Rocky Mountain Acidic Fen
Global Name:
Rocky Mountain Acidic Fen
Global Rank:
G4G5
State Rank:
S3
(see reason below)
External Links
State Rank Reason
These rare habitats are vulnerable as a result of their restricted extent and primarily from warming and drying climate trends.
General Description
This National Vegetation Classification Group is composed of acidic peatlands in montane areas. They occur infrequently in mountainous areas on both sides of the Continental Divide. Acidic fens in particular can be found throughout the northwestern portion of the state to the mountains of southwest Montana. They are most common from north of Missoula and Lincoln to the Swan Valley and north and west to the border. They are confined to specific environments defined by groundwater discharge, soil chemistry, and peat accumulation. In Montana, these fens develop on non-calcareous bedrock such as Belt series argillites and granites of the Idaho Batholith (Chadde etal 1998). The soil chemistry of acidic fens, also known as poor fens, is acidic and nutrients are low except in iron fens where the pH is low (acidic) but nutrients are high. 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). 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. Acidic fens, usually support several herbaceous communities or plant associations dominated by Sedges (Carex spp), Spikerushes (Eleocharis spp), Dulichium arundinaceum, and Pod Grass (Scheuchzeria palustris). Bryophytes are usually abundant and dominated by Sphagnum (Sphagnum species). The surrounding landscape may be ringed with other wetland types: fens often grade into marshes, wet meadows or riparian shrublands, and can be surrounded by conifer swamps or wet to mesic coniferous forests.
This group encompasses a portion of the Rocky Mountain Subalpine-Montane Fen Ecological System.
Diagnostic Characteristics
Peatlands; Poor Fens; Sphagnum-dominated; Herbaceous-dominated; Minerotrophic; Permanently Saturated Organic Soils gen with >40cm Peat, Acidic Soil Water pH; Rocky Mountain Region; Valleys, Montane and Subalpine Zones
Typical Dominants: Sedges (Carex lasiocarpa, Carex limosa and others), Dulichium arundinaceum, Pod Grass (Scheuchzeria palustris), Marsh Cinquefoil (Comarum palustre)
Similar Systems
Range
Acidic fens are very scattered in distribution in the mountainous areas on both sides of the Continental Divide. They can be found throughout the northwestern portion of the state to the mountains of southwest Montana. They are most common from north of Missoula and Lincoln to the Swan Valley and north and west to the border. In the southwestern portion of the state, examples are known from the Bitterroot Mtns, Tobacco Root Mtns, and the West Pioneers.
In MT, G515 occurs within Level III Ecoregions: 15 (Northern Rockies), 16 (Idaho Batholith), 17 (Middle Rockies), and 41 (Canadian Rockies) and potentially extending into the western portion of 42 (Northwestern Glaciated Plains).
In Montana, G515 occurs primarily within these Major Land Resource Areas: 43A - Northern Rocky Mountains, 43B - Central Rocky Mountains, and 44A - Northern Rocky Mountain Valleys.
Density and Distribution
Based on 2025 land cover layer. Grid on map is based on USGS 7.5 minute quadrangle map boundaries.
Mapped Distribution by County
Beaverhead, Blaine, Chouteau, Deer Lodge, Gallatin, Granite, Hill, Jefferson, Madison, Missoula, Park, Powell, Ravalli, Silver Bow
Based on 2025 land cover layer.
Spatial Pattern
Small Patch
Environment
Acidic (Poor) fens are a unique type of montane wetland that support a unique community of plants not found in other types of wetlands. They 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. In Montana, these fens develop on non-calcareous bedrock such as Belt series argillites and granites of the Idaho Batholith (Chadde etal 1998). 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 dependent on more acidic conditions.
Acidic 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 water. They are characterized by a series of linear hummocks oriented perpendicular to the slope. 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 habitats at higher elevations.
In northwestern Montana, fens occur at montane to subalpine elevations, generally ranging from 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.
