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Managing Iowa Habitats

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Managing Iowa Habitats Managing Iowa Habitats Fen Wetlands Introduction Why should I be concerned? Fens are the rarest of Iowa’s wetland commu- Fens are an important and unique wetland nities and of great scientific interest. While type. Not only are the fens themselves rare, but their geology varies, they all are the products they shelter over 200 plant species, 20 of which of the seepage of groundwater to the surface. are Iowa endangered and threatened species. Because the water is rich in calcium and other Many of the plant species have been in these minerals, only a select group of plants is able to areas for thousands of years. The fen’s vegeta- grow there. As a result, fens contain many tion, in turn, shelters wildlife by providing plant species considered endangered or valuable habitat. threatened in Iowa. Fens are valuable to humans as well. They are A few of the oldest fens contain plant remains important as sites of groundwater discharge — that date back 10,000 years, though most Iowa good indicators of shallow aquifers. Vegetation fens are less than 5,000 years old. A few of in all wetlands plays an important role in these “younger” fens may have existed 10,000 recycling nutrients, trapping eroding soil, and years ago, but because of dramatic climate filtering out polluting chemicals such as changes, they may have dried up and lost the nitrates. However, the rarity of fens and their plant remains (by burning or erosion) that relatively small size makes it important could prove their age. When the climate grew to protect them from overloading by wetter again about 5,000 years ago, these fens these materials. Too many nutrients may have reappeared. Most Iowa fens, how- or other chemicals can irreparably ever, are thought to be of rather recent origin, damage fens. As miners watched geologically speaking. It is likely they were their canaries for signs of poisoned formed in the last 5,000 years and are related to air, it is wise for humans to watch the changes in geology brought by the receding closely the health of plant and animal of the last glaciers from Iowa. life in ecosystems such as fens, which are good indicators of environmental quality. The condition of plants and animals in fens can help us mon- itor the quality of groundwater resources. Fringed gentian IOWA STATE UNIVERSITY Extension and Outreach Pm- 1351f | February 1993 What are fen wetlands? Fen characteristics The name fen is not as commonly used in the Fens differ substantially from other wetlands. United States as in Europe where these Some of these differences cannot be directly wetlands are not as rare. In Iowa, fens have observed because they are deep below the soil been called marshes, seeps, side-hill seeps, surface. mound springs, wet prairies, sedge mead- ows, and sloughs. They also have been Geological settings - underground mistakenly called bogs, hanging bogs, and structure hillside bogs. Fens are typically found on Fens can form where a groundwater source hillsides, though occasionally in low areas rich in minerals is exposed to the surface. between hills. Depending on a fen’s location in the state, its foundation was laid either during a glacial Fens differ There are many different kinds of wetlands. period hundreds of thousands of years ago substantially Certain characteristics make fens unique. (and since eroded), or during the most recent from other Typically, the water in fens is neutral to glacial period — about 10,000 years ago. In wetlands. alkaline (the pH is 7 or greater) and rich in either case, the geology provided an ideal nutrients. The water comes from groundwa- setting for the development of a fen. There are ter that saturates the fens’ organic soils — basically two settings: layers of partially decomposed plant material • The most common setting is one in which the called peat. In certain areas this peat is so groundwater source is a buried sand and saturated with water it sometimes is called gravel deposit located in debris left behind “quaking soil” because the ground trembles by glaciers (see Figure 1). Over time, when walked on. Sometimes the water that erosion exposed portions of this sand and saturatesthis soil carries enough dissolved gravel resulting in a seep area — calcium carbonate to form a grayish-white, a place where the groundwater is near the lime-rich substance called marl or tufa when surface. Peat soil is characteristic of fens it comes to the surface. Because of this formed over the seep area. calcium deposit, these wetlands are some- • A less common setting is one in which the times referred to as calcareous. groundwater source is in sand and gravel that was deposited on the surface by Because of the dissolved calcium and other flowing water from the melting glaciers. minerals in the water, fen plants are