Factors affecting Holocene fire dynamics in boreal Europe

In studies addressing temporal variation in regional wildfire, macroclimate is often considered to be the predominant control, whereas the role of forest tree composition is often considered subsidiary. In this study, pollen and charcoal records from 75 soil and two lake sediment cores were used to reconstruct the post-glacial history of fire in boreal Europe. The soil charcoal records exhibit a spatial pattern of deposition that illustrates the importance of climate, whereby the paucity of charcoal from the wet outer coastal region is in marked contrast to the abundance of charcoal from drier inland settings. Regarding the importance of vegetation, the soil charcoal records document that the late-Holocene expansion of Picea abies, a new forest dominant, significantly altered the incidence of fire at a sub-continental scale. The lake charcoal records also reveal that the incidence of fire has changed markedly through time. During the early Holocene interval (ca. 10-7.5 kilo annum [ka]) both of the coring sites were characterized by similar fire regimes, whereas in the mid-Holocene fire disturbance decreased at the southern site commensurate with the expansion of thermophilous vegetation under warm dry conditions. At ca. 5.5 ka, expansion of oak and lime resulted in a brief increase in fire activity, possibly due to increased fuel loads. Similarly, the expansion of spruce at ca. 2.5 ka coincides with a marked increase in fire frequency, illustrating that the expansion of spruce altered the fire regime. However, this result differs from that obtained from the soil charcoal records where the expansion of spruce resulted in a general decrease in fire. The different responses to the expansion of spruce may be related to differences in local microclimatic and topographic conditions. Combined, these findings illustrate that climate, forest composition, and topography are important parameters that regulate the fire regime. Consequently, they must be considered when modeling future fire risk boreal forests.