Type 7 Wetlands: Wooded Swamps

wetland restorationwetland types
Written By Kristin Shardlow

Wooded swamps are a common feature on the Minnesota landscape throughout the state, but especially in central and northern Minnesota. They contribute vastly to the quality of life of every Minnesotan through the ecosystem services they perform, the timber they provide, their benefits to wildlife, their beauty, and the recreational opportunities they afford.

Understanding Wooded Swamps

Type 7 Wooded Swamps are dominated by mature trees generally twenty feet or taller. They are found throughout the state, but especially in east-central and northern Minnesota. Minnesota’s Ecological Classification System recognizes three types of hardwood swamp and nine types of conifer swamp. Geographically, the distribution of hardwood swamps skews southward, while conifer and mixedwood swamps are more concentrated in the north. Wooded swamps occur mostly along slow-moving streams, in old oxbows, on river floodplains and floodplain terraces, on flat uplands, and in very shallow lake basins.

Wooded Swamps are closely related to Shrub Swamps and Carrs (Type 6). Both types may have extensive populations of tall and short shrubs – alders, willows, dogwoods, mountain maple, winterberry, leatherleaf. Both will have saturated soils and often standing water six inches or deeper. But a wooded swamp will have at least 25% tree cover dominated by the species listed above. On a given site a shrub swamp may grade into a wooded swamp, and many species of waterfowl make no distinction between the two in carrying out their activities.

Swamps differ from Bogs (Type 8 Wetlands) in having better drainage thanks to being connected to flowing water bodies. Bogs, on the other hand, are typically fed by precipitation and may be perched above the surrounding landform; the water has minimal outflow, resulting in stagnant conditions with lower oxygen, lower pH, and lower nutrient availability.

Characteristics Specific to Type 7 Wetlands

Soil Profile: The soil of hardwood swamps generally have a layer of peat over mineral soil, with a layer of muck at the very surface. The soils of conifer swamps have poorly decomposed Sphagnum peat over a thick layer of well-decomposed peat. The pH is typically greater than 5.5, so not as acidic as bogs (Type 8 Wetlands). The hollows between Sphagnum mounds are typically saturated or submersed. 

Canopy Vegetation: The hardwood tree species common in the wooded swamps of Minnesota include ed maple, black ash, elms, yellow birch, and balsam poplar. Conifers include tamarack, eastern white cedar, black spruce, and balsam fir. One also finds mixedwood swamps having combinations such as tamarack with yellow birch, or black ash with other hardwoods and a white cedar component. 

Understory Vegetation: The understory of a hardwood swamp is dominated by ferns, grasses (bluejoint, manna grass) and sedges, and flowering plants (forbs). Moss cover is mostly from brown moss species. Pools of standing water can support duckweed and emergent species like the smartweeds. Hummocks and decaying logs may support populations of upland understory plants. The understory of a conifer swamp may have a similar assortment of species. The main difference from hardwood swamps is greater moss cover—from 50% to continuous cover, with Sphagnum and feathermosses on the hummocks and brown mosses often submersed in the hollows. Another difference is the great cover of short shrubs in the heath family (Ericaceae). 

Fauna: The location of wooded swamps along streams and in floodplains means that these wetlands serve as migration corridors. Wood ducks, barred owls, herons, egrets, and various songbirds flock to these areas. Pools in the woodlands provide habitat for insects and amphibians.

Functions & Benefits

As with all wetlands found in Minnesota, wooded swamps provide a number of different ecosystem services and economic opportunities. Highlighted below is a list of benefits that a Type 7 Wetland offers:

Flood Control: A wooded swamp acts like a sponge. When heavy rains occur the swamp is able to absorb the influx of upland runoff. This prevents waters from spilling into the adjacent uplands and reduces the amount of water escaping into the stream or lake that the swamp borders. This minimizes floodwaters downstream. Conversely, during periods of drought, the wetland can serve as a source of water for the lake or river it borders, a process called low flow augmentation.

Erosion Control: The swamp’s trees and other vegetation also anchor the soil, preventing erosion of the streambank or lakefront.

Filtration and Water Quality: Stormwater runoff from upland is captured and stored in wetlands, percolating through the root systems of wetland vegetation. This effectively filters pollutants and excess nutrients accumulated by stormwater and protects the water quality of lakes and streams.

Habitat and Species Diversity: As noted above, wooded swamps along waterways serve as migration corridors for waterfowl. This habitat continuity promotes movement and genetic mixing, which promotes diversity and resilience among each species’ population.  

Recreational and Economic Value: Wooded swamps are home to several timber species important to Minnesota’s forestry industry: black spruce, balsam fir, red and silver maple, black ash, with balsam poplar supplementing the demand for aspen. Pursuits like hunting and trapping contribute to local economies both through production of products for human consumption and through travel and recreation income.

Mitigation Partners, Inc. Founders Dax Dickson & Tory Christensen

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Mitigation Partners, Inc. Founders Dax Dickson & Tory Christensen

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Kristin Shardlow
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Type 6 Wetlands: Shrub Swamps and Carrs
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