Does a grove of giant saguaro cactus constitute a forest? We think of forests as being made up of trees, and trees are defined as "perennial plants with a single stem or trunk that is usually without branches for some distance above the ground."
The question then becomes: Is the saguaro a tree? These prickly giants--which grow in Arizona a small area of eastern California, and the state of Sonora, Mexico--certainly are perennials. They have a single stem with branches, and they can live to be 200 years old.
Still not satisfied? Trees should be woody and have green leaves, you say? OK, agreed. But don't let that fleshy green stem fool you. The saguaro has an internal skeleton made up of anywhere from 12 to 30 woody ribs that support a core of spongy flesh. Once a saguaro dies and the fleshy parts have decayed, this woody skeleton remains as a fixture in the desert for many years. Like all members of the cactus family, the saguaro does not have true leaves. The life-sustaining chlorophyll is contained in the stems of this group of plants.
If a grove of saguaro cactus still doesn't meet your definition of a tree, consider some of the plants with which it is associated. Saguaros grow among the paloverde, with its slender green stems and bright yellow flowers, and the thorny mesquite. Certainly these are trees, and if so, isn't this a forest?
Ecologists refer to the Arizona desert, a subdivision of the Sonoran desert, as an arborescent desert, one with tree-like vegetation. This is one of the most spectacular deserts in North America and contains a great variety of plant and animal life. Tree-like vegetation grows here because the average annual rainfall is about 11 inches and occurs during two seasons--winter and summer.
These giant cacti depend on their close association with small trees like the paloverde and the mesquite to survive. When a saguaro is very young, it cannot tolerate direct exposure to the hot desert sun. It must grow under shade. Not until about age 50 does a saguaro break through the crown of one of these shrubby trees to face the sun directly for the first time.
As you might guess, growth is exceedingly slow during the early life of a saguaro. After its first growing season, it is just a tiny nubbin, no more than 1/10th of an inch tall. After five years, it may be all of an inch tall. Even after 25 years, saguaros average only one to two feet in height. At age 50, about the time they see their first direct sunlight, they are about six feet tall.
The first lateral branches, or arms, appear when plants reach age 75 and are about 10 to 12 feet tall. After that, growth rates increase and they begin to take on their classic form. Mature saguaros can reach 50 feet in height and have myriad arms in a variety of bizarre, sometimes human, shapes.
The saguaro's pleated, fleshy stems absorb water during rainy periods. They swell up as they absorb the life-sustaining moisture, only to shrink back as reserves are depleted during dry spells. A mature saguaro can store up to 200 gallons of water, enough to sustain it for a year.
Saguaros are the cactus condominiums of the Sonoran desert. Their stems often are full of chambers hollowed out by nesting birds. These holes are drilled by the gila woodpecker and the gilded flicker. The saguaro stem makes an excellent nesting site--the temperature inside a nest can be 20 degrees cooler than the outside in summer and 20 degrees warmer in winter.
After the flickers and woodpeckers abandon their nests, the sites are quickly taken over by other birds such as elf owls, sparrow hawks, and purple martins. Other birds, like the whitewing dove, red-tail hawk, and great horned owl, nest in the branches. In suburban Tucson, I watched a mother starling carry food to a nest of chirping youngsters in a large saguaro that was part of a cactus garden.
During April and May, clusters of waxy, white flowers form at the end of the saguaro's branches, to be pollinated by birds, insects, and the wind. In June and July, the flowers produce plum-size red fruits with a pulpy interior containing about 2,000 tiny seeds. If a seed is fortunate enough to land in the shade of a small tree, and there is sufficient moisture in the soil, it may germinate. But the odds of this happening are slim--only about one in every 250,000 seeds becomes a plant.
The sweet, pulpy fruits are tasty and rich in vitamin C. They provide food for many desert inhabitants, including harvester ants, javelinas, coyotes, squirrels, an assortment of rodents, and even humans.
