Black spruce trees dominate Canada's forests. Wildfires and dryer climate are changing that

An increase in wildfire activity across Canada is undermining the resiliency of black spruce trees, and is changing the makeup of boreal forests, according to a new research study.

Black spruce trees are found throughout North America's vast boreal region, which includes all three of Canada's territories and all 10 of its provinces. 

The study, published in Proceedings of the National Academy of Sciences, found black spruce trees dominated most forest sites before a wildfire, but the species would frequently lose that dominance after a wildfire. 

"We aren't seeing the spruce recovery that we expected to see," Jill Johnstone, a Yukon-based researcher, told CBC News. She was part of a team that looked at composition changes in 1,538 forest sites that burned in the region in the last 25 years. The study was led by Jennifer Baltzer, the Canada Research Chair in Forests and Global Change, out of the Wilfrid Laurier University in Waterloo, Ont.

"About a third of black spruce forest plots were no longer recovering back to a trajectory that was going to lead to a black spruce forest again in the future," said Johnstone. 

Spruce regeneration completely failed, according to the study, in 18 per cent of the 1,140 black spruce sites that were studied. 

"Black spruce is a fire-adapted species that has dominated the boreal landscape for millenia," Baltzer said in a media release. "But the cumulative effects of a drier, warming climate plus increased fire activity is pushing black spruce to the point that it's not able to maintain dominance on the landscape. 

"This is really strong evidence of a tipping point in this ecosystem that's likely to shift it to a different state."

Johnstone said a "bunch" of the data used during the study came after the 2014 wildfires in the Northwest Territories. The data shows that more deciduous trees, like birch, poplar and aspen are growing in the area now, she said. 

When there's a forest fire, explained Johnstone, trees release seeds from their cones. Then, in the years that follow, the seedlings grow and the forest is rejuvenated. 

"What we're seeing is that that's still happening for jack pine, but it's not playing out that way for black spruce anymore." 

And the reasons why, said Johnstone, are two-fold. 

"An established tree has ... all these roots, [it] has a big canopy. It's actually pretty resilient to the direct effects of climate. But if you kill that tree, and now it has to regenerate as a tiny seedling, those seedlings are much more vulnerable to adverse, dry, summer conditions and they can die." 

The roots of black spruce trees are, explained Johnstone, often "perched" above pieces of cold and sometimes frozen soil that's close to the earth's surface.