Michigan's Natural Communities

Poor Fen

State Rank: S3

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Overview

Poor fen is a sedge-dominated wetland found on very strongly to strongly acid, saturated peat that is moderately influenced by groundwater. The community occurs north of the climatic tension zone in kettle depressions and in flat areas or mild depressions on glacial outwash and glacial lakeplain.

Landscape Context

Poor fen occurs in flat areas and shallow depressions on sandy glacial outwash and glacial lakeplain, and in kettle depressions on pitted outwash and moraines. Poor fens found in kettle depressions are associated with active or extinct glacial lakes. Within kettle depressions, poor fens can occupy the entire basin or occur as a floating mat along the margin of a remaining glacial lake. Poor fens occurring on former glacial lakebeds and drainageways tend to be more extensive than kettle fens, which are limited in area by the size of the glacial ice-block that formed the basin. Poor fens occur adjacent to other peatland communities, often grading into bog, poor conifer swamp, and muskeg. More minerotrophic systems such as northern fen, northern shrub thicket, northern wet meadow, and rich conifer swamp can occur along the outer margins of poor fens where groundwater seepage from adjacent uplands is prevalent. Upland community types neighboring poor fen typically include fire-adapted communities such as pine barrens, dry northern forest, and dry-mesic northern forest.

Soils

The organic soils of poor fens are composed of peat, which frequently forms a shallow, continuous mat ranging from one to three meters in depth. Organic soils near the surface are fibric peat and very strongly to strongly acid with low nutrient availability. Low levels of groundwater input combined with high water-retaining capacity of fibric peat produce continuously saturated conditions in the rooting zone of poor fens. The water table of poor fens is stable, typically at the soil surface with soils remaining saturated but seldom flooded. The surface waters of poor fens are characterized by very strong to strong acidity, low available nutrients, low specific conductivity, cool temperatures, moderate levels of dissolved organic matter, and anaerobic conditions.

Natural Processes

Poor fens are weakly minerotrophic peatlands, receiving inputs of water and nutrients from both ion-poor precipitation and low exposure to nutrient-rich groundwater. The acidity of poor fens limits the availability and uptake of essential mineral nutrients by plants. Saturated and inundated conditions inhibit organic matter decomposition and allow for the accumulation of peat. Under cool, anaerobic, and acidic conditions, the rate of organic matter accumulation exceeds organic decay. Development and expansion of fens occurs via two different processes in glacial lakeplains and outwash plains versus kettle depressions. Fens develop in glacial lakeplains and outwash where groundwater influence maintains saturated conditions. Peat develops vertically and spreads horizontally. Lake-filling occurs in small kettle lakes with minimal wave action where gradual peat accumulation results in the development of a sedge mat that can fill the basin or form a floating mat in the lake or a grounded mat along the water’s edge. Floating mats of fen sedges, such as wiregrass sedge (Carex lasiocarpa), are able to pioneer open water and submergent and emergent marshes. The interlacing of rhizomes and roots forms a floating mat that is buoyed by the water and accumulates organic matter in the form of sapric peat. Over time fen mats are often invaded by ericaceous shrubs and acidifying sphagnum mosses. The invasion of sphagnum moss into rich fen systems such as northern fen often results in conversion to more acidic communities such as poor fen or bog.

Natural disturbance factors influencing poor fens include fire, flooding, windthrow, and insects. Surface fire can contribute to the maintenance of fens by killing encroaching trees and promoting graminoid dominance. In addition, many of the shrub species resprout following fire. In the absence of fire, a thick layer of leaf litter can develop that stifles seedling establishment and seed bank expression. Fire severity and frequency in fens is closely related to landscape context and fluctuations in water level. Prolonged periods of lowered water table can allow the surface peat to dry out sufficiently to burn. Such peat fires can result in the conversion of peatland to mineral soil wetland. Peat fires within bogs can also release enough nutrients to favor succession to more minerotrophic peatlands such as poor fen or intermittent wetland. Lowering of the water table of fens can also result in the increase in decomposition rates of organic matter and the subsequent accumulation of compact peat that is more conducive to shrub and tree growth. In such a circumstance, poor fen can succeed to poor conifer swamp or northern shrub thicket. Flooding, often caused by beaver activity, can contribute to the maintenance of fens and also result in the conversion of fens to bogs. Roots of peatland trees are physiologically active near the surface and are killed when the water table rises during prolonged flooding. Trees growing in fens are particularly susceptible to windthrow because peat provides a poor substrate for anchoring trees and the anaerobic conditions associated with saturated soils prevent trees from rooting deeply. Tree survival in fens is also limited by insects and parasites. Insect outbreaks of larch sawfly (Pristiphora erichsonii) cause heavy mortality of tamarack (Larix laricina), while the plant parasite dwarf mistletoe (Arceuthobium pusillum) kills black spruce (Picea mariana).

