Part 3

In southern Arizona, the sequence of vegetation types begins with desert scrub at the lowest elevations, and ranges through desert grassland, oak woodland and chaparral, oak-pine woodland, ponderosa pine forest, and Douglas-fir forest, to Engelmann spruce forest on top of the highest mountains. In the national monument, we have all these types except the last—although desert grassland is poorly represented here because the steep slopes squeeze it into a narrow altitudinal band; and Douglas-fir forest is restricted to small areas in canyons and on north-facing slopes at high elevations. A few drainageways (notably Chiminea Canyon, on the south side of the monument) support patches of deciduous trees classified as riparian woodland.

These vegetation zones should not be visualized as nice neat bands on the mountainsides, however, for roughness of topography and differing tolerances of plant species usually lead to gradual changes from one zone to the next. Because of the greater exposure to drying heat and sunlight, each belt occupies a higher range of elevation on south-facing slopes than on north-facing ones. And along draws, where conditions are wetter and cooler than on ridges at the same elevations, the vegetation zones finger down to lower elevations; on ridges the opposite is true.

Animals are less restricted than plants to particular vegetation belts, but they, too, show a zonation with altitude. You must look, for instance, in the paloverde-saguaro community (part of the desert scrub type) for kangaroo rats and cactus wrens; in the oak-pine woodland for Mexican jays; and in the ponderosa pine forest for whitetail deer and tassel-eared squirrels.

In the Tucson Mountains, only the desert scrub type is well represented, although the highest ridges support a suggestion of desert grassland. A small patch of shrub live oaks, relicts of a wetter period, huddles on the north side of Wasson Peak. Zonation is poorly developed here because the mountains are low and small in mass. (Studies have shown that mountains of smaller mass tend to have warmer and drier climates than bulkier mountains of the same height.)

Major Vegetation Types in Saguaro National Monument

Vegetation type Average Elevations Annual Life zone July (feet) rainfall temp. (inches) Prominent species Southwestern Desert 94 2,200 to 4,000 7 to 13 Lower Sonoran Scrub Paloverde Saguaro Pricklypear Cholla Catclaw Ocotillo Creosotebush Mesquite Ironwood Grassland Transition 85 3,500 to 4,500 10 to 15 Upper Sonoran Mesquite Beargrass Gramagrasses Amole Sotol Oak-pine-juniper 74 4,500 to 7,000 12 to 22 Upper Sonoran Woodland and Chaparral Emory Oak Mexican Blue Oak Shrub Live Oak Pinyon Pine Mountain-mahogany Manzanita Skunkbush Coniferous Forest 68 above 6,000 18 to 30 Transition and start of Canadian Ponderosa Pine Gambel Oak Buckbrush Mountain Muhly Douglas-fir White Fir Aspen Snowberry Mexican White Pine

TUCSON MOUNTAIN SECTION

Biologists have developed several systems for classifying assemblages of plants and animals over broad regions, and one of the most widely known is that of C. Hart Merriam. Around the turn of the century, he conceived a system of “life zones,” named for the parts of the continent where they are best developed: Tropical, Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic. In Saguaro National Monument, the Lower Sonoran Zone corresponds to the desert scrub type; the Upper Sonoran includes desert grassland transition, oak woodland, and oak-pine woodland; the Transition Zone is equivalent to the ponderosa pine forest; and the poorly represented Canadian Zone has Douglas-fir and white fir. Because Merriam’s system is so widely used, his terms are included in the tabulation on page 20, which summarizes the main characteristics of plant zones in the monument.

