Fire ordinarily helps the boreal forest's black spruce trees. Now it threatens them, too

Warmer, drier conditions that lead to more frequent fires in Canada's vast boreal forests are threatening the dominance of black spruce trees that for thousands of years thrived in a healthy relationship with forest fires.

Black spruce trees and the thick layer of peat they take root in are great fuel for the fires. So typically, every 100 years or so, a fire would sweep through and take out a stand of these iconic boreal trees. 

That same fire would warm up the black spruces' waxy cones, releasing its seeds that would allow the black spruce forest to regenerate.

But in recent years, climate change has undermined the healthy relationship between black spruce trees and forest fires. More frequent wildfires are pushing large areas of black spruce forests past their recovery point.

As a result they're being replaced by other species, and sometimes the forest doesn't regenerate at all.

"We do see evidence of shifts away from black spruce dominance in more than one third of the sites," said Jennifer Baltzer, the Canada Research Chair in Forests and Global Change at Wilfrid Laurier University. 

This shift away from black spruce dominated-forests could have far reaching implications for the wildlife that depend on them — like caribou — and for the massive amount of carbon these forests store underground.

Baltzer is the lead author of a new study that analyzed more than 1,500 former burn sites across the North American boreal forest, between 1989 and 2014.

"This is one study, in a growing body of evidence, that we're pushing ecosystems toward these tipping points that we don't really know what comes next," Baltzer told Quirks & Quarks host Bob McDonald. 

More frequent fires not giving black spruce a chance

Shorter intervals between wildfires means black spruce forests are not able to reach reproductive maturity, and thus don't have enough seeds available to replace themselves.

In addition, fires that rip through these areas are now burning more deeply into the layers of peat soil that characterize black spruce forests. 

"Because the soil layers are drier," said Baltzer, "[it allows] for more complete combustion of this organic soil layer."

This is changing the condition of the seedbeds — the forest floor that was once springy and spongy — to allow other tree species, like aspen in western North America and jack pine in the east, to establish themselves and outcompete the black spruce.

"The most extreme change that we see is actually a shift from forested to non-forested, where we don't see any tree species recruiting at all, and this leads to a shift toward a grassland or tundra-type state," said Baltzer.

She said across North America, the most extreme changes they documented occurred in western regions. 

"The frequency of loss of black spruce resilience was much more common in western North America, and we found that this was linked with climate conditions," she said. Greenhouse gas emissions have led to temperatures warming faster across the north than in more southern latitudes.

Effects on climate change

Boreal forests sequester 30 to 40 per cent of all terrestrial carbon on the planet, with most of that carbon being locked up in the soils, which is critical to the global climate system.

Black spruce stands in the boreal forest cools the underlying carbon-rich soil, slowing down the decomposition of organic matter and in some cases, in more northerly regions, also insulating the permafrost underneath. 

Replacing the cool and shaded understory that lies beneath the thick canopy in a black spruce forest with a more open deciduous tree habitat could accelerate the thawing of the permafrost even more, generating a warming feedback loop.

Baltzer said her findings are just another big, red flag that we need to take rapid action to reduce fossil fuel emissions.

"There's not a management solution to this particular problem. The management solution is greenhouse gas emissions reduction," she said.

Produced and written by Sonya Buyting. Click on the link above to hear the interview with Jennifer Baltzer.