A five-year study by the Australian National University of more than 130 forest sites around the world found these wet mountain ash forests just over an hour's drive from the Melbourne Cricket Ground store twice the amount of carbon as a tropical rainforest.

The cool, moist temperate forests of the O'Shannassy catchment, which includes the headwaters of the Yarra River, store just under 2000 tonnes of carbon a hectare in their giant 300-year-old mountain ash eucalypts, lush understorey vegetation such as tree ferns, and in dead wood on the forest floor.

But as the trees age, the figure rises, with trees more than 250 years old boosting the carbon sink capacity to just over 2800t a hectare.

The unexpected findings, published online today in a National Academy of Sciences journal in the United States, have critical implications for climate policy in the lead-up to the United Nations climate conference to be held in Copenhagen later this year.

The research paper, by ANU ecologists Heather Keith, Professor Brendan Mackey and Professor David Lindenmayer, over-turns conventional theories on the role of forests in carbon accounting.

Professor Mackey said: "It identifies a gap in climate change policy that Australia needs to address.

"There has been a lot of talk about the need to address tropical deforestation in developing countries, but these results show we must start by recognising the carbon benefits to be gained from protecting our native forests," he said.