The 9000-year histo of vegetation development and disturbance patterns of a swamp-forest in Dalama, northern Sweden

Abstract

Analysis of pollen and charcoal in peat cores together with age-structure data of a swamp- forest site in the western part of central Sweden show the regional vegetation succession and the disturbance patterns over the last 9000 years. Five major vegetation sequences are identified: 1) Betula- Pinus period (c. 7000 to c. 6000 cal. BC); 2) wetland development (c. 6000 cal. BC to c. cal. AD 300); 3) Picea swamp-forest establishment ( c. cal. AD 300-1000); 4) human exploitation (c. cal. AD 1000-1900); and 5) cessation of human impact ( c. cal. AD 1900- present). Between c. 7000 cal. BC and c. cal. AD 1000, 17 fires were recorded in the peat. During the last 900 years, the area has been under the influence of human impact, mainly by animal grazing and trampling, together with deliberate flooding. These activities have affected the structure and dynamic of the swamp-forest by suppressing the regeneration of Picea and thereby creating an open Picea stand. During the last hundred years, a gradual cessation of human activities has led to increased Picea establishment and a denser swamp-forest. Despite these prolonged anthropogenic disturbances, the swamp-forest has qualities that make it unique in the Swedish forest landscape: 1) it has been forested for about 1700 years, during the last c. 900 years of which fire has been of little or no importance; 2) there are no visual signs of logging; and 3) dynamic processes, with trees continuously establishing, ageing and reaching senescence, have produced a forest with a high abundance of dead wood. The results show that the vegetation has been highly dynamic in response to both anthropogenic and 'natural' disturbance, and to the immigration of Picea. The present high biodiversity, and subsequent conservation interest does not result from long-term stability or absence of fire and other disturbance agencies. Conservation policies should therefore, in addition to species preservation, work to create a mosaic-like landscape where many different forest types are represented, containing structures, substrates and processes that are important for many different species. This provides the species the freedom of movement necessary for survival within the forest landscape in a longer perspective.