Detecting the phylogenetic signal of glacial refugia in a bryodiversity hotspot outside the tropics

Introduction Glacial refugia have likely been important in shaping diversity gradients outside the tropics. Many taxa that have high extratropical diversity in the present day, such as mosses, may have persisted in glacial refugia. However, the biogeographical histories of most species within refugia remain unclear. Location Haida Gwaii archipelago, north-west coast of British Columbia, Canada. Taxa Bryophyta (mosses). Methods We reconstructed the regional phylogeny of the mosses of Haida Gwaii, a putative glacial refugium and 'hotspot' of moss diversity, and used phylogenetic comparative methods to examine the macroecological imprint of glacial refugia on the geographic range structure and phylogenetic attributes of present-day moss assemblages. Results We found that many mosses have widespread, but disjunct, distributions, with few close relatives on the islands. We suggest that these features reflect the imprint of glacial history, whereby species within refugia represent isolated populations of previously more widespread species that may have diversified elsewhere. We also observed evidence for phylogenetic overdispersion of species within high-elevation habitats, which best match the climatic regime of the historical glacial refugium. Our results are consistent with the filtering of evolutionarily distinct glacial relicts within these habitats, which contrasts markedly with the patterns of phylogenetic clustering observed across other non-refugial habitat types. Main conclusions The islands of Haida Gwaii represent an extratropical hotspot of bryophyte diversity. Our study illustrates how the present-day phylogenetic structure of mosses on Haida Gwaii may have been shaped by glacial history, and highlights the importance of glacial refugia in maintaining extratropical moss diversity.