Scientists analysed the sediment-rich base of the ice cores
DNA extracted from ice cores show that moths and butterflies were living in forests of spruce and pine in the area between 450,000 and 800,000 years ago.
Writing in the journal Science, the researchers say they believe the DNAs are the oldest pure samples obtained.
The ice cores also suggest that the ice sheet is more resistant to warming than previously thought, the scientists say.
"We have shown for the first time that southern Greenland, which is currently hidden under more than 2km of ice, was once very different to the Greenland we see today," said Professor Eske Willerslev from the University of Copenhagen and one of the authors of the paper.
"What we've learned is that this part of the world was significantly warmer than most people thought," said Professor Martin Sharp from the University of Alberta and a co-author of the paper.
The ancient boreal forests were thought to cover southern Greenland during a period of increased global temperatures, known as an interglacial.
Temperatures at the time were probably between 10C in summer and -17C in winter.
When the temperatures dropped again 450,000 years ago, the forests and their inhabitants were covered by the advancing ice, effectively freezing them in time.
Studies suggest that even during the last interglacial (116,000-130,000 years ago), when temperatures were thought to be 5C warmer than today, the ice persevered, keeping the delicate samples entombed and free from contamination and decay.
At the time the ice is estimated to have been between 1,000 and 1,500m thick.
"If our data is correct, then this means that the southern Greenland ice cap is more stable than previously thought," said Professor Willerslev. "This may have implications for how the ice sheets respond to global warming."
Research by Australian scientists has suggested that a 3C rise in global temperatures would be enough to trigger the melting of the Greenland ice sheet.
In 2006, research conducted by researchers at Nasa suggested that the rate of melting of the giant ice sheet had tripled since 2004.
While in February 2006, researchers found that Greenland's glaciers were moving much faster than before, meaning that more of its ice was entering the sea.
And in 1996, Greenland was losing about 100 cubic km per year in mass from its ice sheet; by 2005, this had increased to about 220 cubic km.
A complete melt of the ice sheet would cause a global sea level rise of about 7m; but the current picture indicates that while some regions are thinning, others are apparently getting thicker.
The new results were obtained from the sediment rich bottom of ice cores.
The 2km long Dye 3 core was drilled in south-central Greenland, whilst the 3km-long Greenland Ice Core Project (GRIP) core was taken from the summit of the Greenland ice sheet.
Samples from other glaciers, such as the John Evans Glacier on Ellesmere island were used as a control, to verify the age of the samples and to confirm that the DNA was from plants that grew in southern Greenland, rather than from plant matter carried by wind or water from elsewhere in the world.
Although the ice contained only a handful of pollen grains and no fossils, the researchers were able to extract DNA form the organic matter held in the silt.
Comparisons with modern species show that the area was populated by diverse forests made up of alders, spruce, pine and members of the yew family.
Living in the trees and on the forest floor was a wide variety of insect life including beetles, flies, spiders, butterflies and moths.
The discovery pushes forward the date when the last forests were known to exist in Greenland by nearly two million years.
Previously, the youngest fossil evidence of a native forest in the region came from fossils found in the Kap Kobenhavn Formation in northern Greenland. There, the fossils date from around 2.4m years ago.
The study paves the way for scientists to probe beneath the ice in other parts of the world.
"Given that 10% of the Earth's terrestrial surface is covered by thick ice sheets, it could open up a world of new discoveries," said Dr Enrico Cappellini of the University of York.