Beringia as a glacial refugium for boreal trees and shrubs: new perspectives from mapped pollen data

Aim Beringia, far north-eastern Siberia and north-western North America, was largely unglaciated during the Pleistocene. Although this region has long been considered an ice-age refugium for arctic herbs and shrubs, little is known about its role as a refugium for boreal trees and shrubs during the last glacial maximum (LGM, c. 28,000-15,000 calibrated years before present). We examine mapped patterns of pollen percentages to infer whether six boreal tree and shrub taxa (Populus, Larix, Picea, Pinus, Betula, Alnus/Duschekia) survived the harsh glacial conditions within Beringia. Methods Extensive networks of pollen records have the potential to reveal distinctive temporal-spatial patterns that discriminate between local- and long-distance sources of pollen. We assembled pollen records for 149 lake, peat and alluvial sites from the Palaeoenvironmental Arctic Sciences database, plotting pollen percentages at 1000-year time intervals from 21,000 to 6000 calibrated years before present. Pollen percentages are interpreted with an understanding of modern pollen representation and potential sources of long-distance pollen during the glacial maximum. Inferences from pollen data are supplemented by published radiocarbon dates of identified macrofossils, where available. Results Pollen maps for individual taxa show unique temporal-spatial patterns, but the data for each taxon argue more strongly for survival within Beringia than for immigration from outside regions. The first increase of Populus pollen percentages in the western Brooks Ranges is evidence that Populus trees survived the LGM in central Beringia. Both pollen and macrofossil evidence support Larix survival in western Beringia (WB), but data for Larix in eastern Beringia (EB) are unclear. Given the similar distances of WB and EB to glacial-age boreal forests in temperate latitudes of Asia and North America, the widespread presence of Picea pollen in EB and Pinus pollen in WB indicates that Picea and Pinus survived within these respective regions. Betula pollen is broadly distributed but highly variable in glacial-maximum samples, suggesting that Betula trees or shrubs survived in restricted populations throughout Beringia. Alnus/Duschekia percentages show complex patterns, but generally support a glacial refugium in WB. Main conclusions Our interpretations have several implications, including: (1) the rapid post-glacial migration rate reported for Picea in western Canada may be over estimated, (2) the expansion of trees and shrubs within Beringia should have been nearly contemporaneous with climatic change, (3) boreal trees and shrubs are capable of surviving long periods in relatively small populations (at the lower limit of detection in pollen data) and (4) long-distance migration may not have been the predominant mode of vegetation response to climatic change in Beringia.