Above the Tree Line in the Greater Yellowstone Ecosystem

Candice Gaukel Andrews January 12, 2021 0

The northern tree line—beyond which the climate is too harsh for trees to grow—circles Earth’s northern landmasses for more than 8,300 miles. ©Candice Gaukel Andrews

I once walked on the moon. It was remote and windy and bare; the kind of landscape that seems to call to me.

This “moon,” though, wasn’t off Earth. It was found just outside Yellowstone National Park in the Shoshone National Forest, above the tree line. Chilly, treeless and windswept, this type of alpine zone covers less than 5 percent of the planet’s surface.

Above the tree line may not be a place that appeals to everyone, but for me—and a very special, tiny herbivore—it feels like home.

The question is, how long will this rare, moon-on-Earth place continue to exist, and how long can its diminutive resident remain?

The 22-million-acre Greater Yellowstone Ecosystem is one of the last remaining, nearly intact ecosystems in the Earth’s northern temperate zone. ©Candice Gaukel Andrews

First forest

Shoshone National Forest is a part of the Greater Yellowstone Ecosystem, a nearly unbroken expanse of federally protected lands encompassing an estimated 22 million acres of varied terrain, ranging from sagebrush flats to rugged mountains. Shoshone is the first federally protected national forest in the United States and one of the first nationally protected land areas anywhere. Originally a part of the Yellowstone Timberland Reserve, this 2,500,000-acre national forest in Wyoming was created by an act of Congress and signed into law by U.S. President Benjamin Harrison in 1891.

With Yellowstone National Park on its western border, the Shoshone National Forest encompasses the area from the Montana state line south to Lander, Wyoming, and includes portions of the Absaroka, Beartooth and Wind River Mountains. The higher elevations are snow-clad most of the year.

The region above timberline makes up 25 percent of the total acreage of the forest; and of that, 13 percent is listed as just barren, ice or rock.

Because American pikas have evolved to live in extremely low temperatures, they are rarely found below 8,000 feet in elevation. ©Candice Gaukel Andrews

Plush pikas

Weighing just five ounces, American pikas are fuzzy, thick-furred, potato-sized herbivores that inhabit the tops of Western mountains. Frantic workers, they collect large piles of grasses and wildflowers during the summer—a process called haying—to eat during the winter. The piles, which can encompass a bushel of vegetation, resemble dried flower arrangements.

With their round bodies, prominent ears and no visible tail, pikas are exceptionally cute. But despite their cuddly appearance, these smallest members of the rabbit family are among North America’s toughest animals. They have to be. Pikas are one of the few mammals in the Lower 48 that can survive their entire lives in alpine terrain, above tree line. Their dense fur and plump bodies conserve heat, and fleecy paws provide traction on snow.

American pikas are found in California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Washington, Wyoming, as well as in western Canada. Telltale signs of pika territory are their hay piles. Pikas are easy to observe, and unlike many other species, you can get somewhat close to them—although they will squeak-yell at you if you come too near. If you sit quietly, you can watch them go about their business.

Many species of trees are now moving to higher, cooler habitats in response to a warming climate. ©Candice Gaukel Andrews

Trekking trees

Over the past century, the interior West, which includes the lion’s share of the nation’s high-mountain habitats, has warmed about 1.5 degrees Fahrenheit. Computer models show the region heating up an additional 4.5 to 14.4 degrees Fahrenheit during the next 100 years.

As the alpine warms, scientists expect snowpack to shrink, a phenomenon already observed in the Pacific Northwest, the Sierra Nevada and the Southern Rockies. Reduced moisture would dry alpine soils, spur the invasion of lower-elevation conifers and grasses, and crowd out native species.

Already, trees are on the move. A paper published in the August 2017 science journal Global Change Biology shows that in the upper regions of the Great Basin, limber pines and bristlecone pines are leapfrogging up mountains in reaction to climate change.

The dominant habitat type in the Greater Yellowstone Ecosystem is taiga, or boreal forest. Open areas—such as above the tree line in the Shoshone National Forest—are rare. ©Candice Gaukel Andrews

Across North America’s alpine, such “conifer encroachment” threatens wildlife species adapted to open habitat, such as pikas.

Trapped at the top

In fact, pikas may be the most vulnerable to global warming. Unlike wildlife that can shift their ranges north or to higher altitudes in response to a changing climate, pikas have nowhere else to go. In some locations, entire pika populations have already disappeared. For example, in the Great Basin—the arid region between California’s Sierra Nevada and the Rocky Mountains—the mammals have recently vanished from seven of 25 locations where they were documented in the early 1900s.

Over the past 30 to 40 years, pikas have also disappeared from some talus slopes—rock piles at the bases of mountains—in Montana’s Bridger Range. As expected, the most recent pika losses occurred at the warmer, southern end of their territories. But while fossils show that pikas have been lost from several Western mountain ranges over the past 10,000 years, the speed at which they are disappearing now is far more rapid than ever before.

While the American pika is not currently listed as an endangered species, it is considered to be an indicator species for climate change. ©Candice Gaukel Andrews

Decreasing snowpack is at least part of the problem. Snow that covers talus slopes in winter insulates pikas from subfreezing temperatures. If they are shivering through winter, however, it affects their fitness. Ironically, global warming could be causing some pikas to freeze in winter.

In the summer, pikas must descend into the cool, moist talus on hot afternoons. As temperatures rise, researchers say, pikas will abandon lower-elevation talus slopes and migrate higher into the mountains until they can go no farther—much like living on the highest point of a sinking island. All other mammal species in continental North America have greater heat tolerances. Other damaging effects related to climate change are the invasion of new predators and pests, and increases in extreme weather events.

In the past, pikas might have been able to disperse between mountain ranges. But warmer temperatures make that journey a death march now. If an isolated population blinks out today, it will be nearly impossible for that habitat to become recolonized.

Pikas breathe life into an otherwise bare-bones place. If they can persist, perhaps we can, too. ©Candice Gaukel Andrews

Missing the moon

In the moonlike environment above the tree line on Earth, pikas provide a link to life. In an otherwise harsh and stark place, they represent hopefulness in the harshness and softness in the starkness. They remind us that if we should one day have to look to live on other planets that may not be as green and nurturing as Earth, it is possible.

For now, some miniature mammals are telling us that dramatic changes are occurring in the alpine. I, for one, would miss these pika places should we lose them. Their plight is really presaging our own.

Here’s to finding your true places and natural habitats,

Candy

Leave A Response »