Thursday, 20 November 2003: 9:30 AM
Fire and spruce bark beetle disturbance regimes on the Kenai Peninsula, Alaska
Edward E. Berg, US Fish and Wildlife Service, Soldotna, AK; and R. S. Anderson and A. D. De Volder
The Kenai Peninsula has a low incidence of lightening and humans have caused virtually all historically documented fires. Nevertheless, our fire history studies show that, on a scale of decades to centuries, fire has been an important process on the landscape, second only to spruce bark beetles. For Paradox Lake we have a 14,000 calendar-year post-glacial record of charcoal abundance in a 9-meter lake sediment core. From this charcoal record we estimate that fire activity to be at a post-glacial minimum during the early tundra period, but increased to ca. 12 fire events/1000 years with the immigration of alder and poplar. By the arrival of white and black spruce 8000 years ago, fire event frequency had declined to 8 to 10 events/1000 years. For the present upland mixed white spruce and birch forest around this lake we estimate that over the last 3000 years the fire rate has been about 5 fire events/1000 years, or a mean fire return interval of 200 years. Fire scar studies in black spruce forests to the east of this site suggest a fire return interval of 90 years, with increasing fire activity after the 1830s due to European settlement.
In the central Kenai Peninsula we used 63 radiocarbon-dated soil charcoal samples to date fires in an upland area of beetle-killed and logged Lutz spruce forest, representing about 80 square miles. These samples indicate a median fire return interval of 600 years, and that the median time-since-burning is also approximately 600 years.
Our dendrochronology studies of spruce bark beetle outbreaks at 23 sites show that over the last 250-300 years (for which we have dateable tree-rings), spruce bark beetles have substantially thinned the forests at intervals of 20 to 60 years. Spruce bark beetle outbreaks thus appear to be the dominant form of disturbance driving the upland forest cycle on the Kenai Peninsula.
We have found no evidence that fire has followed spruce bark beetle outbreaks in the past. A run of warm summers since 1987 has created a spruce bark beetle outbreak of unprecedented scale (4 million acres in southern Alaska) and at least two fires with high rates of spread in recently beetle-killed timber. This suggests that under a warmer climate regime, fire and beetle kill will be more closely associated than in the past.