Vegetation

Poor fens have a unique flora that is intermediate between northern fen and bog. Poor fens are characterized by a graminoid-dominated herbaceous layer of low to moderate diversity. While sedges remain dominant, many poor fens also support a continuous carpet of sphagnum mosses and widely scattered, slightly raised peat ridges or mounds with low ericaceous, evergreen shrubs and stunted conifer trees. Gradients in pH, light, soil moisture, and cation concentrations (i.e., nutrient availability) determine floristic composition of poor fens. Sedges dominate the species-poor herbaceous layer of poor fens. Few-seed sedge (Carex oligosperma) and wiregrass sedge (Carex lasiocarpa) are typically dominant. Other sedges that are characteristic of poor fens include creeping sedge (Carex chordorrhiza), coastal sedge (C. exilis), livid sedge (C. livida), few-flower sedge (C. pauciflora), and mud sedge (C. limosa). Additional graminoids that thrive in poor fens include twig-rush (Cladium mariscoides), three-way sedge (Dulichium arundinaceum), cotton-grasses (Eriophorum spp.), swamp candles (Lysimachia terrestris), white beak-rush (Rhynchospora alba), arrow-grass (Scheuchzeria palustris), and tufted bulrush (Trichophorum cespitosum). The following is a list of prevalent herbaceous plants occurring in poor fen: rush aster (Symphyotrichum boreale), fireweed (Chamerion angustifolium), fringed willow-herb (Epilobium ciliatum), grass-leaved goldenrod (Euthamia graminifolia), wild blue flag (Iris versicolor), bog buckbean (Menyanthes trifoliata), marsh cinquefoil (Comarum palustre), false mayflower (Maianthemum trifolium), bog goldenrod (Solidago uliginosa), and common bog arrow-grass (Triglochin maritima). Insectivorous plants, round-leaved sundew (Drosera rotundifolia), spoon-leaf sundew (D. intermedia), pitcher-plant (Sarracenia purpurea), horned bladderwort (Utricularia cornuta), and flat-leaved bladderwort (U. intermedia), are common features of poor fens. Where a continuous moss layer occurs, it is dominated by sphagnum mosses, especially Sphagnum magellanicum, S. angustifolium, S. capillaceum, S. capillifolium, S. recurvum, S. papillosum, and S. fuscum.

The patchy shrub layer of poor fens is dominated by low, primarily ericaceous shrubs including bog rosemary (Andromeda glaucophylla), leatherleaf (Chamaedaphne calyculata), bog laurel (Kalmia polifolia), Labrador tea (Rhododendron groenlandicum), bog willow (Salix pedicellaris), large cranberry (Vaccinium macrocarpon), and small cranberry (V. oxycoccos). The tall shrub layer of poor fens is less dense than the low shrub layer and is often restricted to the periphery. Tall shrubs typical of poor fens include black chokeberry (Aronia prunifolia), mountain holly (Ilex mucronata), pussy willow (Salix discolor), steeplebush (Spiraea tomentosa), and wild-raisin (Viburnum cassinoides). More minerotrophic shrubs, like bog birch (Betula pumila), Kalm’s St. John’s-wort (Hypericum kalmianum), and shrubby cinquefoil (Dasiphora fruticosa), can occur in poor fens where their roots extend beneath the surface mat to minerotrophic peat. Trees within poor fens are widely scattered (tree cover is typically less than 10%), stunted (seldom reaching six meters), and are often restricted to scattered, low peat mounds. The most commonly occurring trees in poor fens are black spruce (Picea mariana) and tamarack (Larix laricina), with jack pine (Pinus banksiana) and white pine (P. strobus) as occasional associates.

