Desert Plants: The Die-Hards

Saguaros, the giants of the desert, against a stark Arizona sky. Desert Plants: The Die-Hards

When one thinks of the Southwestern deserts the saguaro comes to mind. It is the monarch and "trademark" of our magnificent Sonoran Desert. After all, mature saguaros reach heights of 50 feet, majestically dominating the landscape in parts of this region. But it is the rather lowly and unpopular creosotebush which really rules the Southwestern deserts. This olive-gray shrub is admired mainly by botanists and by rabbits. It doesn't even have anything to do with creosote, that oily, strong-smelling substance used to coat fence and telephone posts. The botanists, in fact, rather hallow it as the species that delineates the Sonoran Desert, ranging as it does all over that geographic zone and the great Mojave and Chihuahuan deserts as well. Across the basins and bare peaks of western and southern Arizona and far south into Mexico, the desert sprawls over thousands of square miles. So completely has evolution fitted the creosotebush, or Larrea, for its place in the sun that it is the very exemplar of desert survival. In common with the cacti, a waxy coating on its leaves conserves, by retarding evaporation, the water this evergreen stores and an underlying lacework of shallow roots soaks up even momentarily-available moisture. As with the whip-like ocotillo, it divests itself of leaves almost altogether during drought. It was the excess of Larrea that once convinced an agricultural chemist in El Paso, Texas that he could turn stems to riches. He was familiar with the failure of earlier efforts at exploiting the regional flora, such as extraction of rubber from guayule along the border between Texas and Mexico (which since has become virtually extinct). But he held to his own idea as more straightforward: the manufacture of broom straw from the fibers of creosotebushes. The fibers proved to be adequate for the purpose, and there certainly was no lack of raw material. What the Texan had failed to perceive, however, was the obstacle, an insurmountable one commercially, of reaping the widely-spaced shrubs.

Some scientists attribute this isolation to a toxin that they believe Larrea and certain other plants, among them brittlebush, turpentine-broom, greasewood, and mesquite, exude to dispel crowding. Other biologists scoff. They contend that it is the dearth of organic debris, or humus, beneath their canopies that discourages the establishment of seedlings. Whatever the explanation (and no one knows for sure) the result is fortuitous for the creosotebush: a larger area from which to draw water! Of the myriad and mysterious ways in which the chain of life insures itself against being broken, none is more fragile than this continuity of desert growth.

Creosotebushes, which bring forth yellow flowers and fuzzy white seedballs each spring, support themselves at some distance from their relatives. Except for occasional intrusions by seasonal neighbors, they also remain fairly aloof from any other vegetation.

The continuity somehow prevails. There are seventy-one cactus species in Arizona alone. And of the hundreds of dry-land plants in the state, 74 may be found nowhere else; there is no place too forbidding for at least a few, it seems. But even more striking than the fact that they are so numerous, or that they endure the most torrid of habitats, is the diversity of their accommodations to the aridity.The odds of seed germination seem to be against our desert plants. Some seeds, for example, can germinate only under conditions brought about by the crashing, drenching violence of a summer thunderstorm!

Such storms are not the least awesome of nature's magical feats. In a flashing, crashing, torrential crescendo of frightening intensity, a tenth or more of the annual allotment of rainfall often slams down in minutes.

Dirt baked to dust by months of drought turns into a mire, and the washes, or arroyos (no more than deep, dry scars on the earth through most of the year) surge with rampant fury.

The phenomenon is as brief as it is tumultuous. With a new dawn, the sun again has its unremitting way. And within hours the land looks, at least superficially, as though nothing has happened. The plains are shriveled, and the washes seem a wasteland. But the appearance deceives, for the cloudburst was simply a noisy overture to the quiet drama of awakening that is its aftermath.

Shoots along the beds of the washes will quickly herald new generations of smoke trees, ironwood, and blue paloverde. All three of these stream-course residents and it may be that they reside there because they can reproduce nowhere else yield seeds cloaked in an armor that requires considerable force to dislodge. This is a precaution against premature emergence in soil not damp enough for sprouting.

But now their long-dormant seeds have been swept up by the flash flooding, scoured and shelled in the swirling of gravel and other desert debris in the wash and finally cast downstream, there to take a tenuous hold.

Not all of them will. The smoke tree seeds, for example those that were washed less than 150 feet from the parent plant won't be smashed sufficiently for the embryos to escape, while those seeds carried more than 300 feet in the swirling water would be ground to nothingness. Only in the distance between is there chance of germination! Again, where water is scarce, the scheme does not permit over-population.

Away from the washes, across the undulating flats and up the weathered

slopes, more of the thunderstorm's legacy may be witnessed and scented. Saturated, the creosotebushes perfume the air with a resinous, musty, almost haunting aroma. A week earlier, they and the other year-rounders (the perennials) would have been alone the cacti, chollas, mesquites, yuccas.

