Range Formation of Beech Forest Understory Herbs

During the Pleistocene ice ages central European landscapes were dominated by tundra and steppe vegetation. Deciduous trees were forced into refugia mainly in southern, but also in central and eastern Europe. In contrast to tree species, little is known about the phylogeography (i.e. the historic dynamics of species ranges explored with molecular genetic approaches) of forest understory herbs. We will reconstruct the Last Glacial Maximum refugia and postglacial migration routes of six herbs, which are strongly associated with beech (Fagus sylvatica), the most abundant deciduous tree in central Europe. By comparing individual phylogeographies of these beech forest understorey species (BFUS) to each other and to those of deciduous tree species we will clarify whether they responded to the climatic changes since the Last Glacial Maximum in an independent or concerted manner. Based on this knowledge we will further aim at explaining why most BFUS occur only in a fraction of the distribution range of beech. To this end we will quantify the relative role of migration abilities and ecological requirements of species as limiting factor for the postglacial spread of BFUS. Our study will, thus, strongly increase the knowledge of the genesis of the biodiversity in forests in general and of the biogeography of temperate forest understory herbs in particular.

Deciduous forests form the dominant natural vegetation of temperate Europe. During the cold stages of the Pleistocene, European deciduous trees were forced into refuge areas. One aim of the project was to reconstruct the postglacial expansion of beech forests by elucidating the spatiotemporal diversification of selected understory plant species of beech forests (BFUS) using genomic and molecular genetic data. Furthermore, we used advanced species distribution modelling to estimate migration rates and population growth rates as well as ancestral distribution ranges of BFUS considering potential ecological barriers to dispersal. Our results support that even during the Last Glacial Maximum not only the major beech refugium in the northwestern Balkan Peninsula, but also the southern margin of the Eastern Alps and even parts of the Northeastern Alps were covered by forests. Specifically, demographic modelling based on genomic data showed that black hellebore (Helleborus niger) has survived in refugia in that area, likely in open subalpine forests dominated by larch and pine species. The central Dinaric Alps were identified as an important refugium for two out of six studied BFUS; from there these two species dispersed further to the Carpathians. Previously, there was no firm palaeobotanical evidence for a refugium of beech in the central Dinaric Alps. Taken together, our data reject the hypothesis that the spatial genetic patterns seen in different BFUS are congruent, which implies idiosyncratic responses of species to climatic oscillations instead of range shifts of entire communities. The range filling approach revealed conclusive indication for strong dispersal limitation of each of the five range-restricted BFUS. First, range filling is low, ranging from 15% to 39%. This implies that the present-day distribution of these species covers only one quarter of the area representing the climatic niche. Individual re-colonization histories were highly idiosyncratic, which is in line with the molecular findings. Consequently, the idiosyncratic patterns of the BFUS seem to be rather the product of chance events than being determined by functional traits of dispersal propagules or the species' niches. The most innovative points of this study and main advances in the field of biogeographic modelling are 1) the uncovering of the high importance of chance events in the formation of species ranges; 2) the inclusion of historic climates in the parameterization of species distribution models; and 3) the high performance of our model enabling us to cover the entire Pleistocene re-colonization from refugia to the current distribution at a high spatial resolution in a spatiotemporally explicit manner. As the project significantly increases the knowledge of the spatiotemporal diversification of BFUS, it helps identifying strongly divergent or genetically variable populations. It may thus provide the foundation of the preservation of genetic diversity, one of the fundamental levels of biodiversity.