Strange Places 6: Beach Pebbles and Moon Rocks
This month I’ll continue discussing micro rather than macro habitats: strange places that exist here and there, rather than large expanses of strange territory such as the Richtersveld or Namaqualand. This month’s topic covers two kinds of micro-habitats; those dominated by gypsum and those consisting of a layer of rounded, water deposited pebbles mixed in with sparse soil. In the North American arid lands, both of these habitats are associated with a number of rare and very interesting cacti, often with very little other accompanying vegetation.
Dry areas marked with deposits of gypsum soil and gypsum outcrops occur here and there throughout the southwest U.S A. and much of Mexico. Of these various formations and deposits, the most visually spectacular consist of nearly shear walls composed entirely of microcrystalline gypsum that looks rather like white, translucent quartz. Somewhat water soluble, it weathers into fantastically ornate shapes, scalloped, tooth-edged scoops and miniature sharp edged ridges. Obviously, not much grows on such almost purely mineral surfaces, but several cacti have managed to colonize these small gypsum cliffs, finding shelter for their roots in the smallest crevices. Accompanying them (as I’ve mentioned in previous columns), are the emerald green, almost perfect sunbursts of the fern-ally Selaginella gypsophila (“phila” signifying “lover of,” thus “gypsum-loving”). Several of the rarest and strangest Mexican cacti grow in these surroundings, including both species of Aztekium, A. ritteri and A. hintoni, as well as the closely related Geohintonia mexicana. I’ve seen a population of A. ritteri climbing along in single file up a crack in the cliff and completely filling it, little furrowed green discs vividly contrasting with the bright white, crystalline gypsum. Although not restricted solely to this habitat, another rare cactus, Thelocactus matudae, has also made its way onto the edges of the gypsum cliff. Gymnocactus (now often considered Turbinicarpus) beguinii, a white, glassy spined cactus that grows in a variety of locations, also may colonize pure gypsum cliffs.
Though the astekiums are restricted to gypsum substrates, their main population grows on parts of the canyon where the gypsum has weathered and degraded from pure white and crystalline to dull yellow-beige, crumbling rock. A very few other plants also manage to cling to the vertical walls alongside the aztekiums. The density of the cacti on these cliffs, however, is hard to believe. Sections of the walls are literally solidly covered with old, multi-headed clumps of Aztekium ritteri, yet the plants exist nowhere in nature except for a few spots in this one small canyon (more of an overgrown gully than a real canyon).
This specificity to a certain type of soil substrate is known as edaphic endemism. Generally, plants exhibiting edaphic endemism are considered capable of surviving on soils where competing plants cannot, rather than the edaphic endemic actually requiring that particular type of soil for its own survival. In my informal observations this usually seems to be the case; for example, aztekiums, aside from being painfully slow growing, aren’t particularly difficult to grow in any quick draining soil pretty much devoid of organic matter, exactly as is the case with many other arid growing cacti. Mammillaria crucigera, another very attractive cactus species, also Mexican, but from far to the south, though slow growing, similarly will grow in any normal cactus mix, under typical cactus cultivating techniques. Strombocactus disciformis (from Hidalgo State in central Mexico) as well, also painfully slow-growing, doesn’t require any dramatically different growing techniques than other arid growing cacti. Although these gypsophilous cacti share rarity, exotic appearance, and the ability to evoke covetousness among cactus growers their edaphic endemic properties don’t seem obligatory.
Other gypsum endemics, however, may possess some deeper tie to their soils, and consequently present daunting challenges to even the most skilled grower. To find them we have to cross the border north into the United States and head for the American southwest, in the Four Corners region. Occasionally, sandy flats in these parts are littered with the “moon rocks” of this column’s title, another form of gypsum, called selenite (which means “moon mineral) in recognition of its pearly luster’s resemblance to moonlight.. It’s another microcrystalline form of gypsum and looks like slightly cloudy, somewhat pitted glass. Given the appropriate geology and climate, it precipitates out of the soil after the rains, and the irregular pieces can be as much as a couple of feet long. It’s an indicator of the right climate, though our American. gypsum cacti don’t grow in it. Instead, they grow in tumps (little mounds) of soft, almost powdery, gray-white soil of a remarkably unfertile nature.
