LATE PLEISTOCENE SHRUB EXPANSION PRECEDED MEGAFAUNA EXTINCTIONS IN EASTERN BERINGIA

The expansion of shrub-tundra vegetation and disappearance of the steppe-tundra (mammoth-steppe) is one of the most important ecological transitions to occur in the northern high latitudes during the Late Pleistocene. In eastern Beringia, the rapid expansion of Betula shrubs is widely recorded in pollen records and coincides with extinctions of megafauna grazers (horse and mammoth). Despite the importance of this major vegetation change, the spatial pattern and precise timing of the Late Pleistocene shrub expansion is poorly resolved. This is largely because of chronological uncertainty in lake records from this region, caused by the scarcity of dateable plant macrofossil remains and reliance on inconsistent bulk sediment ¹⁴C dates. By constraining the timing and spatial pattern of past shrub expansion we can examine its link with mammalian extinctions and better understand the current climate-related shrub expansion underway in the Arctic.

Here, we re-examine the of chronology of Late Pleistocene pollen records for which AMS ¹⁴C dating is available and compare results with a database of published ¹⁴C dates from plant macrofossils, megafauna remains and Arctic ground squirrel middens. Salix spp. consistently increase in abundance before Betula, suggesting either taxon-specific threshold responses to climate amelioration or differing environmental constraints. We find no strong evidence for a time-transgressive Betula shrub expansion in eastern Beringia during the Late Pleistocene, supporting the view that Betula shrubs expanded from isolated glacial refugia; however, it is possible that the lack of a clear pattern may reflect the limited chronological precision of many lake records. The frequency of shrub plant macrofossil remains increases ca. 14,000 cal. BP, which coincides with declines in the abundance of ¹⁴C-dated bones from megafaunal grazers. These results support the hypothesis that Late Pleistocene megafauna extinctions in eastern Beringia were preceded by habitat loss as the steppe-tundra gave way to Betula shrub-tundra. This pattern calls into question the claim that shrub expansion and demise of the steppe-tundra was driven by the loss of ecosystem-maintaining herbivores to human overkill.