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).