Part 4

Saguaros provide not only food for man and beast, but homes for animals. Walk through a giant cactus forest and you will be amazed at the number of holes drilled in these plants. The holes are made by Gila woodpeckers and related gilded flickers, which often relinquish them after one nesting season. The next occupant may be any of a host of desert dwellers, including screech and elf owls, purple martins, and invertebrates. Some birds use the plant as a foundation for their homes. White-winged doves, for instance, often build flimsy stick platforms on the tips of saguaro arms; red-tailed hawks and horned owls construct more substantial nests in the forks.

Although billions of saguaro seeds are produced yearly in the extensive stands of the monument, only a very few find favorable locations for germination and growth. Trees, rocks, dead limbs, pebbles—anything that reduces evaporation in the immediate vicinity of the seed—improve the chance of germination. These kinds of shelter not only provide the necessary moisture conditions, but also hide the seed from armies of ants, rodents, and other animals searching for food.

Early growth is extremely slow. A 2-year-old saguaro may be only one-quarter of an inch in diameter, and a 9-year-old plant may be 6 inches high. These years are the most hazardous. Insect larvae devour the tiny cactuses. Woodrats and other rodents chew the succulent tissue for its water, and ground squirrels uproot the young plants with their digging. In later life, the saguaro must contend with uprooting wind and human vandalism, as well as the earlier foes—drought, frost, erosion, and animals.

In a century of maturity, a saguaro may produce 50 million seeds; replacement of the parent plant would require only that one of these germinate and grow. But in the cactus forest of the Rincon Mountain Section, the rate of survival has been even lower, so that over the last few decades the stand has been dwindling. What is wrong?

Many answers to this question have been advanced, but like all interrelationships in nature, the saguaro’s role in the desert web of life is very complex, and involves past events as well as present ones; a partial answer to the problem may be all we can hope for. The following reasons for the decline of the saguaros have been suggested by researchers.

There is some evidence to suggest that the Southwest has been getting drier since at least the late 19th century, and while the saguaro is adapted to extreme aridity, some of the “nurse” plants that shelter it during infancy are not. If such shrubs as paloverdes and mesquites dwindle, it is argued, so must the saguaro, which in its early years depends on them for shade.

Other culprits in the saguaro problem are man himself and his livestock. Around 1880, soon after the first railroad reached Tucson, a cattle boom began in southern Arizona. The valleys were soon overstocked, and cattle scoured the mountainsides in search of food. By 1893, when drought and starvation decimated the herds, the land had been severely overgrazed. Though the monument was established in 1933, grazing in the Rincon Mountain Section’s main cactus forest continued until 1958. (Elsewhere in the monument, it still goes on.) Compounding the problem, woodcutters removed acres of mesquite and other trees. In the center of the present Cactus Forest Loop Drive, lime kilns devoured quantities of woody fuel. Further upsetting the desert’s natural balance, ranchers and Government agents poisoned coyotes and shot hawks and other predators—in the belief that this would benefit the owners of livestock.

This unrestrained assault on the environment had unfortunate effects on saguaros as well as on the human economy. Overgrazing may have resulted in an increase in kangaroo rats (which benefit from bare ground on which to hunt seeds) and certain other rodents adapted to an open sort of ground cover. Man’s killing of predators, their natural enemies, further encouraged proliferation of these rodents, which some people say are especially destructive of saguaro seeds and young plants. Whatever the effect these rodents have on the saguaros, the removal of ground cover intensified erosion and reduced the chances for the seeds to germinate and grow. And certainly the cutting of desert trees removed shade that would have benefited young saguaros. In the Tucson Mountain Section, which is near the northeastern edge of the Sonoran Desert, freezing temperatures are perhaps the most important environmental factor in saguaro mortality.

Although the causes of decline of the cactus forest lying northwest of Tanque Verde Ridge are still something of a puzzle, several facts are clear: the saguaro is not becoming extinct; in rocky habitats many young saguaros are surviving, promising continued stands for the future; in non-rocky habitats, some young saguaros are surviving, ensuring that at least thin stands will endure in these areas. Furthermore, since grazing was stopped here, ground cover has improved—a plus factor for the saguaro’s welfare. On the negative side, it is possible that, in addition to suffering from climatic, biotic, and human pressures, the once-dense mature stands of the monument are in the down-phase of a natural fluctuation. It is possible, too, that these stands owed their exceptional richness to an unusually favorable past environment which may not occur again. We can hope, however, that sometime in the not-distant future the total environmental balance will shift once again in favor of the giant cactus.

