Postglacial Migration of Forests in Illinois, Wisconsin, and Minnesota

Abstract

1. Taking into consideration all the limitations of pollen analysis, the fossil pollen counts of fourteen bogs in Illinois, Wisconsin, and Minnesota show the succession of the forests during their entire development. 2. Eleven bogs are found within the limits of substage III of the Wisconsin epoch, and three within the limits of substage IV. 3. Abies and Picea pollen were the predominant ones at the lowest levels in all bogs. 4. Dryness and decay of peat often renders it unfit for pollen analysis. 5. The bogs of substage III reveal a succession from Abies-Picea forests to deciduous ones, the oaks being the predominant trees of the latter. The change from Abies-Picea to deciduous forests is generally abrupt, and if the thickness of peat is considered an indicator of age, the Abies-Picea period, especially in the southern bogs, was shorter than the deciduous period. Quercus pollen was more abundant in the southern bogs. 6. The younger northern bogs belonging to substage IV show that the conifers have been the significant trees throughout their history. 7. Climatic conditions remained very uniform throughout the period represented by the upper two-thirds of the pollen diagrams. 8. The results do not correlate with the Blytt-Sernander hypothesis.