Noteworthy Animals

Poor fens provide important habitat for small mammals such as short-tailed shrew (Blarina brevicauda), beaver (Castor canadensis), meadow vole (Microtus pennsylvanicus), mink (Mustela vison), muskrat (Ondatra zibethicus), and masked shrew (Sorex cinereus). Both muskrats and beaver can profoundly influence the hydrology of peatlands. Muskrats create open water channels through the peat and beavers can cause substantial flooding through their dam-building activities. Beaver dams can cause blocked drainage in fens and the subsequent succession of fens to bogs. Numerous butterflies and moths are restricted to bogs and fens because their food plants occur within these open peatland systems.

Rare Plants

  • Carex nigra (black sedge, state endangered)
  • Carex wiegandii (Wiegand's sedge, state threatened)
  • Eleocharis nitida (slender spike-rush, state endangered)
  • Petasites sagittatus (sweet coltsfoot, state threatened)

Rare Animals

  • Alces alces (moose, state threatened)
  • Appalachia arcana (secretive locust, state special concern)
  • Ardea herodias (great blue heron, protected by the Migratory Bird Treaty Act of 1918)
  • Asio flammeus (short-eared owl, state endangered)
  • Boloria freija (Freija fritillary, state special concern)
  • Boloria frigga (Frigga fritillary, state special concern)
  • Botaurus lentiginosus (American bittern, state special concern)
  • Canis lupus (gray wolf, state threatened)
  • Circus cyaneus (northern harrier, state special concern)
  • Clemmys guttata (spotted turtle, state threatened)
  • Coturnicops noveboracensis (yellow rail, state threatened)
  • Emydoidea blandingii (Blanding’s turtle, state special concern)
  • Erebia discoidalis (red-disked alpine, state special concern)
  • Falcipennis canadensis (spruce grouse, state special concern)
  • Falco columbarius (merlin, state threatened)
  • Haliaeetus leucocephalus (bald eagle, state threatened)
  • Lynx canadensis (lynx, state endangered)
  • Merolonche dolli (Doll’s merolonche moth, state special concern)
  • Pandion haliaetus (osprey, state threatened)
  • Phyciodes batesii (tawny crescent, state special concern)
  • Picoides arcticus (black-backed woodpecker, state special concern)
  • Pseudacris triseriata maculata (boreal chorus frog, state special concern)
  • Sistrurus c. catenatus (eastern massasauga, federal candidate species and state special concern)
  • Somatochlora incurvata (incurvate emerald, state special concern)
  • Terrapene c. carolina (eastern box turtle, state special concern)
  • Williamsonia fletcheri (ebony boghaunter, state special concern)

Biodiversity Management Considerations

The primary mechanism for preserving poor fens is to maintain their hydrology and water chemistry. A serious threat to poor fens is posed by off-road vehicle (ORV) traffic, which can destroy populations of sensitive species and significantly alter fen hydrology through rutting. Reducing access to peatland systems will help decrease detrimental impacts. Increased surface water inputs and reductions in groundwater recharge can be prevented by avoiding road construction and complete canopy removal in stands immediately adjacent to fens. Where shrub and tree encroachment threatens to convert open wetlands to shrub-dominated systems or forested swamps, prescribed fire or selective cutting can be employed to maintain open conditions. Silvicultural management of poor fens to preserve open canopy should be employed during the winter to minimize damage to the peat and impacts to the hydrologic regime. Where feasible, fires originating in adjacent uplands should be allowed to burn across poor fens when they do not pose serious safety concerns or threaten other management objectives.

Poor fens are primarily threatened by changes to their hydrology and water chemistry, which may result from road building, ORVs, quarrying, peat mining, creation of drainage ditches and dams, agricultural runoff and nutrient enrichment, or runoff from logging. Fen vegetation is extremely sensitive to minor changes in water levels, water chemistry, groundwater flow, and nutrient availability. A reduction in groundwater flow and subsequent decrease in nutrients in poor fens can result in the shift to less minerotrophic wetlands such as bog. Lowered water tables from drainage allow tree and shrub encroachment into open fens and the eventual succession to closed-canopy peatlands. Conversion to more eutrophic wetlands has occurred as the result of nutrient enrichment and raised water levels, which cause increased decomposition of peat soils. Eutrophication from pollution and altered hydrology has detrimentally impacted fens by generating conditions favorable for invasive species. Monitoring and control efforts to detect and remove invasive species are critical to the long-term viability of poor fen. Particularly aggressive invasive species that may threaten the diversity and community structure of poor fen include purple loosestrife (Lythrum salicaria), narrow-leaved cat-tail (Typha angustifolia), hybrid cat-tail (Typha xglauca), reed (Phragmites australis subsp. australis), reed canary grass (Phalaris arundinacea), European marsh thistle (Cirsium palustre), glossy buckthorn (Frangula alnus), and multiflora rose (Rosa multiflora).

