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

Glacial refugia have likely been important in shaping diversity gradients outside the tropics. However, the biogeographical histories of most species within glacial refugia remain unclear. We examined the geographic range structure and phylogenetic attributes of the mosses of Haida Gwaii, a putative glacial refugium and ‘hotspot’ of moss diversity off the northwest coast of British Columbia. We found that many species have widespread, but disjunct distributions, typically 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. The phylogenetic dispersion of species within high elevation habitats, which best match the climatic regime of the historical glacial refugium, is consistent with the filtering of evolutionarily distinct glacial relicts, and contrasts markedly with the patterns of phylogenetic clustering observed across other habitat types. Our study illustrates how the present-day phylogenetic structure of species composition and diversity can reveal the signal of glacial refugia, and help explain why some taxa are more diverse outside of the tropics.