With the late July and August deluges, though, come the stirrings of the summer annuals, their seeds buried for nearly 11 months. Soon there will be pigweed, the purslane, the edible greens; and the beige earth is transformed into a palette by the amethyst of the aster, the nocturnal white trumpet of the thorn apple, the violet horse nettle, and the yellow Arizona poppy.

It is a colorful, albeit transitory, performance. They will blossom for perhaps six weeks and then, dying, entrust their heritage to the seeds of another season.

The ribs of the thirsty saguaro have protruded like those of a lost prospector. But as it drinks deeply, the accordion pleats unfold, and again the giant cactus is plump. The ocotillo, void of leaves in drought, quickly refoliates, as it may do half a dozen times in a year of intermittent rain.

And so the cadence is eternal. With each week of autumn, this world becomes drier until the usually more gentle, soaking showers of December and January. It is upon these that the "winter" annuals, which actually await the warmth of spring before budding, rely chiefly for reincarnation. If the precipitation has been generous, and the temperatures cooperate, the floral spectrum of spring will be much more profuse than that of summer. The horn-shaped purple penstemon and the apricot-hued broad bells of the globe-mallow should appear at higher elevations in March, along with the white of the slender-stemmed strepanthus and the butter-gold bladderpod mustard.

How do the seeds of such short-lived species, so long entombed, "know" when to arise? Why do they stay somnolent some years and yet not in others? What apparently activates them is a delicate combination of moisture, temperature and light, and among the different organisms the particular requisites differ a great deal.

Larrea seeds burst to life when rain is followed by temperatures that do not fall below 59 degrees; they won't sprout if the temperature drops below that. But winter annuals watered by that same rain will sprout in temperature as much as 13 degrees cooler. The seeds are remarkably temperature-sensitive.

Those annuals will not sprout when the soil is warm, nor will summer germinators emerge from cool soils.

The volume of rainfall is critical, a half-inch usually being the barest minimum. Were this not so, an insignificant shower might induce seedlings into an environment too dry to sustain them. It may be that some seeds are coated with an "inhibitor" substance that only adequate moisture can dissolve.

When all the factors are somehow right, thousands of tiny shoots frequently edge up into the same incredibly crowded space. Dr. Frits Went, the Dutch-born botanist who has conducted classic research in germination, has noted that when this occurs the vegetative competitors do not destroy one another, they merely fail to attain their normal sizes.

And in this he sees a lesson for humans: "If there is not enough for all to grow tall and strong, then all remain smaller."

At any rate, of all the desert flora, only the annuals, by programming their short lifespans for the more amenable months climatically, effectively escape the most torturous times. The other, more permanent plants somehow must cope with the excessive heat and inadequate moisture, and at least one of them, the ubiquitous creosotebush, does so with bizarre efficiency (many peopleconfuse creosotebush with the true greasewood, a plant more at home in northern Arizona). The creosotebush, as it expends the last of its rainy-season resources, sometimes sheds the olive-gray of its older, lower leaves. The smaller budding leaves it keeps, but these become brown and limp. And then, inexplicably, there follows a virtual shut-down of the photosynthetic and other processes.

Creosotebushes have been known to be wiped out by drought; but in "hold" condition, somewhat analogous to hibernation in animals, it can survive almost total dehydration for prolonged periods. In that ability to directly tolerate extreme aridity, it and a scant few other specimens (like the pigmy-cedar, which apparently can live without any water, and the white-leaved desert holly) are unique.

Most of our other perennials, unable to accomplish that, have had to evolve methods for avoiding the consequences of devastating dryness for instance, by storing more water and using less of it.

Sedum, a leaf succulent, has been endowed with enlarged water-storage cells, and many of the plants that live in sand possess a greater proportion of woody tissues than the same plants elsewhere, which allows them too to retain more water.

Much of the moisture which plants lose is transpired from the leaves. Saltgrasses decrease this loss by curling their leaves, and the brittlebush, like the ocotillo, often drops them altogether. Here, some species have an architectural advantage in that their leaves are very small (the paloverde) or they simply have none (the cacti). But the succulents are, by design, the premier hoarders of water. They bank it in their stems, as the cacti do, store it in subterranean chambers in the manner of the night-blooming cereus, or keep it in their stems, like the century plant.

The spiny pads and fruit of the prickly pear cactus. The fruit can be made into a palatable jam. WESTERN WAYS

Plants must photosynthesize in order to live, and this requires carbon dioxide. They absorb carbon dioxide through their infinitesimal pores, or stomata, but these are two-way valves, so to speak. When they open, water vapor escapes. The cacti, again, have minimized this loss. They take in carbon dioxide only at night. Because it is cooler then, and the humidity is higher, less moisture disappears through the stomata.

Roots obviously are basic to all seed plants. Nonetheless, one would expect them to show special modifications where water is so scarce, as they certainly do.