The two best known of these are Pediocactus sileri and Sclerocactus mesae-verdae, members of two of the most unusual cacti genera. They don’t necessarily look odd, just small barrel cactus types. P. sileri (once considered the sole member of the genus Utahia, although it doesn’t actually grow in Utah) resides in a few places in the Arizona Strip, that part of the state north of the Grand Canyon. It sometimes grows on eroded mounds of bentonite clay, but most typically on soft white, almost pure gypsum soil. It can reach close to a foot in height (generally considerably smaller), and despite its size and bristling armor of sharp white, black-tipped spines, still manage to be almost impossible to see against the white soil. The tallest specimens are somewhat bullet shaped, but more often they’re quite spherical. Their decent sized, pale-yellow flowers have slightly fimbriate (fringed) petals. Farther east, in northwest New Mexico, Sclerocactus mesae-verdae (once known as Coloradoa) slowly makes small clumps of four-or five inch gray-colored globular bodies, relatively lightly armored and typically without a central spine. It has white flowers and seems to have selected the least appealing habitats possible as a place to live, barren, eroded, blazingly hot in summer, bitterly cold in winter. Many sclerocacti specialize in miserable habitats; another species, S. spinosior often grows in the extremely saline, alkali covered beds of dry lakes.
All sclerocacti and pediocacti are difficult in cultivation, especially in places with humid winters, but the two gypsum plants take the cake. They may survive for a year or two, but unless grafted (which often leads to rather unnatural lush growth) they typically do not survive for long in cultivation. In particular, Pediocactus sileri has almost never been successfully cultivated on its own roots.
Sclerocactus wrightii, another gray-bodied plant, may grow in bentonite clay, or alkali, or sometimes, in gypsum. From Utah, it’s another small barrel cactus type (as are all sclerocacti) which, when a seedling, may have as much as ninety percent of its body mass beneath the soil in the form of a tuberous root stock. It’s not restricted to gray soiled places though, as it also grows in the other classic habitat of Sclerocactus and Pediocactus, gentle slopes with a covering of rounded, alluvial pebbles.
Gypsum outcrops and patches, never very large or very common, occur here and there in the Southwest. Bentonite clay, on the other hand, comprises a major component of the multi-layered geography that becomes quite impossible to ignore if you spend much time in that part of the country. Often purple gray, sometimes streaked with white or red, it erodes into domes and makes the Painted Desert painted. In many places the bentonite layer is covered by a layer of ancient lake pebbles, rounded by long exposure to waves and currents millions of years ago. These pebbles, often made of quartz and other erosion resistant minerals, develop a kind of sheen. If you’re looking for rare cacti in the United States and you see a gentle slope covered with a slight glitter, it’s a good place to stop. Several rare cacti may grow in such places, including S. wrightii and, even rarer, Pediocactus peeblesianus var. peeblesianus, one of the scarcest and potentially most endangered of American cacti. It lives in northern Arizona, on a few little flat-topped hilltops covered with old lake bed pebbles, with individual plants not as big as the pebbles themselves and hiding between them. The plants themselves are really great looking, tiny, tapering barrel cacti rarely more than an inch tall, with thick, corky, completely harmless spines. Most of the year the plants are pretty much invisible, pulled very low to the ground by contractile roots, but they’ll show themselves briefly after the early spring rains. Even better at concealing itself, another alluvial pebble resident, Pediocactus winkleri, from Utah, pulls itself completely underground after its brief, early Spring growing season. It’s a species that wasn’t discovered until the end of the 1970s, by a hunter walked past some plants in flower completely by chance. P. winkleri is great at hiding. The first time I saw it, in the company of five other people, we searched for a couple of hours and found a total of half a dozen plants, causing us to fear that the site had been largely cleaned out by “cactus-nappers.” A couple of years later I was back at the spot, with