Repeat fires tip the scale on near-surface permafrost in Wrangell-St. Elias National Park and Preserve

Fire ecology crew from Eastern and Western Alaska Fire Management teams. From Left to right: Sarah Brey, Fleur Nicklen, Sarah Martin, and Olivia Lawrence.

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Fire ecologists study wildfire impacts to learn how often to conduct prescribed fire. In Alaska’s interior, prescribed fire is done so rarely that not much is known about fire effects.

In July 2024, the Bipartisan Infrastructure Law funded a study in an area of Wrangell St. Elias National Park and Preserve that burned twice in seven years. First in the 2009 Chakina Fire, and again in the 2016 Steamboat Fire. This frequency of fire is not typical for most of Interior Alaska which normally only burns every 60-150 years. The study area has an even longer fire interval burning every 150-200 years.

This area also retains some of the southern-most permafrost, which is typically within a few degrees of thawing. Permafrost is ground that remains frozen year-round. The layer above it thaws and refreezes each year, but thawing permafrost is a serious concern with various environmental implications.

Mean surface temperature in July 2024 of plots that were unburned, once burned, and twice burned.

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Fire ecologists sampled soil characteristics in plots that:

1) were not burned for over 100 years

2) burned once in 2009

3) burned in 2009 and 2016

The ecologists found that ground surface temperature increased from not burned to twice-burned. The crew found frozen ground within 1 m of the ground surface in all unburned plots. Yet, only found frozen ground in 54% of the once-burned plots, and in a mere 8% of the twice-burned plots.

Photos of examples duff plugs showing the organic mat depths from an unburned control plot (left), a plot burned in 2009 (center), and a plot burned in 2009 and 2016 (right).

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Specifically, the organic mat depth was, on average, 24 cm thick at the unburned plots, 10 cm at the once-burned, and just 4 cm at the twice-burned plots. In winter, these insulating mats can have the opposite effect, where thicker mats help keep soil warmer. However, this winter effect depends on snow depth, which acts as another insulator.

This black spruce forest was burned in 2009 by the Chakina Fire and burned again in 2016 by the Steamboat Creek Fire. Photos from left to right are: 1 year post fire (2010), 1 year post two burns (2017), and 8 years post two burns (2024). A distinct shift to deciduous dominance is likely here.

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Based on current models, near-surface permafrost in this region is expected to decline in the coming decades. This study demonstrates how fires, especially repeated fires, can speed up the loss of near-surface permafrost in these vulnerable areas.