Vegetation
Acidic fens closely resemble the vegetation of bogs, with Sphagnum mosses and ericaceous shrubs common. Dominant species often include Carex aquatilis, Carex livida, Carex lasiocarpa, Dulichium arundinaceum, Ledum glandulosum, Trichophorum cespitosum, Spikerushes (Eleocharis spp), Pod Grass (Scheuchzeria palustris) and Marsh Cinquefoil (Comarum palustre) (Cooper 1986b, Windell et al. 1986, Steen and Coupe 1997). They often support a large number of rare and uncommon bryophytes and vascular plant species. Acidic fens, usually support several herbaceous communities or plant associations often dominated by Sedges (Carex spp) and Sphagnum (Sphagnum species) within a single site. In Montana, wet, floating Sphagnum-dominated mats are associated with open water edges or depressional areas of fens. Bryophyte floating mats often consist of Meesia triquetra, Scorpidium moss (Scorpidium species), Sphagnum magellanicum and Sphagnum fuscum. The bryophyte floating mat supports a very minor component of sedges such as mud sedge (Carex limosa) and smaller sedges such as Carex aurea, Carex disperma) and Carex interior, as well as cottongrass species (Eriophorum species). Fen indicators such as Scheuchzeria palustris, sundews (Drosera species) and Buckbean (Menyanthes trifoliata) occur on these floating mats.
The surrounding landscape may be ringed with other wetland types: fens often grade into marshes, wet meadows or riparian shrublands, and can be surrounded by conifer swamps or wet to mesic coniferous forests.
In Montana, currently only 2 Alliances and 2 Associations are attributed to this group within the National Vegetation Classification. Additional vegetation types certainly occur within the state and further review and documentation are needed.
National Vegetation Classification
Download the complete NVC hierarchy for Montana
TP1 B08 Palustrine Wetland
TP1.c S71 Bog and Fen
TP1.c1 F140 Boreal, Temperate and Montane Peat Bog
TP1.c1.Na D343 North American Boreal-Subboreal Bog
TP1.c1.Na.2 M876 North American Boreal and Subboreal Bog and Acidic Fen
TP1.c1.Na.2.c G515 Rocky Mountain Acidic Fen
A2142 Pinus contorta Treed Fen Alliance
A3437 Carex lasiocarpa - Carex livida - Dulichium arundinaceum Acidic Graminoid Fen Alliance
CEGL001810 Carex lasiocarpa Fen
CEGL001831 Dulichium arundinaceum Shore Fen
*Disclaimer: Some Alliances and Associations are considered provisional. Some require further documentation to verify their occurrence in the state
and some may be modified or deleted in future revisions after collection of additional data and information.
Dynamic Processes
Montane fens act as natural filters, cleaning ground and surface water. They maintain stream water quality through denitrification and phosphorus absorption. 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. 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.
Management
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.
Restoration Considerations
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 habitats because Rocky Mountain fens build peat so slowly (8 to 11 inches per 1,000 years). In fens 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).
Cattle use in a fen can alter the hydrology by damaging soils within the fen habitat. Soil compaction and pugging within the peat layer will change surface water flow. 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.
Species Associated with this Community
- How Lists Were Created and Suggested Uses and Limitations
Animal Species Associations
Please note that while all vertebrate species have been systematically associated with vegetation communities, only a handful of invertebrate species have been associated with vegetation communities and invertebrates lists for each vegetation community should be regarded as incomplete. Animal species associations with natural vegetation communities that they regularly breed or overwinter in or migrate through were made by:
- Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Werner et al. 2004, Adams 2003, and Foresman 2012);
- Evaluating structural characteristics and distribution of each vegetation community relative to the species' range and habitat requirements;
- Examining the observation records for each species in the state-wide point observation database associated with each vegetation community;
- Calculating the percentage of observations associated with each vegetation community relative to the percent of Montana covered by each vegetation community to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use. Species that only overwinter in Montana were only evaluated for overwintering habitat use. Species that only migrate through Montana were only evaluated for migratory habitat use. In general, species are listed as associated with a vegetation community if it contains structural characteristics known to be used by the species. However, species are not listed as associated with a vegetation community if we found no support in the literature for the species’ use of structural characteristics of the community even if point observations were associated with it. If you have any questions or comments on animal species associations with vegetation communities, please contact the Montana Natural Heritage Program's Senior Zoologist.
