Exploring Holocene continentality changes in Fennoscandia using present and past tree distributions

With global warming, the distribution of warmth through the year is likely to change in the future and comparable changes may have occurred over the course of the Holocene. Its effect on vegetation composition and species distribution can be compared to that of a continental versus an oceanic climate. The distribution of five major tree species along a continentality gradient was studied in Fennoscandia based on distribution maps and on their proportions of pollen in surface-sediment samples. Both analyses indicate that the five arboreal species show similar patterns of response to a continentality index in the order Ulmus glabra, Corylus avellana, Quercus robur, Tilia cordata, and Picea abies from the most oceanic to most continental. Continentality, growing degree days above 5 °C, and January temperature were reconstructed quantitatively from four pollen diagrams using transfer functions based on a combined Fennoscandian pollen surface-sample data-set. Quantitative reconstructions indicate that the climate in Fennoscandia has become increasingly more continental over the last 7000 years, and this is largely an effect of winter cooling. Early Holocene vegetation composition has poor analogues to the present vegetation in Fennoscandia, which hampers quantitative reconstructions. Qualitative reconstructions suggest that the early Holocene in Fennoscandia was the most oceanic period, but probably with a high variability in temperature.