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