only one other person, and through sheer luck we hit it just right. Going into automatic counting mode we finally stopped at one hundred; there were so many it was simply ridiculous to keep counting. In later years I’ve been back and found as many as a few dozen plants, often fewer. It all depends on the weather. I’ve even caught plants, as it were, in the act of sucking themselves underground, descending into a little crater of their own making. Compressed into a couple of minutes, a time lapse film of these cacti over the course of a year would be pretty entertaining. There’d be nothing visible, then, for what would seem an instant, (really lasting for several weeks) the plants would suddenly pop up from underground, bloom (pretty salmon-pink flowers almost as big as the cactus), set seed and produce flat-topped cylindrical little fruit, and then (by early June), start retreating back underground until they had disappeared entirely from view, covered up by blowing sand and grit. Late summer thunderstorms may provoke an even briefer, secondary growing period. Since their habitat ranges from 110 degrees F in summer to 30 below in winter, being underground for most of the year provides obvious survival advantages, particularly for such small, delicate, plants defended only by tiny, harmless spines.
Hiding under pebbles must be a good habit, as many other pediocactus species share it, some growing amid chunks of marble, others under pavements of limestone and quartzite pebbles. Both sclerocacti and pediocacti have remarkably inefficient seed distributions habits—their drab, dry fruit split, and their seeds either remain on top of the plant or roll down to its base. Some populations, which live near dry water courses, may have their seeds dispersed by very infrequent but massive flash floods, but in the right places the almost omnipresent winds (responsible for the towering rock spires and strange rocky “hoodoos” and “goblins” in the region) could easily blow cactus seeds along the ground for mile after mile. Finally, just a slight rise covered with the tiny domes of rounded pebbles might provide a spot for seeds to lodge and, with luck, to germinate and grow. This idea might seem far-fetched, unless, of course, you’ve tried to camp in these windstorms, which regularly blow over eighteen-wheelers, or attempted to sleep in a wind-shaken tent with the racket so intense you have to shout to be heard from two feet away.
Many of the mysteries of pediocactus populations remain unsolved. P. peeblesianus var. peeblesianus is so restricted in distribution that apparently identical habitats no more than a half mile away don’t support any plants at all. P. winkleri, and its close relative, P. despainii, have surprisingly wide distributions in central Utah, either in pebble topped soil or gypsumy bentonite, almost always in bleak though picturesque places. Finally, another extremely rare, little known cactus, Sclerocactus brevispinus, from northeast Utah, also grows in soil covered by split and broken rock rather than water-deposited pebbles. The plant, very squat, with small though sharp spines clusters, resembles some kind of Gymnocalycium more than a sclerocactus, and it’s so rare and so interesting that it deserves mention (there are some other sclerocacti at least as rare, but from utterly different habitats—maybe I’ll talk about them some other day).
As I mentioned, the Mexican gypsum lovers are not that hard to cultivate, any more than other very slow, arid-growing cacti. The American gypsum plants are next to impossible to keep alive, but the pebble growers, though also pretty impossible in an area with weather like ours, will grow with care in parts of the Southwest, the same general area where they originate. None of them are plants for beginners.
Perhaps surprisingly, some sclerocacti are extremely common in parts of Utah and New Mexico, and in the right altitudes and soils, Pediocactus simpsonii grows almost everywhere (as far north as Montana, as far west as Nevada, and as far south as New Mexico). In the Bay Area, however, survival in an ungrafted state is pretty much not going to work. U.C.B.G. has some nice examples of Mexican gypsum growers and some grafted examples of the American plants. We have been known to offer seedlings of some of the Mexican species, but on an infrequent and irregular basis—a few specialist growers can offer some of them more regularly.
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