Vegetationsutveckling och brandhistorik i Tyresta under 9000 år : en pollenanalytisk studie av en skvattramtallmyr i Tyresta nationalpark, Södermanland

Sammanfattning: Analyses of pollen and charcoal in peat cores, together with age-structure data of Pinus sylvestris from a Ledum-Pinus-mire in the Tyresta National Park (south-east Sweden) show the regional vegetation succession and fire history over the last 9000 years. Five major stages of mire development are identified: 1. Lake (c 7000-6400 cal BC); 2. Wet fen (c 6400-6000 cal BC); 3. Deciduous fen (c 6000 cal BC to c cal AD 700); 4. Open Sphagnum-mire (c cal AD 700-1100); 5. Pine bog (c cal AD 1100-present). The relationship between the occurrence of Ledum-Pinus-mires (a swampforest-type frequent in the area) and fire is discussed. Climate, fire and human activity are the three most important factors that have influenced the vegetation succession in the Tyresta area. Changes in climate have affected the species composition of the forest. Fires have been common from c 5600 cal BC to c cal AD 1650 in both the forest and the fen. They have strongly influenced the mire development and fen vegetation. A positive correlation may exist between fire and the occurrence of Ledum palustre. The reduction of forest fires in the 17th century was due to a change in the utilisa-tion of the forest by man. The commercial use of the forests for fuel and timber, as well as the forest law of 1647, which prohibited deliberate burning of the forests, led to a partial cessation of deliberate burning and an initiation of fire control. The past 350 years during which there has been a low fire influence, might be atypical for the 7000 years of fire history. It might therefore be appropriate to reintroduce forest fire as an ecological disturbance factor in order to preserve the natural properties of the National Park.