In the Arms of the Desert: The Study of Sonoran Saguaros, One of the World's Tallest Cacti, Uncovers Much about the Survival of Other Plants in This Extreme Climate

Article excerpt

It is a warm, sunny winter afternoon on Tumamoc Hill, on the west side of Tucson, Arizona. The sun lights up a multitude of saguaros and other Sonoran Desert plants that dot this twenty-three-hundred-foot hillside Here grow large saguaros, many with multiple armlike branches that curve out and up, candelabra-style, toward the clear blue sky. Others have branches that twist and turn, mostly up bus some down, and a few up and then down. Still other smaller and younger ones have no arms yet, just a single column shaped like a baseball bat. Whatever their size or shape, though, the saguaros glisten a golden yellow as the sun reflects off their sharp spines.

"Saguaros are the quintessential symbol of the Southwest," says Tony Burgess, an ecologist and assistant professor at Columbia University's Biosphere 2 in Oracle, Arizona, who has studied saguaros mid other plaits at the Desert Research Laboratory in Tucson. "Saguaros are linked to our image of the desert. They dominate the landscape because they draw the human eye. We are trained to see them because their structure is similar to our own. They even seem to speak Spanish."

Looking at saguaros while listening to Burgess, one begins to appreciate why so many people are fascinated by these tall cacti. Saguaros are virtually everywhere on Tumamoc Hill and many other places in the Sonoran Desert. Indeed, they grow so dense in some spots around Tucson and northern Mexico as to form thick stands. More than their numbers, as Burgess suggests, by their size and shape saguaros possess an exotic appearance that dominates our view of the desert and commands our respect.

Beyond the power to capture our imaginations, saguaros also tell us something about how cacti and other desert plants survive in a hot, dry environment. "Saguaros are an indicator species," says Mark Dimmitt, director of natural history at the Arizona Sonora Desert Museum in Tucson. As such, they provide glimpses of the relationships between plants, animals, and people in the desert. Not only have saguaros survived the desert's intense summer sun and lack of water, but also, so far at least, the consequences of human changes as well as the worries of some observers who predicted their demise.

Because of their size and shape, saguaros are easily the most studied and most recognizable Sonoran Desert plant. They range from Phoenix and Kingman in Arizona south through Sonora into northern Sinaloa in Mexico. Typically reaching thirty to forty feet in height and sometimes fifty feet or more, saguaros tower over most other desert flora. They are the tallest cactus in the United States. Although some columnar cacti in the Tehuacan Valley southeast of Mexico City are as tall, only the similarly shaped cardon of the Baja Peninsula and western Sonora is bigger.

There are about twelve hundred (or eighteen hundred, depending on which expert you ask) species of cacti, all but one native to the Americas. The Sonoran Desert alone boasts three hundred species. Although cacti are usually thought of as living in deserts or semi--arid regions, some South American species grow treelike with branches and leaves in rain forests. A few have vinelike branches draped around or hanging from trees. Others live as high as twelve thousand feet on mountains where freezing temperatures and snow are common in winter. And still others inhabit tropical Caribbean islands, especially on exposed southern shores or where rainfall is seasonal.

Saguaros belong to a group of cacti with a columnar- and candelabra-like shape. In the Sonoran Desert, these include saguaros, cardons, and a smaller, cardon-like species called the hecho, or hairbrush cactus (for the sharp spines on its fruit). All have a trunklike column rising from the ground and from one to a dozen or more arms at maturity, growing, from the column. Three other Sonoran Desert species--the pitahaya, organ pipe, and senita--lack a central column. …