Clearing a Fire Break Deepened Permafrost Thaw by About 30 cm in Interior Alaska

Protecting a place from fire can change the ground beneath it. When firefighters cleared every tree from a fuel break in Yukon-Charley Rivers National Preserve, the soil above the permafrost there thawed about 30 centimeters deeper than in the untouched forest next to it — a measurable cost that National Park Service ecologists are now tracking.

The Eastern and Western Area fire ecology teams at the southeast corner of the Biederman allotment fire break. Credit: Mariana Perez, NPS.

A place fire keeps finding

A few miles up the Yukon River from its confluence with the Charley River lies the Biederman allotment, historic and privately owned. Wildfire has threatened it three times in the last 16 years; before that, the area burned in 1969 in a fire of nearly 500,000 acres.

In 2014, the NPS Eastern Area Fire Management and a Tanana Chiefs Conference fire crew cut a shaded fuel break around it. The NPS prefers shaded fuel breaks, which thin the forest but keep some trees, because they reduce wildfire risk while disturbing ecosystems least. During the most recent fire, though, firefighters lit a backburn and cut the remaining trees, turning the break "unshaded."

Ground held together by ice

The forest around the allotment is mostly black spruce over largely continuous permafrost. The active layer — the soil that thaws each summer above the permafrost — is about 50 cm thick here. Near-surface permafrost decides which plants grow and strongly controls how fast trees grow. It is the physical foundation of these plant communities, and it holds large amounts of carbon in organic-rich soil. As the climate warms, soil temperatures here are at or near thawing, and more disturbance, such as an unshaded fuel break, is likely to speed that up.

Installing temperature and moisture loggers. Credit: NPS.

What the plots showed

The new break was a chance to measure what this kind of fuel break does to plant species, active layer depth and microclimate. In their first year, the Eastern and Western Area Fire Management Ecology teams installed paired plots in the break and in the surrounding natural forest — and one in the small piece of shaded break that remained — along with air and soil temperature and soil moisture loggers.

The plant surveys are still being analyzed and the loggers are still recording. One result was clear at once: the depth to permafrost was about 30 cm greater in the unshaded break than in the natural forest, a significant difference caused by thawing permafrost.

Active layer thickness in natural forest (green), shaded (blue) and unshaded (orange) fuel breaks. Credit: NPS.

The results will help land managers choose fuel break types as they weigh protecting allotments from wildfire against the park's other values.

Sources

Based on "Alaska fire ecology teams monitor ecological shifts in fire breaks in Interior Alaska" from the National Park Service (Yukon-Charley Rivers National Preserve), a work of the United States government in the public domain.