Fens occur infrequently throughout the Rocky Mountains from Colorado north into Canada. They are confined to specific environments defined by groundwater discharge, soil chemistry, and peat accumulation. This system includes poor fens, rich fens and extremely rich fens. Fens form at low points in the landscape or near slopes where groundwater intercepts the soil surface. Groundwater inflows maintain a fairly constant water level year-round, with water at or near the surface most of the time. Constant high water levels lead to accumulation of organic material, usually greater than 40 centimeters (15 inches), except on sites underlain by limestone bedrock. In addition to peat accumulation and perennially saturated soils, extremely rich and iron fens have distinct soil and water chemistry, with high levels of one or more minerals such as calcium, magnesium, or iron. Fens maintain stream water quality through denitrification and phosphorus absorption. They are among the most floristically diverse of all wetland types, supporting a large number of rare and uncommon bryophytes and vascular plant species, and provide habitat for uncommon mammals, mollusks and insects. Fens usually occur as a mosaic of herbaceous and woody plant communities. In herbaceous communities, there are several plant associations dominated by sedges (Carex species), spikerushes (Eleocharis species), and rushes (Juncus species). Bryophyte diversity is generally high and includes sphagnum (Sphagnum species). Forb diversity is especially high in extremely rich and iron fens. The woody community is typically composed of willow (Salix species) and birch (Betula nana) carr shrubland. The surrounding landscape may be ringed with other wetland systems: fens often grade into marshes, wet meadows or riparian shrublands, and can be surrounded by conifer swamps or wet to mesic coniferous forests. In very rare cases, fens can occur within prairie grasslands in the glaciated Great Plains. Fens are found in scattered locations along the Rocky Mountain Front, in the Rocky Mountains and and intermountain valleys, in the small isolated central mountain ranges, and at higher elevations on the Beartooth Plateau in the southern portion of the state.
This system occurs infrequently throughout the Rocky Mountains from Colorado north into Canada. In Montana, small fens are found in scattered locations in the glaciated plains and in the small isolated mountain ranges of the central part of the state. The Swan, Stillwater and Flathead valleys have numerous rich and extremely rich fen systems due to the prevalence of limestone bedrock in the Whitefish, Mission, and Swan mountain ranges. Similarly, rich and extremely rich fens are found along the limestone-rich Front Range east of the Continental Divide. East of the Continental Divide, both small and large rich and extremely rich prairie fens occur on the extreme western Great Plains bordering the Rocky Mountain Front (Lesica, 1986) and rarely, within the northwestern mixed grass prairie (Heidel et al, 2000). Further south in western Montana, poor fen systems are more common in the Bitterroot, Lolo, and Beaverhead ranges, and are found in the granitic, isolated central Montana island ranges and the Beartooth Plateau in southwestern Montana.
The montane-subalpine fen ecological system is a small-patch system composed of mountain wetlands that support a unique community of plants not found in other types of wetlands. Fens are confined to specific environments defined by groundwater discharge, soil chemistry, and peat accumulation of at least 40 centimeters (15 inches), although peat accumulations in areas overlain by gravel, cobble or bedrock may be less. Soils are typically organic histosols with 40 centimeters or more of organic material if overlying a mineral soil, or less if overlying bedrock, cobbles or gravels. Histosols range in texture from clayey-skeletal to loamy-skeletal and fine-loams. Fens form at low points in the landscape or near slopes where groundwater intercepts the soil surface. Groundwater inflows maintain a fairly constant water level year-round, with water at or near the surface most of the time. Constant high water levels lead to accumulations of organic material. Rich and extremely rich fens are found in areas underlain by limestone. Water chemistry ranges from only slightly acidic to alkaline and is usually distinctly calcareous. Marl deposits (precipitated calcium carbonates) are common in these systems. Tufa deposits or terraces can be seen in some rich fens and are composed of virtually pure calcium carbonate at the soil surface, formed by continuous discharge and evaporation of calcite saturated groundwater. In northwestern Montana, pH values usually range from 5.9 to 8.4 (Chadde et al., 1998). Poor fens are more common in the northern Rocky Mountains and occur in areas overlain by non-calcareous bedrock, e.g., argillites and granite. These are usually flat, acidic, and saturated to the surface, sometimes with standing water. Iron rich fens are more rare in occurrence, and can be strongly acidic (as low as pH 2.98) and associated with geothermal features and bedrock of weathering pyrite, as found in some occurrences in the Yellowstone Plateau (Lemly, 2007). Iron rich fens support a diverse bryophyte community, typically have less vascular plant diversity, and are composed of species dependant on more acidic conditions.
