SAN FRANCISCO -- Sixteen thousand years ago, woolly mammoths, Beringian lions and short-faced bears roamed a grassy steppe that stretched across North America into Alaska.

Two thousand years later—the equivalent of an eye blink in geologic time -- the animals were extinct in the area that is now Alaska. The grasslands had been replaced by peaty soil covered with moss, sedge and lichen.

Scientists presenting new research here at the fall meeting of the American Geophysical Union say warm, wet conditions helped flip the switch that turned the mammoth steppe into the Alaskan tundra of today -- and that episode hints at how rapidly an ecosystem can transform.

The question they're asking now is whether climate change is pushing Alaska toward another major biome shift, similar to the abrupt transition from grassy steppe to peaty tundra that took place thousands of years ago.

Daniel Mann, a geologist at the University of Alaska, compares those abrupt changes to suddenly flipping a canoe.

"We're at a time of rapid environmental change, and basically, we want to know what kind of canoe we're riding in," he said. "Some of them have a much more sudden threshold to flipping over."

Such scenarios also raise questions about the limits of adaptability: Which species can survive an abrupt change in their habit? Which species thrive? And which species will be left behind?

During the shift from steppe to tundra in Alaska's distant past, mammoths, lions and short-faced bears went extinct. Horses and bison disappeared from the region, although populations of those animals survived in other parts of the world. But caribou and musk oxen managed to make the transition into the new tundra landscape, where they persist today.

Looking for the tipping point

Andrea Lloyd, a biologist at Middlebury College, is using tree rings to study changes in Alaska's boreal forest. Her research suggests that, like the woolly mammoth, the spruce and aspen forests found in Alaska's interior will have a hard time coping with future climate change, as their habitat becomes warmer and drier.

The question Lloyd is trying to answer is just when that change will occur.

"We're trying to see if we can use tree-ring data to see that regime shift coming," she said. "The thing about rapid ecological changes is that it's easy to see them in the rear-view mirror. ...It's a lot harder to see them when they are happening."

The northern edge of Alaska's boreal forest is creeping upward as the climate warms, making areas that were once too cold for trees to thrive hospitable. Lloyd's work focuses on the boreal forest's southern edge, where thick stands of spruce give way to aspens and then to grasslands.

Her research shows that both types of trees grow best in cooler, wetter years, and a relatively small amount of warming can reduce that growth.

With interior Alaska projected to grow warmer and drier throughout this century, "there is the potential for the loss of both species, and replacement by droughty, sagebrush, grassy areas," Lloyd said.

More frequent wildfires and insect infestations—themselves influenced by climate change -- can help speed that transition, said Pieter Beck, an ecologist at the Woods Hole Research Center.

Beck's work, which combines satellite observations of vegetation growth with tree-ring data, shows that while the northern reaches of the boreal forest are thriving, the growth of trees south of Alaska's Brooks Range is slowing due to drought stress.

"We're seeing the ideal envelope for boreal forests move north," said Beck. "I don't have an answer at what time scale the tipping canoe and widespread changes in regime would occur, but I think we're at a stage now where the question is whether they could occur on the time scale of the 21st century."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500