Other Common Cactuses

Many other cactuses share the saguaro’s environment. The BARREL CACTUS is sometimes mistaken for a young saguaro, but can easily be distinguished by its curved red spines. Stocky and unbranching, this cactus rarely attains a height of more than 5 or 6 feet. It bears clusters of sharp spines, called “areoles,” with the stout central spine flattened and curved like a fishhook. In bloom, in late summer or early autumn, this succulent plant produces clusters of yellow or orange flowers on its crown. The widely circulated story that water can be obtained by tapping the barrel cactus has little basis in fact, although it is possible that the thick, bitter juice squeezed from the plant’s moist tissues might, under extreme conditions, prevent death from thirst. Desert rats, mice, and rabbits, carefully avoiding the spines, sometimes gnaw into the plant’s tissues to obtain moisture.

The group of cactuses called opuntias (oh-POON-cha) have jointed stems and branches. They are common and widespread throughout the desert and are well represented in the monument.

Those having cylindrical joints are known as chollas (CHO-yah), while those with flat or padlike joints are called pricklypears.

Chollas range in size and form from low mats to small trees, but most of those in the monument are shrublike. TEDDY BEAR CHOLLA, infamous for its barbed, hard-to-remove-from-your-skin spines, forms thick stands on warm south- or west-facing slopes. Its dense armor of straw-colored spines and its black trunk identify it. Because its joints break off easily when in contact with man or animal, this uncuddly customer is popularly called “jumping cactus.” A similar species is CHAIN FRUIT CHOLLA, notable for its long, branched chains of fruit, which sometimes extend to the ground. Each year, the new flowers blossom from the persistent fruit of the previous year. There is a common variety of this species that is almost spineless. STAGHORN CHOLLA, an inhabitant primarily of washes and other damp places, is named from its antler-shaped stems. This cactus’ scientific name—Opuntia versicolor—refers to the fact that its flowers, which appear in April and May, may be yellow, red, green, or brown. (Each plant sticks with one color through its lifetime.) Among the smaller chollas, thin-stemmed PENCIL CHOLLA grows from 2 to 4 feet high on plains and sandy washes. DESERT CHRISTMAS CACTUS, almost mat-like in form, blooms in late spring but develops brilliant red fruits which last through the winter.

PRICKLYPEARS, like many of the chollas, produce large blossoms in late spring. Those on the monument are principally the yellow-flowered species. The reddish brown-to-mahogany colored edible fruits, called tunas, attain the size of large strawberries. When mature in autumn, they are consumed by many desert animals.

Some of the smaller cactuses are so tiny as to be unnoticeable except when in bloom; examples are the HEDGEHOGS, the FISHHOOKS, and the PINCUSHIONS. Blossoms of some of these ground-hugging species are large, in some cases larger than the rest of the plant, and spectacular in form and color. All add to the monument’s spring and early summer display of floral beauty.

Non-Succulents

For the diversity of devices for adaptation to an inhospitable environment, the many species making up the non-succulent desert vegetation provide an absorbing field for study. As we have seen, there are two ways to survive the harsh desert climate; one is to avoid the periods of excessive heat and drought (“escapers”); the other is to adopt various protective devices (“evaders” and “resisters”). Short-lived plants follow the first method; perennials, the second.

Perennials

Chief among the requirements for year-round survival in the desert is a plant’s ability to control transpiration and thus maintain a balance between water loss and water supply. In this struggle, the hours of darkness are a great aid because in the cool of the night the air is unable to take up as much moisture as it does under the influence of the sun’s evaporating heat. Therefore, less exhaling and evaporating of water occurs from plants, and both the rate and the amount of water loss are reduced. This reduction in transpiration at night allows the plants to recover from the severe drying effects of the day. One biologist may have been close to the truth when he stated, “If the celestial machinery should break down so that just one night were omitted in the midst of a dry season, it would spell the doom of half the nonsucculent plants in the desert.”

