Letter From The Desert: the Immortal Cholla

Above: Burton Frasher tourist postcard, 1946


I've spent the last decade or so marveling at the very long spans of time desert plants can stay alive. Joshua trees, often considered incredibly long-lived, generally max out at less than 300 years, but the unprepossessing blackbrush at the Joshua tree's base can reach more than twice that, and the ephedras, turpentine bushes, buckhorn and pencil cholla, Mojave yucca, and a few other plants can live more than 1,000 years without much fuss. Creosote bushes famously live for extremely long times: there are a few near our house, which Heart waters regularly, that are likely in excess of 1,500 years in age, and one a half-hour's drive away we've visited a few times has been calculated at 11,700 years old and change. Not far away, a clonal clump of Palmer oak in the Jurupa Maountains near Riverside is probably 13,000 years old, but might be more like 30,000, and Pando, a clonal forest of aspens in Utah, is estimated at around 80,000 years old.

Each individual stem of Pando lives maybe a couple hundred years max, so this isn't an obviously ancient-looking forest we're talking about here, as an entire forest of timberline bristlecone pines might be. King Clone, that ancient creosote ring, is a stand of unremarkable small creosote bushes that just happened to catch a botanist's eye a generation ago. DNA tests confirmed that all the bushes in the ring were genetically identical, thus clones, thus sprung from the same seedling almost 12,000 years ago.

Pando is probably an individual plant, a whole lot of trunks springing from a single massive forest-wide root system. (There are probably clonal clumps of Joshua tree, especially in the west edge of the trees' range near Los Angeles, that have been growing for some thousands of years, with individual "trees" growing and dying over the course of a century or two.)

But it's not about the "individual" plants, in King Clone's case. Most of the shrubs in that clonal ring probably haven't been connected to each other by roots for centuries. That may also be true of the Jurupa oak. Calling those two groups of plants "individuals" might seem to be stretching it. Sure, they all sprang from the same union of one ovule and one pollen grain, but so do a very large number of human twins each spring in paired zygotes from a single union of zygotes, and they tend to get testy when you insinuate the two of them make up a single individual. Don't ask me how I know.

So I've been straining the last few months to wrap my head around how to think of the teddy bear cholla, Cylindropuntia bigelovii, whose colorful description by dead white male writer J. Smeaton Chase I referred to last week.

You see, I learned recently — though botanists have actually known this for decades — that teddy bear cholla is triploid. "Triploid" means possessed of three sets of chromosomes. "Diploid," which is what many plants and almost all humans are, means having two sets of chromosomes. "Haploid" means one set of chromosomes, which, as the story goes, explains what the H stands for in "Jesus H. Christ." (Unsubscribe link is at the bottom of this email.)

Plants are pretty likely to have more than two sets of chromosomes. Creosote bushes in the Chihuahuan Desert and south are generally diploid, but something happened as they moved into the Sonoran Desert and they doubled up their chromosomes and became tetraploid, then grew another pair when migrating into the Mojave, and Mojave creosotes are thus mainly hexaploid.

Two, four, six, eight, plants can thus still procreate. But odd ploidy numbers make sexual reproduction awkward, because each plant makes gametes with half the usual number of chromosomes, and odd numbers don't divide well into half, which makes the chances of viable seed fairly slim.

Thus triploid plants, like Cylindropuntia bigelovii and a few other cholla species, as well as some watermelons and apples and bananas and other friendlier plants, mostly do not reproduce by seed. In the case of chollas, they reproduce by letting go of wickedly thorny segments of themselves, which — after attaching to pant leg or dog snout or other such mobile agent of dispersal — hit the ground, some of them eventually to root in new spots and grow to make new wicked propagules.

Tourists in Joshua Tree National Park regularly visit a spot called the Cholla Garden, a patch of the Pinto Basin several dozen acres in extent that's covered in teddy bear cholla. In all likelihood, the immense patch of cholla all came from a single cholla stem, perhaps imported stuck in the skin of a mastodon.

That's straightforward enough. But if teddy bear cholla rarely if ever sets viable seed, and if we apply the King Clone definition of "single-organismhood" to chollas, that means that not only is the Cholla Garden a single individual, but it's one that likely extends far beyond the Pinto Basin, to north and south of the National Park, into nurseries and home gardens and university arboreta, wherever one of the Cholla Garden segments may have ended up and taken root.

And that's a little hard to wrap my head around.


A week ago tonight I met a few friends for dinner, and between the main and dessert courses we trooped out into my friend's backyard to watch the desert bighorn on the ridge behind her house. There were eight of them, by the count of the person with the best eyesight there, though we didn't need particularly good eyesight to see their faces. They were as frankly curious as we were. And then we had ice cream.

It's now two weeks since my Facebook account was deleted for good, at least as regards my ability to resuscitate it. (I'm sure Facebook and the NSA still have everything I ever posted, crossreferenced with every photo someone tagged me in without asking first.) I expected to have more of a wrenching cold turkey experience, but no: aside from wondering idly how I'll stay in touch with a couple people who seem to exist only on Facebook, I have had nothing but additional free time and lower blood pressure. I highly recommend the experience.

Errata

Two readers were kind enough to send notes reminding me last week that Acacia greggii, the main topic of the last letter, has been renamed Senegalia greggii since around 2005. I also reminded myself, though unfortunately not before hitting "send," that Forsythia blooms appear some time in spring before the leaves emerge, and so my comparison of foliated Senegalia stems to "Forsythia before the bloom" was woefully inaccurate. Hey, I said it'd been a couple decades since I've seen one.

Thanks for writing, with corrections or not. I may not reply to every last note, but I do read them all and enjoy all of them, even the mean ones. (There haven't been any mean ones yet.)

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