Holocene fire disturbance in the boreal forest of central Sweden

Abstract

Holocene fire disturbance and vegetation history were reconstructed using macroscopic charcoal and pollen accumulation rates from two lake sediment records (Holtjarnen and Klotjarnen) collected in the boreal forest of central Sweden. The records were used to examine the potential drivers associated with changes in fire regime. Climate, vegetation and human activity were all identified as factors variously influencing the fire regime. In the early Holocene, near bicentennial fire return intervals were regionally widespread, suggesting that fire disturbance was largely regulated by climate at that time. In the mid- and late Holocene, vegetation exerted an important control on the fire regime. During the mid-Holocene, the expansion of thermophilous broadleaf vegetation offset the influence of warmer climate by altering the local microclimate and by changing the structure and flammability of the available fuels. During the transition to the late Holocene, thermophilous vegetation decreased in abundance and Pinus increased, resulting in a more flammable forest even though the climate was cooling and moistening. Fire disturbance correspondingly increased. The modern boreal forest was established in the late Holocene as Picea expanded regionally as the climate cooled, moistened, and became increasingly continental. Although no change in the frequency of fire was apparent at this time, increased stand densities likely facilitated greater fuel consumption in subsequent fires. Within the last millennium, human action markedly modified the forested landscape, altering the fire regime.