Six Thousand Years of Fire at Bear Lake, Rocky Mountain National Park

Visitors near Bear Lake, Rocky Mountain National Park. NPS.

The question

Fire frequency and intensity depend on climate — temperature, humidity. Does the prehistoric record at Rocky Mountain National Park show fire, vegetation and climate changing together over the Holocene, the last ten thousand years?

The research

With no written weather records, scientists use proxies. Jim Doerner of the University of Northern Colorado took sediment cores from Bear Lake and analyzed their layers for:

• radiocarbon age, which dates each layer;
• pollen, which shows the plants growing then — and so the climate;
• charcoal: larger and more numerous particles mean fires closer to the lake, so particle size, analyzed statistically and matched to the dates, separates local fires from background charcoal.

Results

The core. The longest was 255 cm (100 inches); its bottom layers are about 6,100 years old (radiocarbon years before present).

Pollen: twenty-eight kinds.

Climate: warmer than today from 6,100 to 3,200 years ago; cooler from 3,200 to 1,900; then warming again.

Fire: about two to five fires every 1,000 years near Bear Lake over the whole 6,100 years, in three phases:

What it means

Fires were more frequent in the warmer middle Holocene, less frequent through the cooler transition to the late Holocene, and increased again as warming returned. Climate plays a major role in the park's long-term fire history: more fire in warm periods, less in cool ones. The findings help explain how today's landscape formed and how ecosystems may respond to climate change.

Sources

Based on "Fire History and Climate Change," by Bert Cushing, Rocky Mountain National Park, National Park Service (2008), summarizing research by Jim Doerner, University of Northern Colorado; a work of the United States government in the public domain. The summary is not a statement of park policy. The original's summary calls the middle-to-late transition "cooler, drier" yet "likely warmer than now"; its pollen results place the cool period at 3,200–1,900 years ago, which is followed here.