Long-term sustainability of a northern boreal deciduous swamp forest in northern Sweden: succession in the absence of fire

Abstract

In Fennoscandian boreal forests the general succession pattern after a stand-replacing fire includes the pioneer tree species Betula, Populus, Salix and Pinus. The primary colonizers are subsequently replaced by secondary species; on mesic and moist soils commonly by Picea abies. Many swamp forests have developed from deciduous Betula/Alnus fens to Picea abies swamp forests during the last 3500 years, since Picea became common in this region. Our objective was to study the long-term vegetation development, fire history and succession patterns of a boreal swamp forest in N Sweden. The area was characterized by Betula trees, and there were no signs of recent fire. We wished to investigate whether Betula dominance was a result of succession after disturbance other than fire, or an exception to the general rule that normally Picea abies is the dominating secondary species in this region. The study included analyses of pollen and charred particles along with AMS dating. Our results revealed that Betula has been the dominant tree species in the swamp forest for 8000 years. The vegetation developed from a deciduous wet forest, via forest fen, to the present swamp forest; Betula spp. has been both pioneer and secondary species. The scarcity of charred particles suggests that severe fires have been rare in the Betula swamp forest. The vegetation development at the study site has been governed by long-term mire development, local nutrient conditions and the low impact of fire. The lack of stand-replacing fire at the study site is probably the reason why Betula still dominates the swamp forest investigated, although Picea had become common in other parts of the region c. 3500 years BP. The study has shown that is is not possible to apply a general succession model to all situations during the Holocene but that succession patterns depend highly on site-specific conditions.