very distinctive. The most common plants in the These geological settings of fens are important fen are sedges that often hide some rare and because they relate to the likelihood of con- beautiful wild flowers. Other plants that live tamination by chemicals and other pollutants. in the fen include grasses, forbs (broad-leaved Fens with groundwater sources near the plants), and a few shrubs and trees. surface (second setting) can be contaminated by surface-derived pollutants much more easily than fens whose source of groundwater is buried (first setting). However, the glacial deposits surrounding the buried groundwater plants sources of eastern Iowa fens are old and glacial debris weathered. This makes them more fractured, pools allowing pollutants moving through the peat ground to enter the fen easily and rapidly. Thus eastern Iowa fens are more likely to be seep area sand and gravel deposit water flow contaminated by surface pollutants that move into and through the fractures than similar western Iowa fens, whose glacial debris geologic setting is younger and not as weathered. Figure 1. Generalized cross-section diagram of an Iowa fen. 2 Soils was first thought to Fen soil contains a large quantity of decompos- reflect an east-west ing plant and animal material called humus. gradient across the state. Soils with a high humus content are called Currently, researchers organic soils. Fen organic soil is saturated with feel it is more likely due water, and the humus is so rotted that it is to differences in the difficult to discern any bits of plant or animal nature of the geology material. Such organic soil is called muck. In of the sites. contrast, the true peat soils of bogs show recognizable bits of plant stems and other Water-response to material. Two soil series are typical of Iowa rainfall. fens: Palms muck and Houghton muck. How- A fens’s response ever, some exceptions exist; these soil series do to rainfall is related to its not always have fens, and some fens are found geological setting. Moist- on other soil series. ure levels in eastern Iowa fens tend to reflect the Water — pH current quantity of rainfall. A particular water chemistry also characterizes Most eastern fens are a fen. For example, its pH is alkaline, neutral, drier during drought or only slightly acidic, whereas a bog is very periods and recover quickly acidic. Fen soils are saturated with water after wet periods because coming directly from groundwater sources. the geological materials Grass of Parnassus This groundwater supplies most of the nutri- around the groundwater ents needed by fen plants. Classification of sources allow water to move Iowa fens is based on the quantity of available rapidly through to the fens after nutrients and the water pH. There are three a rainfall. At the same time, distinct types: these materials do not hold water • nutrient-poor (slightly acidic), well enough and the groundwater • nutrient-rich (neutral), and storage volume is not large enough to maintain • very nutrient-rich (alkaline). moisture levels during droughts. Only two nutrient-poor fens are known to exist Some western Iowa fens have geological in Iowa. One of these is located in Pilot Knob materials surrounding them that cause water State Park. Nutrient-rich fens are located to move through more slowly. These materials chiefly in the types of geological settings hold water better and the groundwater sources typical of eastern Iowa. Most of the very can store more water. These conditions can nutrient-rich fens are found in northwestern help maintain a more constant moisture level Iowa. Very nutrient-rich fens also may be and cause a more delayed response to changes characterized by calcium carbonate (lime- in rainfall patterns. stone) deposits of marl or tufa. Plant life Water — hardness Certain species of plants, such as beak-rush, The hardness of water is largely defined by the Grass of Parnassus, and brook lobelia, can be quantity of dissolved calcium and magnesium used as indicators of fens. (See page 8 for ions in the water. The more dissolved ions scientific names of plants.) Of those plant present, the harder the water. Hard water tends species peculiar to fens, 80 percent occur in to be more alkaline. As a result, slightly both eastern Iowa and western Iowa fens. alkaline, very nutrient-rich fen have higher These include upright sedge, inland sedge, bull concentrations of dissolved calcium carbonate, sedge, purple-stemmed aster, flat-topped white sulfate and silica than neutral, nutrient-rich aster, Muhly, and Grass of Parnassus. Most fens. The different water chemistry among fens fens in Iowa are dominated by upright sedge. 3 However, some well-developed western fens plant species found in fens are valerian, brook contain a variety of dominant sedges that lobelia, tall cottongrass, and sage or hoary include water and beaked sedge.
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