Snow Daze

For people caught on the wrong mountain slope at the wrong time, avalanches can be lethal. But for wildlife, they can be a benefit, carving out habitat and increasing biodiversity.

By Tom Yulsman


Even by snowy Colorado's standards, the blizzard that blanketed the state's Front Range between March 17 and 20 of 2003 was extraordinary. With a hurricane-like eye 100 miles across, the storm dumped an astounding seven feet of snow in places—not counting the drifts. And on the slopes of Pendleton Mountain west of Denver, that was just too much.

At 2 a.m. on March 23, a slab of wet snow at 12,000 feet released its tenuous grip on the peak and began to slide. The resulting avalanche scoured up all of the season's snow from the slope and swelled quickly to a width of 900 feet. As it raced downhill the white monster snatched boulders and snapped trees in half before slamming onto a frontage road along Interstate 70. When cleanup crews arrived, they found snow, rock, and splintered trees piled as high as a two-story building and covering 500 feet of the road.

Without question the avalanche, which carved a new meadow from the forest on Pendleton Mountain, was an example of nature's destructive side. But strange as it might sound, a little destruction—or “disturbance,” as ecologists call it—is not always a bad thing. What we see as devastation actually yields many ecological benefits. By redistributing water and nutrients, and opening up new wildlife habitat, avalanches boost the diversity of plant, insect, and animal species.

“Avalanches may seem like unusual or catastrophic events, but they can have a positive outcome for ecosystems,” says Steward T. A. Pickett, a plant ecologist with the Institute of Ecosystem Studies in Millbrook, New York. By sculpting meadows out of the forest, “they provide resources for plants and animals that require open habitat, and they increase the patchiness of the mountain landscape.” And as research by Pickett and other scientists has shown, a patchy landscape is often a more biologically diverse landscape.


How do avalanches happen in the first place? To get the answer, I ventured into the backcountry near Silverthorne, Colorado, just below the Continental Divide, which is prime snow-slide territory. My guide was Brad Sawtell, an avalanche forecaster and educator with the Colorado Avalanche Information Center. During snow season he checks conditions and produces daily avalanche forecasts.

We set out on a gray day in March—a risky period for slides in Colorado, arguably the avalanche capital of the United States. While Sawtell skied, I trudged along on snowshoes under blossoming cumulus clouds.

Avalanches typically release only on slopes with an angle greater than 25 degrees. So it was important to know the steepness of the terrain we were passing through. Sawtell's reading of 25 degrees showed that we were in a vulnerable area. Whether gravity succeeds in tearing a layer of snow free—helped, perhaps, by a passing skier—depends on the quality of the layers making up the snowpack and on the strength of the bonds between them. To test these factors, Sawtell and I dug a snow pit big enough to stand in. He then used tongue depressors to mark 13 visible layers of snow in the pit's walls. Each layer was deposited during a different storm and contained greater or lesser amounts of water.

“The snowpack is like a layer cake, with some layers consisting of pound cake and others of angel food cake,” Sawtell explained. “Imagine putting dense pound cake atop fluffier angel food cake and then tilting the whole concoction on end.” Whether the dense pound cake slides off will depend on several factors, including the angle of tilt and the shear strength of the icing between the layers. (Avalanches certainly are possible earlier in the season under different conditions, but in Colorado, spring is the time of greatest risk.)

On Pendleton Mountain, the record-setting spring snowfall was incredibly wet and dense, and it accumulated atop a thin layer of drier snow. All that weight sitting on such a weak layer was a recipe for a catastrophic slide. And as Sawtell and I dug our snow pit, we wondered whether we were dealing with a similar kind of tilted layer cake. To answer that question, Sawtell looked at snow crystals under a lens, pried individual layers out into the pit with a shovel to assess the strength of the bonds between them, and conducted other tests. The verdict: Wet, dense snow (pound cake) was indeed sitting atop drier, fluffier winter snow (angel food cake)—with very slippery icing in between. “The snowpack is telling me that it is capable of avalanching,” Sawtell concluded.

Thanks to his experience as a backcountry skier and an avalanche forecaster, Sawtell knows how to avoid slide-prone areas. And as someone who regularly writes up reports on avalanches that kill people, he is well aware that pushing the envelope can be lethal. So after digging our snow pit, Sawtell decided it was time to retreat back down the slope and head for home. We made it safely. But between 2000 and 2005, 222 people in the United States and Canada did not—they lost their lives in avalanches.

