"We must be coming up to the Great Divide," I yelled over the engine noise of the old Dodge van. It was chugging in low gear over the sixteenth ridge in a row, this one covered with pines, the others below it with junipers and oaks; below that, we had begun our day in the cactus-studded warmth of the Sonoran Desert near Tucson. Now we were some 200 miles southeast, in the northernmost thrust of the Sierra Madre Occidental, the Mother Mountains."How can you tell we're close to the divide?" Anne wondered. It was her first time in the Sierra, and she was trying to take it all in."There to the west of us, look, the washes all seem to be draining away to the southwest. That's the Rio Bavispe watershed. They'll coalesce with other arroyos draining the western slopes of the Sierra, become the Rio Yaqui, and spill into the Sea of Cortez. I'm not sure about these to the east of us -- right now they're heading northeast, maybe to tributaries of the Rio Grande ... Aggie, can you see them on the map?"Aggie Haury, our septuagenarian navigator, sat in the seat behind Anne and me, maps on her lap, eyes scanning the horizons. "We must be in Chihuahua already," she surmised, "and if we're east of the Animas Mountains, which should be in New Mexico to the north of us, they either dump into playas or into the Rio Grande."A last scent of the pines wafted in as I rolled up my window. It was getting cold, for although the light was still flooding the ridge in front of us with a brilliant golden wash, we were within a half hour of sunset and more than six thousand feet above sea level. We inched up over the ridge and began our descent onto the high, tree-stippled plateau of far western Chihuahua. This was a relief, for I was feeling nauseated from lurching around so many turns, swerving and plunging to follow so many switchbacks. If I had not been hanging onto the steering wheel, I would surely have lost my lunch by now.There was another reason for my near-nausea. Compared to my traveling companions, I had burdened myself with an almost impossible goal for our week-long journey in the Sierra. Anne Fitzgerald had come here to document the history of the Americas according to the tortilla, the Mesoamerican bread of life, in all its varied cultural manifestations. Aggie Haury had come to learn more of the cultural connections between the U.S. Southwest and Mesoamerica as manifested in Casas Grandes, a prehistoric trade center between the two regions.And I had come to search for the ghost of Aldo Leopold, who sixty years before had heard the song of the Rio Gavilan clear as a mountain stream in these Sierra. He had come here late in his career, and it was perhaps the first and the last large tract of healthy landscape he would see before he died. Toward the end of his life, he admitted that it was in the Sierra Madre that he "first clearly realized that the land is an organism, that in all my life I had seen only sick land, whereas here was a biota still in perfect aboriginal health."In the two weeks of his first trip here, he saw no overgrazing, but ample signs of wolves and mountain lions. He saw no fire suppression, but scant brush and ample spacing of pines where wildfires regularly moved through the forests. The guacamaya, or thick-billed parrot, nearly extirpated from the adjacent United States, gave the forest's music its cadence.Perhaps most telling of all indicators was Leopold's sense that the watershed of the Rio Gavilan was intact, "a picture of ecological health," with slow, clear-running streams of water suitable for drinking. Yet he noticed something curious about the watercourses draining into the Gavilan: they were all punctuated with check dams of dry masonry put in place by prehistoric Native Americans. Although Leopold was not aware of recent Indian affairs -- the last Apache raid in the Rio Gavilan had been only the year before he first visited the watershed -- he felt a strong affinity with its earlier indigenous inhabitants:There once were men capable of inhabiting a river without disrupting the harmony of its life. They must have lived in thousands on the Rio Gavilan, for their works are everywhere. Ascend any draw debouching on any canyon and you find yourself climbing little rock terraces or check dams, the crest of one level with the base of the next. Behind each dam is a little plot of soil that was once a field or garden, subirrigated by the showers which fell on the steep adjoining slopes ... the deer love to lie on these little terraces. They afford a level bed, free of rocks, upholstered with oak leaves, curtained by shrubs. One bound over the dam and the deer is out of sight of an intruder.In nine days, Leopold and his friends saw more than 180 deer among those terraced slopes; a year later, visiting with his brother and son, he saw another 250 deer in sixteen days. Though he ostensibly took both trips for sports hunting, Leopold himself did not shoot a single deer during his brief immersion in the Sierra Madre. Instead, he wrote notes that eventually wove their way into two of the most sonorous essays within A Sand County Almanac: "Song