The best way to appreciate the biotic changes that occur with elevation is to walk or ride a horse to the top of the Rincons. But if your time or energy is limited you can get a quick view of these sequences by driving up nearby Mount Lemmon, in the Santa Catalina Mountains.

desert plants

When you look out over the cactus forest, in either part of Saguaro National Monument, you may think there’s a sameness to it in all directions—saguaros standing amid scattered shrubs. But look harder, or walk about, and you will discover variations in the scene. First, there is a gradual change in the vegetation from the mountain foot down the bajada or pediment to the valley floor, as saguaros and paloverdes (green) become sparser and creosotebushes (smaller and brownish) take over. (The monument itself does not extend far enough from the mountain bases to include extensive creosotebush communities, but these cover large areas in the lowest parts of the valleys.) Then there is the luxuriant growth along washes, where mesquites and paloverdes grow to tree size and there are more kinds of plants. And if you are observant you may notice slight variations with each change in slope on the rolling hills—for example, more grass growing on their north sides. On another scale, you can see separate little communities of plants in special situations, as under shrubs or on rocks.

These patterns are due to variations in the environment. For the desert is hotter and drier in some places than in others. The gradual downslope changes in vegetation reflect the decrease in the amount of soil moisture available to plants—a condition caused by the decreasing size of soil particles and consequent shrinkage of water-holding space between particles. Desert washes encourage plant growth because they channel water and cold air. Strangely enough, night temperatures are often lower in a desert valley than farther up the slopes. This “inversion” is due to drainage of cold air down mountainsides, forming cold “pools” in valley bottoms, especially in winter. Cold air is heavier than warm, and (since air flows much like water) it is channeled down the draws—a fact that will strike you if you walk into a wash near the mountains at night or early in the morning. Add this phenomenon to the great amounts of moisture that lie beneath the surface of washes, and you can see that desert drainageways are really linear oases. More subtly, desert hills reflect in their vegetation the differences in soil moisture from north slopes to south slopes caused by increasing exposure to sunlight.

One controlling factor, then, is dominant in the desert: the scarcity of water. This results not only in the unusual forms and adaptations of desert plants, but also in a distinctive type of plant community. In wetter climes, plants compete mostly for sunlight. They can grow close together, as long as they receive adequate illumination. During the regrowth of forests, several distinct sets of plants appear, each succeeding group more tolerant of shade than the last, as the forest canopy closes. In the desert, there is no such succession. Clear a patch of desert vegetation, and the same species will reappear—spaced out, with bare ground between them as before. For here sunlight is abundant (we might say overabundant), but there is not enough water to allow plants to cover the ground.

Dr. Forrest Shreve, who was a botanist at the Carnegie Institution’s Desert Laboratory near Tucson and a master student of deserts, defined a desert as “an area in which deficient and uncertain rainfall ... has made a strong impression on the structure, functions, and behavior of living things.” The distinctive characteristics of desert plants and animals have evolved through millions of years, in a trial-and-error process in which only the better-fitted forms have survived. It would be enlightening to know how many of the species and varieties of plants that developed during the past 60 million years or so have failed to adapt to Sonoran Desert conditions. It is fascinating to study the hundreds of forms that have succeeded and to try to determine what structures they have perfected and what methods they have originated that enabled them to maintain themselves in such a harsh environment.

Desert plants can be classified as “escapers,” “evaders,” and “resisters,” according to their means of adaptation to water shortage. Escapers, such as the annuals, avoid the problem entirely by waiting out the dry periods as seeds, to sprout and reproduce only when the rains come. Evaders, such as the ocotillo, reduce their water loss during droughts by such methods as dropping their leaves or going into a state of dormancy. Resisters, however, “hang in there” all year, taking the desert’s worst. The cactuses, prime examples of this group, rapidly soak up water from each rain and store it for use during drought; the mesquite’s deep roots tap a more constant source of moisture.

Succulents

A large group of desert plants conserve water for use in periods of drought by storing it in specialized tissues during the wet season. Some of these “succulents,” principally the yuccas, eschevarias, and agaves, have developed water-storage tissues in their leaves. A few, notably the NIGHTBLOOMING CEREUS (see appendix for scientific names of plants), have slender stems but an enormous, carrot-shaped root in which the moisture-storage tissue is located. The GOURDS also have large, thick, moisture-retaining roots, as does the WILD-CUCUMBER.