The saguaro's fruit is a traditional food of the Tohono O'Ohdam Indians (formerly known as the Papago). So important is the saguaro in their culture that the annual harvest of its fruits marks their new year. They collect the fruits by pulling them from the branches with a "kuipad," a tool made from the cactus' woody ribs. The pulp is cleaned, mixed with water, and boiled over an open mesquite fire.
After about an hour of cooking, the seeds are strained and the remaining juice is boiled into a thick syrup, which is used to make candy, jam, and a wine for religious festivals. The oily seeds are ground and used for food.
The best examples of saguaro "forests" in the United States are in Arizona's Saguaro National Monument and Tucson Mountain Desert Park near Tucson. In the 1940s, an article in National Geographic referred to the specimens in Saguaro National Monument as "grotesque giants of the Sonoran desert." But most of those old giants are gone today, victims of old age, wind, and lightning. Recurrent winter freezes, a frequent phenomenon in eastern Arizona, which is near the northern limit of the saguaro's natural range, have also taken a toll on both the very young and the very old plants.
Some plant ecologists believe that the saguaro is in trouble. Today its worst enemy is man. Extensive development in desert basins where this cactus does best has eliminated a lot of habitat. Livestock grazing is also hard on saguaros, especially the young ones. In the hot desert, animals normally bed down in shaded spots, the only areas where the young saguaros can grow. They easily kill the young nubbins that have recently germinated. Another factor in the plight of the saguaro is cactus rustlers, people who dig them up and transplant them in residential areas.
Elimination of grazing in some parts of the saguaro's range has allowed young plants to once again become established. In Sabino Canyon north of Tucson, I saw many young saguaros growing under the shade of mesquite and paloverde trees. However, these plants were in the one- to six-foot range, making them at least 25 years old. Were the younger plants too difficult to spot, or were they just not present? That's a question I can't answer.
So, is a grove of these gentle giants of the Sonoran desert really a forest? That's something I can answer: I believe it is. And I hope that through effective protection of the remaining saguaro groves, future generations will also be able to debate this question.
MEET THE NEIGHBORS
Amid the majestic stands of saguaros, there exists a flora that is as fascinating as it is varied. Here are descriptions of just a few of the plants found in the Sonoran desert.
The most abundant cacti are the chollas and prickly pears, easily recognized by their jointed stems. The joints of the chollas are round while those of the prickly pears are flat. More than 200 species are known in the western hemisphere, and they occur from the praries of northern Canada to the southern tip of Argentina.
Two of the more common prickly pears seen growing among the saguaros are the brown-spined prickly pear (Opuntia phaeacantha), a species with a low, spreading form, and the Engelmann's prickly pear (O. engelmannii), which has a more upright form. Both sport bright yellow flowers in spring. The fruit and stems of some pricky pears are edible and are popular delicacies in Mexico.
The joints of the jumping cholla (O. fulgida) are loosely connected and break off easily. Don't walk too close to this cholla or you're bound to pick up several joints in your shoe or leg. Once embedded, its barbed spines are hard to pull out. The teddy bear cholla (O. bigelovii) may look soft with its dense covering of golden spines, but its joints also detach with little effort.
Another common cactus in the area is the Arizona barrel cactus (Ferocactus wislizenii). It somewhat resembles a young saguaro. This barrel-shaped species is armed with stout, hooked spines. It often leans toward the sun; sometimes specimens lean so far they eventually fall over.
The robust hedgehog (Echinocactus fasciculatus) is a low-growing cactus with large, showy magenta flowers and a tasty fruit. Another low cactus is the Arizona fishhook (Mammillaria microcarpa), which has grayish-green stems and in April produces clusters of delicate pink flowers.
One of the most spectacular plants of the Sonoran desert is not a cactus. The flaming ocotillo (Fouquieria splendens) has true leaves that appear only during rainy periods. Bunches of brilliant scarlet flowers crown clusters of long, spiny stems and can appear even if the branches don't have leaves.
Bill Ciesla is a freelance writer living in Rome, Italy.
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|Title Annotation:||includes related article; saguaro cactus|
|Date:||Mar 1, 1993|
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