The southern coastal Beringian land bridge: cryptic refugium or pseudorefugium for woody plants during the Last Glacial Maximum?

Aim: The Bering Land Bridge (BLB) connected Asia and North America during glacial periods, supported a diverse ecosystem of now-vanished megafauna, and is a proposed glacial refugium. This study tests whether southern coastal Beringia was a refugium for woody taxa during the Last Glacial Maximum (LGM) and hypotheses about habitats available on the BLB before and after megafaunal extinction. Location: St. Paul Island, Alaska. Methods: We analysed sediment cores from the Lake Hill, with a new age model anchored by 18 radiocarbon dates and multiple palaeoecological indicators (sedimentary ancient DNA [sedaDNA], macrobotanical fossils, and pollen) for the presence/absence of four woody genera: Picea, Betula, Alnus and Salix. We reconstructed vegetation history and compare St. Paul tundra composition to mainland counterparts. Results: St. Paul has been continuously occupied by graminoid-forb tundra with prostrate shrubs (Salix, Ericaceae) since 18,000 years before present (yr bp). Fossil pollen of Picea, Pinus, Betula and Alnus is present in the Lake Hill sediments at low relative abundances and accumulation rates, consistent with long-distance transport. Macrobotanical fossils and sedaDNA analyses do not support Picea, Betula and Alnus presence. The St. Paul modern and fossil pollen assemblages are compositionally unlike mainland counterparts, but most closely resemble Arctic herbaceous tundra. Stratigraphically constrained cluster analysis indicates no major change in the vegetation after woolly mammoth extinction at 5600 yr bp, although Poaceae, Cyperaceae, Equisetum and forb abundances increase. Main conclusions: This study strongly indicates that St. Paul and, by implication, southern coastal Beringia were not refugia for woody taxa during the LGM. The persistence of prostrate shrub-graminoid tundra supports interpretations that herbaceous tundra prevailed on southern Beringia during the LGM, whilst not ruling out the possibility of mesic shrub tundra in the interior. This herbaceous tundra supported an island refugium for woolly mammoth for 8000 years, showing no major vegetation composition changes after extinction.