TEMPERATE ZONE FENS OF THE GLACIATED MIDWESTERN USA James P

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.

Temperate zone definitions do not necessarily fit well within the context of boreal fens
Locally derived definitions are often inconsistent with one another
Climatic differences and geomorphic setting impose different limits on fens

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

The geologic setting determines the hydrogeologic and chemical characteristics of the fen.
Fens occur at stratigraphic and/or topographic breaks that create hydrologic gradients that cause groundwater to reach the surface. Temperate zone fens are often associated with glacial deposits, although glacial stratigraphy is not a requirement for fens to develop.
Fens can and do form over other aquifer types including consolidated deposits.
We recognize a few topologies which represent the majority of forms seen in our studies (Figure 2), but caution that numerous
intermediate morphologies are possible.

Figure 2. Typical fen settings

Some Midwestern fens

FEN HYDROLOGY

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

Fens show a relatively constant water level in the central portions of a peat mound and on parts of hillside fens where evidence of groundwater discharge is most obvious (Table 1).
Water levels generally do not annually fall below about 75 cm in depth (in measurements not made in transitional areas or areas affected by obvious drainage such as tiling, ditching or incised creeks).
While upward gradients are often used as characteristics of fens one should not expect upward gradients in every portion of the fen system.
Inundation by water above land surface is not commonly seen except in isolated, very shallow depressions.
Surface discharge is variable
The water balance is controlled by the regional hydrogeologic and climatic setting.

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

Peat accumulation has often been used a determinant of fen condition.
Many temperate-zone sites are characterized by a sapric peat or muck at the high end of the humification scale
Fens may also accumulate calcium carbonate or other precipitates
Minerals precipitating from groundwater discharge should be treated separately from clastic sediments brought into the fen
Accumulation of precipitated minerals causes peat at temperate sites to contain high ash contents. (Table 2)

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 ºC)	(n=42)	Average 33.8%	range <1 to 85%

** after compensation for carbonates

FEN VEGETATION

Temperate zone fens are graminoid dominated
Ours study does not support the use of indicator species to describe or define fens of the temperate zone
Only a small percent of species found in fens are common to most fens and many of these are not unique to fens
Botanically based fen definitions used regionally do not work when applied to the entire temperate zone

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

Tend to have non-acid pH
Have high conductivity

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 ºC)	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

Fens are wetland communities dependent on groundwater, which moves through and maintains saturation of the root zone throughout most of the year, but without long-term above-ground inundation. In adjoining non-fen communities water levels are either ponded for long duration (marsh) or are less constant and vary according to season (wet prairie, sedge meadow).
The substrate is carbon-accumulating ranging from muck to peat to carbonate, thus including some wetlands that might not be peatlands by the strictest definition of peat.
The typical chemistry is a near-neutral pH with a conductivity over 100S/cm, but this is highly location-specific and lithology dependent.
Fens are typically graminoid-dominated and contain plants which may be common over wide geographic areas but may also contain indicator species which can be useful on a local basis. Absence of such species should not be used to define the absence of a fen.

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

Better field indicators for defining fen communities that may have more utility over a wide geographic range of fens.
Continued studies of the hydrology of fens and similar, often transitional ecosystems such as sedge meadows, wet prairies, and marshes. Long-term data is needed to fully understand the nature of water-level fluctuations, degree of saturation, and percentage of time influenced by standing water.
Water budgets for these communities would help to further quantify the relative importance of groundwater, surface water, and precipitation.
Research into the factors that define the type and patterning of vegetation seen in fens and their transitional wetland communities.
An understanding of the sensitivity of existing fens to anthropomorphic alterations.