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Old King of the Green Desert
| Article
# : |
19934 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
4 / 1992 |
2,311 Words |
| Author
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Henry Lansford
Henry Lansford is a free-lance writer and communication
consultant based in Boulder, Colorado. He has been writing
about the natural resources of the Rocky Mountain West for the
past twenty-four years. He is also scientific writer-editor
for the Atmospheric Sciences Research Center of the State
University of New York at Albany. |
The Sonoran Desert, which extends up from the Mexican state of Sonoran across much of southern Arizona, is unique among North American deserts: Here, the predominant color is green. It comes in many shades, from the dusty olive foliage of mesquite trees to the fresh, verdant hue of tiny leaves emerging from the seeds of ephemeral annual plants that spread a flowery carpet across the rocky desert floor in early spring.
Ecologists classify the Sonoran Desert as an arboreal, or tree-type, desert. It supports a forest of highly adapted plant species that can survive on a mean annual average rainfall of 10 to 12 inches--much less in times of drought--with summer temperatures that can stay well above 100ºF for long periods. It also has a diverse animal community that includes kangaroo rats, Gila monsters, rattlesnakes, bobcats, coyotes, desert tortoises, and runty wild pigs known as javelinas, as well as roadrunners, Gila woodpeckers, white-winged doves, and a myriad of other birds.
This unlikely forest contains a number of scrubby but genuine trees such as the mesquite and paloverde. But it is the bizarre shapes of many varieties of cacti that give it a quality of eerie unreality for most people seeing it for the first time. The largest is the tree-sized saguaro (Carnegiea gigantea), which reaches high above the crown of the true trees. Partly because its arm-waving shape is roughly humanoid and easy to cartoon, the saguaro has become the symbol of the North American desert. However, its habitat in Sonoran, southwestern Arizona, and a little sliver of southern California constitutes only a fraction of the total desert area of our continent.
A Tree Turned Inside Out
The saguaro has been described as a tree designed by someone who never saw a tree, but in reality it's more like a tree turned inside out. The hard, woody portion of the saguaro, which corresponds to the outer layers of a tree trunk, is incorporated into a skeleton of tough ribs. The softer, living tissue, which is at the heart of a tree trunk, surrounds this skeleton, and the whole think is wrapped in a waxy, spiny, pleated hide Saguaros may live as long as two centuries; a mature specimen stands 40 to 60 feet high and weighs about two tons. The saguaro's growth rate varies with habitat--a 12-foot saguaro in Saguaro National Monument east of Tucson is probably about 50 years old, and a 30-foot one is about 100.
Rainfall in the Sonoran Desert is biseasonal, with
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