The cactuses store water in their stems and thus are called stem succulents. In the course of its evolution the cactus has eliminated leaves, and their function has been taken over by the green outer covering of the stems. Thus the amount of transpiration (moisture loss through plant breathing pores) has been greatly reduced.

The cactuses are thought to have evolved from relatives of the rose family in the West Indies, beginning some 18,000 to 20,000 years ago. From there they spread to most parts of the Western Hemisphere, but particularly to the drier regions, changing their forms to meet new conditions. One of the youngest of plant families, the cactuses are still evolving rapidly. This doesn’t make the task of classifying them easier for the taxonomists.

In varied forms, cactuses enliven the paloverde-saguaro community. In size they range from tiny button and pincushion types, some of which weigh only a few ounces, to the giant saguaro, the bulkiest of which may weigh several tons.

Cactuses as a group are easily recognized, although many people mistakenly believe that any desert plant with spines or thorns is a cactus. Shreve describes their main characteristics thus:

Several structural features have served to give the cacti their outstanding appearance, so unlike that of other plants. Most general have been the loss of the leaf as a permanent organ, the enlargement of the stem to accommodate water-storing tissue, and the development of local spinebearing structures known as “areoles.” In several genera, the stem is segmented into sections which are flat and somewhat leaflike; in others the stem is round, much branched, and the surface occupied by close-set tubercles. In a large group, including massive erect forms, as well as slender climbing ones, the stem is grooved or fluted and thus able readily to accommodate its surface to great fluctuations in the water content of the tissues.

If you are trying to identify species, however, cactuses can be annoying, since they often hybridize. You must expect to find some individuals that don’t fit the book descriptions.

The Saguaro—Monarch of the Monument

Of all the species of cactus recorded in Saguaro National Monument, the giant SAGUARO (pronounced sah-WAH-roe) holds the center of interest. From the visitor’s standpoint, all other plants, no matter how bizarre in appearance or peculiar in living habits, are merely stage scenery for setting off the star of the desert drama. For size, this vegetable mammoth tops all other succulents of this country; heights of more than 50 feet and weights of more than 5 tons have been reported. There have been specimens with more than 50 arms, or branches. Although no accurate method of determining saguaro age has been devised, it is estimated that an occasional veteran may reach the two-century mark.

Structurally, the giant cactus is well adapted to meet the stern requirements of its habitat. Its widespread root system, as much as 70 feet in diameter, lying close to the surface of the ground, anchors and holds the heavy plant erect. The shallow root system quickly and efficiently collects and channels to the main stem any moisture that may penetrate the topsoil. The trunk and branches have a cylindrical framework of long slender poles or ribs fused at the constricted base. This woody skeleton supports the mass of pulpy tissue, the whole being covered with tough, waxy, spine-bearing “skin.” Numerous vertical ridges, like the pleats of a huge accordion, permit the stems and branches to expand in girth as the tissues swell with water during wet weather and to shrink during times of drought.

So efficient is the saguaro’s water-storage system that, even after years of extreme drought, the plant retains enough moisture in reserve to enable it to produce flower buds. The buds appear in vertical rows at the tips of the main stem and branches, a few opening each evening over a period of several weeks in May and June. The flowers, up to 4 inches in diameter, have waxy white petals. This beautiful blossom is the State flower of Arizona. The egg-shaped fruits mature in late June and July, splitting open when ripe to reveal masses of juicy, deep-red pulp filled with tiny black seeds. Pulp and seeds are consumed by several kinds of birds, especially white-winged doves. Many fruits that fall to the ground are promptly eaten by rodents and other animals.

Indians, too, eat the fruits. European explorers who followed Coronado’s expedition into this region found peaceful Papagos and Opatas living here, hunting animals and utilizing many native plants. Among the most dependable of Papago food sources was the fruit of the giant cactus. So important was this fruit harvest in their economy that they designated this season as the start of the new year. Today, in some parts of the desert, Pima and Papago Indians still harvest the fruits. The word “saguaro” is believed to derive from a Spanish corruption of a Papago word for the big cactus.