Postglacial recolonisation of plants in the western Alps of Switzerland

During the Quaternary glaciations, alpine plants could only persist in scattered ice-free areas located either within the Alps or at their periphery. From there, species recolonised the Alps after the retreat of glaciers, and efforts have been made to reconstruct the main migration pathways using either floristic information (i.e. patterns of species distribution) or studies of infraspecific genetic variation (i.e. distribution patterns of genotypes or lineages). This review article compiles floristic and molecular information about the recolonisation of the western Swiss Alps. Three main pathways of recolonisation have been suggested in the literature: the Rhodanian pathway (from south-western refugia, along the Rhone valley), the transalpine southern pathway (from southern refugia, crossing the line of main summits through high-altitude passes), and the transalpine eastern pathway (from eastern refugia, along the Rhine valley and across the northern Alps). Floristic data mainly support the Rhodanian pathway, as many western Alpine species occur along this route, as well as the transalpine southern pathway, as some southern species occur in the vicinity of the relevant passes. In contrast, few species show distribution patterns consistent with the transalpine eastern pathway. Recent molecular studies have supported the floristic patterns. In particular, distinct genetic lineages of the widespread alpine species Biscutella laevigata were distributed along the Rhodanian pathway and transalpine southern pathway, and genetic diversity was especially high in the central area, where these recolonisation pathways met. Knowledge about past migration routes of plants is essential to understand the response of species to climate change and the present patterns of biodiversity within the Alps.