The last glacial maximum extended into Iowa as far south as Des Moines 18,000 years ago. The vegetation directly south of the glacier was similar to the modern tundra steppe of the Canadian Arctic. As the climate changed and the glacier receded, spruce forests migrated north and covered much of Iowa. Analysis of pollen from lake and marsh sediments as well as river deposits indicates that prairie was first established in north-central Iowa by about 10,000 years ago.
Between 10,000 and 7,000 years ago, the climate continued to warm in the Midwest, and spruce forests migrated farther north. Prairie expanded across much of Iowa, with oak and elm hardwoods lining rivers and streams. In the northeast corner of Iowa, spruce forests were replaced by mixed coniferous forest with some hardwoods.
The warmest and driest period —7,000—5,000 years ago— known as the Holocene Thermal Maximum, prairie vegetation dominated the landscape across Iowa. Much of the forests along rivers and streams disappeared during this time. While pollen records are not available for much of southern Iowa, soil analysis indicates a dominance of prairie vegetation.
Climate in the Midwest became cooler between 5,000—2,000 years ago, and deciduous trees again lined river valleys with prairie dominating uplands. Deciduous trees also migrated south into northern and northeastern Iowa during this period.
While humans have been influencing vegetation since they appeared on the landscape, fire was the first large-scale impact promoting the expansion of prairie. Later, during Euroamerican settlement, draining wetlands, deforestation, plowing for agriculture, and suppressing fire eliminated 99.9% of Iowa’s natural prairie.