An updated phylogenetic bioregionalization for the European fern flora

Biogeographic regions have been extensively used as reference units in macroecological studies and to prioritize biodiversity conservation efforts. However, classical bioregionalizations were delineated based on taxonomical similarity across space, and thus the importance of historical factors may have been underrated. This limitation may be particularly relevant for lineages with long and complex evolutionary histories, such as ferns. Here, we drew on an exhaustive distribution dataset including all fern species and subspecies of Europe (661 grid-cells of c. 110 x 110 km each), as well as a nearly complete molecular phylogeny to define fern phyloregions based on their phylogenetic relatedness. Also, we quantified the degree of specificity of individual phylogenetic clades to the phyloregions using a new index of geographical confinement based on phylogenetic diversity. Six distinct phyloregions were identified, with a primary divide between north-eastern and south-western Europe. Both phylogenetic beta diversity and clade specificity were overall low, supporting the idea that dispersal limitation is not a major driver of fern distribution. Yet, the phylogenetic specificity analysis revealed that ancient fern lineages show preference for northern latitudes, which explained the northeast to southwest split of the territory. More than 40 years after the only bioregionalization analysis for the European fern flora, our study provides a fresh regional delineation that takes into account the evolutionary history of the group. In addition to classical bioregionalization approaches, our phylogenetic specificity index allowed us to elucidate the identity of the clades that ultimately shaped the bioregions, which might otherwise had remained obscure.