Description

Wildland fire is a keystone disturbance in the boreal forest, affecting everything from public safety, to woodpecker populations, to permafrost. How settlement by European people impacted wildland fire regimes in Alaska is poorly understood because paleo-fire records near population centers are rare. High-resolution records of past fires are needed to detect recent changes in the tempo of burning in the boreal forest because the era of human settlement and fire suppression are relatively short there. Here, we describe the annually resolved record of burning in the watershed of a thermokarst lake near the city of Fairbanks, Alaska (established AD 1902). We use the sedimentary archive preserved in Ace Lake to reconstruct the frequency of local wildland fires over the past 500 years. Clastic-biogenic varves consisting of silt and algae are deposited in deep-water zones (>5 m) of the lake and preserved by meromictic conditions. These annual layers provide the age control for paleo-fire activity. To identify individual fires and to describe their relative magnitudes, we measured charcoal area as viewed in epoxy thin sections. Results show that the timing of charcoal peaks in the varve record correlates with ages of fire scars on spruce trees growing around the lake, as dated by tree-ring analyses. Four fires occurred after AD 1900, the last one in 1962. European settlement of the area was accompanied by a striking increase in fire frequency. During the 400 years prior to AD 1900 - before Fairbanks was founded - fires occurred on average every 58 years (35-98 year range). After European settlement, local fires became roughly twice as frequent (mean 21 years, 10-28 year range). This period of anomalously frequent fires followed by fire suppression starting in the 1960s has caused fuel loads to become synchronized around Fairbanks today. Our paleo-records suggest this large area of uniform fuel loading will soon be overdue to re-burn.