Fire and ice: Impacts of wildfires on permafrost and peatlands

“We’re at a national preparedness level five out of five,” said Dan Thompson. “We have a bunch of fires in Northwest Canada, many of which are in the permafrost zone. So, this isn’t just an abstract risk, it’s very much real and has major circumpolar impacts that are particularly relevant this year, as it seems to be almost every year now.”

Despite the busy fire season, Dan answers our questions about how fires impact peatlands and permafrost before he dives back into his work modeling fire growth for this year’s fires.

Can you describe what peatlands are?

Peatlands are a type of wetland where the soils consist of organic matter that don’t completely decompose, which forms layers of peat. In Scandinavia, you tend to see more open peatland systems. These open peatlands tend to contain soggy mosses and leafy broadleaf shrubs that are less flammable. In North America we have open peatlands too, but we also have a lot of naturally forested peatlands. Spruce trees are common in Canadian peatlands, which are extremely flammable and can carry intense crown fires where the entire tree is torched up in flames in a dramatic fashion. Over half of Canada’s peatland systems live within the boreal forest, and the combination of trees and peat coexisting turns into a fairly flammable ecosystem.

Why are peatlands important ecosystems?

When they’re not burning, peatlands provide extensive ecosystem services. Peatlands have been storing carbon from the atmosphere over many millennia. Very little carbon is actually stored in trees, it’s mostly underground, invisible in the peat layers. So, these boreal peatlands are really important in the global scale of carbon sequestration.

In terms of their cultural significance, peatlands are a cornerstone of a variety of traditional food sources. Many berries grow in peatlands, and they also serve as a habitat for mammals such as caribou that are important for local food security. Peatlands can also serve as travel corridors particularly in winter.

Peatlands are an important habitat for some of Canada’s protected and endangered species such as the boreal woodland caribou. The older, semi-forested peatlands – many of which contain permafrost – are an ideal habitat for these caribou in terms of foraging and also protection from predation.

Peatlands are also really important in the water cycle. They store water from snowmelt for example, and slowly release it during the summer dry periods. Often peatlands are the origin point of many smaller streams in the boreal forest, so they are an important source of water for smaller fisheries habitats. They also serve as a moderator of the hydraulic cycle. Keeping them intact is important.

How are fires impacting permafrost and peatlands?

To touch on the scale, a typical North American northern peatland is often two, sometimes three meters thick of peat. So it's more peat depth than most people are tall. When a fire burns through highly-flammable spruce trees with permafrost below, it’s going to burn only about 10 to 20 centimeters of that two meter deep peat. So the typical depth of burn is only a small fraction – around 10% or less – and the vast majority of the peat remains intact after a fire.

However, after a fire, vegetation grows very slowly in the Arctic, and there’s no more tree cover to shade the surface. Temperatures in the boreal interior of North America can reach over 30° Celsius. The combination of these freshly burned surfaces and the hot summer temperatures leads to a strong increase in both the temperature of the peat and the depth of the peat’s active layer [the annually unfrozen layer above the permafrost]. This has a profound impact on the rate of permafrost loss. In addition, we also see that the permafrost active layer becomes thicker and that the permafrost itself becomes less influential to the ecosystems and the frozen soil becomes more vulnerable and warmer overall.

If we had a static climate, peatlands and permafrost would recover because the trees and shrub layer would grow up and provide shade and shelter to the lost surface over time. So there is a natural cycle involved with re-aggregation of permafrost after fire, but with polar amplification and global warming, it gets complicated. Most fire-impacted permafrost would naturally come back to its pre-fire state within 50 years. But 50 years from now we’ll have a such a warmer planet that will not be supportive to new permafrost formation in southern permafrost areas exposed to fire.

What could be the impact of increasing permafrost thaw due to fires?

A big impact of fires is that when burned, peatlands and permafrost release large amounts of carbon they store back into the atmosphere. We have actually measured the age of the carbon dioxide emitted from the peat. There’s some evidence that suggests that even though wildfires may burn to a depth of only 15-20 centimeters, the warming starts to transmit further down into the deeper peat column. And even if it’s just slightly warmer temperatures, it increases the rate of carbon loss beyond the burned area in deeper but still unfrozen peat. It's not causing massive loss in the deep layers, but it's waking them up and making them more sensitive to temperature changes. So there are some minor signals that fire disturbance actually increases deeper carbon losses.

There’s also impacts due to the dynamics of changing snow cover. Snow coverage is a bad thing for permafrost because it insulates the peat layer during the coldest part of winter, [sheltering it from freezing temperatures that reinforces the permafrost]. Trees are good at intercepting snow fall on the ground, so when fires burn off trees in permafrost areas, snow is able to cover the permafrost and acts like a thick blanket keeping the cold away. So it’s not only warmer in the summer, it’s also warmer in the winter.

Permafrost thaw can also have a profound impact on air quality. When peatlands burn, they smolder and there’s a lot more particulate matter produced compared to burning trees. This has a huge impact on air quality in northern Canada, especially because many people in this region don’t have air conditioning and are used to keeping their windows open all summer long.

To flip the script, there’s also impacts of permafrost loss on fire. After fire, we go from this relatively dry tree system on top of the permafrost to a sort of soup of wet mosses from the thawing ground ice. So ironically, fire on permafrost peatlands can breed a wetter ecosystem in its wake. If you fast forward to 20 or 30 years following a fire, these previously burned peatlands that are now much wetter environments could serve as a barrier to new fire. What we’re seeing now with this accelerating fire cycle is that in peatland permafrost, fires aren’t moving nearly as fast as they did when the landscape had mature timber on it. Nature is finding a little way to push back and slow down these fires.

What is your involvement in CAFF’s ArcticFIRE project?

I sit as a supporting technical specialist to the CAFF ArcticFIRE project. While I’m not involved in the project’s week-to-week activities and planning, I work to support my fellow Government of Canada project members by retrieving and analyzing data to help move the project forward. There are so many facets around fire in the Arctic and boreal of the northern hemisphere: the human dimension, climate change, ecology, just to name a few. It really helps to have folks like myself working on the sidelines to support this important international effort.