TEMPERATE ZONE FENS OF THE GLACIATED MIDWESTERN USA
James P. Amon, Wright State University, Dayton, OH
Carol A. Thompson, Tarleton State University, Stephenville, TX
Quentin J. Carpenter, University of Wisconsin Institute for Environmental Studies - Madison, Madison WI 
James Miner, Illinois State Geological Survey, Champaign, IL 

WHAT IS A FEN ANYWAY?

Fens represent some of the rarest, biologically diverse wetlands in the temperate zones of the United States, but are not well-recognized.

GOALS
Produce a definition which fits the observed aspects of most sites generally recognized as fens in the Midwestern temperate zone of the USA
Eliminate from the definition the characteristics which make them locally specific and thus confusing to identify. 
Definitions should be process- based

WHY? 
We need consistent definitions to assure that regulatory officials can protect these temperate wetlands
Definitions must be simple enough for wetland protection, but detailed enough to allow communication between wetland scientists

 

STUDY AREA
Our core area of study stretches from eastern Ohio to western Iowa (Figure1). To the west we have investigated fens in eastern North Dakota and north central Nebraska. The northern border of this zone is approximated by the vegetative tension zone as described in Eggers and Reed (1997) and by Curtis (1959). The southern border is approximately the most southward influence of the Wisconsinan glaciation. For this report we visited or, in a few cases, obtained data on fens in over 70 sites from Nebraska, South Dakota, North Dakota, Minnesota, Iowa, Missouri, Wisconsin, Illinois, Indiana, Michigan and Ohio.

                                                        Figure 1. Study Area 

 

 

FEN GEOMORPHOLOGY

 


                                                            Figure 2. Typical fen settings

 

                            Some Midwestern fens

 

MOmomnd.jpg (200995 bytes)                       slmound.jpg (223729 bytes)

                 

 KLfen2.jpg (127521 bytes)                    lkedge.jpg (156517 bytes)                 

               

FEN HYDROLOGY

All Midwestern temperate zone fens are supported by similar hydrologic factors that may help discriminate fens from non-fen wetlands. 

 

Table 1.  Water Depths During Growing Season (cm)

Negative values are below land surface and negative values are above land surface. The root zone, approximately 0 to 20 below the surface, is maintained moist or saturated during the growing season. Measurements in cm.

Average

Minimum

Maximum

n

Iowa (June – September)

-0.02

-40

41

30

Wisconsin (April – September)

-15

-51

2

6

Illinois (April – September)

-15

-40

10

5

Ohio (May through August)

-8.6

27.9

0

15

 

FEN SUBSTRATES

 

Table 2. Physical Characteristics of Fens

Fen soils are variable in composition and saturation is not universally seen. Data from Nebraska, Iowa, Minnesota, Illinois, Wisconsin, Indiana and Ohio.

Marl present (visible or soil produces bubbles with HCl)

31 of 42 sites

Saturated to land surface during growing season

24 of 45 sites

Peat

18 of 34 sites

Muck

30 of 34 sites

 

Bulk density

(n=24)

Average 0.374

range 0.159 to 1.266

Organic matter**

(n=24)

Average 66.9%

range 21 to 99 %

Ash (at 450 ēC)

(n=42)

Average 33.8%

range <1 to 85%

** after compensation for carbonates

 

FEN VEGETATION

 

Table 3. Number of species present and those in common among temperate zone fens

While individual fens have 200 to over 500 species, the diversity represented in temperate zone fens is much greater. Species reported in fens vary from state to state and the commonality of species decreases with distance between fens. FACW and OBL refer to wetland categories in the National List of Plants that Occur in Wetlands (Reed 1997). OBL and FACW species are selected to emphasize those restricted to sites most likely to represent fens with continuously moist conditions.

 

Total species

OBL + FACW species

All Fens (Includes Colorado, Dakotas, Missouri)

1341

653

Midwestern temperate fens of 8 states combined

   

(Ohio, Indiana, Illinois, Iowa, Wisconsin, Nebraska, Michigan and Minnesota)

1156

551

Ohio, Indiana, Illinois

1030

474

Species shared by Ohio, Indiana, Illinois

163

119

Indiana, Illinois, Iowa

742

349

Species shared by Indiana, Illinois, Iowa

98

84

Illinois, Wisconsin

548

262

Species shared by Illinois, Wisconsin

100

84

Nebraska, Ohio

755

404

Species shared by Nebraska, Ohio

56

52

Ohio, Indiana, Illinois, Iowa

1050

486

Species shared by Ohio, Indiana, Illinois, Iowa

90

77

Ohio, Indiana, Illinois, Iowa, Wisconsin

1052

487

Shared by Ohio, Indiana, Illinois, Iowa, Wisconsin

61

55

Ohio, Indiana, Illinois, Iowa, Wisconsin, Nebraska

1075

507

Shared by Ohio, Indiana, Illinois, Iowa, Wisconsin, Nebraska

21

21

 

FEN CHEMISTRY

Water chemistry of fens is not consistent enough to distinguish them from other non-fen wetlands of the temperate zone

 

Table 4 Chemistry of fens in the temperate Midwest

Based on over 70 sites visited in 9 states (Nebraska, Iowa, North Dakota, Minnesota, Illinois, Wisconsin, Missouri, Indiana, Ohio) and published data. Many readings† are below detection levels so averages are from measurable samples only. BD indicates most samples were below detection limits.

 

Average

Range

n

Groundwater

     

Electrical Conductivity (m S at 25 ēC)

624

146 to 1523

70

pH

7.29

6.31 to 8.29

70

Calcium (mg/L)

101

50 to 292

76

Magnesium (mg/L)

34

9.7 to 88.0

76

Ca/Mg ratio

2.4

   

Nitrate (mg/L)

2.5†

< 0.04 to 15.83

60

Ammonia (mg/L)

0.3†

< 0.02 to 1.35

60

Dissolved Phosphate (mg/L)

BD

< 0.04 to 0.2

42

Iron (mg/L)

variable

< 0.1 to 598

70

Sulfate (mg/L)

123

< 1 to 870

44

Soil

     

Soil Phosphate (mg/L)

2.9

0.4 to 9.0

45

Eh (mv)

-103

+263 to -435

25

 

MIDWEST TEMPERATE ZONE FEN DEFINITION




CONCLUSION

Our intent is to summarize the broad range of functional characteristics of temperate-zone fens showing that the present classification systems do not adequately address these systems well and are not based on features held in common with fens throughout their range. The broad geographic, hydrogeologic, topographic, and climatic setting of these wetlands means that features such as vegetation, chemistry, and soils vary over a broad range. Because of the gradational nature of these wetland communities, it is not possible to precisely define a value for pH or calcium concentration nor precisely define the expected plant community. This complicates field recognition of fens by those unfamiliar with fen architecture who rely on prescribed definitions as a means of identification. Our contention, however, is that fens can be separated from other temperate-zone wetland types such as marshes and wet meadows even though those communities are often inter-gradational. Figure 4 shows the range of characteristics that we believe have the most utility for separating these communities. The figure is not meant to imply precisely defined regions, but includes the most likely range of characteristics of each wetland category. From this diagram, we can develop a definition of temperate-zone fen that shows that the surface vegetation of fens are based on their hydrologic features, which in turn are controlled by the geologic setting. 

 

RESEARCH NEEDS