James J. Miner

TEMPERATE ZONE FENS OF THE GLACIATED MIDWESTERN USA

A study of more than 70 fens in the Midwestern United States and a review of the literature indicates that these temperate zone wetlands may differ from fens of the boreal zone and are not adequately differentiated from the by present classification systems. Fens of the Midwestern temperate zone 1) are wetlands with high botanical diversity, 2) are supported in part by ground water with conductivity > 100mS/cm and circumneutral pH, 3) contain water in the root zone during most of the growing season yet are not usually innndated, and 4) accumulate organic and/or carbonate substrates. Individually, none of these descriptors is adequate to distinguish fens from other wetland communities of the Midwest such as marshes, sedge meadows, and wet prairies; yet, when they are taken together, such discrimination is possible. While fens of this zone share many species, our study does not support using indicator species because too few are both faithfully represented and geographically widespread. Midwestern temperate fens are sustained by forces of climate, landscape, and geology, which permit ground water to seep continuously into the root zone in a focused location. Since water availability in the temperate Midwest is less than in the boreal zone, continuous discharge is needed to maintain the saturation conducive to peat formation.

DYNAMICS OF PEAT ACCUMULATION AND MARL FLAT FORMATION IN A CALCAREOUS FEN, MIDWESTERN UNITED STATES

The age and sequence of peat accumulation were investigated at a calcareous fen in northeastern Illinois, USA. The purpose of this study was to identify the processes that form and sustain marl flats, which are areas of marl or tufa substrate within the fen that contain numerous rare plant species. Geomorphic, stratigraphic, and radiocarbon evidence was used to establish the processes and chronology of peat accumulation and erosion adjacent to each marl flat. The age of the base of the peat deposit varies greatly throughout the fen, ranging from 14,679 calibrated years before present (cal. years BP) to nearly modern, indicating that colonization of the sand and gravel substrate by peat occurred throughout the period from the Late Pleistocene to present. Adjacent to one marl flat, trends in basal peat age and thickness show that peat accumulation has progressed laterally inward from both sides, suggesting that the marl flat has been infilling with peat progressively by accumulation at the margins since at least 5,370 cal. years BP or longer. A second marl flat in the fen is surrounded by older, thick peat of differing ages on either edge and is bounded by fresh scarps, indicating that the marl flat currently is expanding laterally by erosion into the preexisting peat blanket. These two examples suggest a continuously repeating process, where erosion of the accumulated peat blanket forms a marl flat, which is later covered by peat accumulation. Trends in basal peat age elsewhere in the fen suggest that other marl flats may have existed in the past that have been completely infilled with peat. This study suggests that marl flat formation is a natural process that has been occurring for millennia, continuously creating habitat for the rare plant species that occupy marl flats. There is no evidence that the marl flats at this site are indicative of anthropogenic disturbance, so that management options for these areas are limited to maintaining the quality and quantity of ground-water discharge that supports both peat formation and erosion.

A Simplified Soil-Zone Monitoring Well

in lllinois. W are fundamentally a hydrologic feature (Winter, 1992), and variations in hydrologic conditions are a major factor controlling the distribution of wetland vegetation (Gosselink and Turner, 1978; Niering, 1987a; Niering, 1987b; Weller, 1987). Measuring various aspects of site hydrogeology is an increasingly common practice in wetland restoration and management projects. However, because past hydrogeologic studies often did not focus on wetlands, as several authors have noted (see for example Gosselink and Turner, 1978; LaBaugh, 1986; Doss, 1995), the standard techniques used in hydrogeology are not well known to wetland managers. Also, in hydrogeologic studies, each site commonly requires an individualized approach (Nielsen, 1991), so that the design of a monitoring well and methods for its installation are generally determined at each well site by a hydrogeologist. Natural-areas workers without training in hydrogeology are generally not aware of the pitfalls that can adversely affect water-level measurements if adjustments in design are not made. Because many of the hydrogeologic conditions that complicate well design occur at depth, wells designed to measure water levels in the soil zone avoid many hydrogeologic complications, and can be installed using standard designs. This in turn makes it possible for natural-areas workers with little training in hydrogeology to measure water levels in the root zone, which is often the only hydrogeologic data required to make restoration and management decisions, without needing to alter the construction of each well. Another advantage of a standard design is that identically constructed wells allow direct comparison of water-level changes at different sites. While carrying out a number of wetland studies in Illinois over the last several years (Miner et al., 1994; Miner et al., 1996; Miner et al., 1997; Simon et al., in press), we developed a monitoring well for the specific purpose of measuring the elevation of the water table in the soil zone. We designed the wells to be as shallow as possible while still containing the standard features that ensure the integrity of a well (see for example ASTM, 1990; Nielsen, 1991 ). In designing them we took advantage of several distinctive features of soilzone hydrology. The most important of these is that, in the soil-zone, there is less chance of encountering the complex hydrogeologic conditions, such as confined aquifers, that often occur at depth. Also the soil zone typically includes many interconnected macropores, such as root channels and soil structures, which permit the free flow of water through the soil. Wells that intercept these macropores will accurately reflect the level to which soil saturation can occur. These facts lessen the need to tailor wells to the specific geologic conditions of the site, so that a standard design can be used. To test our well in the field, we compared water levels in a well at one site to the saturation measured in a set of nested tensiometers installed nearby, and found that the data showed similar levels and trends. Data collected from these wells can be used in planning and monitoring the restoration or management of natural areas.