One of the common trees in the desert part of the monument is the MESQUITE (mess-KEET). In general appearance it resembles a small, spiny apple or peach tree with finely divided leaves. Its roots sometimes penetrate to a depth of 40 or more feet, thus securing moisture at the deeper, cooler soil levels, from a supply that remains nearly constant throughout the year. This enables the tree to expose a rather large expanse of leaf surface without losing more water than it can replace. A number of mechanical devices help the tree reduce its water loss during the driest part of the day (10 a.m. to 4 p.m.). Among these are its ability to fold its leaves and close the stomata (breathing pores), thereby greatly reducing the surface area exposed to exhaling and evaporating influences. In April and May, mesquite trees are covered with pale-yellow, catkinlike flowers which attract swarms of insects. These flowers develop to stringbeanlike pods rich in sugar and important as food for deer and other animals. In earlier days, the mesquite was also a valuable source of food and firewood for Indians and pioneers.

Another desert tree abundant in the monument is the YELLOW PALOVERDE. It is somewhat similar in size and general shape to the mesquite. Lacking the deeply penetrating root system of the mesquite, the paloverde (Spanish word meaning “green stick”) has no dependable moisture source; but it has made unusual adaptations that enable it to retain as much as possible of the water collected by its roots. In early spring the tree leafs out in dense foliage, which is followed closely by a blanket of yellow blossoms. At this season the paloverdes provide one of the most spectacular displays of the desert, particularly along washes, where they grow especially well. Blue paloverde, growing in the arroyos, blooms well every year. Yellow, or foothill, paloverde, a separate species, blooms only if the soil moisture is high following winter rains.

With the coming of the hot, drying weather of late spring, the trees need to reduce their moisture losses. They gradually drop their leaves until, by early summer, each tree has become practically bare. The trees do not enter a period of dormancy, but are able to remain active because their green bark contains chlorophyll. Thus, the bark takes over some of the food-manufacturing function normally performed by leaves, but without the high rate of water loss.

Carrying the drought-evasion habits of the paloverde a step further, the OCOTILLO (oh-koh-TEE-yoh) comes into full leaf following each rainy spell during the warmer months. During the intervening dry periods it sheds its foliage. The ocotillo, a common and conspicuous desert dweller, is a shrub of striking appearance, with thorny, whiplike, unbranching stems 8 to 12 feet long growing upward in a funnel-shaped cluster. In spring, showy scarlet flower clusters appear at the tips of the stems, making each plant a glowing splash of color.

A number of desert shrubs fail to display as much ingenuity as the paloverde. Some of these evade the dry season simply by going into a state of dormancy. The WOLFBERRY bursts into full leaf soon after the first winter rains and blossoms as early as January. Its small, tomato-red, juicy fruits are sought by birds, which also find protective cover for their nests and for overnight perches in the stiff, thorny shrubs. In the past, the berrylike fruits were important to the Indians, who ate them raw or made them into a sauce.

Commonest of the conspicuous desert non-succulent shrubs is the wispy-looking but tough CREOSOTEBUSH, found principally on poor soils and on the desert flats between mountain ranges. It is also sprinkled throughout the paloverde-saguaro community in the monument. A new crop of wax-coated, musty-smelling leaves, giving the plant the local (but mistaken) name “grease-wood,” appears as early as January. The leaves are followed by a profuse blooming of small yellow flowers and cottony seed balls. During abnormally moist summers or in damp locations, the leaves and flowers persist the year round; but usually the coming of dry weather brings an end to the blossoming period. If the dry spell is exceptionally long, the leaves turn brown, and the plants remain dormant until awakened by the next winter’s rainfall. Pima Indians formerly gathered a resinous material, known as lac, which accumulates on the bark of its branches, and used it to mend pottery and fasten arrow points. They also steeped the leaves to obtain an antiseptic medicine. Ground squirrels commonly feed on the seeds.

A large shrub of open, sprawling growth usually found along desert washes in company with mesquite is CATCLAW. Its name refers to the small curved thorns that hide on its branches. In April and May, the small trees are covered with fragrant, pale-yellow, catkinlike flower clusters that attract swarms of insects. The seed pods were ground into meal by the Indians and eaten as mush and cakes.

In lower elevations of the Tucson Mountain Section, the gray-blue foliage of IRONWOOD is a common sight, but the species does not range farther eastward. Its wisterialike lavender-and-white flowers blossom in May and June. The nutritious seeds are harvested by rodents and formerly were parched and eaten by Indians. The wood is so dense that it sinks in water; Indians used it for making arrowheads and tool handles.