A tale of two conifers: Migration across a dispersal barrier outpaced regional expansion from refugia

Aim: Understanding how climate refugia and migration over great distances have facilitated species survival during past climate changes is crucial for evaluating contemporary threats to biodiversity, particularly in the face of dispersal barriers. We address this longstanding question on the refugial origins and post-glacial development of mesic forests. Location: Pacific Northwest, North America. Taxon: Mountain hemlock (Tsuga mertensiana) and western redcedar (Thuja plicata). Methods: Range-wide genotyping-by-sequencing (ddRADseq) of both study species and a pollen reconstruction of mountain hemlock presence over the last 20,000 years. Results: Mountain hemlock occurred in two coastal populations (Oregon and Washington) during the glacial maximum, each of which dispersed to the interior (Idaho and British Columbia) during the Holocene. These populations spread in the direction of dominant winds across a barrier of dry, rain-shadowed valleys. In contrast, for western redcedar, we infer four disparate refugia during the glacial maximum: southern (California), central (Washington), interior (Idaho), and northern (Haida Gwaii islands). Main conclusions: Despite the presence of pre-dispersed refugial populations, the majority of the redcedar distribution was colonized by the central population. The history for these two key conifers contrast with many recent studies emphasizing the role of cryptic refugia in colonizing modern species ranges.