Montana Field Guide

Fens occur infrequently in mountainous areas on both sides of the Continental Divide. Alkaline fens, in particular, occur from the valley bottoms and the foothills of the Rocky Mountain Front into the montane and subalpine zones. They are most common from northwest Montana east to Glacier National Park and south throughout the Swan Valley and east to the Rocky Mountain Front. They are confined to specific environments defined by groundwater discharge, soil chemistry, and peat accumulation. 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). In Montana, alkaline fens develop on calcareous bedrock, primarily limestone. The water chemistry of alkaline fens, also known as rich and extremely rich fens ranges from slightly acidic to alkaline and is usually distinctly calcareous with higher concentrations of dissolved minerals. Marl deposits (precipitated calcium carbonates) are common in these systems. Fens 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. Alkaline fens are floristically diverse and usually support several herbaceous communities or plant associations commonly dominated by Sedges (Carex buxbaumii, Carex limosa, Carex utriculata and others), Kobresia spp, Beaked Few-flower Spikerush (Eleocharis quinqueflora), Bog Birch (Betula glandulosa), and Hoary Willow (Salix candida). In contrast to acidic fens, which usually have abundant cover of Sphagnum moss, alkaline fens are more commonly dominated by true or brown mosses (Chadde etal 1998).

This Group encompasses a portion of the Rocky Mountain Subalpine-Montane Fen Ecological System.

Peatlands; Rich and Extremely Rich Fens; Calcareous Fens; True Moss (Brown Mosses)-dominated; Herbaceous-or Shrub dominated; Minerotrophic; Permanently Saturated Organic Soils gen with >40cm Peat, Neutral to Alkaline Soil Water pH; Rocky Mountain Region; Valleys, Montane and Subalpine Zones

Typical Dominants: Sedges (Carex buxbaumii, Carex limosa, Carex utriculata and others), Kobresia spp, Beaked Few-flower Spikerush (Eleocharis quinqueflora), Bog Birch (Betula glandulosa), Hoary Willow (Salix candida)

Alkaline Fens are scattered in distribution in the mountainous areas of Montana on both sides of the Continental Divide. They occur from the valley bottoms and the foothills of the Rocky Mountain Front into the montane and subalpine zones. They are most common from northwest Montana east to Glacier National Park and south throughout the Swan Valley and east to the Rocky Mountain Front. South of this area in Montana, alkaline fens are rare.

In MT, G516 occurs within Level III Ecoregions: 15 (Northern Rockies), 17 (Middle Rockies), and 41 (Canadian Rockies) and extending into the western portion of 42 (Northwestern Glaciated Plains) along the Rocky Mtn Front.

In Montana, G516 occurs within these Major Land Resource Areas: 43A - Northern Rocky Mountains, 43B - Central Rocky Mountains, and in limited extent within 44A - Northern Rocky Mountain Valleys, 44B - Central Rocky Mountain Valleys, and 46 - Northern and Central Rocky Mountain Foothills.

Small Patch

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. Alkaline (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).

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 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.

Alkaline fens are generally dominated by dense stands of Sedges (Carex buxbaumii, Carex limosa, Carex utriculata and others), as well as Kobresia spp, Few-flowered Spikerush (Eleocharis quinqueflora), Beaked Spikerush (Eleocharis rostellata), and Cottongrasses (Eriophorum spp). Shrubby species are present in scattered patches or they may form shrub-dominant peatland communities called carrs. Typical species include Bog Birch (Betula glandulosa), Hoary Willow (Salix candida), and Alnus incana. Mosses are common, but in contrast to acidic fens, which usually have abundant cover of Sphagnum moss, alkaline fens are more commonly dominated by true or brown mosses (Chadde etal 1998). Typical species include Aulacomnium palustre, Bryum pseudotriquetrum, Campylium stellatum and Limpritchia revolvens. Alkaline fens support a number of rare mosses and vascular plants that are not found in any other habitats.

In Montana, 3 Alliances and 11 Associations are attributed to this group within the National Vegetation Classification. Additional vegetation types may occur within the state and further review and documentation are needed. The Kobresia myosuroides - Kobresia simpliciuscula Alkaline Graminoid Fen Alliance is attributed for Montana but no finer scale vegetation types at the Association level are currently attributed to the state; this requires further review.

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