What happened to Alaska’s soils during the 2019 fires?

Alaska was on fire in the summer of 2019.

It was the hottest summer on record for the state, and the driest on record in southcentral Alaska. For the first time ever, “extreme drought” conditions were declared. A typical summer in Alaska sees rain in May, a drier June, followed by increasing rain until winter. It’s why I recommend people visit our lovely state around the 4^th of July: the mosquitoes are fewer and you might stay a little dryer.

Unfortunately for many last summer, instead of clear skies, people were choking on smoke.

Plumes of smoke from the McKinley fire, 2019. Credit: Robert Sheldon.

Fire season began in April 2019, a full two months earlier than in the past. Fires burned 2.6 million acres in 2019, of which approximately 1.6 million were in interior Alaska, where permafrost is prevalent.

Due to growing concern over the release of massive amounts of carbon from thawing permafrost, one might ask how fire affects permafrost. It is a complex topic, and one that is not fully understood.

In permafrost soils, up to three feet of the surface thaws in the summer, but under that, the ground stays frozen. The frozen ground prevents water from draining, and the cold, wet soil limits which plant species grow. A typical boreal forest is composed of stunted black spruce with a deep layer of moss on the ground.

A heavy layer of moss protects much of the soil surface during a fire. There are changes, though. A lighter green surface reflects light, but the dark, charred surface absorbs light, and thus, more heat. Credit: Lorene Lynn.

Many fires will burn the surface plants, but not penetrate through the deep moss, protecting the soil. There are changes, though. A lighter green surface reflects light, but the dark, charred surface absorbs light, and thus, more heat. Charred plant material also acts to repel water, increasing the likelihood that rain will run off the surface.

The loss of much of the moss reduces the insulating layer at the surface. It exposes the soil to greater thaw in the summer, but conversely, a colder freeze in the winter. In the past, the burn area the following summer would be colonized by fireweed, an aptly named plant that paints whole mountainsides in glorious fuchsia.

Shifts in vegetation community are common following fire. Changes are dependent on the burn severity (depth and heat of burn) and available plant colonization sources (nearby seed sources or regeneration from underground plant material). Most often, coniferous forests shift toward deciduous species (cottonwood, aspen, and birch) after a fire. Over time, spruce will again move in and the permafrost returns to its previous temperatures.

Thermokarst thaw slump. As the ground thaws and falls away, ice wedges in the ground are exposed. They are what look like vertical ridges. The exposed area then warms and thaws, and the cycle continues. Credit: Lorene Lynn.

As our climate warms, some are concerned that large burned areas may not refreeze. This could lead to massive thawing and slumping of the permafrost, called thermokarst. Those of us who work in the arctic are already seeing this happen across the landscape, even where fire has not existed.

In southcentral Alaska, I am working on a 35-acre restoration project at a gravel pit that was grandfathered into the Kenai National Wildlife Refuge. We had planned on harvesting vegetation mat from areas that would be mined to use for restoration purposes. Our planning was thwarted by the 600,000-plus acre Swan Lake Fire. We watched this fire approach our work site, communicating with the firefighters daily. We eventually were evacuated from the restoration site, and not allowed to return for over a month.

Kenai National Wildlife Refuge restoration work. The day after this photo was taken, we were evacuated from the site and not allowed to return for over a month as the fire raged on. What I found upon return was a little disheartening. Credit: Lorene Lynn

Upon return to our work, what I found was a little disheartening. Most of the four acres of vegetation mat we had harvested and stored burned in the fire, along with an additional five acres we had yet to harvest. It was encouraging to see that regeneration had already begun in the surrounding forest, indicating that the fire had not burned entirely through the soil.

Aspens suckers were already three feet tall just two months after the fire. Soil nutrient availability is often enhanced by the charred material at the surface. Our work plan at the restoration site is shifting toward cultivation of plants from seed and cuttings to supplement the loss of native vegetation. Prior to the fire, we collected soil samples from which to extract mycorrhizae which we will use to bolster plant propagation.

The forests will grow again. We just need to hope the permafrost will stay frozen.

Answered by Lorene Lynn, Soil Scientist/Restoration Ecologist, Red Mountain Consulting LLC, Alaska

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