How Hot Does the Ground Get Under a Burning Brush Pile? A Denali Experiment

At Denali National Park & Preserve, wildland fire crews cut brush and small trees near buildings and stack the slash in piles, to be burned later on snow. The work, largely funded through the Bipartisan Infrastructure Law, lowers fire risk to nearby structures — and it has become an experiment in how pile burning affects subarctic soils and what lies in and beneath them.

Slash cut and piled in summer, ready for burning in winter. Photo by the National Park Service

Slash piles burning on snow. Photo by Chris Kopek, National Park Service

Sensors under the fire

In March 2023, Matt Behrens, a graduate student at Northern Arizona University (NAU), worked with the NPS Alaska Western Area Fire Management fire ecology team to bury high-temperature sensors under several piles, recording how heat moved down into the soil.

• The fire effects team dug a trench under each pile site.
• Behrens laid sensors at different distances from the pile's center and at different depths through the soil beneath it.
• The trench was refilled with its own organic soil, and the cut trees and branches re-piled on top before burning.

NPS fire effects staff Chris Kopek and Mariana Perez dig a trench for the temperature sensors; the soil on the blue tarp goes back into the trench. Photo by Sarah Stehn, National Park Service

The branches on the green tarp were piled back over the trench before the burn. Photo by Sarah Stehn, National Park Service

After the burns, crews used measuring tapes to lay out sampling points, dug down to mineral soil to measure the depth of the organic layer, and took soil cores for nutrient analysis.

First results

• Heat took several hours to work down through the thick, organic-rich duff.
• There was little soil heating beyond the edge of the pile.

Next steps

Behrens planned to instrument more piles in fall 2024, to compare fall and spring burning and different fuels and soil types. Knowing how deep and how hot soils get will help fire crews across Alaska protect permafrost, soil seed banks and cultural resources that haven't yet been found.

Research and management together

The work is funded by a Joint Fire Science graduate research grant, NAU, and infrastructure law money through a Cooperative Ecosystem Studies Unit agreement between NAU and the Park Service. Just as important is coordination on the ground among the area's fire management officer, prescribed fire specialist, fuels crews and the Alaska Region fire ecology team, so the right areas are cut and burned in an order that leaves time to instrument the piles — keeping the science aimed at real fire-management questions.

Sources

Based on "Connecting research to management: Investigating pile burn effects on soil heating," Denali National Park & Preserve, National Park Service; a work of the United States government in the public domain. The article dates the piling to the summers of 2023 and 2024 but the first sensors to March 2023, so the years of piling are not given here.