A 600-year forest fire record in a varved lake sediment (Ristijärvi, Northern Karelia, Eastern Finland)

Charcoal particle and pollen influx values in the annually laminated sediment of a boreal forest lake surrounded mostly by mesic soils were used to reconstruct the history of forest fires in Eastern Finland for a 600-year period between 100 BC and AD 500, prior to any significant human influence. Six to seven local fires were detected during this period, at an average interval of 90—115 years. The corresponding fire rotation time is estimated to be 180—230 years. The local fire interval and Picea pollen percentages at the site were of the same magnitude as those recorded at two sites with dry soils in eastern Finland, suggesting that the average fire return interval may have been similar at dry and mesic sites. The intensity of fires was estimated from the declines in influxes of spruce and pine pollen. Pine can survive surficial fires of low intensity, but spruce trees are killed by virtually all fires. The contemporary declines in spruce and pine pollen influxes suggest that about half of the fires were strong ones, causing stand replacement.