Variation

Species composition of poor fens varies depending on gradients in nutrient levels and water chemistry. The degree of minerotrophy of a given fen and within a fen depends on a variety of factors including the amount of groundwater discharge and its chemistry; degree of dilution from precipitation; the characteristics of the bedrock and/or glacial deposits the groundwater has percolated through (i.e., older glacial sediments have less dissolved minerals due to prior leaching); the distance the water has traveled through the peatland; and the thickness and character of the peat. Fens occurring on former glacial lakebeds and drainageways tend to be more extensive than kettle fens, which are limited in area by the size of the glacial ice-block that formed the basin.

Similar Natural Communities

Bog, coastal fen, intermittent wetland, muskeg, northern fen, northern wet meadow, patterned fen, poor conifer swamp, prairie fen, and rich conifer swamp.

Relevant Literature

  • Amon, J.P., C.A. Thompson, Q.J. Carpenter, and J. Mines. 2002. Temperate zone fens of the glaciated Midwestern USA. Wetlands 22(2): 301-317.
  • Bedford, B.L., and K.S. Godwin. 2003. Fens of the United States: Distribution, characteristics, and scientific connection versus legal isolation. Wetlands 23(3): 608-629.
  • Cohen, J.G., and M.A. Kost. 2008. Natural community abstract for poor fen. Michigan Natural Features Inventory, Lansing, MI. 16 pp.
  • Eggers, S.D., and D.M. Reed. 1997. Wetland plants and plant communities of Minnesota and Wisconsin. U.S. Army Corps of Engineers, St. Paul, MN. 263 pp.
  • Glaser, P.H., J.A. Janssens, and D.I. Siegel. 1990. The response of vegetation to chemical and hydrological gradients in the Lost River Peatland, northern Minnesota. Journal of Ecology 78(4): 1021-1048.
  • Heinselman, M.L. 1963. Forest sites, bog processes, and peatland types in the Glacial Lake Region, Minnesota. Ecological Monographs 33(4): 327-374.
  • Riley, J.L. 1989. Southern Ontario bogs and fens off the Canadian Shield. Pp. 355-367 in Wetlands: Inertia or momentum, ed. M.J. Bardecki and N. Patterson. Federation of Ontario Naturalists, Don Mills, ON. 426 pp.
  • Schwintzer, C.R. 1978. Vegetation and nutrient status of northern Michigan fens. Canadian Journal of Botany 56: 3044-3051.
  • Schwintzer, C.R. 1981. Vegetation and nutrient status of northern Michigan bogs and conifer swamps with a comparison to fens. Canadian Journal of Botany 59: 842-853.
  • Siegel, D.I., and P.H. Glaser. 1987. Groundwater flow in a bog-fen complex, Lost River Peatland, northern Minnesota. Journal of Ecology 75(3): 743-754.
  • Vitt, D.H., H. Crum, and J.A. Snider. 1975. The vertical zonation of Sphagnum species in hummock-hollow complexes in northern Michigan. Michigan Botanist 14(4): 190-200.
  • Zoltai, S.C., and D.H. Vitt. 1995. Canadian wetlands: Environmental gradients and classification. Vegetatio 118: 131-137.

For a full list of references used to create this description, please refer to the natural community abstract for poor fen.

More Information

Page Citation

  • Kost, M.A., D.A. Albert, J.G. Cohen, B.S. Slaughter, R.K. Schillo, C.R. Weber, and K.A. Chapman. 2007. Natural Communities of Michigan: Classification and Description. Michigan Natural Features Inventory, Report No. 2007-21, Lansing, MI.

Page updated on 11-26-2014

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