The Data Trail
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. The sun ascends with us as we continue up the slope of what was, 65 million years ago, the caldera of a volcano. Although the morning remains cool, not more than 60 degrees yet, there is no shelter from the sun. Bertelsen moves at a careful, steady pace, though he’s a bit slower now, he says, than before his triple-bypass surgery in 2004, the same year in which he broke a leg and had to be helicoptered off his favorite mountain trail one night. He prefers to start hiking around midnight — nocturnal activity being a sensible strategy for any desert mammal — and will walk through the night and into the following afternoon without sleeping. He’s sturdily built, wearing a black fleece jacket, khaki pants, sunglasses, and a broad-brimmed hat over his straight gray hair. Thousands of tons of water surround us, sequestered in a forest of tall, green living columns; a single mature saguaro might hold as much as eight tons. Water, water, everywhere, but a lost hiker — or an illegal immigrant — would not find a drop to drink in a saguaro grove; the cactus binds its water in a viscous, slimy fluid. The recent rain wasn’t enough to save some saguaros. Paloverdes — thorny-branched trees with green photosynthetic trunks and limbs that shed their leaves during winter — are also suffering. As we wend up the flank of Wasson Peak, which rises 4,639 feet above sea level, Bertelsen’s count of dead or dying plants ticks steadily upward. I ask him if we’re seeing the impact of the Southwest’s protracted drought. "No question," he says. "The last time I was on this trail, maybe 10 years ago, I didn’t see any dead saguaros, certainly no dead paloverdes. That’s one of the reasons I wanted to come here. I want to see what’s happening. That one’s dying," he adds, nodding at the blackened top of a tall saguaro, multi-armed like some cactus incarnation of Vishnu. Saguaros typically don’t grow arms until they’re at least 75 years old. Judging by the size of its limbs, this one must have been growing for more than a century. It beat many odds during its life, starting as one of the few survivors of the tens of millions of pinhead-size black seeds produced by its parent. Like most saguaros, it probably grew in the lifesaving shade of a nurse plant — a paloverde, acacia, or ironwood tree. After 10 years it would have been just over an inch high; by 30 it would have reached two feet, its growth accelerating exponentially. It endured a drought lasting several years in the 1950s. But the current 14-year drought — the longest in at least a century — is killing it. Saguaros may take two or three years to die. Some remain majestic even in death, standing fully upright, their gray, tubular woody skeletons flensed of all flesh. They’re easy to anthropomorphize. The Tohono O’odham, one of the more than a dozen indigenous cultures of the Sonoran Desert, use the same word — O’odham — for both "people" and "saguaros." ( Tohono means "desert.") In one of their old stories, the first saguaro appeared when a young girl, neglected by her mother, was transformed into a cactus, her arms forever raised to the sky. The vagaries of life here — one saguaro dies while others on the same hillside stand replete with water — suggest to Bertelsen a biotic complexity that defies any sort of generalized explanation. "What makes something appear and disappear? Maybe a sixteenth of an inch of rain, maybe something that is so subtle we’ll never be able to figure it out," he says. "I think it’s too simple to try to explain everything in terms of temperature and precipitation. Maybe rain a day earlier or later makes a big difference. There’s a whole bunch of stuff that I’ll see once, and then see 10 years later. I call them desaparecidos , the disappeared ones. Everything doesn’t bloom every year. You’ve got to watch over a long period of time to see what’s happening." As we walk, Bertelsen keeps up a running commentary on nearly everything we pass: ocotillo, a spindly, spiny shrub with astonishing flame-red flowers; desert mistletoe, a parasite lodged in the branches of paloverdes and other trees; barrel cacti, some of which lean so far toward the sun they uproot themselves; bunches of native grasses — threeawn, bush muhly, tanglehead — that look nearly identical to me; at least two kinds of prickly pear cactus — Engelmann and mojave, their broad pads often gouged by pack rats; teddy bear cholla, singularly uncuddly, even for a cactus — five feet of "don’t even think about touching me," with stubby, plump, jointed arms completely covered with barbed yellow spines