Research shows northern forests in Alberta, Saskatchewan and Manitoba are drying up and shrinking from drought caused by climate change, while the eastern boreal forest is holding its own.
A paper published Monday suggests the forests in those provinces are already emitting more greenhouse gases than they absorb.
The finding could overturn assumptions that global warming would improve growing conditions for trees in the North.
"We found the boreal east and the boreal west is a totally different story," said University of Montreal biologist Changhui Peng, lead author of the paper in the Proceedings of the National Academy of Science.
Peng and colleagues examined tree plots in 96 different locations across Canada. They were carefully chosen out of 20,000 candidates.
The chosen stands had to be mature, yet undisturbed by fire, insects or industry. Long-term data on trees and weather had to be available.
Using that data, the researchers calculated "biomass change" — the total combined growth of the trees — for every year from 1963 to 2008.
They found that in both the East and West, trees were dying sooner. But in eastern forests, the surviving trees were growing faster, which evened out the effect of increased mortality.
In the West, trees were growing slower as well as dying younger. Peng and his colleagues concluded that western boreal forests were producing two tonnes of wood less per hectare in 2008 compared to the early 1970s.
The trend appears to be picking up speed.
"After 2000, there's a bigger jolt," said Peng. "That's really obvious."
Statistical analysis links the decline to drought. Western Canada has suffered both reduced precipitation and rising temperatures at a faster rate than further east.
The findings are bad news for those counting on forests to continue to suck more carbon out of the atmosphere through growth than they emit as trees die and decompose. Peng's data suggests eastern forests emit about as much carbon dioxide as they absorb, but that's no longer the case in the West.
Boreal forests there became a net carbon source in about 2003, although that has improved somewhat since then.
Peng's study also questions assumptions that global warming will encourage tree growth and expand forests further north. Unless warming temperatures are accompanied by higher precipitation -- which hasn't happened -- the forestry industry may have to re-examine projections.
"Yes, global warming may be beneficial for the shrub and tundra area," Peng said. "But in the southern boreal, the summer water deficit is a huge problem, particularly in Alberta, Saskatchewan and Manitoba."
That could eventually create concern if declining forests are no longer able to support annual harvest allocations, said Peng.
"Maybe your goals for the year are not sustainable. You need to take into account these climate-change effects."
More detailed calculations are needed to determine how close current allowable cuts are to the forest's shrinking productivity, Peng suggested.
His overall results are echoed in other forest systems. Recent studies have found similar trends in other drought-stricken forests, such as in the Amazon basin, temperate regions in the U.S. and aspen forests in Canada.