Notes on the postglacial spread of abundant European tree taxa

The reaction of vegetation to past climate change provides important insights for vegetation responses to future climate change. A key problem for projections into the future is obtaining estimates of the rates at which plants are able to spread as their environment changes. To address this uncertainty, we review the palaeoecological and phylogeographic literature to estimate the range of observed rates of spread for the major European trees and discuss aspects of their postglacial spread. The review is illustrated with isochrone maps depicting the time when particular thresholds in pollen proportion were reached in pollen diagrams available from the European Pollen Database. We find that rates of at least 1,000 m year⁻¹ were realised by early colonisers including Corylus and Ulmus, while trees spreading later into established woodlands, e.g. Quercus and Tilia, achieved rates of around 500 m year⁻¹. Phylogeographic investigations are available for most of the abundant European trees, often indicating that populations in the central and southern parts of the three south European peninsulas were not the origins for the postglacial colonization of central and northern Europe. In some cases, the results of these studies clearly show the direction of postglacial spread, while generally providing new information to help in interpreting pollen data. Phylogeographic results for Alnus suggest that the high apparent rates of postglacial spread are due to an initial spread at low population density and a later expansion. This decoupling between spread and population expansion is also seen for late expanding trees such as Picea, Fagus and Carpinus. Here, population expansion was probably not delayed by dispersal, but by a limiting climate as assumed by von Post. While the late Holocene expansion of Picea and Fagus in Sweden was important as a dating tool in the development of pollen analysis by von Post 100 years ago, we remain unable to determine which particular driver caused the late expansion of these two trees.