that make the whole plant gleam in the bright morning light. Bertelsen carries a comb with him on his hikes in case he brushes a teddy bear — the spines will lodge in your fingers if you try to remove them from elsewhere on your body. "You know a true desert rat because they always have a comb," he says. "Not for their hair." Bertelsen’s careful observations have been honed over the years as what started as casual hikes became something more. "I had read something by Thoreau," he says, "where he wrote how you could tell the time of month by what was blooming. So I started keeping track of blooms — not for anyone, just for curiosity. I always had a journal, but it quickly became obvious that I needed something more. So I started using a checklist. I started making comments about drought in 1994. Plants had been moving up in elevation, but it happened so gradually it was hard to see; you’re too close to it. That’s why stepping back and looking at the data is so useful. I was doing this because I thought it was interesting. I never thought it would be important." "I just about fainted when Dave explained what he had. There was so much information there waiting to be mined." I’m meeting with Theresa Crimmins, an ecologist at the University of Arizona in Tucson. She’s recounting how she and her husband, Michael, a climatologist at the university, first met him five years ago after a talk Michael gave on climate change. "Dave came up to me after the talk and said, ‘I have a big data set. I don’t know if you would be interested,’" Theresa says. "He’d been doing this for 20 years. A paid scientist could never collect something of this magnitude." When I mention Bertelsen’s striking observation that as many as 75 percent of the desert species he’s tracking don’t seem to be adapting to climate change, she offers a more measured judgment. "Dave has an incredible data set," she says, "but it is segmented by mile; there are a lot more species that could be showing more subtle responses on smaller scales that we’re not able to catch right now. " Michael Crimmins echoes his wife’s admiration for Bertelsen’s dedication. "We’ve worked with him to tease out the patterns in his data," he says. "A data set of, first, that quality and, second, that breadth, just doesn’t exist. You would never see this in a funded project. The NSF" — the National Science Foundation — "might give you five years. You couldn’t plan to collect data like this, and Dave is just so good at it, paying attention and being systematic — way better than some field scientists or grad students. Dave does it because he loves it. "What we’ve found is that indeed some plants that bloomed at lower elevations when Dave began are now blooming at higher elevations," Michael continues. "But additionally, we’ve found a very complex dance of species. Some are responding strongly to climate change, some not so strongly. Some are blooming at a lower elevation instead of higher. The true complexity of an ecosystem is that species respond individually to climate change. They’re not going to get together in a forum and decide as a biome or ecosystem that they’ll do this together; they all have unique strategies to deal with climate. The response of many different species to climate change is wrapped up in Dave’s data set, and it’s very complex. "We’re fighting a bit of conventional wisdom here: that species will move upslope, following an envelope of perfect climate for them which is constrained by temperature. You’d expect that as it gets warmer lower down, species will move up. That works for some species, but not for all. The rate of change is the big story with climate change. When you talk with people, one of the arguments they’ll throw back at you is that the climate has always changed, and that is absolutely right. It’s the rate of change that is the problem right now. It’s changing so quickly that it exceeds the adaptive capacity of some species." Some adaptations are straightforward, Theresa tells me, and others less so. Warmer temperatures have allowed saguaros, for example, to expand their flowering range to higher elevations. But the flowering range of other plants — ragweed, wild carrot, and greenspot nightshade, to list just a few — has contracted, their upper-elevation limits remaining unchanged while the lower boundaries of their ranges have moved higher. "For the species showing contractions of their flowering ranges," she explains, "what we think might be going on is that warmer temperatures are becoming increasingly intolerable at the lower ends of their distributions and low-temperature triggers for signaling