Plant Species Associations
Please note that while diagnostic, dominant, or codominant vascular plant species for a vegetation community have been systematically assigned to those communities and vascular plant Species of Concern were systematically evaluated for their associations with vegetation communities, the majority of Montana’s vascular plant species have not been evaluated for their associations with vegetation communities and no attempt has been made to associate non-vascular plants, fungi, or lichens with vegetation communities. Plant species associations with natural vegetation communities were made in a manner similar to that described above for animals, but with review of Lesica et al. (2022) and specimen collection data from the Consortium of Pacific Northwest Herbaria. If you have any questions or comments on plant species associations with vegetation communities, please contact the Montana Natural Heritage Program's Program Botanist.
Suggested Uses and Limitations
Species associations with vegetation communities should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning. These potential lists of species should not be used in place of documented occurrences of species or predicted habitat suitability models (this information can be requested at: https://mtnhp.mt.gov/requests/), or systematic surveys for species and onsite evaluations of habitat by trained biologists. Users of this information should be aware that the land cover data used to generate species associations is based on satellite imagery from 2016 and was only intended to be used at broader landscape scales. Land cover mapping accuracy is particularly problematic when the vegetation communities occur as small patches or where the land cover types have been altered over the past decade. Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections). Finally, although a species may be associated with a particular vegetation community within its known geographic range, portions of that vegetation community may occur outside of the species' known geographic range.
Literature Cited
- Adams, R.A. 2003. Bats of the Rocky Mountain West; natural history, ecology, and conservation. Boulder, CO: University Press of Colorado. 289 p.
- Consortium of Pacific Northwest Herbaria. https://www.pnwherbaria.org/ Last accessed May 30, 2025.
- Dobkin, D. S. 1992. Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34. Missoula, MT.
- Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
- Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998. Montana atlas of terrestrial vertebrates. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT. 1302 p.
- Hutto, R.L. and J.S. Young. 1999. Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32. 72 p.
- Lesica P., M. Lavin, and P.F. Stickney. 2022. Manual of vascular plants, 2nd Edition. Brit Press. 779 p.
- Maxell, B.A. 2000. Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species. Report to U.S. Forest Service Region 1. Missoula, MT: Wildlife Biology Program, University of Montana. 161 p.
- Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath. 2004. Amphibians and reptiles of Montana. Missoula, MT: Mountain Press Publishing Company. 262 p.
- Species of Concern Associated with this Community
Vascular Plants
Mammals
Birds
Amphibians
Invertebrates
- Diagnostic, Dominant, or Codominant Plant Species for this Community
Vascular Plants
- Other Native Species Commonly Associated with this Community
Vascular Plants
Original Concept Authors
G. Kittel (2015)
Montana Version Authors
S. Mincemoyer, T. Luna, L. Vance, C. McIntyre
Version Date
12/5/2024
References
- Literature Cited AboveLegend:
View Online Publication
- Additional ReferencesLegend: View Online Publication
Do you know of a citation we're missing?
Bedford B.L., and K.S. Godwin. 2003. Fens of the United States: distribution, characteristics, and scientific connection versus legal isolation. Wetlands 23:608–629.- Hansen, P. L., R. D. Pfister, K. Boggs, B. J. Cook, J. Joy, and D. K. Hinckley. 1995. Classification and management of Montana's riparian and wetland sites. Montana Forest and Conservation Experiment Station, School of Forestry, University of Montana, Miscellaneous Publication No. 54. 646 pp. + posters.
- Mitsch WJ, Gosselink JG. 2000. Peatlands. In: Wetlands. 3rd Edition. John Wiley and Sons, Inc. 920 p.
- Web Search Engines for Articles on "Rocky Mountain Acidic Fen"
