| Resumo: |
A pollen diagram from a lake in the former bed of the eastern arm of Lake Agassiz in northern Minnesota records a vegetation of spruce forest followed by immigration successively of Pinus banksiana and (or) P. resinosa at 10 000 B.P., then Abies and Pteridium, and still later Alnus. Between 8000 and 7000 B.P. prairie and (or) Quercus savanna prevailed on the uplands, followed by deciduous forests of mainly Quercus, Ostrya virginiana, Fraxinus pennsylvanica, and possibly Populus sp. Slightly later, Pinus strobus migrated into the area, resulting in a gradual decline of pollen of deciduous forest types. Betula pollen, however, rises, and there is an indication of a return to prairie conditions prior to 3000 B.P. During the 8000-7000 B.P. dry interval the lowland vegetation consisted of fens of Typha latifolia, Dryopteris thelypteris, and Cyperaceae. Later paludification and lateral expansion of the peatland gave rise to rather rich swamps of Picea mariana, Larix laricina, Alnus rugosa, and Thuja occidentalis. There are some conspicuous peaks of Myrica in the pollen diagram. The time after 3000 B.P. is characterized by much Pinus strobus pollen and minima of deciduous trees and herbs. In the lowlands, formation of raised bogs and poor swamps and fens began, indicating a shift in climate towards wetter conditions. The arrival of white man in the area is reflected by the rise of Ambrosia. The shifts in overall peatland types are clearly accompanied by changes in the species composition of Pediastrum in Myrtle Lake, indicating corresponding changes in the lake waters.
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