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Desert Vegetation, The Principal Attraction

WHEN YOU FIRST VISIT this National Monument, you will be profoundly impressed with the unusual forms of plantlife. Here, through many centuries of adaptation, plants have developed physical and functional means which enable them to thrive under the rigid requirements of restricted and uncertain rainfall, high temperatures, drying winds, and various types of soil.

Coville creosotebush in flower and fruit.

As you become accustomed to the desert, you will begin to notice striking differences in the type and density of vegetation in various parts of the monument. Responsible for this are the physical and chemical properties of the soils, amount and distribution of moisture, degree and direction of slope, exposure and elevation, and also the influence of other plants and animals.

The striking variety of luxuriant plant growth that gives significance to Organ Pipe Cactus National Monument stems from its geographic location at the head of the Sonoyta Valley—a broad lowland stretching northeastward from the Gulf of California. Centuries ago, the tiny Sonoyta River flowed almost due west, but extensive volcanic activity in the region of the present Pinacate Craters blocked the river's course and turned it southward, where its scant waters now become lost in the desert sands.

The desert areas of North America. Organ Pipe Cactus National Monument is located at approximately the second "o" in "Sonoran." From "Plant Life of The Sonoran Desert," by Forrest Shreve.

Thus, only here, where the Sonoyta Valley brings northward a fingerlike extension of a southern climate, do a few species of plants typical of the central gulf coast phase of the Sonoran Desert cross the international boundary. Only here will you find in any abundance within the United States such plants as sapium, a relative of the Mexican jumping-bean, the pudgy elephanttree, Mexican nettlespurge, the rare senita, and huge clumps of the spectacular organpipe cactus, from which the monument derived its name.

But this extension of Mexican plants is only part of the story. The monument also contains a highly scenic section of that widespread region in which plants of the drought-ridden California microphyll desert blend with those of the more liberally watered Arizona succulent (upland) desert. In this unique meeting place of three desert vegetative types, and in a magnificent setting of basin-and-range topography, the manifestations of the great Sonoran Desert reach an unexcelled peak.

Calfornia Microphyll Desert

The term "microphyll" means small leaved. Reduction of the surface areas of their leaves, through which transpiration (loss of water) occurs, is one of the principal adaptations of plants to dry environments. In general, plants of the wide flatlands of the lower Colorado River valley of southeastern California and southwestern Arizona have this characteristic. In the monument, California microphyll desert dominates the area west of the Growler Mountains and Quitobaquito Hills.

The late Dr. Forrest Shreve for many years carried on investigations for the Carnegie Institution of Washington regarding the vegetation of the Sonoran Desert. His studies show that over much of the microphyll desert, creosotebush and bur-sage provide up to 80 percent of the plant population. Usually the two are found together. This somewhat monotonous scene is occasionally broken by small groves of the tall, stiff-branched ocotillo and the leafless holacantha, or crucifixion-thorn. (For pronunciation of unfamiliar words, see glossary.) Within sandy washes the graceful, plumelike crowns of the smokethorn demand attention. Along the borders of these washes is a shrub or small tree called mescat acacia, with the slender, sprawling stems of the holycross, or rattail, cholla growing beneath its shelter. Along the bases of the Agua Dulce Hills, the landscape is dotted with clumps of a red-sapped shrub called nettlespurge, or sangre-de-drago (blood-of-the-dragon). Pencil cholla, a compact dwarf tree cactus, often with a distinct trunk, is found singly, or occasionally as small thickets, along the western bases of the Growler Mountains and Puerto Blanco Mountains.

Although the vegetative cover of the microphyll desert is, in general, unspectacular, a number of the plant species are of particular interest and noticeable when in bloom.

Of these, the ocotillo, among the best known and most widespread of Sonoran Desert shrubs, is common throughout the monument. Its long, whiplike stems grow as an inverted cone rising 4 to 15 feet above the ground. After a rain, these stems become covered with a dense growth of small, green leaves and from March to May produce at their tips large, showy clusters of bright-red flowers. As soon as the soil dries, the plants drop their leaves and remain semidormant as bare, thorn-armed stems until the next shower stimulates refoliation. This cycle may be repeated several times during a season. Thus, owing to spotty showers, the plants may be in full leaf in one part of the monument while those only a short distance away may be bare of foliage.

Ocotillos usually occur sparsely in the microphyll desert, but they are abundant on the upper piedmont plains (the sloping, gravelly bajadas, or alluvial fans, spreading out from the mouths of canyons) and on rocky hillsides where succulents abound. In such locations, ocotillos often mingle with tree chollas and stands of organpipe cactus.

The holacantha, or crucifixion-thorn, grows singly or in small thickets. It is restricted to the flatlands in the tight soil of the outwash plains far from the mountains, where it sometimes rises above the other vegetation. Flowers are greenish yellow and inconspicuous, but the harsh masses of brown fruit-clusters add to the plant's strange appearance. Lack of leaves, whose food-making function has been taken over by chlorophyll-containing bark, has lowered the holacantha's water requirements.

Common throughout desert regions of the Southwest, Coville creosotebush is the dominant shrub of the microphyll flatlands. Local people call it greasewood because of the waxy, moisture retaining covering of the leaves. It gives off a musty odor when wet. The pale-yellow flowers, abundant in spring, are superseded by round, fuzzy white fruits which give the bushes a frosted appearance. Ideally fitted for life in a hot, dry environment, creosotebushes grow at evenly spaced intervals. Their widespread roots are so grasping of moisture that only the shallow-rooted, short-lived annuals can compete with them. Recent investigations indicate that creosotebushes may deposit a toxic substance in the ground that discourages the growth of other plants. However, following soaking winter rains, the topsoil between plants may be covered for a few weeks with a dense growth of desert Indian-wheat, while after summer showers these spaces may be taken over briefly by the annual, shallow-rooted grama grasses. Shreve's studies indicate that an individual creosotebush might live for more than 100 years.

