Natural Community Systems -- Photo Guide

Medium level fen system

a medium level fen system south of Ossipee Lake (photo by Ben Kimball for the NH Natural Heritage Bureau)
a medium level fen system south of Ossipee Lake (photo by Ben Kimball)

medium level fen system at Greenough Pond (photo by Ben Kimball for the NH Natural Heritage Bureau)
medium level fen system at Greenough Pond (photo by Ben Kimball)

medium level fen system at Bradford Bog (photo by Ben Kimball for the NH Natural Heritage Bureau)
A medium level fen system at Bradford Bog; this example is more wooded than many. (B. Kimball photo)


DescriptionMedium level fens are open, acidic peatlands with more minerotrophic influence than poor level fen/bogs due to the effects of upland runoff, exposure to lake and stream water, or limited groundwater seepage.  Weakly to moderately minerotrophic conditions prevail, with pHs ranging from the low 4s to mid 5s.  They occur in a variety of landscape settings, but mostly along stream and lake borders where the nutrient levels and seasonal fluctuations of water levels are greater than in poor level fens, but less than in emergent marshes (thus allowing peat to accumulate over the long term).  They are most frequent around relatively stagnant ponds and lakes and drained depressions in glacial outwash or ice-contact deposits.  Peat is moderate to deep and moderately well decomposed in the upper layers, with a well-developed hummock - hollow topography in many of its constituent communities.  They are more common than kettle hole bog and poor level fen/bog systems.

These systems are a mosaic of open, sedge-dominated fens, dwarf to medium-height shrublands, and open moss lawns, carpets, and pools.  Tall shrub fens are also common.  In shrubby areas, vigorous patches of Myrica gale (sweet gale), Spiraea alba (eastern meadowsweet), and sometimes Chamaedaphne calyculata (leatherleaf) are prominent and usually more than 20" (50 cm) in height (leatherleaf tends to be shorter in poor level fen/bogs).  Ilex verticillata (winterberry), Toxicodendron vernix (poison sumac), Acer rubrum (red maple), and Larix laricina (eastern larch or tamarack) indicate weakly to moderately minerotrophic conditions in areas that have tall shrubs and trees (these species are sparse or absent in poor fens).  Robust, tall sedges, like Carex lasiocarpa (wire sedge / hairy-fruited sedge), Carex utriculata (bottle-shaped sedge), and Carex stricta (tussock sedge), are also common, and may dominate large areas individually or in mixtures with other species.  Lagg (moat) areas along the upland margin and lawns, carpets, and pools near water bodies often support aquatic peat mosses and herbs such as Sphagnum torreyanum, S. cuspidatum, S. pulchrum, Carex canescens (silvery sedge), Vaccinium macrocarpon (large cranberry), Rhynchospora alba (white beak-rush), and Dulichium arundinaceum (three-way sedge).

A typical natural community sequence from the upland border towards the center of the basin, channel, or water-margin is as follows: a moat or lagg zone; a tall shrub fen zone; a dense medium-height shrub zone with sweet gale; sedge fen; and open moss carpet areas closest to the water’s edge.  Moss carpets or lawns are typically not present or well developed in fens along streams, but are more common in lake border or floating mat settings.

Diagnostic natural communities:

   Moss lawns and sedge/medium-shrub fens

      • Sweet gale - meadowsweet - tussock sedge fen (S4)

      • Wire sedge - sweet gale fen (S3)

      • Large cranberry - short sedge moss lawn (S3)

      • Bog rosemary - sedge fen (S3)

   Tall shrub fens

      • Highbush blueberry - sweet gale - meadowsweet shrub thicket (S4)

      • Winterberry - cinnamon fern wooded fen (S4)

      • Sweet pepperbush wooded fen (S2)

      • Alder wooded fen (S3S4) 

Peripheral or occasional natural communities:

      • Floating marshy peat mat (S3S4)

      • Marshy moat (S4) 

      • Sedge meadow marsh (S4) 

      • Water willow - Sphagnum fen (S3) 

      • Alder - lake sedge intermediate fen (S2S3)

Landscape settings: sluggish stream, pond or lake borders, open headwater basins, drained depressions in glacial outwash or ice-contact deposits

Soils: deep, poorly- to moderately well-decomposed peat; weakly to moderately minerotrophic; pHs generally in low 4s to mid 5s; topogenous and limnogenous (limited soligenous influence)

Spatial pattern: small to large patch (5100+ acres); irregularly circular or linear; irregular or banded zonation parallel to stream or pond border; streams often pass through peatland (has inlet and outlet)

Physiognomy: sparse woodland, tall shrub, medium-height shrub, moss carpets/lawns

Distribution: broadly distributed in New Hampshire

Characteristic species:
Indicators of weakly to moderately minertrophic conditions found in medium level fens (mostly absent or limited to marginal areas of kettle hole and poor level fen/bogs):
      Carex lasiocarpa (wire sedge / hairy-fruited sedge)
      Carex utriculata (bottle-shaped sedge)
      Carex stricta (tussock sedge)
      Carex lacustris (lake sedge)
      Carex canescens (silvery sedge)
      Lysimachia terrestris (swamp candles)
      Dulichium arundinaceum (three-way sedge)
      Triadenum virginicum (marsh St. John's-wort)
      Osmunda cinnamomea (cinnamon fern)

Shrubs and trees
      Myrica gale (sweet gale)
      Spiraea alba (eastern meadowsweet)
      Spiraea tomentosa (steeple bush)
      Chamaedaphne calyculata (leatherleaf) -- >20” and usually closer to 36” high
      Vaccinium macrocarpon (large cranberry)
      Ilex verticillata (winterberry)
      Alnus incana ssp. rugosa (speckled alder)
      Acer rubrum (red maple)
      Larix laricina (eastern larch)
      Toxicodendron vernix (poison sumac)

pHs generally in low 4s to low 5s

Many species of kettle hole and poor level fen/bog systems may also be found in medium level fens (see list in kettle hole bog system)

Associated natural community systems:  This system can co-occur in large peatland basins with poor level fen/bogs.  It is typical for medium level fens to have small portions of the peatland dominated by oligotrophic conditions and communities.  When these areas are limited in extent or constitute a small proportion of the wetland, they are considered inclusions within the medium level fen system; when they are more extensive or constitute a substantial proportion of the peatland, the peatland may best be treated as having both poor and medium level fens systems within the same wetland site.  Conversely, poor level fen/bogs can have areas with more minerotrophic influence with medium level fen communities.  These are treated in the same way.

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