of the Gavilan" and "Guacamaya." In these essays he goes furthest toward defining ecological health in tangible terms."The Sierra Madre," he mused, "offers us the chance to describe, in actual ecological measurements, the lineaments and physiology of an unspoiled mountain landscape." But there is a nuance missed by most of Leopold's philosophical disciples over the four decades since the Sand County and Round River essays did much to forge the developing ethics of the so-called environmental movement. Like other montane ecosystems, which collectively cover one-fifth of the world's land, the Sierra had not only been heavily peopled at one time but their inhabitants had also managed them in a way that did not deplete their diversity. Leopold suggested that indigenous people must instead have contributed to its ecological health and stability. The obvious human manipulations of the watershed did not keep Leopold from calling it "unspoiled." For him, the term "unspoiled wilderness" took on new, meaning, one that could include the appropriate presence of cultural features set in place by any inhabitants, not only "Indians." This entire notion remains a contradiction in terms for most wilderness advocates today, who cannot imagine any appropriate cultural presence in "the wild."Although we would be traveling to the north, south, and east of the Gavilan watershed where Leopold had hunted for ecological health, I was still hoping to catch an earful of that same music. What was the benchmark of health that Leopold had found, more precious than the fountain of youth sought by Ponce de Leon centuries before? Could I still hear, as Leopold apparently had, how human voices might blend with that larger harmony rather than transforming the concert into cacophony? At this point in my life, perhaps more than any other, I needed such a benchmark of health. I had begun to come out of the malaise of a three-year bout with chronic fatigue; at the same time, my marriage was suffering from innumerable pressures, and my work conserving the native seeds of the Sierra Madre was suffering as well. I needed to remember why wildness had mattered to me to begin with: mountains and rivers, compadres and partners, Native American traditions and natural forests, wild chiles and ancient beans, breathtaking barrancas and prehistoric terraces where century plants were formerly cultivated. I needed to be wooed again by the sheer diversity of life found in one pocket of this continent, to be grateful again about having glimpsed the mountains that mothered much of this diversity. And I knew it was a diversity facing pressures of unprecedented scale. As Derek Dennisten has warned, "there is a dangerous disproportion between the great importance of mountain ecosystems and their cultures and the attention they receive from national governments and international organizations -- a disparity that increases the risks that now confront both the mountains themselves and all those who rely on them."Just after dawn one cool December morning, Aggie, Anne and I walked out onto a precipice overlooking la Cascada de Basaseachic, watching water fall for close to a thousand feet, atomizing into mist before it hit a large plunge pool on the canyon floor. We shivered as we stood on a huge boulder of volcanic tuff, a mix of conifers above us, wind whisking through their needles. Apache pine. Chihuahua pine. Ponderosa. And my favorite, with its downturned needles forming a crown of frowns: pino triste. We were less than fifty miles from the continental divide, perched at seven thousand feet above sea level, wrapped in multiple layers of clothes. Still, the wind whipped through these layers, chilling our skin into goose-bump-pocked hides.As I scanned the cliff face across from us -- now partially veiled in mist, fog and wood smoke -- I remembered a previous visit I made to Basaseachic, during early fall. With a Tohono O'odham linguist and a Portuguese-American zoologist, I had climbed down a near-vertical set of rotting log steps to see the cascade from below. I was struck by the presence of so many plants other than pines, positioned in "window boxes" along the cliffs where they would be relatively protected from freezes. I recognized five kinds of century plants, including the mescalito the Indians call taiehcholi, which is restricted to middle elevations of the northern cordillera and found nowhere beyond it. And three little grayish-blue succulents in the crassula family clung to rock faces even though I associate them more with the foggy coasts of Baja California. Then there was a strange species of sotol nearby, akin to those of the Durango highlands. Lower down, I believe I spotted a handful of bromeliad species, relatives of pineapples and Spanish moss, life-forms with an orientation toward the tropics.As we looked down on the barranca from pine-topped plateau to deeply carved canyon bottom, I realized that we were facing a microcosm of the entire Sierra. Its steep elevational gradients allowed strange juxtapositions of tropical and subalpine life within a matter of miles; palms and