Plant formations different from any extant today apparently were widespread in North America and Europe during the last deglaciation, produced by the independent biogeographic responses of plant taxa to climate change. Dissimilarity analyses of modern and fossil pollen samples in eastern North America show that the unique plant associations centered around the Great Lakes at 14 000 calendar years before present (yr BP), with high dissimilarities during 17 000–12 000 yr BP. The late-glacial fossil pollen assemblages are characterized by (1) high abundances of boreal conifers such as spruce and larch relative to their Holocene values, (2) high abundances of herbaceous types (sedge, sage, and ragweed), (3) high abundances of broad-leaved deciduous types (ash, hornbeam, poplar, hazel, and willow), and (4) the low abundance or absence of pine, alder, and birch. When the fossil pollen samples are assigned to biomes using the affinity score technique, the late Pleistocene pollen samples are assigned to mixed parkland, a biome that is not extant in North America today. The fastest vegetational changes occurred 13 000–11 000 yr BP, when the late Pleistocene vegetation reorganized into the Holocene biomes, which have persisted to today. Simulations by the Community Climate Model, version 1 (CCM1), suggest that late-glacial climates were also unlike modern climates, featuring a “hypercontinental” mixture of colder-than-present winters, warmer-than-present summers, and lower-than-present precipitation. Dissimilarity analyses of the pollen data and CCM1 simulations for 21 000, 16 000, 14 000, 11 000, and 6000 yr BP show that (1) the temporal and spatial distribution of high dissimilarities in the vegetation (relative to present) coincide with dissimilarities in simulated climate, (2) the timing and spatial distribution of changes in the vegetation and simulated climate also agree, and (3) the largest climatic and vegetational changes follow the peak period of dissimilarity from present. Taken together, these three lines of evidence support the hypothesis that the no-analogue plant associations were in equilibrium with orbital- and millennial-scale climate change. Nonclimatic factors such as low atmospheric CO2 concentrations and the presence of now-extinct megafauna species may have increased the openness of the Pleistocene vegetation, but by themselves cannot explain the observed mixture of boreal, temperate, and herbaceous taxa in the no-analogue pollen assemblages, nor can they explain the prevalence of no-analogue pollen samples during the late glacial period.
Williams et al. (Sat,) studied this question.