Montana Field Guide

Fens 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 systems: 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.

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)

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.

Small Patch

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 systems 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.

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 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 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.

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, 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).

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. 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.

G. Kittel (2015)

S. Mincemoyer, T. Luna, L. Vance, C. McIntyre

12/5/2024