Much of the ground surface between clumps of creosotebush is covered with a mosaic of pebbles, called desert pavement. It is largely a result of wind and sheet-water erosion, which have removed the light surface soil, leaving a tightly packed accumulation of small stones. This desert pavement resists the further action of the agencies that produced it.

Along with creosotebush, bur-sage is among the most common and widespread plants in both the microphyll and succulent deserts, but it is relatively inconspicuous. Similar to common ragweed, to which it is closely related, bur-sage also resembles the common cocklebur. White bur-sage is the close companion of creosotebush in the microphyll desert, giving place to triangle bur-sage among the succulents farther east. Both species occur in Organ Pipe Cactus National Monument. Bur-sage, which flowers in March and April, is one of the annoying hay-fever plants of the desert.

Although often mixed with other vegetation, the gray-green cattle saltbush is sometimes found in nearly pure stands where flooding has resulted in moderate amounts of alkali. This shrub grows 3 to 5 feet high. Its flowers, which appear in summer and early autumn, are inconspicuous, but the plants themselves attract attention and are often mistakenly called sagebrush. Its leaves provide excellent browse for deer and pronghorns. In former times, Papago Indians parched the seeds and ground them into meal.

Grasses are notably absent over much of the desert floor. In some sandy locations, however, clumps of big galleta remind us that certain grasses have become adapted to desert conditions. After summer rains, fast-growing but short-lived annual grasses cover many desert flatlands with a blanket of green.

Found along the washes which cross desert flats and also on gravelly slopes of the upper alluvial fans is California beloperone, or desert-honeysuckle. This is a low-growing, distinctly leafy shrub with brilliant red flowers that attract hummingbirds. Common throughout southeastern California, this species ranges into the monument but it is rarely found east of the Ajo Mountains.

Where gravelly washes wind across creosotebush-covered flats, their otherwise brush-hidden courses are often revealed from a distance by narrow belts of blue paloverde mixed with mesquites and an occasional tesota, or desert-ironwood tree.

Tesotas, which grow to be 30 feet tall with a trunk diameter of 18 inches, are easily identified by their evergreen leaves. Foliage is browsed by bighorn (mountain sheep), while the nutritious seeds are harvested by rodents and were formerly parched and eaten by Indians. The wisteria-like lavender-and-white flowers are abundant in May and June. The wood is so hard and dense that it sinks in water. Indians used it for arrowheads and tool handles.

Whiplike stems of ocotillo (upper left) rise beside organpipe cactus. Buds and blossoms of ocotillo (below) are bright red. Strange-looking, leafless crucifixion-thorn (upper right) grows only in the tight soil of desert flats.

Within recent years, many tesotas in the western part of the monument have become heavily infested with parasitic mesquite American-mistletoe, which has killed branches and some entire trees. Fruits of the mistletoe are eaten by several species of birds, one of which, the phainopepla, relies on them as a major item of its winter diet. Use by birds helps to spread the parasite to other trees. Paloverde and mesquite also are subject to mistletoe infestation.

Velvet mesquites are widely distributed throughout the desert part of the monument, particularly bordering washes. In April and May the pale-yellow, drooping flower-clusters attract swarms of insects. In former times, mesquite beans were a mainstay of life for the Pima and Papago Indians. The long, sweet pods were ground into meal which served as a staple food. Branches and trunks of mesquite trees were the principal source of firewood for the Indians.

Mesquites usually do not drop their leaves as a means of lowering water losses. Instead, by noon they fold their leaves along the mid-rib, thereby reducing the area of leaf surface affected by dry air currents. The extremely long taproot, reported to reach a depth of 40 to 60 feet, takes advantage of ground moisture inaccessible to other plants. Thus, the mesquite resists desert dryness, with almost as much of the tree below ground as above.

Two noticeable plants grow sparingly along the bottoms of sandy washes. One is the smokethorn (smoketree), an important member of the vegetation of the California microphyll desert that reaches the extreme eastern limit of its range within the western part of the monument. Leafless, the slender ashy-gray branches form a dense, somewhat plumelike crown that, at a distance, looks like smoke from a campfire. In May and June the tree is covered with small, violet-to-indigo flowers. Individuals in the monument are dwarfed and shrubby, although trees of this species in desert washes near the Colorado River reach a height of 20 feet.

(Above) Following summer rains, the desert floor is carpeted with green, short-lived annual grasses. (Below) Tesota, or ironwood, is an indicator of a generally frost-free climate.

The other rare invader, this one coming from the Mexican desert to the south, is the senita, or sinita, cactus. Superficially it resembles the organpipe cactus. Senita clumps are easily identified by the deeply ridged 5- or 6-ribbed stems, rarely exceeding 10 feet in height. Clusters of long gray spines cover the tips of the older stems, giving the plant the local name "whisker cactus." Found only along the extreme southern edge of the monument, which is its northern range limit, the senita spreads by the rooting of parts of stems buried in sand. Senita clumps, with 50 or more stems, sometimes spread over many square yards along the bottoms and edges of sandy washes and gravelly flats. The senita is one of the spectacular plants of the gulf coast part of the Sonoran Desert. Its small, pale-pink flowers open at night in May and June.

A number of other plants help to form the vegetation characteristic of drainageways that cross the flats and lower bajadas. Among them are broom baccharis (desert broom, or rosinbrush), white burrobrush, and desert tobacco.

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Last Modified: Sat, Nov 4 2006 10:00:00 pm PST

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