pines, columnar cacti and junipers grew together in some canyons. Balconies and window boxes along canyon walls maintained refugia of plants and animals hundreds of miles from their core distributions. Rich Spellenberg, Toutcha Lebgue, and Rafael Corral, who have compiled records of plants collected at Basaseachic over the decades, have tallied up 825 species inhabiting an area of less than twenty square miles. That is perhaps one-fifth of all the kinds of plants now identified from the northern Sierra Madre.Yet what humbled me in the face of this barranca was not merely its depth, its size, its precipitousness. It was how little we actually fathom of life in the Sierra. When botanists Richard Felger and Robert Bye helped me nominate the Sierra Madre Occidental to the World Conservation Union and World Wildlife Fund as one of the few centers of megadiversity for plants anywhere in North America, we had to concede that botanists have probably identified as yet only five out of every eight plants that may grow in the region. My Desert Museum coworker Tom Van Devender is discovering two or three new species there every year, even though he is not primarily a plant taxonomist. When all the region's plants have been catalogued and identified -- if that mythic moment ever occurs -- it is likely that four thousand kinds of plants will be tallied from the northern reaches of the Sierra Madre. In an area less than a quarter of the size of Texas, these mountains mother about the same number of plant species as the whole state does. Japan, too, harbors roughly the same number of plants, but its islands cover twice the area of the northern Sierra. The Sierra that stretched before me clear up to the spot where Leopold camped in the Rio Gavilan harbor the northernmost blending of Mexico's two richest floras -- and Mexico ranks among the top three centers of bio diversity in the world.The current preoccupation with assessing the plant diversity of various regions was not in vogue when Aldo Leopold entered the Sierra. Diversity in and of itself did not factor into his definition of ecological health. But he knew when an area's richness of plant life had been depleted by cows, sheep or goats grazing at unsupportable densities; he knew when their cropping of understory vegetation resulted in fire suppression and diminished heterogeneity among forest patches. Leopold probably didn't know that Apaches, who had kept the forests of the Gavilan burning until just the year before his first trip there, had made raids that kept ranchers from overstocking the watershed, as other ranchers had overstocked the ranges just north of the Chihuahuan border.Leopold's forays into the Sierra were as short as my own, a couple of weeks at a time at most. That's not enough time to grasp the full richness of its flora and fauna. I could identify by sight just about 500 of its species -- a pittance of its total diversity. Leopold probably never saw the Sierra's flora in complete leaf, for his visits were during the cool season, months after the widest range of wildflowers, insects and birds are at their peak. Somehow, though, that wildlife biologist raised in the more monotonous Midwest sensed that the Song of the Gavilan was filled with many voices.It is also filled with many tastes, as Leopold himself did recognize: "To the superficial eye the Gavilan is a hard and stony land ... But the old terrace-builders were not deceived; they knew it by experience to be a land of milk and honey. These twisted oaks and junipers bear each year a crop of mast to be had by wildings for the pawing. The deer, turkeys, and javelinas spend their days, like steers in a corn field, converting this mast into succulent meat. These golden grasses conceal, under their waving plumes, a subterranean garden of bulbs and tubers, including wild potatoes."I have eaten those wild potatoes myself, along with many of the other 250 wild crop relatives that still occur in the region. Although I love the tastes and textures of the wild potatoes, chiles, tomatillos, teosintes, beans and strawberries of the Sierra, they are but a small component of comestible cornucopia there. More than 400 plant species are eaten by the tribes of the northern Sierra Madre; historically, the Tarahumara alone utilized at least 220 kinds of native plants as food. But perhaps these numbers mean nothing; Bob Bye has estimated that by the time modern ethnobotanists entered the region with all the tools needed to identify and analyze these nutritional resources, less than 40 percent of the plants prepared as food in previous centuries remained in use.Just as the forests of the Gavilan have not remained fully intact since Leopold's death, life in the remaining indigenous communities to the south -- in a spur range known as the Sierra Tarahumara -- has also undergone rapid change in this century. Most of the 66,000 Tarahumara living in the Sierra still converse in their native tongue, but their degree of dependence on-the land's diverse plant resources is diminishing. When Aggie, Anne, and I detoured into the heart of Tarahumara country for several