Montana Field Guide

This National Vegetation Classification Group is composed of alkaline peatlands on the plains and prairies. They occur within a grassland mosaic and 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). The water chemistry of alkaline fens, also known as rich and extremely rich fens ranges from neutral to alkaline and is usually distinctly calcareous with higher concentrations of dissolved minerals. Marl deposits (precipitated calcium carbonates) are common in these habitats.

These habitats are rare in MT and have been documented from only a handful of sites from the Rocky Mtn Front to the North Dakota border including on the Blackfeet Reservation (Luna and Bahls 2017) and in Sheridan County (Heidel, Cooper and Jean 2000).

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; Great Plains Region

Typical Dominants: Sedges (Carex nebrascensis, Carex pellita, Carex simulata, or Carex utriculata), Eleocharis spp. (Eleocharis quinquefolia, Eleocharis palustris)

Alkaline Fens are very widely scattered in the Great Plains region, extending from the Rocky Mtn Front to the North Dakota Border and primarily north of the Missouri River.

In MT, G1216 occurs within Level III Ecoregions: 42 (Northwestern Glaciated Plains) and likely also in 43 (Northwestern Great Plains).

In Montana, G516 occurs or likely occurs within these Major Land Resource Areas: 46 - Northern and Central Rocky Mountain Foothills, 52 (Brown Glaciated Plains), 53A (Northern Dark Brown Glaciated Plains), 53B (Central Dark Brown Glaciated Plains), 58A (Northern Rolling High Plains, Northern Part).

Small Patch

This group occurs in the Great Plains where the climate is semi-arid to temperate. Sites occur within a grassland context where groundwater is close to the surface and saturates the soils year-round, resulting in organic soils consisting of peat or muck. Groundwater pH is neutral to alkaline. Sites can be found along at the headwaters of other wetland types such as streams or lakes, or along hillsides where groundwater emerges.

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 habitats. 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.

Sites are typically dominated by medium or low-statured, rhizomatous graminoids. Open to dense graminoids less than 1 m tall comprise the majority of the vegetation. Forbs are often less dominant in height and cover. Bryophytes often provide a significant ground cover. Pools of water can provide microsites for aquatic vascular plants and bryophytes. Low to medium- statured shrubs are often scattered but rarely dominant. Dominant vegetation may vary by zones within a single site. Common species include sedges (Carex aquatilis, Carex nebrascensis, Carex pellita, Carex simulata, or Carex utriculata) and spikerushes (Eleocharis quinquefolia and Eleocharis palustris). Other graminoids present often include rushes (Juncus spp.), bulrushes (Schoenoplectus spp.), Triglochin maritima, Glyceria striata and cattails (Typha latifolia). In open pools, Chara spp. and Utricularia spp. may be found. Forb species are highly variable rangewide but often include Lobelia spp., Helianthus spp., Mimulus spp., Scutellaria spp., Symphyotrichum boreale and Viola spp. Equisetum spp. are often present as well. Low to medium shrubs may be present and include willows (Salix exigua, Salix bebbiana and Salix serissima), red-osier dogwood (Cornus sericea), and birch (Betula pumila).

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.

Land uses surrounding fens can potentially alter the hydrology and nutrient inputs of these habitats, 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.

These fens may be susceptible to invasive plants that occur in nearby wet prairie or wet meadows including Agrostis stolonifera, Phalaris arundinacea, and Sonchus arvensis.

Cattle use in fen habitats can alter the hydrology by damaging soils within the fen. 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.

Vinge-Mazer et al. (2025)

S. Mincemoyer

7/5/2025