Nature Lab

Karst Fen Surveys

Researchers evaluate features of karst fens, create ecological site description

By Dianne Van Dien

Mucky soil, sedges, mosses, and forbs — these are things you will see if you come across a fen. But what exactly is a fen?

“Fens are unique wetlands because their soils are consistently saturated,” says MDC Wetlands Systems Manager Frank Nelson.

In the Ozarks, fens are part of the same geology — known as karst — that creates caves, sinkholes, and springs. These features occur in areas where groundwater dissolves parts of the calcium-rich bedrock. With fens, the groundwater seeps out slowly above certain layers of bedrock and keeps the soil soggy. Too wet for trees, fens are small areas where specific types of non-woody plants grow.

MDC, the U.S. Forest Service, and the nonprofit NatureCITE have partnered to evaluate 30 fen sites in the Ozarks to create an ecological site description (ESD) for this unique habitat. ESDs help land managers and private landowners better understand and care for their land, but fens had no ESD. In 2020, researchers began surveying the soil and plants at these sites to gather reference criteria.

The lack of oxygen in fens’ cool, wet soils slows decomposition, resulting in the buildup of organic plant matter over time. “With this project,” says Kyle Steele, forest ecologist at Mark Twain National Forest, “karst fen soils will become the first organic soils to be formally described in Missouri.”

Fens also have “a distinct assemblage of plants,” Steele says. “These plants have the unique ability to handle soggy soils with a high amount of calcium.”

Some of these plants typically grow in cool, wet climates farther north. Due to the unique plants and wet conditions, Nelson says, fens support a diversity of smaller animals, including dragonflies, pollinators, crayfish, and snails.

“A lot of fens are small in size,” says Steele, “but they’re so unique and contribute to the broader biological diversity of the region.”

At a Glance

No two fens are exactly alike, but they share common traits. Fens in the Ozarks:

• Have organic soil properties, which are very rare in Missouri
• Contain unique plant groupings that use wet, high-calcium soils
• Support unique insect species
• Tend to be small (0.1 to 30 acres)
• Have consistent water table at or near the soil surface

In some fens, sedges are the main vegetation, while others look prairielike with a variety of flowering plants, mosses, and sedges, in addition to some grasses.