days, we were amazed just how deeply this dependence had been shattered.On a crisp and brilliant morning, we awoke not far from the railroad yards of Creel, a lumber town that is blanketed in wood smoke and fog at this time of year. Shafts of light touched down as the fog broke up, illuminating the piles and piles of pine logs ready to leave the Sierra by rail. Nearly all of the surrounding coniferous forests have already been cut, some as many as four times over the past century. The old growth remaining nearby occurs in isolated patches and is under constant threat. Not far away, in the town of Anahuac, a massive pulping mill has recently been renovated and expanded to the tune of 350 million dollars; to repay their loan, the mill owners will turn thousands of acres of mixed pine forests into toilet paper destined for American markets.The loss of forest canopy nearby did much to reduce the abundance of edible plants in the understory, and a recent four-year drought diminished them even further. As we were leaving Creel for the hinterlands, I ran into Padre Verplanken, the priest who had organized much of the food relief effort for drought-stricken communities bereft of both wild and cultivated crops."At least 119 communities were affected," Verplanken reported. "We received 190 metric tons of foodstuffs by train, but the hard part was ensuring distribution out to remote areas of the Sierra."It was hard to imagine that a place with so much natural abundance had been so devastated in just a few years. The drought was obviously not the sole cause of the hunger; it was primarily an aggravating factor once the land's bounty had been depleted. As we drove out of Creel, we saw slope after slope with no more than immature pine saplings on them; the formerly rich carpet of other wildlings had been frayed and desiccated. I have since learned that the Sierra Madre forests are one of seven global hot spots with at least half their area more than a mile high where the endemic plants are threatened by imminent destruction.We headed out toward the Tarahumara community of Panalachic, stopping now and then to talk with farmers, pottery makers, loggers and herders, many of them mestizo rather than Tarahumara. Along the way, we kept seeing a car that would pass us, head off on a spur road, and then, later, pass us again. Finally, I flagged the driver down, curious to see what he was up to."I'm selling factory-made flour tortillas and white bread to all the ranchos out this way. The maize crops have almost completely failed for several years running; I don't know how it will be this year, but I bet not too good. Before the shipments of dry masa and other flour starting coming in, people had hardly any grain. So I started this route that I run every few days. It used to be that just the Mexicanos would buy the bread and flour tortillas, because the Indians still prefer corn tortillas. But now they've gotten used to it, and the Indians buy bread as well."Several hours later, when we arrived at the fields of Manuel Torres Lerma, his entire Tarahumara family was out harvesting what little blue corn his sloping field had produced that season. The women, all dressed in brightly colored skirts, blouses, and scarves, with children wrapped onto their backs, moved between the corn rows, husking the ears and tossing the fuller ones into gunnysacks carried away in a wheelbarrow. Manuel stripped an ear of a corn stalk and husked it to show us what they were up against."The rains came too late, not until early August," he explained. "Look, all the grain on the ends of the ears is shriveled up, because we planted so late that they could not mature once the late-season drought came again. See those plants lower down on the slope? They're the only ones with fully filled ears. We fertilized that area with goat manure, and that is the only place where the plants matured before the late-season drought ... Those ears are longer, brighter, richer in blue. The others already look bleached by the sun."Maize cultivation has gone on in the Sierra for centuries, but perhaps it was done on a smaller scale before the introduction of draft animals and the walking plow. Now, where forests are cut upslope from fields, and fields are fallowed a good portion of the year, soil erosion can be severe. In many of the more extensive fields found near Tarahumara villages, there is a curious lack of terracing to control soil and water flow, although some terracing can be found in small orchards and dooryard gardens. I thought of the ominous warning from the Worldwatch Institute regarding montane farming cultures: "Time-tested mountain farming practices, many of which go back centuries, are being abruptly threatened by population growth, the fast-growing global economy, and the overwhelming cultural influences of the plains."The infrequency of terracing today is surprising, considering how extensive terrace building was prehistorically. Aldo Leopold was neither the first nor the last to marvel at them. Archaeologists such as Aggie's deceased husband, Doc Haury, along with his flamboyant