For a snowshoer in the backcountry, the prospect of an avalanche like the one on Pendleton Mountain is justifiably terrifying. But from the long-term perspective of ecology, they are, as Steward Pickett notes, an important part of how ecosystems in mountainous areas work.

Thirty years ago ecologists were less aware than they are today of the important role disturbances play in shaping ecosystems, and they tended to focus narrowly on particular kinds of “pure,” undisturbed ecosystems—a closed-canopy forest, say—and how each functioned in isolation. Today ecologists take a broader view. “When you see that there are closed patches of forest, open patches, avalanche chutes, and other kinds of landscapes, you can put the whole picture together,” Pickett says.

The whole picture he sketched for me nicely describes what's known in ecology as the forest-tundra ecotone. This is the transition zone between the conifer forests blanketing the lower slopes of mountains and the high meadows of grasses, sedges, and flowers of the alpine tundra. On Pendleton Mountain and elsewhere in the Rockies this transition zone is something of a patchwork quilt, where the closed-canopy forest fragments into tree islands surrounded by meadows. Helping to shape this patchy landscape are the vertical, grassy boulevards carved by avalanches.

Ecotones are places where different ecosystems come together. And research shows that these overlapping areas host a richer diversity of species than those of the individual ecosystems that meet there. Like a culturally diverse neighborhood in a thriving city, the higher elevations of western mountains are places where plant, insect, and animal communities mix, or at least exist in close proximity.

But this isn't the only way a patchy landscape enhances species richness. Pickett and his collaborators have shown that the edges between patches also are important. “These places are biological hot spots in the landscape,” he says. Flowering and fruiting often are more abundant in the boundaries, in part because of species that specialize at living on the edge, and in part because of the mix of forest and meadow species present there.

These effects are evident throughout the Rockies, but nowhere have I seen them on more dramatic display than in Montana's Glacier National Park, where up to half of high-elevation basins have been disturbed by snow avalanches. Many of the resulting chutes are home to shrubby vegetation, such as alder, false huckleberry, and hawthorn.

According to Steve Gniadek, a wildlife biologist for the National Park Service at Glacier, this kind of ecological patch provides cover for many species, nesting sites for birds, good hunting for keen-eyed hawks and eagles, and a veritable feast of berries and other tasty plants for hungry grizzlies. “These shrub fields are habitats that generate high biodiversity as a result of the mosaic effect and the edges between patches,” he says. “Many bird species use the avalanche paths to hunt for insects, or specialize in working the edges between the shrub fields, meadows, and forests.”

For example, in Glacier National Park a number of warblers, such as the Wilson's and the MacGillivray's, nest in shrubby areas. In addition, the Townsend's solitaire, which likes open habitat, “would definitely forage in avalanche chutes,” Gniadek says.

Shrub-filled avalanche paths are attractive to a variety of species because the paths exhibit a diverse structure: a tall stratum of vegetation, such as alder; a middle level that may consist of chokecherry and serviceberry; and a short understory of small shrubs such as bearberry and snowberry, plus an abundance of grasses and flowering plants. This vegetative structure nurtures a variety of insect species as well as the different insect-eating birds that prey on them. “Birds respond to the kind of landscape diversity found in avalanche paths—vertical and horizontal diversity—simply by having more food available, more nesting sites, and more cover from predators,” Gniadek says. “It becomes a pretty productive habitat.”

Large birds also find ways to exploit avalanche paths. “I see bald and golden eagles checking out avalanche chutes for carrion after the snow has melted out,” Gniadek says. “They try to beat bears to the carcasses of elk or goats that got caught in winter avalanches.” Moreover, golden eagles in particular will use avalanche paths to hunt for ground squirrels, marmots, and other prey. Adds Gniadek, “In spring this may be an important food source for golden eagles migrating through.”


At the top of glacier's food web are grizzly bears. On my visits to the park, I have frequently seen them moving through the shrubby vegetation growing in avalanche chutes. “Bear elevators” is how Dan Fagre, a U.S. Geological Survey ecologist, described these chutes during a hike we took to the park's Grinnell Glacier. That's because the bears that start foraging at the bottom work their way to the top, feasting on berries and other delectables as they go.

