Exploring climatic and biotic controls on Holocene vegetation change in Fennoscandia

1. We investigated the potential drivers of Holocene vegetation changes recorded at four Scandinavian pollen sites, two in Sweden and two in Finland, at a time when they were largely free of anthropogenic influence. 2. We used the generalized dynamic vegetation model LPJ-GUESS forced with climate anomaly output from an atmospheric general circulation model to simulate tree species dynamics from 10 000 years ago to the present. The model results were compared to high-resolution pollen accumulation rates gathered at the sites. 3. Our results indicate that both the observed northern distributional limits of temperate trees, and the limits of Pinus sylvestris and Alnus incana at the tree line, are a result of millennial variations in summer and winter temperatures. The simulation of several distinct trends in species occurrence observed in the pollen record indicates that a time lag due to the slow spreading of species need not be invoked for most species. 4. Sensitivity studies indicate that competition, natural disturbance and the magnitude of interannual variability play key roles in determining the biomass, establishment and even the presence of species near their bioclimatic limits. However, neither disturbance due to fire nor limits on establishment due to drought were likely to have been major determinants of the observed trends on the timescales considered. 5. We were unable to limit the modelled occurrence of Picea abies at the study sites to the periods at which it was observed in the pollen records, indicating that we have still not completely understood the driving or limiting factors for Holocene changes in Picea abies abundance. 6. Synthesis. This study shows that by combining quantitative vegetation reconstructions with a modern, process-based dynamic vegetation model, we may gain new insights into the potential biotic and abiotic drivers of Holocene vegetation dynamics, and their relative importance. This knowledge will be crucial in enabling us to assess more confidently the response of northern European vegetation to future climate change.