Fens develop successionally through lake-filling, flow-through successional processes or by paludification (Chadde et al., 1998). Lake filling occurs in depressions and is often characterized by the presence of floating mats and a ring of carr vegetation on the outer margin of the peatland. Flow-through fens are the most common in the northern Rocky Mountains. They occur along springs, streams, slopes and benches with a constant inflow and outflow of calcium-rich water. They are characterized by a series of linear hummocks oriented perpendicular to the slope. Carr shrubland is well developed in flow-through fens due to well-aerated, nutrient-rich water near the inflow and outflow zones. Usually there is an open, nutrient- poor community in the central portion of the fen. Paludification occurs when fens expand due to a rise in the water table caused by peat accumulation. This process is most often observed near seeps and springs or adjacent to closed basin peatlands where peat accumulation causes wetter conditions along the outer edges. Higher water tables kill existing trees. In the northern Rocky Mountains, this successional process is limited due to prolonged summer droughts; however it may be seen in some fen systems at higher elevations.
In northwestern Montana, fens occur at montane to subalpine elevations, generally ranging from 985-2,165 meters (2,500-5,500 feet). In southwestern Montana, subalpine and alpine fens occur at higher elevations (Heidel and Rodemaker, 2008). These communities typically occur in seeps and wet sub-irrigated meadows in narrow to broad valley bottoms. Surface topography is typically smooth to concave with lake-fill peatlands or with slopes ranging from 0 to 10 percent in flow-through fens.
Floristically, rich and extremely rich fens support the greatest vascular plant species diversity of all peatland types in the Rocky Mountains. Extremely rich fens are characterized by high species diversity and a mosaic of herbaceous and woody plant communities. In contrast, poor fens have scattered vascular plant cover and lower species diversity but are characterized by a nearly continuous and diverse cover of mosses and other bryophytes.
Several vascular plants have been identified as extremely rich or rich fen indicators in Montana, and some of these species are uncommon or rare. Indicators include: sage-leaf willow (Salix candida), simple bog sedge (Kobresia simpliciuscula), Bellardii bog sedge (Kobresia myosuroides), Rolland’s small clubrush (Trichophorum pumilum), little green sedge (Carex viridula), northern single spike sedge (Carex scirpoidea), pale sedge (Carex livida), bulblet-bearing water hemlock (Cicuta bulbifera), slender cottongrass (Eriophorum gracile), green keeled cottongrass (Eriophorum viridicarinatum), beaked spikerush (Eleocharis rostellata), northern bog violet (Viola nephrophylla), pale bog laurel (Kalmia polifolia), Kalm’s lobelia (Lobelia kalmii), and yellow widelip orchid (Liparis loeselii). Other orchids such as giant helleborine orchid (Epipactis gigantea) are found in open sedge-dominated portions of the fen system, while one-leaf orchid (Ameorchis rotundifolia), sparrow’s egg ladyslipper (Cypripedium passerinum) and small yellow ladyslipper (Cypripedium parviflorum) occur on raised sphagnum hummocks around trees and shrubs near the perimeter of the fen. These species are found almost exclusively in fens or wet forest habitats bordering fens. Poor fens often include species found in more acidic conditions such as pale bog laurel (Kalmia polifolia), rannoch rush (Scheuchzeria palustris) and sundews (Drosera species).
In extremely rich and rich fens, the herbaceous community is often dominated by beaked sedges (Carex utriculata or Carex rostrata), water sedge (Carex aquatilis), mud sedge (Carex limosa), woolyfruit sedge (Carex lasiocarpa), spikerush (Eleocharis species), cottongrass (Eriophorum species), rushes (Scirpus species and Trichophorum species) and bulrushes (Shoenoplectus species). Other frequent species include Buxbaum’s sedge (Carex buxbaumii), analogue sedge (Carex simulata), northern bog sedge (Carex gynocrates), bristly-stalked sedge (Carex leptalea), poor sedge (Carex paupercula), yellow sedge (Carex flava), hair sedge (Carex capillaris), silvery sedge (Carex canescens), lens sedge (Carex lenticularis), Baltic rush (Juncus balticus), northern rush (Juncus alpino-articulatus), dagger leaf rush (Juncus ensifolius), threadleaf rush (Juncus filiformis), common spike rush (Eleocharis palustris), and few-flowered spike rush (Eleocharis quinqueflora). Common grasses include bluejoint reedgrass (Calamagrostis canadensis), tufted hairgrass (Deschampsia cespitosa), and fringed brome (Bromus ciliatus).
