Type 6 Wetlands: Shrub Swamps and Carrs

wetland types
Written By Azalea Hallin-Graber

As their locations and landscape positions are similar to those occupied by wooded swamps, shrub swamps have similar soils and perform similar ecosystem services. Thanks to their location along waterways and sequences of lakes, they serve as migration corridors for wildlife and are particularly valuable as breeding and nesting grounds for a wide range of bird species.

Understanding Shrub Swamps and Carrs

Shrub Swamps and Carrs are closely related to Wooded Swamps (Type 7), often occupying an intermediate position between these and Type 2 (Wet Meadow) wetlands. This intermediate position may either be spatial or temporal—spatial when it occurs in a zone situated between these two wetland types on a site; temporal when it marks the colonization of an open meadow by woody species, first by shrubs and eventually by trees.

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. Both types of  swamps may have extensive populations of tall and short shrubs—alders, willows, dogwoods, mountain maple, winterberry, and leatherleaf. Both will have saturated soils and often standing water six inches or deeper. The general distinction between shrub swamps (Type 6) and wooded swamps (Type 7) is that shrub swamps have less than 25% cover from mature tree species and at least 50% cover from tall shrubs—as opposed to the shorter, stiff-leaved ericaceous shrubs often found in coniferous swamps and bogs. 

Broadly characterized, the two main types of shrub swamps found in Minnesota are alder thickets dominated by speckled alder (also called tag alder) and willow carrs dominated by species in the genus Salix (Minnesota has 20). Other tall shrub species common in this wetland type are dogwoods, meadowsweet, highbush cranberry and other Viburnum species, winterberry, and false indigo.

Characteristics of Type 6 Wetlands

Soil Profile: The soil in shrub swamps typically is covered with up to six inches of water throughout the growing season. They have very pooerly drained mineral soils that contain grayish colors or red soiled with mottles. Organic soils tend to be 2 to 51 inches thick. 

Vegetation: The understory of a shrub swamp contains many willow species such as meadow willow, Bebb’s willow, tea-leaf willow, and pussy willow. Some other species are redosier dogwood, meadowsweet, highbush cranberry, tag alder, and currants. Canada blue joint grass, joe-pye weed, smooth goldenrod, sensitive fern, wild mint, bedstraw, and sedges make up the herbaceous species. In disturbed shrub swamps, canary grass and redtop grass are present. 

Fauna: The location of shrub 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 a habitat for insects and amphibians.

Functions and Benefits

As with all wetlands found in Minnesota, shrub swamps provide a number of different ecosystem services and economic opportunities. Highlighted below is a list of benefits that a Type 6 Wetland offers:

Flood Control: A shrub swamp acts like a sponge. When heavy rains occur the swamp is able to absorb the influx of upland runoff. This prevents water 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 shrub 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, shrub 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: Shrub swamps are home to several timber species important to Minnesota’s forestry industry: black spruce, balsam fir, red and silver maple, and black ash, with balsam poplar supplementing the demand for aspen. Pursuits like hunting and trapping contribute to local economies both through the production of products for human consumption and through travel and recreation income.

Mitigation Partners, Inc. Founders Dax Dickson & Tory Christensen

Sources

Azalea Hallin-Graber
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Type 5 Wetlands: Inland Open Freshwater
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Type 7 Wetlands: Wooded Swamps