Those of the cacti, by and large, including some chollas, stretch out close to the surface in order to capture even the most modest rains that may fall their way. The shallowest of all are those of the pencil cholla and barrel cactus, neither more than two inches deep.

Saguaros are an exception. Besides a superficial network not unlike those of the smaller cacti but radiating out a distance of four or more times the height of the trunk, they also drive a larger branch straight down a foot or two. That may have been intended as an anchor, but it grows more slowly than the laterals, making the laterals increasingly important for support. Should disease strike the laterals, or if they are severed, the behemoth may topple.

Another sort of system consists almost solely of a taproot which stabs far underground for moisture. The crucifixion thorn has such a taproot, and that of Mormon tea extends just as deep as the plant is high. A number of plants, finally, put out both horizontal and vertical roots, among them ocotillos, creosotebushes, brittlebushes, and, with the most aston-ishingly opportunistic tentacles of all, the mesquites. Mesquites are thorny legumes, or members of the pea family, with fernlike leaves and a thirst unbecoming a The unique root system of a desert plant is a key to its survival. Left to right: Creosotebush depends on long taproots and a widespread system of laterals which reach out farther than the height of the plant; Saguaro - has a short taproot mainly for stability, and shallow laterals that radiate out four or more times the height of the trunk; Mesquite - utilizes some laterals, But absorbs most of its moisture through deep, vertical roots, and Ocotillo equally important vertical and lateral roots, both relatively shallow. JOANNA MC COMB

►A good place to view the many unusual varieties of desert vegetation is Organ Pipe Cactus National Monument on the U.S. Mexico border. — JOSEF MUENCH

desert native. Ranchers used to say (as some still do) that wherever they thrive there is certain to be groundwater.

The digging of more than a few wells has been based on that belief, which is not completely without basis. The mesquites are, if anything, adjustable. Their size depends upon the proximity and extent of water. Thus on dry, gravelly slopes they are no more than squat, shapeless bushes. But on a floodplain, where they can send their roots snaking down into the aquifer, commonly 30 feet or more, they are transformed into thick-trunked trees that may thrust upward to 50 feet.

According to theory, these roots may make their way to great depths because they develop more rapidly than the bush above them. How far they conceivably could penetrate is a fascinating question. A few years ago, excavators at an open-pit copper mine near Tucson uneartheda chunk of one still moist -175 feet below ground level!

The shrubs (or tree, as the case may be) has invaded rangelands extensively, in part because livestock eat the beans it produces and then transmit the living seeds through their digestive tracts. On the Papago Indian Reservation, narrow lines of mesquites flare out like spokes from many water tanks marking the trails that cattle, creatures of habit that they are, follow to the tanks to drink.

No less aggressive is the silvery-green salt cedar, or tamarisk. Nothing that grows in the low country seems more at home there. Despite this, it (like the tumbleweed) is an alien. It is uncertain when the first salt cedars were brought to the United States from Eurasia. Apparently there were few in the Southwest in 1900. Because they love heat and are oblivious to alkaline or salty soils, they were coveted, and still are by some people, for shade and as windbreaks. But when they escaped cultivation, they inexorably began to infiltrate. They spread along streambeds particularly, often grouping into vast thickets as dense as jungles.

Salt cedars now have spread across a million Western acres more than nine thousand of those acres being along the flood plain of the Gila River near Globe, Arizona, alone and their requirements for water are prodigious. Those beside the Gila dissipate, in a single summer month, more moisture than that which falls as rain all year, draining the watertable of irreplaceable water.

Consequently, engineers and other experts charged with delivering more water for agricultural and urban use regard them as a plague. On the other hand, outdoorsmen defend them, because they attract and feed wildlife.

The root systems that allow the salt cedars such proliferation are not so deep, usually, as those of mesquites, but generally are more extensive.

Most of what is known about roots in southern Arizona derives from a study unsurpassed still that was done early in the 1900s by William Austin Cannon, an investigator work-ing at the Carnegie Institution's old Desert Laboratory on Tumamoc Hill in Tucson.

Cannon discovered that a majority of plants in that area sprouted hairy filaments on their roots during the rainy months, the more productively, pre-sumably, to seek out moisture.

And he weighed, measured and otherwise assayed the substructures of every species he could spade up. Among other things, he found that the roots of most annuals only rarely reach lower than eight inches, and that the root area of a 37-inch-high creosotebush encompassed 538 square feet!

Scientists like Cannon are busy still, for much remains to be learned about desert plants. Most of the researchers who have pursued such mysteries pre-sumably have been beguiled by them simply because they are mysteries.

But Arizona members of the Soil Conservation Society have taken a new path. They have written a book on how to utilize such vegetation around your very own home, where to acquire it, how to plant it and utilize it (Landscaping With Native Arizona Plants, University of Arizona Press, Tucson, $4.95). They suggest some 300 examples that can beautify yards and (no minor matter in the Southwest) will require far less water than green lawns and other implants from “back home.”