Invasion of Norway spruce (Picea abies) and the rise of the boreal ecosystem in Fennoscandia

1. Norway spruce (Picea abies), one of the dominant tree species in Eurasia, has spread slowly westward in northern Europe, invading eastern Finland about 6500 calibrated years ago (cal. years BP), eastern central Sweden about 2700 cal. years BP and southern Norway about 1000 cal. years BP. Its spread is the most recent and best constrained invasion of a main tree species in northern Europe and allows an assessment of colonization patterns and associated competitive replacement processes. 2. We analysed five selected high-resolution pollen accumulation rate (PAR)-records along a 700-km long transect in the direction of P. abies invasion from eastern Finland to central Sweden across the present P. abies- and Pinus sylvestris-dominated southern boreal zone. 3. Our results show that the P. abies population increased in size from the time of the initial expansion to levels comparable with the modern in 100-550 years. At each site P. abies invaded a dense, intact Pinus-Betula-Alnus forest, mixed with temperate deciduous taxa, particularly Tilia cordata and Corylus avellana. The resident mixed forest provided no or weak resistance to the colonization of P. abies, and the variable population growth rate was therefore not caused by compositional differences in the resident forest but by other, possibly local edaphic factors. 4. Of the taxa that formed the resident forest, T. cordata responded most strongly to the invasion of P. abies. This suggests that the mid-Holocene T. cordata population decline was not directly climate-induced but resulted from competitive replacement due to overlapping ecological niches with P. abies, a stronger competitor. 5. Synthesis. The rise to dominance of P. abies was caused not only by its rapid population growth but by associated competitive suppression of other taxa, leading to a major ecosystem change from a mixed conifer-deciduous forest to the modern P. abies- and P. sylvestris-dominated boreal conifer forest in central Fennoscandia. This competitive suppression by P. abies is still reflected in the scattered occurrence and generally weak performance of T. cordata in the boreal zone of Europe and may influence its distribution and abundance patterns under predicted future climate scenarios.