Greenpeace called for a moratorium on new logging in areas of the forest that still have large, unfragmented blocks of older-growth trees, and warned it had similar fears about the boreal forests that stretch across Russia and northern Europe.

"Research is starting to show that the forest is tipping from being an annual carbon sink to being an annual carbon source," said Christy Ferguson, Greenpeace's forests campaigner in Toronto.

The "Turning Up the Heat" report, prepared by researchers at the University of Toronto, surveyed a variety of separate scientific studies on the boreal forest in recent years but did not include any new research itself.

Canada's boreal forest, characterized by the predominance of conifers like pine and spruce, stretches in a vast curve across the country below the Arctic, from the Yukon territory in the northwest to the Atlantic coast of Newfoundland.

Studies estimate it stores about 186 billion tonnes of carbon, equal to about 27 times what the world produces from burning fossil fuel each year. Two-thirds of the carbon is stored in the forest's soil.

Greenpeace says the carbon released as trees are harvested contributes to climate change. That, in turn, threatens the northern forest with problems such as insect outbreaks and large forest fires that destroy more trees.

A large fire creates the scenario of a "carbon bomb" or the sudden release of massive amounts of carbon into the atmosphere, similar to what happened in 1997 when peat fires broke out in Indonesia, Ferguson said.

The Forest Products Association of Canada agrees the boreal forest needs more protection, but said Greenpeace's report does not consider the replanting of trees and that much of the carbon remains stored in the timber products produced.

"The only way to look at the carbon footprint with integrity ... is to look at the full life-cycle," said president Avrim Lazar. "Sustainable logging in Canada is a very responsible activity."

Lazar said the association's members are well aware of the impact of climate change on the northern forest, which is why they pledged to make their operations "carbon neutral" by 2015 without buying carbon offset credits.

Consumers are better off buying Canadian-made lumber products and paper than those from other parts of the world where deforestation and illegal logging are rampant, Lazar added.

But Ferguson said the report found studies that indicate carbon neutral logging might not be possible, and the soil in some logged areas continues to release carbon for up to a decade after the trees are cut.

"As much as we would want it to be the way it works, that just isn't what happens," Ferguson said.