Other paleobotanists have suggested that Picea glauca (white spruce) migrated northwards across the northern Great Plains of North America with the retreat of the Laurentide Ice Sheet. These researchers proposed that white spruce forests quickly colonized recently deglaciated landscapes in the Dakotas and Minnesota in the United States, eventually reaching southern Saskatchewan and Manitoba in Canada by about 12,000 14C yr B.P. New paleobotanical and radiocarbon evidence, presented here, concurs with this migratory scenario, but question previously published chronologies and differs in its paleoevegetation interpretation. Previous chronologies were based on the 14C dating of organic sediments, which are known to provide erroneously older ages. A revision of the timing of white spruce occupation of the northern Great Plains during the late Pleistocene has been compiled based on the analysis of over 80 pollen and 200 plant macrofossil samples, and 12 associated 14C ages (obtained from terrestrial plant macrofossils) from six sites located in the northern Great Plains of the United States and Canada.

The fossils and 14C ages from the Wendel site, situated on the Glaciated Till Plain, indicate that a parkland of Picea glauca and prairie herbs colonized southeastern North Dakota by 11,500 14C yr B.P. These spruce trees probably occupied the shorelines of numerous ponds and lakes, and were subsequently replaced by grassland at about 11,000 14C yr B.P., as a result of greater regional aridity. Further west on the Missouri Coteau upland, a belt of hummocky moraine with numerous kettle lakes, the Picea glauca parkland phase occurred later, 10,800 to 10,600 14C yr B.P. in southeastern North Dakota (Coldwater Lake), and 10,300 to 10,000 14C yr B.P. further north in Saskatchewan (Andrews, Kyle, Beechy, and Neufeld sites). The results from this study indicate that a non-analog vegetation colonized the northern Great Plains immediately following deglaciation, as has been reported elsewhere. The anomalous mixture of prairie herbs, species of which exist today in this mixed-grass prairie region, and white spruce, a boreal taxa, suggest that the vegetation was not that of a boreal forest, as earlier proposed. Consequently, regional temperatures during the late Pleistocene were probably not as cool as previously thought.