dormancy are not being reached." The Crimminses published their analysis of Bertelsen’s plant data last year in the journal Global Change Biology . They’re only beginning to study his animal observations. "We need to get that into a database, primarily his information on birds," says Theresa. "He’s saying, just anecdotally, that he’s seeing massive declines in the number of species." Climate change is exacerbating another, more imminent threat to the Sonoran Desert: an invasive species called buffelgrass. "We have an invasion by an African grass that’s capable of unhinging the Sonoran Desert," says Julio Betancourt, a paleoecologist with the United States Geological Survey. "It’s more disastrous than anything climate change can throw at the desert." Betancourt, whose office is just down the hall from Theresa Crimmins, has spent most of his career studying climate change and deserts. Buffelgrass has been introduced to the southern United States as a fodder crop at various times since the late 1800s. It has now spread across southern Arizona and into Mexico, where it outcompetes many native plants. Because it evolved in a part of the world characterized by seasonal fires, it quickly reestablishes itself in areas that have been burned. Betancourt and other scientists worry that the expansion of the range of such a fast-growing species to higher altitudes as a result of global warming could convert the Sonoran Desert into a flammable savanna. The Sonoran Desert has been essentially fire-free for 10,000 years or more. Stands of vegetation tend to be separated by wide stretches of bare, rocky ground, which limits the extent of fires. Very few Sonoran flora are adapted to fire — saguaros die after even small blazes. "Before buffelgrass was here, you could douse a paloverde with gasoline and the fire wouldn’t spread," Betancourt says. In parts of the desert where buffelgrass has covered formerly bare ground, that’s no longer the case. Buffelgrass and other invasive species provided much of the fuel for the fires in 2005 that burned a million acres of desert in Arizona and Nevada. The fires in Arizona, which were sparked by lightning, killed some 80 percent of the vegetation on 250,000 acres, including the largest known saguaro cactus, called the Grand One. "That’s rivaling forest fires," Betancourt says. "I think the premise of conservation will change in the Sonoran Desert. It used to be that you would try to preserve open spaces and let nature take care of itself. That’s no longer the case. If you can’t manage the resource, in a few decades you could have a flammable grassland instead of the saguaros and paloverdes and Gila monsters. The buffelgrass is a test. If we don’t solve it, a lot of things will be moot." "That’s where God lives," Bertelsen says, pointing to a peak on the southwestern horizon. We’re on the summit of Wasson Peak, some 2,000 feet above the trailhead. "It’s Baboquivari Peak. The Tohono O’odham say a god lives there." Tucson spreads below us; its distant edges shimmer in the warm air and seem to lap against the base of the Santa Catalina Mountains to the northeast, the site of Bertelsen’s 1,270 hikes. On the way back down the trail, Bertelsen quizzes me every 20 minutes or so, asking me to identify various grasses. By mid-afternoon I’ve managed only two correct answers. "What’s that?" he asks, flicking a walking pole. Long seconds pass while I ponder a dry grass tipped with stiff bristles. "Threeawn?" "All right!" He seems genuinely pleased. "You’ve recognized three things. I do that every time I bring someone out, try to get you to recognize three things that you wouldn’t have recognized before. Saguaros don’t count." Later, after pulling a few tufts of buffelgrass in a dry wash, we rest beneath a rock overhang, and the shade sharpens my appreciation of the role nurse plants play in protecting young saguaros. As we drink water, I ask Bertelsen what compels him to walk the same 10-mile trail week after week, year after year. "I don’t think I’m compulsive," he says. "I’m drawn to it. I don’t feel I have to — I just really want to. Every trip there is always something different. Always." I tell Bertelsen his words remind me of a quote from John Burroughs, the nineteenth-century American naturalist: "To find new things, take the path you took yesterday." "Oh, I haven’t heard that!" he says. "I like that. I always tell people, it’s not 1,270 hikes. It’s a hike of 12,700 miles. It’s one journey. It’s not a separate thing. That’s exactly the way I feel about it. It’s one long, continuous walk."
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The Data Trail