for National Geographic News
High in the Canadian Arctic, large tracts of tundra have given way to forests of spruce trees and bushes in response to a spike of warming temperatures nearly a century ago, according to a new study.
The transition took place more quickly than scientists thought, suggesting that tundra could keep shrinking as temperatures continue to warm.
"With the type of warming we are seeing now, the potential exists for real and rapid change," said Ryan Danby, a biologist at the University of Alberta in Edmonton, Canada.
Danby and colleague David Hik made the findings by studying treeline, the transitional habitat where trees and bushes give way to tundra, a mixture of hardy shrubs, grasses, mosses, and lichens.
This transition zone is sensitive to temperature, Danby explained.
Danby and Hik used tree rings and other data to measure changes over the past 300 years in the density and altitude of treeline forests at six sites in the southwestern Yukon Territory (see Canada map).
"From 1700 to 1925 we saw no change at all at treeline. It was a very stagnant, very stable environment," Danby said.
"And then all of a sudden during that second quarter of the 20th century it was like somebody just flipped on a switch and something happened in the system."
In that 25-year period treeline shifted as much as 280 feet (85 meters) higher in elevation on warm, south-facing slopes, and tree density increased by as much as 65 percent on cooler, north-facing slopes.
These rapid changes pose challenges for caribou and other animals that depend on the tundra's scrubby landscapes, Danby said.
Danby and Hik report their findings in the March issue of the Journal of Ecology.
End of Little Ice Age
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