Calcareous Fens: Understanding this rare, special and sensitive wetland type

wetland restorationconservation
Written By Kristin Shardlow

A fen is a unique wetland that is primarily fed by input from groundwater flowing through the aquifer from higher-elevation landforms. Recharge from groundwater rather than rainfall is favored by position along a slope, where water moves downward, preventing stagnation and keeping nutrient levels relatively high.

What is a Calcareous Fen? 

The word “calcareous” means that the fen is high in calcium carbonate. This happens when the aquifer, where groundwater is stored, consists of limestone glacial till or is otherwise formed from limestone parent material. Thus, calcareous fens are high in pH. Depending on the fen’s topographical position, it may be called on the one hand a slope or apron fen; or, on the other hand, a peat dome or mound fen.

Though they range in area from a tenth of an acre to dozens of acres, most calcareous fens are less than four acres. Because they are fed by groundwater flow-through, their water levels tend to remain consistent throughout the growing season, with influx being balanced by plant uptake and surface evaporation. 

Calcareous fens appear as accumulations of peat – spongy, dense, and relatively cold due to the groundwater origin. Because the organic soil is constantly waterlogged, the fens are low in oxygen and thus also low in nutrient availability. However, because of the down slope flow-through, which replenishes the water supply on an ongoing basis, calcareous fens are higher in oxygen and nutrients than, for example, sphagnum bogs with their stagnant and acidic conditions.

Rare, Special, and Sensitive 

Calcareous fens are rare because they arise from a specific combination of geological and hydrological conditions. In the U.S. they are known to occur in only ten states. In Minnesota, only 200 calcareous fens remain, located in 37 of our 87 counties.

They are special because they are home to plant species adapted to their stable hydrology and high-pH, low-oxygen, low-nutrient conditions. Ten of Minnesota’s rare and endangered species are specialists to calcareous fens and similar habitats, with hairy fimbry (Fimbristylis puberula) found exclusively in calcareous fens. 

Calcareous fens are sensitive because their unique conditions can be readily disrupted by human activities. Land use changes, even from elsewhere on the landscape, can alter their specific hydrology with profound impacts on the vegetation. Pollution from runoff can introduce nutrients, leading to invasion by opportunistic and non-native species such as reed canary grass. The soft, spongy peat is easily disturbed and destroyed by humans and livestock moving through them.

Protecting Calcareous Fens in Minnesota

Calcareous fens, once destroyed, cannot be restored or replaced. Because they are a niche habitat for many plant and animal species, those species are likewise vulnerable to environmental degradation.The Minnesota Department of Natural Resources maintains a listing of known calcareous fens. The state’s Wetland Conservation Act prevents any alteration or degradation of calcareous fens, in whole or in part, except under strictly regulated circumstances as approved by the commissioner of natural resources (MN Statute 103G.223). Furthermore, the ten rare and endangered species found in calcareous fens are protected under Minnesota’s Endangered Species Law (MN Statute 84.0895) from being collected or trafficked.

Mitigation Partners, Inc. Founders Dax Dickson & Tory Christensen

Sources

Kristin Shardlow
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