Rich and extremely rich fens also support high forb diversity. Common species include showy pussytoes (Antenarria pulcherrima), bog orchid (Plantanthera species), buckbean (Menyanthes trifoliata), elegant death camas (Zigadenus elegans), grass-of-parnassus (Parnassia species), beautiful shooting-star (Dodecatheon pulcherrinum), pink elephant’s head (Pedicularis groenlandica), arrow-grass (Triglochin palustris), and Siberian chives (Allium schoenoprasum). At subalpine elevations, common butterwort (Pinguicula vulgaris) often occurs near seeps or springs, in areas where there is marl accumulation or on tufa deposits or terraces.
In Montana, wet, floating Sphagnum-dominated mats are associated with open water edges or depressional areas of fen systems. Bryophyte floating mats often consist of Meesia moss (Meesia triquetra), Scorpidium moss (Scorpidium species), Magellan’s peatmoss (Sphagnum magellanicum) and brown peatmoss (Sphagnum fuscum). The bryophyte floating mat supports a very minor component of sedges such as mud sedge (Carex limosa) and smaller sedges such as grape sedge (Carex aurea), softleaf sedge (Carex disperma) and inland sedge (Carex interior), as well as cottongrass species (Eriophorum species). Fen indicators such as pale laurel (Kalmia polifolia), rannoch rush (Scheuchzeria palustris) and sundews (Drosera species) occur on these floating mats. Buckbean (Menyanthes trifoliata) is a late seral species from the sedge mat phase and is often present on floating mats.
Fens are frequently bordered by willow-bog birch (Salix species-Betula nana glandulosa) dominated carrs. Carr shrubland is well developed in flow-through fens due to highly-aerated nutrient-rich water near the inflow and outflow zones or the perimeter of basin fens. Sageleaf willow (Salix candida) is an indicator species, and sometimes the dominant willow species. Other willow species include autumn willow (Salix serrissima), Bebb’s willow (Salix bebbiana), Drummond’s willow (Salix drummondiana), plane-leaf willow (Salix planifolia), wolf willow (Salix wolfii), and undergreen willow (Salix commutata). Other common carr shrubs include alder (Alnus species), bog birch (Betula nana), alder buckthorn (Rhamnus alnifolia), shrubby cinquefoil (Dasiphora fruticosa), and western Labrador tea (Ledum glandulosum). Engelmann spruce (Picea engelmannii) is the most frequent conifer species associated with fens and forested fen margins of these systems (Hansen and others, 1996).
Mountain fens act as natural filters, cleaning ground and surface water. Fens also act as sponges by absorbing heavy precipitation, then slowly releasing it downstream, minimizing erosion and recharging groundwater systems. Persistent groundwater and cold temperatures allow organic matter to accumulate, forming peat, which allows classification of wetlands within this system as fens. Peat accumulates at the rate of 8 to 11 inches per 1000 years, making peatlands a repository of 10,000 years of post-glacial history.
Land uses surrounding fens can potentially alter the hydrology and nutrient inputs of these systems, thus changing their underlying processes. Increased land use within 100 meters has been found to be correlated with increased nutrient levels in peatlands in Montana, suggesting that setbacks should be 100 meters or more for adequate protection (Jones 2003). Draining, heavy cattle use, and irrigation practices can also alter hydrology and result in the loss of species diversity. Localized peat mining may occur on private lands.
The degree of damage that has occurred in a fen has a significant impact on the prospects for restoration. Peat mining will cause irreversible damage to fen systems because Rocky Mountain fens build peat so slowly (8 to 11 inches per 1,000 years). In fen systems where water has been drained or altered, the original hydrology of the system must be restored before any vegetation restoration can be considered. If water levels are restored, re-growth and re-colonization of peat mosses can occur, although this is a very slow process. In deeper waters, regeneration depends on whether residual peat layers will become buoyant. Regeneration largely depends on water chemistry and residual peat layer quality. When peat quality is inadequate, shallow inundation is recommended (Smolders et al., 2002).
Heavy cattle use in a fen system can alter the hydrology by damaging soils within the fen system. Soil compaction and pugging within the peat layer will change surface water flow. Heavy cattle use can also alter the successional processes within the sedge- dominated area of a fen. Cattle hoof action can lead to pugging and hummocking, creating microsites where shrubs can become established, changing the sedge-dominated meadow to carr shrubland.
Lemly, Joanna M. 2007. Fens of Yellowstone National Park, USA: regional and local controls over plant species distribution.
Click to download offline versions of our field guide.