sometimes-crony, sometimes-rival Charlie DiPeso, were altogether astonished by the extensiveness of the trincheras (terracing) left all over northern Mexico and adjacent Arizona by prehistoric peoples. One time, at a conference I attended on regional history where Doc Haury and Charlie DiPeso were the featured speakers, DiPeso went to the podium wearing a bright red cochineal-dyed cape he had purchased somewhere in Mexico, and presented a slide of ruins at the foot of the Sierra, with dozens and dozens of stone terraces reaching beyond them for as far as the eye could discern. Waving his hands up in the air as if he were scaling a mountain, DiPeso cried out, "Not a single drop of water fell within the entire watershed above that prehistoric village without having had its erosive force broken by terraces and check dams, then channeled into canals for agricultural and domestic uses. Every slope of that watershed was managed to conserve water and soil!"DiPeso's predilection for hyperbole was well known to his bemused audience, which appeared more comfortable with Doc Haury's scientific precision and more modest behavior. But both men had documented extensive water and soil control structures during their half century of archaeological excavations. Why did such a commonsense practice atrophy within the Sierra?Aggie, Anne, and I knew that we could not answer this question, but we did want to take a look at prehistoric terraces. We headed north toward Casas Grandes, Chihuahua, where DiPeso had devoted much of his career to the archaeological excavation and interpretation of the Paquime ruins. An old friend of mine, Paul Minnis, had been following up on DiPeso's work at the nearby village of Mata Ortiz, situated on the high grassy plains at the foot of the Sierra. He had given me a map showing how to get to some terrace fields a few miles west of Mata Ortiz, but once again, I needed the help of Aggie's good eye and map-reading skills to help us find these prehistoric treasures."We go down into this drainage and cross a little wash ... Yes, this must be it ... Then, drive upslope past a corral ... There, I think those may be some rock alignments off to the left! Don't you think they could be what we're after?"I pulled the van over to one side of the rutted track across the grassy slope, and we opened our mud-caked doors to a fierce wind howling down out of the Sierra. Each of us spotted a different line of cobbles extending across the plain, perpendicular to the prevailing gradient of slope. The wind's penetrating chill found its way beneath my bluejean jacket as I ambled along the edge of a hundred-yard-long terrace. Nothing had been planted above its lip of volcanic cobbles for several centuries, but it was as obviously a man-made feature as the terraces Leopold had seen many miles west of where we stood. And unlike the Tarahumara fields we had seen a few days before, these had not been plowed or planted with short-lived herbaceous crops; they had been home to thousands of rosettes of hardy perennial century plants, whose roots held soil in place year-round. We were looking at the remnants of one of the steadiest forms of agriculture in the world; it reminded me of how I still sought such stability in my own life.I turned my back to the wind and stood above the westernmost terrace trying to count the other half-buried linear alignments of cobbles that I could see on the gently sloping plain below. As I turned my eyes to the next row of cobbles, a black-tailed jackrabbit suddenly appeared, as though it had melted out of one of the volcanic rocks. It had been perfectly camouflaged, with its ears pulled back and its white flanks hidden by a furtive crouch. The jackrabbit ran downslope from the terrace, then froze again, assuming an even lower crouch against the ground, blending in with the buff color of the grama grasses surrounding it. Within another ten paces, I scared up two additional jackrabbits, both of which ran downwind and out of sight.When I rejoined Aggie and Anne, we had all found different paths to the same dry watercourse, which showed evidence of hand-dug canals jutting off from it, draining toward the terraces themselves. Along the widest portion of the watercourse, there was an island-like bench of extremely fertile soil set in the midst of one drainageway; the entire "island" had been carefully terraced and partitioned into irrigable patches, each lined with larger, broader cobble borders than the ones we had seen up on the grassy plains. We marveled at the careful reading of contours and flow patterns that the prehistoric agave farmers must have made to ensure that this field mosaic captured sufficient moisture for its crops year after year. And although no agaves remained and no humans fed themselves from these terraces' bounty, I was gratified to know that the lush grassy cover resulting from the soil and water conservation accomplished by the terraces still fed a few jackrabbits and sheltered them from the winds.For millennia, the forests of the Sierra have offered the same ecological services that the terraces provide on a microcosmic scale: