DISCUSSION
Ice-cover during the last glacial maximum (LGM; 26.5-19 kya) displaced most high-latitude species, forcing them into ice-free glacial refugia (Hultén 1937; Hewitt 2000, 2003; Bennett & Provan 2008; Clark et al. 2009). The largest documented LGM macro-refugium in North America was located south of the Laurentide and Cordilleran ice sheets in the contiguous U.S., evidenced by fossil data, climatic modeling, and phylogeographic signatures (Graham et al. 1996; Jackson et al. 2000; Holliday et al. 2002), although the extent and complexity of this refugium requires further resolution. Additional LGM refugia are hypothesized in Beringia, the area of exposed continental shelf connecting Alaska to eastern Siberia (Hultén 1937; Abbott & Brochmann 2003; Hope et al. 2013), and multiple smaller micro-refugia are proposed among the archipelagos of the North Pacific Coast (Heaton et al. 1996; Hewitt 2000; Carrara et al. 2003, 2007; Lacourse et al. 2003; Mathewes & Clague 2017), although the duration and possible cyclic recurrence of these refugia remains uncertain. Coastal refugia within the Alexander and Haida Gwaii archipelagos could explain high levels of endemism in this region (Cook & MacDonald 2001; Dawson et al. 2007) and clustered phylogenetic breaks separating insular and continental populations (Colella et al. 2018c; Sawyer et al. 2018) are hypothesized to result from shared biogeographic histories of isolation and post-glacial expansion limited by secondary contact with closely-related, previously-allopatric taxa (Hewitt 2000). Paleoendemic refugial persistence would also explain the rapid reestablishment of complex biotic communities along the coast following deglaciation (Lesnek et al. 2018; Ager 2019).