TfflSLAND 



Summerby Swamp, Michigan 



by Robert Mohlenbrock 



Summerby Swamp, in Hiawatha Na- 

 tional Forest, is among the countless wet- 

 lands that dot northern Michigan, northern 

 Minnesota, and adjoining parts of Canada. 

 Bisected by Michigan Highway No. 123, 

 the swamp covers about three square 

 miles. On one side of the road, the swamp 

 is rather soupy looking, with hummocks 

 of vegetation forming hundreds of tiny is- 

 lands in shallow, standing water. On the 

 other side it is forested with northern white 

 cedar trees. The contrast in vegetation is 

 related to differences in soil chemistry and 

 drainage. This type of variation in wet- 

 lands is also a clue to how these habitats 

 gradually change from one type to an- 

 other, or even into a dry habitat, as a result 

 of plant growth. 



Bird's-eye primrose, above, grows in 

 Summerby Swamp, but is more commonly 

 found farther north. Right: Cattails and 

 flowering asters border the swamp. 



Rod Planck; Photo Researchers, Inc. 



20 Natural History 3/94 



Terms such as bog, fen, marsh, and 

 swamp are often used interchangeably, 

 even by professional botanists. But biolo- 

 gist Howard Crum, in his book A Focus on 

 Peatlands and Peat Mosses (1988), pro- 

 poses a more precise terminology. One of 

 the differences he emphasizes is between 

 peatlands, where sphagnum (peat moss) 

 grows and accumulates, and nonpeatlands. 

 Peatlands develop where the ground is 

 water-soaked throughout the growing sea- 

 son, causing the sphagnum to grow faster 

 than its dead remains can decompose. The 

 built-up deposit is known as peat. 



Peatlands vary depending on the degree 

 of acidity. Fens, according to Crum, are 

 peatlands that are rich in minerals and low 

 in acidity or even sUghtly alkaline. They 

 develop where water near the surface of 

 the wetland is well aerated and suppUed 

 with minerals such as calcium. Northern 

 Michigan has "rich fens" that have abun- 

 dant calcium and a pH value between 6.0 

 and 7.5. (On the pH scale, 7 is neutral, val- 

 ues from 7 to 14 indicate increasing alka- 

 linity, and values from 7 down to indi- 

 cate increasing acidity.) Where the 

 calcium is low, a sedge-dominated "inter- 

 mediate fen" will develop, with a ten- 

 dency to become increasingly acidic. 

 Crum designates a wetland a "poor fen" 

 when the pH is between 4 and 6 and the 

 vegetation, dominated by sphagnum, is 

 still in contact with groundwater. If the pH 

 falls to 3 or less, it is a "bog." 



Crum notes that peatlands form in low- 

 lands that have a constant water supply 

 and may even encroach on open water. In 

 a fen, where the water is well aerated and 

 not too acidic, the habitat will support a 

 diversity of plants, often dominated by 

 sedges. But sphagnum mosses are the key 

 to the peatiand ecosystem: usually several 

 species are present, and they may come to 

 dominate, depending on conditions. 



In some calcium-rich fens in Michigan, 

 spring flooding or other changes in water 

 level may restrict the growth of sphagnum, 

 which is a perennial. Such locales may be 

 invaded by white cedars to become cedar 

 swamps. But in fens where peat accumu- 

 lates rapidly, the water flow is restricted. 



