Plant Migration in the Southern Limits of Wisconsin Glaciation in Indiana

Potzger, J; Friesner, Ray

1. A comparison is made between the advance of the boreal forest along its northern border in Alaska and the comparable retreat along its southern border of the Early Wisconsin glaciation in Indiana. 2. Relic colonies, fossil logs and fossil pollen from seven bogs are cited as evidences. 3. Evidence from fossil pollen indicates that migration resulted in changing the post-glacial forests of Indiana from boreal to a more temperate broad-leaved forest. 4. The advance of the broad-leaved genera was coincident with the retreat of the boreal genera. 5. Dominance by boreal genera was maintained for a few thousand years after the retreat of the glaciers. 6. Loss of dominance by the boreal species occurred relatively suddenly, i.e. over a period of a few hundred years. 7. In all cases the boreal elements of the forest held on as relics for many centuries after loss of dominance and in the case of Pinus, Larix, and Thuja (and in some areas Picea also) persisted to within a few hundred years of the time of settlement of the land by white man. 8. Boreal species still present in Indiana are dependent on microclimate to persist in small isolated colonies. Nowhere in Indiana do these species make aggressive advances. 9. Boreal relics persisting in Indiana (Pinus strobus, Thuja, Larix) indicate that they are following a path of extinction similar to that followed by the same genera in bogs where they had become extinct comparatively shortly before the appearance of civilized man. 10. Some boreal relics have disappeared from Indiana since the time of occupancy by civilized man. Comparatively slight modifications of the habitat eliminated them. 11. The slow process of contraction of the boreal forest on its southern fringe during post-glacial time is not unlike that of its expansion on its northern fringe in Alaska, in reverse order.

Plant Migration in the Southern Limits of Wisconsin Glaciation in Indiana

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