Late Quaternary (from the last glaciation to present) forest history is inferred from the Cedarburg Bog fossil pollen record. Analysis of fossil pollen samples extend over 4 meters of continuous core recovered from near the center of the bog. The deepest and oldest of the fossil pollen assemblages (ca. 12,000 years ago) suggest open spruce woodlands unlike any in the contemporary boreal ecosystem. Pollen from the Pleistocene-Holocene transition (11,000 years ago) is marked by a number of abrupt changes in forest composition related to rapid climate change, species immigration, and progressive soils and ecosystem maturation. By 9,000 years ago most of the modern forest plant species were established. These mixed deciduous forests persisted until the historical deforestation.
Fredlund, G.G., J.R. Brozowski, and J.W. Oh. 1995. A late quaternary pollen record from Cedarburg Bog, Wisconsin. Field Station Bulletin 28(2): 11-17.