An archipelago in the Arizona sky

These walls, steep, chiseled and orange-red, rise a hundred feet above us. When we look up, we can occasionally catch a glimpse of the sunlit mountain rising another 4,500 feet above the canyon. Several such canyons run through the Galiuro Mountains in southeastern Arizona, some 50 miles northeast of Tucson. I have come here with Dr. Jim Malusa, a University of Arizona botanist, and our climb will take us up into one of the Arizona desert's sky islands.

The Sonoran Desert stretches from Mexico into Arizona, a desiccated plain of rock inhabited by only the hardiest species: mesquite, palo verde and cactus; birds, rodents, reptiles and coyotes. If you are from the East, where most hiking is done in lush forests, the desert vistas of southern Arizona take some getting used to, both visually and psychologically. The word wasteland might be the first that comes to mind.

While one can appreciate the lively shadings of a pine or oak forest while driving by in a car, comprehending the luxuriousness of the desert means going into it on foot. After only a few short walks on the many desert trails that surround Tucson, I found myself less wary than contemplative, first admiring the smart adaptations that have enabled life to survive, and finally appreciating the beauty -- the Keith Haring-esque silhouettes of giant saguaro cactus that can take anywhere from 20 to 70 years to develop a strong enough interior structure to support the growth of arms, that weigh 10 tons, blossom with large sweet-nectared flowers and, over their 200-year life span, depend predominately upon a single species, the lesser long-nosed bat, for pollination.

But in every direction a range of mountains truncates the desert vista -- the Santa Catalinas, Rincons and Santa Ritas on the outskirts of Tucson; the Huachucas and Patagonias that lie at the border of Mexico. Some two dozen mountain ranges stand between the Sierra Madre Occidental in Mexico and the Galiuros northeast of Tucson. These are high, massive arcs of ancient rock with peaks up to 11,000 feet. But unlike the Rockies, Cascades or Appalachians, these ranges are not continuous, but rather form an archipelago amid the 20-million-acre sea of desert that surrounds them.

Before the climate in the Southwest began to change some 15,000 years ago, forests stretched from the Rockies down into the Mexican Sierra Madres. Wolves, black bears and jaguars as well as squirrels, birds, reptiles and insects moved freely north and south along this lush and wooded corridor. Tree and plant species readily spread. But with the end of the ice age, temperatures rose and precipitation dropped, the desert to the south spread north. The species that needed water and cooler temperatures, like squirrels and fish, soon found themselves marooned at high elevations by widening arid gulfs between themselves and other animal life. After thousands of years of isolation the result is that, as Darwin once reported from the Galapagos, "different islands to a considerable extent are inhabited by a different set of beings."

But while the islands in the Galapagos at least share the same equatorial climate, in this sky island archipelago, every thousand feet up that you climb takes you into a climate that's more like one some 500 miles north -- a few degrees cooler and with a few inches more precipitation. So as we hike through the Galiuros we're among native Arizona saguaro cactus at 2,000 feet, grasslands at 3,000 feet, juniper at 4,000, oak at 5,000 and Rocky Mountain ponderosa pine forest at 6,000. In the loftier islands, the forests above 7,000 feet resemble those of coastal British Columbia, dominated by Douglas firs.

The trees and plants that surround us as we zigzag through the canyon are a vivid but motley assortment. Perennial water alders and sycamores line the stream bank. In their shadow the undergrowth is thick. Just above our heads, however, agaves sprout from the sun-drenched canyon walls; alongside them, columbine. There's a lone aspen, a black walnut, a few conifers clinging to the rock in a shaded alcove. Where the canyon widens and the sun penetrates, the only water, cold and clear enough to see it alive with small desert fishes, lies in deep pools in rock crevices. We filter it and fill our water bottles.

Only once do we run into another group of hikers. The sprawl of these canyons and the difficulties in negotiating them ensures solitude and silence. That night, we lay out our sleeping bags next to the stream, make a small fire, listen to the tree frogs whose calls echo through the canyon like the knocking of woodpeckers and watch the constellations pass across the narrow stretch of sky above us. The temperature drops from 80 to 40. Malusa, whose job it is to hike the desert, says that in the morning sun the temperature gradient can be so sharp that he's been able to measure a 15-degree difference between air around his feet and the air around his head.

After breakfast and the investigation of a blind canyon that -- had Malusa not had a good topographical map -- we might have mistakenly thought to be our way out, we begin to climb. The going at first is fine. There's soil beneath our feet and shade above our heads. But soon we come out into the full afternoon sun. For a few hundred feet we're surrounded by saguaros, but soon we've passed their narrow bandwidth and there's none to be seen. I know by now we're at 3,000 feet. Two thousand more to climb.

The trail becomes rocky and steep, thorny shrubs of catclaw tear at our arms and clothes, drawing blood. I notice that I'm not perspiring, the dry air draws off the moisture as quickly as it comes. At the crest of our climb we stop and drink and look back down toward the canyon out of which we've climbed. But it's gone, a narrow fissure lost in the sprawling mountain vista.

Facing west, we look out onto the vast sea of desert that separates us from the next mountain island: a 20-mile plain of mesquite; a hillside covered with the flowers of yellow mustard; foothills dense with saguaros, fruiting cholla cactus and flowering ocotillo.

After two days of climbing across rock, it takes a while to get back our flatland legs. But we don't mind the slow trek on wobbly knees back to the Jeep, for as we glance backward, we see that the face of the mountain has been set aglow by the setting sun.