Quirks & Quarks

Intense boreal forest fires may change tree species, and lead to more carbon uptake

As boreal forests recover from unusually intense wildfires in places like Alaska, they are experiencing a shift in tree species from black spruce to deciduous trees like aspen and birch.

Super intense boreal fires burn up soil and create conditions for different tree species to move in

A boreal forest in Alaska after a wildfire in 2015. The black spruce trees fall because the intensity of the fire burns the top layer of soil and deep into the roots (Jill Johnstone)

As boreal forests in places like Alaska recover from the unusually intense wildfires we've seen in recent years, they're experiencing a shift in tree species from black spruce to faster growing deciduous trees like aspen and birch.  Although the shift alters the ecosystem significantly, research suggests the new forests can store more carbon than black spruce dominated boreal forests over time.

Jill Johnstone surveys tree regeneration 2 years after fire in a black spruce stand burned in the 2004 fires in Alaska. (Submitted by Jill Johnstone)

Intense fires not only burn the trees, but also the often thick layer of organic material below the ground's surface. This has been a major concern because this releases long-stored carbon that has accumulated over long periods of time and is not immediately re-absorbed as the trees regrow.

From carbon sink to carbon source

Researchers have been concerned that this could mean that boreal forests could switch from being a carbon sink to a carbon source, thereby accelerating climate change in what is referred to as 'runaway feedback'. 

But after a 15 year study of boreal forest sites in Alaska that had experienced these intense, soil-burning fires, Jill Johnstone, a forest scientist affiliated with both the University of Alaska and Yukon University, and her colleagues noticed that the composition of the forests was changing. The organic soil layer usually favours black spruce, but after an intense fire, that is not the case. A depleted organic layer favours deciduous trees like aspen and birch.

Thirty years after a wildfire, this burned back spruce forest has come to be dominated by birch trees (Jill Johnstone)

Good news for carbon storage

That shift in tree species may actually be good news for climate change. Because they are more efficient at using soil nutrients, deciduous trees are faster growing.  Because of their speed of growth, they can store carbon at nearly five times the rate of black spruce forest over a the period of forest regeneration. 

Johstone said this does not mean the end of boreal forests as we know them, but could mean a significant shift in what forest types become more common.  Even though the study was specific to Alaska, wildfires will likely result in similar changes to boreal forests across Canada.

This burned boreal forest now favours deciduous trees like aspen and birch (Jill Johnstone)




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