keeping soil in place, slowing the flow of water and filtering it along the way, and offering an abundance of food to wildlife and human inhabitants alike. The rivers that flow out of the northern Sierra Madre provide irrigation water to several of Mexico's major breadbaskets: the Yaqui Valley of Sonora, birthplace of the Green Revolution; the coastal plain of Sinaloa, where most winter vegetables destined for the western United States are grown; and the lower Rio Grande, which provides the Midwest with many of the tomatoes and salad greens blessing its grocery shelves every winter.Should more of the forest cover of the Sierra be reduced to stubble, these ecological services will dry up with the same rapidity as the understory edibles. In the early 1990s, I assisted my Native Seeds/SEARCH cofounders, Mahina Drees and Barney Burns, in their efforts to halt a massive forest-harvesting project in the northern Sierra that was being promulgated by the World Bank. Fortunately, prominent ecologists in Mexico -- Exequiel Escurra, Luis Bojorquez, Robert Bye and others -- marshaled considerable evidence to convince the World Bank that the Sierra Madrean forests were more valuable when managed intact than when cut down and flushed away.When I received a call from a scientist inside the World Bank who confided that the multi-billion-dollar project was being scrapped indefinitely, I was elated. The same type of logging that had impoverished the Rio Gavilan since the time of Leopold's death would be halted -- or at least, postponed -- in the Sierra Tarahumara. But that assumption was naive, for it did not take into account that many American industries had heard about the Sierra's forestry resources in the meantime.Over the past five years, since the World Bank was forced to scrap its grand plan for the Sierra, one remnant forest after another has been placed on the chopping block. The kind of integration of human activities within the forest community that Aldo Leopold envisioned in the Rio Gavilan is less and less possible, for small-scale uses of non timber resources are being overwhelmed by massive private-funded extractive industries.Not long after Aggie, Anne, and I returned home, I began to receive notices of new field surveys determining that more than 99 percent of the old-growth forests of the northern Sierra Madre have now been cut -- forests that once covered an area the size of Denmark. The thick-billed parrot is retreating farther and farther south into Mexico as more and more Chihuahua pine stands are felled. The Mexican government has recently established El Carricito as Mexico's first Important Bird Area in the southern Sierra Madre near the Durango border, in part because it offers a refuge for the thick-billed parrots, Mexican spotted owls, and other birds now under siege in Chihuahua.From the terraces on grassy plains near Mata Ortiz, the Tarahumara gardens near Panalichic, and the protected wooded canyon below Basaseachic Falls, I could triangulate to feel what the Sierra must have been like when human settlements still had the capacity to contribute to ecosystem health. But by the time of my visits, something was missing from these northern Sierra habitats, something that Aldo Leopold believed to be essential:It is easy to say that the loss is all in our mind's eye, but is there any other ecologist who will agree? He knows full well that there has been an ecological death, the significance of which is inexpressible in terms of contemporary science. A philosopher called this imponderable essence the numenon of things. It stands in contradistinction to phenomenon, which is ponderable and predictable, even to the tossings and turnings of the remotest star ... Be that as it may, I here record the discovery of the numenon of the Sierra Madre: the Thick-Billed Parrot.Leopold recorded that numenon late in his life -- and late in the life of healthy forest habitats within the Rio Gavilan. His son, Starker Leopold, and many others who have entered that watershed since then, have not had the same opportunity to hear roistering flocks of such velvety green, scarlet and gold-ornamented aerial delights. The Chihuahua pines are more even-aged today than they were before, and old ones are tougher to find. In some places, I am told, the prehistoric terraces have been blown out by floods and, in others, buried by layers of sawdust and silt.I have a gut feeling that the old partnerships between nature and culture have momentarily slipped out of our reach. It is the same feeling I have about the shattered intimacies and tattered landscapes that haunt my own home.I am one of many biologists who simply hope that we have not surveyed such areas well enough to know that the likes of thick-billed parrots and imperial woodpeckers are truly gone for good. If they are, I only hope that those who venture back into the Sierra will hear another kind of call in their stead: the ghost of Aldo Leopold wailing in the wind, mourning the loss of a large tract of land that once had its ecological health intact, a health in which humans formerly played an essential part.