An extensive new study has found that trees in old-growth forests across western North America are dying faster than they can grow back and that climate change is probably the cause.

The conclusion raises questions about how forests are responding to global warming and raises fears that decomposing trees could become another source of greenhouse gases, said Lori Daniels, one of the co-authors of the paper that appeared Thursday in the journal Science.

"This study is providing, certainly circumstantial and consistent evidence, that regional warming is having impacts on our forests," she said. "These trees are stressed and are susceptible.

"I think it's going to raise quite a bit of concern."

The study examined 76 widely varying forest plots in the western United States and southern British Columbia that have been surveyed regularly since 1955. Seven of the plots were in B.C.

The survey data suggests the death rate throughout the forests for trees of all ages and sizes has doubled from one to two per cent per year. No increase in regrowth was recorded, meaning trees are now dying faster than they can replace themselves.

"In all these forest types, we find an increase in mortality."

Trees studied included hemlocks, cedars, ponderosa and lodgepole pines, Douglas firs, spruces, redwoods and sequoias. The forests, which have all been undisturbed for at least 200 years, were dotted throughout the west from B.C. to Idaho to California to Arizona.

The death rate didn't seem to depend on whether individual stands experienced frequent forest fires or were afflicted with insects or harmful fungi.

"We went through a process of elimination to try to find possible factors that caused these changes in mortality," said Daniels.

The common factor in all the forests was a warming climate. Between 1970 and 2006, temperatures in the study area increased up to 1.5 degrees.

"We did ... find a positive and significant correlation between our mortality rates and regional warming."

Daniels said that small increase in temperature has had a major impact on the availability of water, which is already scarcest in summer when trees need it most. And warming trends have increased the length of the season.

"Our trees are already suffering water deficits even without changes in temperature," said Daniels. "Even subtle changes in temperature, and those water deficits are stressing the trees and making them vulnerable."

Over time, forests will become smaller and younger, suggests the study. Ultimately the average age of trees in old-growth stands will be cut in half. That in turn will change the habitat they offer other plants and animals and eventually affect their economic value.

Another concern is that the higher mortality could see dead, rotting trees release decades worth of stored carbon.

"We're really concerned that western forests could become net sources of carbon to the atmosphere, maybe contributing more to global climate change," Daniels said.

The current study has direct implications for forests in B.C. and for some in southern Alberta.

But Daniels said it also raises concerns about the state of the boreal forest, the vast carpet of green that stretches across the northern regions of most of the provinces, and which is considered to be one of the largest intact ecosystems remaining on the planet.

"It raises concerns in general about how trees are responding to warmer temperatures."

Daniels said researchers are already trying to answer those questions.

"This study raises questions that are important for us to explore and answer about our boreal forest as well."