The walking tree islands provide homes for animals and stability for us

Shaped by wind, ice, and cold, the trees of the alpine tundra evolve into an array of forms called krummholz, a German word that means "twisted wood." Found in the transition zone between the forest and treeless tundra in alpine and Arctic areas, krummholz varies depending on latitude and altitude. In the far north, it stretches in a swath, often hundreds of miles wide, across North America, Siberia, the Europe. In the Appalachians of the eastern United States, krummholz is found right at rimberline, spread only a few feet across. The reason: Weather conditions, even at high elevations, aren't harsh enough to force a transition from forest to tundra. In the West, krummholz is extremely common in the higher Rockies and the Pacific coastal mountains.

At the edge of the timberline, trees are thrashed and ice-blasted. Such trees may brow only inches high, rivaling even the most assiduous pruning of a bonsai expert. But krummholz can also send out ground-hugging branches great distances, sometimes forming a sprawling cushion that can be as much as 50 feet in diameter. These stunted trees often huddle together for protection in tree islands that stand alone amid the wide-open meadows of the treeless tundra.

The close clumping of vegetation in krummholz tree islands protects more than the trees themselves. For animals such as voles that frequent tundra meadows, the islands provide quick cover from predators. For birds such as white-crowned sparrows, they offer cozy spots for nesting in an environment that otherwise provides scant shelter. For wintering animals such as gophers, tree islands give shelter from howling winds. And for us, krummholz provides some stability:

The tenacious vegetation helps fasten snow and rock on steep mountain slopes, reducing the risk of avalanches and landslides. Krummholz is also important for scientists monitoring the effects of global warming. Several studies have already shown a shift from krummholz to ordinary forest coinciding with warming temperatures. For example, researchers at Laval University in Sante-Foy, Quebec, have found that since the late 1800's, the forest line has moved more than two and a half miles north, most likely in response to milder winters.

Condos, towns, and other high-country outposts of civilization in North America might also be toast if it weren't for krummholz. On this continent, timberline and krummholz are intact. Their root systems are doing what isn't being done in Europe: stabilizing snow and fragmented rock. Timberline trees in Europe still have not recovered from grazing and woodcutting in medieval times. As a result, snow, soil, and rock on steep slopes are more apt to slide. They've been denuded, with disastrous effects.

As we head up the ridge, the spruce and fir become increasingly stunted and wind-sculpted. Higher still, we emerge into the realm of the mobile tree islands. Scattered across the tundra, some are as small as a Volkswagen Bug, others as big as a bus. At the windward side of these clusters, the trees attain a height of no more than six inches or so. Toward the back, the trees rise and the island widens. At the very rear, the trees are about chest high. In profile, the grove looks strangely like the front of a bullet train.

At the front, facing toward the winde, there's considerable dieback of vegetation, where the island is eroding away. It seems abundantly clear how an island of trees like this can migrate downwind, away from the nasty assault of wind and cold.

The soil in the "wake" of an island loses about 25 percent of the carbon it once contained. Tundra soils contain among the highest concentrations of carbon in thw world. If global warming were to cause trees to enroach on the tundra, a substantial amount of that carbon would be lost from the soil. Part of the carbon would likely wind up in decayed plant material on the ground's surface. The rest would be incorporated into the tissues of the new trees growing on former tundra.

By: Tom Yulsman, Audubon