Fire in the Noatak Tundra: 6,000 Years of Lake-Sediment Charcoal and Pollen

In the summer of 2010, 37 tundra fires burned more than 106,696 acres (43,180 hectares) of Noatak National Preserve in northwestern Alaska, the most fires in the area since record keeping began in 1950. Was that unusual, or something the tundra has seen before? Five researchers, Philip E. Higuera, Jennifer L. Barnes, Melissa L. Chipman, Michael Urban and Feng Sheng Hu, read the preserve's lake sediments to find out. They found that fire has been part of this tundra for thousands of years.

One of the 37 tundra fires that burned in Noatak National Preserve in the summer of 2010. The Kaluktavik River fire burned more than 23,000 acres in July (National Park Service photograph).

The question

More than 5.4 million acres (2.2 million hectares) of Alaska tundra have burned over about 60 years, showing that tundra is flammable in warm, dry weather. Fires affect vegetation, permafrost, nutrient and carbon cycling and wildlife, yet little is known about how often tundra burns naturally or how that links to climate and vegetation, which hinders land-management planning. Evidence suggests Arctic change is altering tundra fires. The 2007 Anaktuvuk River Fire on the North Slope more than doubled the area burned north of 68° N in Alaska since 1950, and it was associated with record-high temperatures and record-low precipitation and sea ice, and the first burning of that area in at least 5,000 years. Denser shrubs may also be changing how flammable tundra is: ancient birch-shrub tundra in the Brooks Range burned readily, which raises the possibility that more shrubs mean more burning.

How the record was read

Pollen from plants and charcoal from fires settle into lake-bottom sediment, where a lack of oxygen preserves them, and layers build up over time. The team cored four lakes along an east–west line in the preserve. Down-valley sites have slightly warmer, drier summers and more tussock-shrub tundra and white spruce. Each core was sliced at 0.10 to 0.20 inch (0.25 to 0.50 cm) intervals, about 15 years on average, and dated with carbon-14 and lead-210. The researchers counted pollen at intervals of roughly 250 to 500 years and charcoal pieces larger than 0.0071 inch (180 µm) in every layer, then used statistics to pick out fire events and calculate the fire-event return intervals (FRIs), the years between fires.

What they found

Charcoal gave unambiguous evidence of burning throughout the past 6,000 years, with individual return intervals from 30 to 840 years, varying by place and over time.

Down-valley sites burned more often than up-valley ones, which the researchers link to climate: cooler summers and more precipitation up-valley limit fuel drying and so the chance fires start and spread. Little Isac Lake had the longest median interval but burned in 2010, its second fire in 26 years, and the record shows intervals as short as 30 years there before.

The charcoal intervals are shorter than earlier estimates based on 30 to 60 years of observation, which put fire cycles at 175 to 193 years for down-valley sites and 480 years up-valley. The authors suspect it is hard to estimate cycles longer than 150 years from under 60 years of data, though real changes not yet detectable, or too many local fires in the charcoal record, might also play a part.

Over the 6,000 years, vegetation and fire shifted with changes in moisture. Spruce pollen rose across all four records, indicating white spruce spread around 2,000 to 3,000 calendar years before present, and alder pollen mostly fell from 4,000 to 6,000 years ago. Most sites burned less often before about 2,000 years ago, when alder was higher and spruce lower. Alder was the pollen type most consistently tied to fire, with more alder going with fewer fires, while spruce, grasses, sedges and birch were generally linked with more fires.

What it means

Long-term mean return intervals were often shorter than estimates from the past 60 years and statistically indistinct from those in Alaska's boreal forests. The authors read this as showing that tundra has been resilient to relatively frequent burning for 6,000 years. They note that more tundra fire could hurt winter forage for the Western Arctic Caribou Herd, but that if caribou used this area over those years, they have lived with fairly frequent fire. They also expect shrub growth under warming to change tundra fire: birch could raise the odds of fire and alder could lower them.

Sources

• Philip E. Higuera, Jennifer L. Barnes, Melissa L. Chipman, Michael Urban and Feng Sheng Hu, a National Park Service article on fire history in Noatak National Preserve, The Burning Tundra: A Look Back at the Last 6,000 Years of Fire in the Noatak National Preserve, Northwestern Alaska; authors' words and figures not federal, so paraphrased and the figures left out except the National Park Service photograph; rewritten in hubnx's own words.