In comments on this post, Spyder offered a plaintive observation:
In the year-end surveys on global climate change, one of the big 12 CA issues for the future was the potential loss (eventual extinction) of Joshua Trees. If i can dig up the research report (i believe it came out the middle of December), it compiled an overview of how global climate change will likely impact CA over this century. There were 12 worst case scenarios, and one of them regarded the demise of the species. Say it ain’t so!
It is in fact so, and CRN reported here on the possibility that climate change may render Joshua trees extinct, due to the trees’ relative inability to disperse their offspring into viable new populations, meaning that the trees can’t migrate north to escape the worst of the augmented desert heat. As I mentioned in that post, the trees likely evolved to depend on large mammals such as the giant ground sloth to move their seeds to new places, and when the megafauna went extinct, so did the tree’s main method of dispersing its population. That’s conjecture, if well-supported. It’s not conjecture, however, that the trees depend on yucca moths in order to set seed at all. Joshua trees thrive well to the north of their native range: there’s a nice old one growing as a street tree in Fallon, NV, 200 miles north of the tree’s northern native limits. But unless populations of Joshua trees and yucca moths migrate in tandem, each population will go extinct.
From that previous CRN post, describing a poster Dr. Kenneth Cole, of the Colorado Plateau Research Station in Flagstaff, and several other scientists presented at a 2005 climate science workshop:
First, Cole and colleagues took what we know about current Joshua tree distribution:
...which is fairly accurate at this point: scientists have covered nearly every square mile of the plant’s habitat on foot, which wasn’t always the case. Then they took projections of future Mojave desert climate in a world with twice the atmospheric carbon dioxide we had in the 20th century, calculated the likelihood of Joshua tree survival due strictly to climatic factors, calculated the rate at which the trees disperse new seedlings to potentially more welcoming habitat, and mapped the species’ projected range in the mid-late 21st century.
Or they tried to. But the projections didn’t include any Joshua tree stands large enough to show up on the map. So they assumed that the trees would somehow disperse their seeds ten times more efficiently than they have been found to, and they got this map:
Even in this extremely optimistic scenario, the Joshua tree will become extinct in Arizona and Utah. No Joshua trees will survive in Joshua Tree National Park, nor will these at my usual campsite on Cima Dome.
Soon after posting that a bit more than a year ago, I learned I’d misspoken. The data Cole and his associates were working with was, apparently, suspect. I took a closer look at the maps in the poster Cole et al. presented. It’s hard to gauge just where the green splotches are in the above maps, as only state and county lines are portrayed. So I took a portion of Cole et al‘s map and superimposed it on a National Geographic map of the same region:
At first glance, the map looks roughly accurate: the trees along I-40 between Needles and Kingman are there, as well as the Pearce Ferry Road and Joshua Forest Parkway stands, a swath from Mesquite, NV to Beaver Dam, AZ at the top margin of the map, and some near the southern corner of Nevada by Laughlin.
Those last are problematic, though. There aren’t many Joshua trees that close to Laughlin. There are big stands about thirty miles north and east of there, but not in the Newberry Range where that splotch sits. And look more closely at the little spot just east of Yucca (and the BIG spot just east of Yucca is wrong, too, but let’s leave it):
That small patch centering on Wikieup is perplexing. There is one Joshua tree in Wikieup, that I know of, and it was planted by the owners of the general store to make their parking lot look more Wild-Westy. Another patch visible on the bigger map just east of that, on Burro Creek? No Joshua trees there.
And this is the kind of criticism I’ve heard of Cole’s report: the data are not ground-truthed. Much of what Cole’s maps show of the western part of the tree’s range is more or less accurate, though he has the trees heading too far south in Joshua Tree National Park, and there are a number of other discrepancies.
What does this mean for Cole’s conclusions? Well, given that his study was based on data that was unjustifiably optimistic in its assumptions about how many Joshua trees exist now, and given that he had to fudge by a factor of ten the ability of those Joshua trees to disperse, it means that his study, as bleak as it may seem, is almost uselessly optimistic. Given current thinking about climate change, unless we can figure out a way to assist the migration of both Joshua trees and their moth partners, as Connie Barlow is trying to do with Florida Torreya, it seems almost certain that Joshua trees will be extinct in the wild by the end of this century.
Unless we stop burning carbon the way we’re doing. But that would require actual inconvenience, so I don’t hold out much hope.
