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The Data Trail

Dave Bertelsen examines new buds on an Engelmann prickly pear cactus in the hills above Tucson.
The Sonoran Desert reveals itself to a passionate observer

Editor's note: This story has been selected for inclusion in The Best American Science Writing 2011(Ecco/Harper Collins) edited by Rebecca Skloot, author of the acclaimed book The Immortal Life of Henrietta Lacks, and her father, Floyd Skloot, an OnEarth contributor.

There's something different about the desert this morning. Something's missing. I don't notice it at first, but my companion, who has hiked in the Sonoran Desert every week for nearly 30 years, stops on the trail ahead of me and cocks his head.

"Listen... not a single bird," Dave Bertelsen says. "We should be hearing cactus wrens, canyon wrens, curve-billed thrashers, Phainopepla" -- a crested desert songbird -- "Gambel's quail, Gila woodpeckers. Even in the dead of winter there are birds. This is totally unique. We should be able to just walk along talking and hear birds. To stop and listen hard -- I've never had to do that before."

We're climbing a winding path on a rock-strewn slope in Saguaro National Park, a few miles west of Tucson's city limits. The sun, just four days shy of the winter solstice, will be rising soon. As the world pirouettes out of darkness, a diffuse pink light hides the stars and temporarily softens a hushed landscape in which almost everything seems to be barbed, sharp, or hard. In the still, cool air, a hundred million giant saguaro cacti from here to northern Mexico brace for the dawn, getting a few last gulps of carbon dioxide before sealing their pores and holding their breath all day long to minimize water loss.

Bertelsen doesn't know what to make of the absence of birds on this mid-December morning. For now it's another datum, brand new, puzzling, and disturbing. Besides, we're not on his favorite trail, north of the city in the Santa Catalina Mountains, the one he has walked 1,270 times -- and counting -- since 1981. During that span Bertelsen has amassed an enormous amount of information on the elevation, distribution, and bloom dates of some 600 plant species and subspecies; in 1997 he began keeping equally detailed records of the reptiles and mammals he has encountered during his weekly 10-mile hikes. Last year he added birds. "I now have 195,000 observations," he tells me as we saunter among saguaros, some of them as tall as four-story buildings. "It's a pretty substantial data set."

The decades spent walking this landscape have made the 68-year-old retired probation officer a leading expert on the Sonoran Desert's unique flora and fauna. Bertelsen's mile-by-mile notes of his treks are so precise and voluminous that a team of scientists at the University of Arizona in Tucson is using them to study the effects of global warming here. His records clearly show that about 25 percent of the plant species he has tracked have shifted their ranges to higher, cooler elevations, a response to desert summers that are now close to 2 degrees Fahrenheit warmer than they were 20 years ago. The change is significant, but Bertelsen worries more about stasis.

"To me what's interesting is not the 25 percent of plants that have adapted by moving up. It's the 75 percent that are not moving up," Bertelsen says. "Twenty-five percent is a lot, but 75 percent aren't adapting. That has big implications. It means most of the desert is not adapting to climate change. Since I started my hikes, the flora have declined 19 percent -- that's species in bloom per mile that I actually see when I'm hiking. For fauna it's a 43.5 percent decline per mile. We're going to lose a tremendous amount."

Compared with other besieged but more luxuriant ecosystems, deserts might seem to be relatively hardened to damage, harsh places inhabited by species already used to living on the edge. What, after all, could it matter if a desert, of all places, becomes a little warmer?

By one definition the Sonoran Desert isn't a desert at all. With 11 or 12 inches of rainfall in a good year, parts of it can exceed the 10-inch limit sometimes used to designate a desert. More generally, though, a desert is defined as a region where water scarcity imposes drastic constraints on life, and the Sonoran Desert easily meets that criterion. It covers approximately 100,000 square miles, from southern Arizona and southeastern California to Mexico's northwestern coast, including most of the Baja peninsula. Of North America's four deserts, the Sonoran contains by far the greatest diversity of plant and animal species. Unlike the Mojave and Great Basin deserts to the north and the Chihuahuan Desert to the east -- which all have cold winters and one rainy season -- the Sonoran has mild winters and two rainy seasons, one resulting from winter storms in the Pacific and another from summer monsoons that blow in from the Gulf of California.

Without that second pulse of moisture, the Sonoran Desert would blend almost seamlessly with the continent's other deserts. Low shrubs would dominate the terrain; some annuals would bloom in exceptionally wet years; trees would be scarce. Instead, the extra rain nurtures life found nowhere else in the world. Saguaros, the iconic cacti with great upraised arms, grow only here, along with more than 2,000 other plant species. More than 350 bird species, 60 kinds of mammals, 100 different reptiles, 30 types of freshwater fish, and hundreds of thousands of invertebrates live in the Sonoran Desert.

A winter storm watered the desert a few days before my first hike with Bertelsen, and it shows, if you know how to look. Saguaros, like everything here, have evolved to take maximum advantage of intermittent rains. The trunk and arms of a saguaro have vertical pleats, so the entire cactus can inflate like a bellows and store the water absorbed by its roots, which lie just three inches or so below the surface. The roots spread to a distance about equal to the height of the cactus and can guzzle 200 gallons from one rainfall, liquid life that will sustain a saguaro for a year.

"This one is full of water," Bertelsen says, pointing with one of his walking poles to a 30-foot-tall saguaro. The waxy surface of the tumescent cactus has become smooth and even.

image of Tim Folger
Tim Folger, an OnEarth contributing editor, has been writing about science and the environment for more than 20 years. In 2007 he won the American Institute of Physics science writing award. His work has appeared in Discover, National Geographic, Sci... READ MORE >