The temperature at the base of the mountain was 100 degrees, not unusual for June in Arizona, but hail began to fall in the tiny wetlands called Bear Wallow Spring, at an elevation of 10,500 feet, on the tallest of the sky islands.
The white granulation fell straight down, caroming from the lush green stalks of the corn lily plants, bending the blossoms of the Franciscan bluebells. No trees interrupted the hail because the forest canopy had been incinerated the summer before. Just spruce trunks with peeling bark stood in the circle of the wet, while all around the oasis, the ground was black, and the woods were splintered and charred. Soon the decapitated trees would tip over.
The fire had burned so hot that it had bleached and split the rocks and strewed their fragments like seashells on black sand. Nothing was alive but spiders—everywhere spiders hauling their egg cases in and out of holes in the ashes. Yet within the magical ring of Bear Wallow Spring (magical to the Apache who used to drink here), hip-high spruce and fir prepared to recolonize the peak of this sky island.
Sky islands is the popular term for a dozen disconnected mountain ranges in southeastern Arizona. A few of the ranges spill over into New Mexico, and others are located south of the border. Ecologists refer to the archipelago of mountains as the Madrean Archipelago; the U.S. Forest Service knows them as parts of the Coronado National Forest. Arising from the Sonoran Desert, the cactus sea that links them, each range has its own biological personality. But they all burn by the same rules.
Physical separation has caused their flora and fauna to diverge. Like the finches observed by Darwin in the Galápagos Islands, genetically distinct subspecies of squirrels, lizards, ants, and lichens have evolved on different outposts. Other differences are due to latitude. For instance, the crest of the Pinaleno range, where the 2004 fire occurred, hosts Engelmann spruce and cork-bark fir, a combination found also in Alaska. The trees are a relic population from the Ice Age. You won't find cork-bark fir at the top of the Chiricahua Mountains, 50 miles farther south. Conversely, neotropical hummingbirds and Apache pines, to name some Chiricahua species, aren't seen in the Pinalenos. Large animals like black bears, cougars, and bighorn sheep used to migrate between the mountain ranges, but human development has cut off their corridors and reinforced each range's biological isolation. As a result, most of the natural action in the sky islands takes place in the vertical dimension, including the action of wildfires.
As islands they seem remote, not because they're far away but because they're far above. Climbing them, you start in Mexico and end up in Canada, each 1,000-foot gain in elevation the ecological equivalent of a 500-mile trip north. The mountains pack more environments into a shorter vertical distance than any terrain in the United States. On the ranges with roads, like the Pinalenos, the Chiricahuas, and the Santa Catalinas, a botanical and meteorologic inventory of western North America can be completed in a half hour.
"Up here you can dial up any kind of weather at any time," said David Hodges, as he brushed hail from his jacket at Bear Wallow Spring. Hodges works for the Sky Island Alliance, a Tucson-based environmental group. "The high elevations are magnets for monsoon storms," he added. Monsoonal rain in Arizona occurs in midsummer, when south winds draw moisture from the Gulf of Mexico and fling thunderheads and lightning against the rims of the sky islands.
Hodges led the way up a crunchy slope still smelling of smoke to an overlook called Hawk Peak. As if on cue, a lightning bolt struck nearby, producing a big flash at eye level and a simultaneous bang, causing him to duck and leap at the same time. Most wildfires here are started by lightning early in the monsoon season, before the rains come to put them out, like the Nuttall fire in 2004, which began on a shoulder of the Pinalenos and catapulted up the mountain, eventually devouring 29,000 acres.
That blaze became an emergency for the Forest Service because of the Mount Graham International Observatory. Hodges pointed from the overlook to a boxy telescope, still under construction. Pale green, it towered over a phalanx of unburned conifers. Two smaller telescopes lay behind it. An irony of the fire and others like it is that Forest Service crews ignited a rival conflagration, trading on favorable winds. They "backfired" the woods near the telescopes before the main fire could get there. The burnout spread fiercely. When it was over, the greatest damage to the forest was man-made.
There's more to the story, however. Why did the Nuttall fire blow up, given that lightning strikes are a dime a dozen in these parts? Normally, spruce-fir woods are insulated from fire by cool temperatures and soggy ground. Although the trees burn easily, they don't burn often, perhaps once every 200 or 300 years. Conditions have to be just right. What made conditions perfect in 2004?
One factor was an outbreak of insects that began in the late 1990s, concurrent with a drought. Sapped by attacks from an exotic aphid, a moth, and two species of bark beetles, the spruce-fir zone in the Pinalenos was scrofulous and drier than normal. Second, the vegetative zone below the summit had become a tinderbox. Pine and Douglas fir populate the mixed-conifer zone; it was overstocked with woody fuel. Also, Engelmann spruce and cork-bark fir had migrated downward in recent decades, nestling beneath the taller conifers like so much kindling.
"Things were out of whack," summarized a Forest Service biologist. Not that the agency was caught unawares. Its own policies had created the conditions.
Ten years earlier, scientists with the Laboratory of Tree-Ring Research at the University of Arizona published a paper on the fire history of the Pinalenos. Analyzing fire scars in the growth rings of old wood, the researchers determined that the last widespread blaze on the upper elevations of the mountain had taken place in 1685. The blaze probably cleared the spruce and fir from the peak, because no trees in the current stand are older than 300 years.
