The last ice age hit northern Europe hard. At its height, about 20,000 to 25,000 years ago, glaciers up to 3000 meters thick covered some 6 million square kilometers of Scandinavia, the United Kingdom, Germany, Poland, and western Russia. It's hard to believe that any trees could survive this onslaught, and most scientists assumed that Scandinavia in particular was treeless until it thawed out about 9000 years ago. Now analysis of ancient DNA has found evidence that conifer trees like pine and spruce were alive and well in Norway as early as 20,000 years ago. The findings suggest that trees may migrate slower than thought when faced with climate warming, which could have important implications for models predicting the effects of current global warming.
Researchers have traditionally relied upon indicators such as fossil pollen from lake sediments to map the extent of trees and other vegetation in ancient times. But some scientists have argued that these methods don't provide the full picture because trees don't always produce pollen when they are at the edge of their range and their survival is challenged.
Back in 2002, for example, physical geographer Leif Kullman of Umeå University in Sweden radiocarbon dated fossilized pieces of the trunks, roots, and cones of spruce, pine, and birch in Sweden's Scandes Mountains to as early as 14,000 years ago, when ice sheets still covered Scandinavia. But some other researchers argued that the samples might have been contaminated.
To try to resolve the issue, a team led by Eske Willerslev, an ancient DNA expert at the University of Copenhagen, analyzed lake sediments and pollen from the Trøndelag region in central Norway and lake sediments from Andøya Island in the country's north. As the team reports in this week's issue of Science, it was able to recover spruce mitochondrial DNA at Trøndelag in sediments dated as early as 10,300 years ago. And in sediments from Andøya Island, Willerslev and his colleagues found spruce chloroplast DNA dated to 17,700 years ago and pine chloroplast DNA dated to about 22,000 years ago, at the height of the last ice age. The team considered the possibility that young DNA might have contaminated older sediments but rejected it because lake sediments generally immobilize organic compounds in place.
The team concludes that the trees probably survived in small "ice-free refugia," known to have existed in western Scandinavia. The results confirm earlier suggestions that trees and other vegetation "did not need to migrate south during a glacial period in order to survive, as previously thought," Willerslev says.
The researchers also argue that their results have implications for modeling how vegetation will respond to global warming. Such models are based largely on reconstructions of past plant distributions, and many modelers have assumed that conifers spread quickly into Scandinavia after the ice sheets retreated about 9000 years ago. "Current models suggesting that conifer forests spread very fast" after a warming episode "may also be wrong," Willerslev says. It looked like trees spread quickly because they were already there, he says.
Kullman, who was not involved in the current work, says he finds the results "not entirely surprising," adding that they challenge "orthodox interpretations" resulting from pollen analysis. And Thompson Webb, a paleoclimatologist at Brown University, says that it is "astounding" that conifers were able to survive as far north as Norway during the height of the ice age. He calls the study "impressive, ... one of those surprising phenomena from the past that was waiting to be discovered."