Abstract
The disjunct temperate rainforests of the Pacific Northwest of North
America (PNW) are characterized by late-successional dominant tree
species western redcedar (Thuja plicata) and western hemlock (Tsuga
heterophylla). The demographics of these species, along with the PNW
rainforest ecosystem in its entirety, have been heavily impacted by the
geological and climatic changes the PNW has experienced over the last 5
million years, including mountain orogeny and repeated Pleistocene
glaciations. These environmental events have ultimately shaped the
history of these species, with inland segments potentially being
extirpated during the Pleistocene glaciation. Here, we collect genomic
data for both species across their ranges in order to develop multiple
demographic models, each reflecting a different hypothesis on how the
ecosystem dominant species may have responded to dramatic climatic
change. Results indicate that inland and coastal populations in both
species diverged an estimated ~2.5 million years ago and
experienced a decrease in population size during glaciation, with a
subsequent population expansion. Importantly, we found evidence for
gene-flow between coastal and inland populations during the
mid-Holocene. It is likely that intermittent migration in these species
has prevented allopatric speciation. In conclusion, the combination of
genomic data and population demographic inference procedures involving
machine learning establish that populations of the ecosystem dominants
Thuja plicata and Tsuga heterophylla persisted in refugia located in
both the coastal and inland regions, with populations expanding and
contracting in response to glacial cycles with occasional gene-flow.