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.