Postglacial Migration of Five Forest Genera

Sears, Paul

The relative order of appearance of pollen of Quercus, Tsuga, Carya, Fagus, and Tilia in 111 peat deposits within an area extending from Iowa to the Atlantic, and from Tennessee to the St. Lawrence is here reported. Resultant inferences concerning order, direction, and extent of migration are discussed in relation to supposed changes in postglacial climate. The general order of appearance in the northeastern part of the area was apparently: Quercus, Tsuga, Fagus, Carya, and Tilia; in the southwestern part: Quercus, Carya, Tilia, Tsuga, and Fagus. The early and general dispersal of Quercus seems to have occurred when Pinus replaced Abies and Picea. This shift in composition coincides with a supposed dry period, possibly resulting from rising temperature rather than from decreased precipitation. Tsuga appears to have accompanied Quercus into eastern Canada at this time in the relatively moister climate of the Atlantic region. In the interior, Quercus was frequently accompanied by Carya. Later the ranges of Tsuga and Fagus apparently expanded, not only within southeastern Canada, but farther west into the area beyond the present limits of these genera. This is regarded as an indication that climate in the continental interior was then more humid than it had been just previously. Tilia also seems to have moved westward, during this more humid time, toward its present limits. After that, Carya evidently extended north of its present general limits. This extension is definitely correlated with a decrease in Tsuga and Fagus and with a maximum of Quercus in the coniferous bogs at the northern edge of the area. It is regarded as indicating a second dry and warm period, now past its climax. Important early centers of dispersal appear to have existed in the southeastern part of the area and in Wisconsin, a considerable part of which was not covered by the last glaciation. This does not preclude the existence of other early dispersion centers and routes which may be revealed when the necessary intensive local studies are made. Data from the lower Ohio Valley should be of especial importance.

Postglacial Migration of Five Forest Genera

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