The bear elevators illustrate one of the direct ecological impacts of avalanches. But there are indirect impacts, too. For example, by moving large amounts of debris, water, and chemicals from higher to lower elevations, avalanches redistribute resources in ecologically significant ways.

In the Rockies and other mountainous regions, the redistribution of resources begins with winds that carry snow from exposed areas and dump it downwind in protected ones, such as in gullies and on the lee sides of ridges. A large part of the landscape thus receives little water, whereas other areas get a lot more. And it is often in these areas that avalanches begin. On Pendleton Mountain, snow slides often have begun in just such an area near the top of the peak; this area was, in fact, where the 2003 avalanche started. But these zones don't just receive a water subsidy. They also get a windfall—literally—of nutrients, such as nitrogen, carried in by wind and precipitation. When an avalanche occurs, these compounds are thrown downhill along with the snow.

“Avalanches push materials further down the mountain, and by scouring vegetation and soils along the way, add additional organic and inorganic goodies to the mix,” says Timothy Seastedt, an ecologist at the University of Colorado, Boulder. “Avalanche chutes are therefore in a constant state of scour and recovery, and areas subsidized with this material are enriched with nutrients.”

One ecological goody is the snow itself, which can accumulate in great depths at the bottom of avalanche paths. “Avalanches can create almost permanent snowfields at the bottom of the chutes that won't melt out until the end of the summer, shortening the growing season in those spots,” Seastedt says. Only a limited set of plants can complete their life cycle in that short a period—plants that ordinarily live in the tundra, where the growing season comes and goes in a few short weeks. “So alpine-adapted species move into areas they wouldn't otherwise colonize.” And, once again, patchiness increases.

This can benefit one of the West's most charismatic animals: the elk. In the middle of the summer the vegetation growing at the bottom of avalanche chutes is essentially spring growth, which elk savor. “This is high-quality browse—high in protein,” Seastedt says.

But there can be too much of a good thing—particularly nutrients. And here is where avalanches can actually amplify a significant impact we humans are having on some high mountain environments in the West.

In Colorado's Front Range, nitrogen pollution from vehicles, industry, and feedlots is carried by winds from densely populated areas up into the mountains, where it is deposited by snow and rain in the alpine tundra—including within avalanche start zones. Research in Rocky Mountain National Park has shown that approximately four pounds of nitrogen compounds are accumulating on each acre every year—about 10 times more than would occur naturally. The National Park Service believes this is beginning to damage ecosystems, causing uncommon types of algae to bloom in pristine alpine lakes, altering the species makeup of high alpine plant communities, changing soil chemistry, and flushing nitrogen that can't be absorbed by plants into waterways. Avalanches can compound this effect. “They can funnel large amounts of snow, with their load of nitrogen pollution, directly into streams and rivers, bypassing the soils and plants that would otherwise filter some of it out,” Seastedt says. Left unchecked, the continued buildup of nitrogen is expected to acidify lakes and streams in some high mountains, leading to even greater impacts on ecosystems.

But today this flip side of avalanches is invisible on Pendleton Mountain. Here a chorus of insects and birds can be heard arising from the long, flower-filled avalanche path carved from the forest by the 2003 slide. This meadow represents a new, species-rich patch stitched into a landscape quilt of varying plant and animal communities, ranging from forest to meadow to wind-raked alpine tundra. And as happens after a forest fire, new life is taking hold. Here and there, aspen saplings—early colonizers of ecologically disturbed sites in the Rockies—are already reaching for the sun beneath the remains of stripped and decapitated trees.


Tom Yulsman is co-director of the Center for Environmental Journalism at the University of Colorado, Boulder.


© 2006 National Audubon Society

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The Nature of Slab Avalanches

Each winter's record of snowstorms, windstorms, and sunny days is stored as a series of layers in the snowpack. As a new snow layer forms, it bonds with the layer below, creating a coherent structure. This structure is altered when the temperature changes rapidly or when the snowpack must support a new load, such as the weight of another snowstorm, or a snowmobile or skier. If these changes occur quickly, a layer deep in the snowpack can buckle under the weight of the snow slab above it. Unsupported, the slab resting above the collapsed layer crashes down the hill as a deadly avalanche.

—Ethan Greene