitat for Joshua trees in the park - but not eliminate it.

SHOGREN: Cornett believes the trees would still grow in the higher, cooler elevations of the park. Still, as we hike across the desert on a gorgeous day, Cornett says Joshua trees have done poorly since he started tracking them. He stops to show me something. When he began his research, a magnificent Joshua tree stood here.

CORNETT: And now, it's dead. And this one is dead. And that one over there is dead. I could find a dozen trees that have died during the study and only one tree that came up.

SHOGREN: The trees in some hotter sections of the park have fared even worse. But is climate change the reason? Cornett says there are other explanations.

CORNETT: All the decline that I have seen I can attribute to drought in the last decade and fires.

SHOGREN: Cornett says it's possible climate change is playing a role. He says one thing's for certain: If Joshua trees go, the whole ecosystem will suffer.

CORNETT: Let's walk a little bit further.

SHOGREN: Cornett takes me to a big branch that's recently fallen off a Joshua tree onto the ground. Its leaves have clearly been nibbled, he says by a thirsty jackrabbit. During the driest times, the only way many animals can find moisture is by gnawing through the live trees' bark.

CORNETT: Things like the antelope ground squirrel, the desert wood rat and the blacktail jackrabbit are now all known to utilize that moisture during times of extreme drought. And of course, if those animals survive, then the coyotes and the foxes and the hawks continue to have animals to prey on and then they survive, as well. So the Joshua tree is like the great canteen of the desert.

SHOGREN: It's also a great draw for tourists. Loretta and Jim Gilbert were visiting the park to celebrate Jim's retirement.

And what do you think about a Joshua Tree without no Joshua trees?

(SOUNDBITE OF LAUGHTER)

LORETTA GILBERT: Well...

JIM GILBERT: This is great(ph).

GILBERT: ...then it wouldn't be Joshua Tree then, any longer. It would just be space. Yeah, Joshua trees are very special, something I will hope that the future generations would be able to see and enjoy.

SHOGREN: Whether or not that would be possible is beyond the power of park officials. They say that's because stopping or slowing global warming will take a worldwide effort.

Elizabeth Shogren, NPR News.

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