6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 496 



The Vegetation of the Lake Michigan Area 



The Lake Michigan drainage area lies within two major forest regions of 

 eastern North America . The northern half of the lake lies within the Great Lakes- 

 St. Lawrence division of the Hemlock-White Pine-Northern Hardwoods region, 

 and the southern part lies in the Beech-Maple forest region (Braun, 1950) . The 

 Great Lakes-St. Lawrence forest is a mixture of coniferous and hardwood species 

 dominated by pines {Pinus) . Locally abundant taxa include oak {Queraus) , aspen 

 (Populus ) , hemlock (Tsuga) , spruce [Pioea) , fir {Abies), maple {Acer), ash 

 {Fraxinus ), and birch {Betula) . In this region of low relief and glacial topog- 

 raphy, drainage is an important control over the local vegetation. 



The southern part of Lake Michigan lies within Braun's Beech-Maple forest 

 region. The dominant trees in this area are beech (Fagus) and sugar maple {Acer 

 saooharwn ). Other locally abundant trees include ash, oak, elm {Ulmus), hickory 

 {Carya) , and sycamore {Platanus ). Immediately south and southwest of the Beech- 

 Maple forest region lies the Prairie Peninsula section, a mosaic of prairie and oak- 

 hickory forest. Although the prairie vegetation does not at present border the lake, 

 it probably was an important source of pollen in the lake sediments, especially 

 in early Holocene times. 



At present the area bordering the northern part of the lake basin is pre- 

 dominantly forested, whereas the southern part of the lake is bordered by farm- 

 land and deforested regions. These areas of intense human use are the source 

 areas for much of the ragweed pollen being deposited at present in the lake. 



Lake Michigan drains an area of approximately 117,600 sq. km. The 

 largest rivers in the drainage basin flow into the eastern side of the lake from 

 the lower peninsula of Michigan (fig. 2) . As those rivers are major sources of 

 pollen flow into the lake, the pollen record within the lake sediments reflects to 

 a great extent vegetation changes of southern Michigan. 



METHODS 



Three 67-mm diameter cores (1000-3C, 1001-3A, and 1002-3A) of bottom 

 sediments were collected with a "Benthos" gravity corer from the R. V. Inland 

 Seas in May 1973 (table 1). These cores were sampled on board ship with a 

 vertical extruder so that the sediments would remain in an upright position and 

 the semifluid mud-water interface would not be destroyed. Generally, the corer 

 caused little disturbance of the delicate surface sediments. 



To determine the depth of the initiation of the Ambrosia increase as closely 

 as possible, the upper 20 cm of each core was removed in 1-cm segments. Below 

 20 cm, a 1-cm sample was removed every 10 cm. The remainder of the core was 

 saved for radiocarbon analysis. All pollen samples were placed in plastic bags 

 and frozen until processed. 



In addition to the 67-mm cores, a 47-mm diameter core (969-2A), which 

 had been collected in October 1972 and stored under refrigeration, was sampled 

 in the laboratory. The upper 5 cm of sediment in this core, having been disturbed 

 by transportation of the core before it was subsampled, was discarded. The 

 remainder of the core was cut into 2 -cm sections to a depth of 1 m; below that 

 depth, 2-cm sections were removed every 10 cm. 



Each sediment sample was oven-dried for 24 hours to determine the dry 

 weight of the sediment. The dry weight varied considerably between samples 



