The Spread of Fagus grandifolia Across Eastern North America During the Last 18 000 years

The spread of Fagus grandifolia in eastern North America illustrates the assumptions behind the mapping of species distribution changes. Recognition of the distinction between population expansion and arrival of a species is crucial for interpreting pollen diagrams. F. grandifolia probably occurred during the Wisconsin full-glacial (18 000 BP) as a scarce tree over much of southeastern North America, and did not have a localized 'refugium'. By 14 000 BP it had begun spreading north, still at very low population densities. Between 14000 and 10000 BP it reached high densities locally in the southeast, probably as the result of population expansion while the climate was temporarily favourable. F. grandifolia had reached approximately to its present geographic limits by 7000 BP, but continued to spread, at a lower rate, in some areas until the present. The eastern Great Lakes region, where F. grandifolia is most abundant today, had the most rapid population expan- sions, at about 7000 BP, attributed to locally favourable environmental conditions. The spread of F. grandifolia across the continent was achieved at very low population densities. The detection and tracking of such a spread is only marginally possible with current pollen-analytical techniques.