Contemporaneous Alnus decline and the beginning of Iron Age cultivation in pollen stratigraphies from southern Finland

Declines in Alnus coinciding with the first signs of Iron Age (A.D. 0-1150) human activities were found in the pollen stratigraphies of five small lakes in southern Finland. One lake did not show a clear minimum. Three of the lakes were investigated with close-interval analyses which showed that the Alnus minimum lasted for several centuries. The results were compared with 41 previously published pollen diagrams with evidence of Iron Age human activity from southern Finland. These diagrams were classified in three ways: (1) showing no Alnus minimum; (2) cases where a minimum was unclear; (3) showing a clear minimum in Alnus. The different types were found randomly scattered around southern Finland suggesting that Alnus minima were a local phenomenon. In most cases the Alnus minimum took place between ca. A.D. 600 and ca. A.D. 1000, A.D. 1300 being the latest date for the end of the minimum. The results do not suggest a pathogen outbreak over the entire area. The beginning of the minimum clearly coincides with the onset of Iron Age anthropogenic activities suggesting that these were the probable cause. Pollen analysis provides little information as to why trees were felled thus archaeological evidence is needed. However, the Alnus decline may prove a new and useful indicator of the onset of Iron Age anthropogenic activity in pollen diagrams.