The record was more complicated in the mixed-conifer zone below. Burn marks appeared in the tree rings at roughly 10-year intervals for several hundred years. Except for the 1685 event, fires in this zone probably did not reach the top. But the marks ceased after 1880, when people took command of the mountains and declared war on wildfire. Thus for a century the fuels accumulated in the conifer belts of the Pinalenos. The university scientists warned of severe fires. "It is not a matter of if such fires will occur but when," they wrote, and the Nuttall fire proved them right.
All of which points to the central paradox of fire in the sky islands: Big wildfires happen when small wildfires are put out. Big fires have erupted in the 21st century because firefighting in the 20th century was too aggressive. Regular, modest doses of fire, scientists now say, keep the sky islands healthy. Fire maintains the ecological structure, the stacked life zones each in its place, and also the diversity of species, because many animals and plants benefit from the patches of new habitat created by low-intensity burning. Withhold fire and things get "out of whack."
The same is true all over the Southwest. Since the 1970s, state and federal agencies have tried to encourage small fires in the forests and parks. To smooth the return of the old regime, brush and saplings have been cut, and prophylactic burns have been set. Nevertheless, the number, the size, and the intensity of wildland fires are increasing, such is the backlog of fuels. The Forest Service is like a weak government negotiating with a powerful political exile. Fire will come back on its own terms whenever it wants.
Here is a list of the major wildfires in sky island ranges since 1994: Rattlesnake fire (Chiricahuas, 1994, 27,500 acres burned); Clark Peak fire (Pinalenos, 1996, 6,300 acres); Bullock fire (Catalinas, 2002, 30,000 acres); Aspen fire (Catalinas, 2003, 85,000 acres); Nuttall fire (Pinalenos, 2004, 29,000 acres); Florida fire (Santa Ritas, 2005, 23,000 acres). These fires took place against the backdrop of even larger blazes elsewhere in Arizona. The Cave Creek Complex fire burned a quarter million acres last year, helping to make 2005 the worst fire season in state history. That is, in its modern history.
The prettiest approach to a sky island is along a flat highway through a yellow desert toward a blue mountain on the horizon. That's how the Chiricahuas look on the drive east from Tucson. By the side of the road the agaves bloom: bulbous, golden flowers on 10-foot stalks indicating the death rattle of the plants after decades of life.
The goal of this expedition with David Hodges was to study the vegetative zones of a sky island from the bottom up and to pull apart their interlocking, inflammable histories. The Sonoran Desert, where the saguaro live, is the lowest vegetative zone, and it resists burning. But since exotic grasses have invaded—cheatgrass, Lehmann love grass, and buffel grass particularly—the fire risk has gone up. The grasses become thatched and dry at the start of fire season.
Ranching is responsible for the environmental transformation of the desert zone and of the next zone higher, known as semidesert grassland. Alien seeds have been imported along with the alien cattle, gaining a foothold when slower-growing bunchgrasses were cropped back. Although it's hard to imagine now, Arizona was almost lush with waving grass in the 18th and early 19th centuries.
Those native grasslands burned all the time. Million-acre fires may not have been uncommon. Some of the fires were sparked by lightning; others were set by Indians, who found that game could be sighted better when the cover was removed. The Apache used fire and smoke to confuse their enemies. After the Apache chief Geronimo surrendered to the Army in 1886, settlers and their herds of cattle and sheep flooded into southeastern Arizona.
As Hodges says, fire exclusion set the stage for fire suppression. The livestock changed the fire regime by eating up the grassy fuels. Fire could no longer roam the valleys and radiate into the mountains. Lightning still caused big fires in the upper elevations, until the Forest Service intervened. The Chiricahuas had a massive fire the year that Geronimo surrendered, but there were no others of consequence for a century afterward.
The grasslands are all but gone. Mesquite, manzanita, and other woody plants, which cattle don't care for, have moved in from above. The shrubs draw more water from the soil than grass does, and their woody biomass magnifies the fire risk. Meanwhile cholla, cactus, catclaw, and other desert plants have migrated upward. Biologists don't believe that the grassland vegetation can ever be restored to what it was. So they have set their sights higher, on the forested portions of the mountains, where fire acts as a double-edged sword.
Leaving the desert, the road mounted through scrub and thin, hardwood timber. In this zone, Hodges said, there are as many as 17 types of oaks. To him oaks are the "totem species" of the sky islands. Wildfires on the lower slopes usually burn everything, but the oaks sprout from their roots and bounce back.
As you come to 7,000 feet in the Chiricahuas, ponderosa pines appear, and slants of shade. The ponderosa pine is the premier tree of the Southwest. The aboriginal forests of Arizona were roomy underneath, and the tall trees stood well apart. Every 5 or 10 years a fire would meander along the ground, excising brush without blowing up into the crowns. A mature ponderosa pine sheds its lower limbs, and its checkerboard bark is fire-resistant, two factors that deny flames a "ladder." Over time the fire-adapted ponderosa pine will drive out all competitors.
Stopping in a glade, Hodges showed a characteristic black, triangular "catface" scar at the base of a tree. A surface fire, smoldering in the twigs and needles that collect on the uphill side of the pine, instigated the scar. The next fire perpetuated the scar and likewise the next, and all the while the tree continued to grow. "Scars are a sign of regular firings," said Hodges. "They predate the fire suppression era." After sporadic logging began in the sky islands, the Forest Service nipped most fires in the bud.