The Late—Wisconsin spruce forest that characterized most of Minnesota (and the rest of the Middle West) differed from the modern Boreal Forest of Canada not only in the absence of pine but also in the apparent admixture of black ash, oak, and elm and in the presence of openings in which Artemisia was abundant. Close—interval pollen analyses at 3 sites in Minnesota show some of the details in the transformation of this spruce forest to pine or hardwood forests. At Kirchner Marsh in southeastern Minnesota, the spruce forest was replaced abruptly about 10 000 yr ago birch, ald and temperate hardwoods before pine (jack pine?) entered the region for a short period. At Wolf Creek in central Minnesota, pine arrived from the east virtually at the moment when spruce declined. At Lake of the Clouds in northeastern Minnesota, spruce forest had succeeded tundra about 10 000 yr ago, but simultaneously pine (jack pine?) and the temperate hard woods (especially elm) appeared in quantity. The spruce and hardwood components then declined about 9300 yr ago as pine (red pine?) increased to dominance during the next 400 yr. Charcoal counts for the 3 sites provide no support for the hypothesis that increased fire incidence explains the forest transformation from spruce to pine. Climatic change must instead be the primary cause. The apparent openness of the spruce forest can be attributed to the continued presence of the wasting ice sheet just to the north, which favored a dry and windy climate, perhaps with relatively mild winters. As the ice sheet withdrew, its influence decreased. Differential rates of tree immigration as well as progressive leaching of upland soils may have been additional factors effecting changes in forest composition.