Peatlands, bogs and fens

Description | Landscape areas | Peatlands, bogs and fens

Coniferous forest location in Minnesota: northeast. Though prehistoric in origin, the peatlands have only recently "arrived" in public awareness. Most are relatively remote, undeveloped, and not suited to casual exploration. As a result, our understanding of peatland ecological systems has just begun to develop. Minnesota's peatlands have achieved special recognition as a world resource, and deserve special attention.

Figure 5.1. Peatland Distribution in Minnesota
Peatland distribution in Minnesota: northeast and north central.

Within the coniferous forest's rugged, coniferous terrain lie the vast lowlands of Minnesota's glacial lake plains. Unique soils, natural communities, climate, and water conditions in these areas have combined to form some of the world's largest peatlands. Smaller peatlands occur in scattered kettle-hole basins throughout the Coniferous Forest region. Figure 7 shows peatland and distribution within the state.

Formation

Peat formation requires steady, stagnant, low-oxygen conditions that prevent normal decomposition of plant debris. These conditions exist on all continents and at all latitudes, including tropical marshes and swamps. Variations in climate, hydrology, native species, and other factors result in the considerable range of peatland communities to be found around the world.

Minnesota's peatlands formed in areas of poor drainage and cool climates, with little evaporation during summer months. The water table lies at or near the surface in these areas, limiting the oxygen needed for decomposition of plant debris. Year after year, these organic materials accumulate, forming the partially decomposed mass of organic material, dark brown or black in color, that is known as peat.

Peatlands are fascinating ecosystems, about which we will undoubtedly hear more in the future. A useful source of information on this subject can be found in the book, The Patterned Peatlands of Minnesota, edited by H.E. Wright, Jr., Barbara Coffin, and Norman E. Aaseng, published by the University of Minnesota Press in 1992. Most of the information in the following discussion comes from this source.

Significance

Minnesota's peatlands are regionally and internationally significant for a variety of reasons. One reason is their extent: Minnesota has more peatland area (over 6 million acres) than any other state in the U.S. except Alaska. These peatlands serve as an important water reservoir, the significance of which has yet to be fully understood.

In addition, Minnesota peatlands present?in nearly pristine condition?unique developmental stages and landforms for research by the world's scientific community. They demonstrate the intricate hydrological patterns and landforms that develop over large areas with diverse water chemistry, flow patterns, and developmental processes. As an added aid to research, Minnesota's peatlands are among the few in the world that are free of permafrost. Permafrost makes it harder to investigate the groundwater and subtle drainage systems so essential to peatland formation and development.

Peatlands also offer excellent research opportunities regarding the complex adjustment of living organisms to their environment. Harsh environmental conditions in the peatlands present dramatic challenges to the survival and adaptation of plant and animal species. In addition, Minnesota peatlands occur at the edges of three biomes, resulting in unique species assemblages, including 24 species that are endangered, threatened, or of special concern in the state.

Minnesota's peatlands present other research advantages as well. Other world peatlands have been utilized commercially (such as northern Europe) or exist in areas even more difficult to reach (such as Siberia and the Hudson Bay Lowlands). In comparison, Minnesota peatlands are relatively accessible, yet relatively free of development.

In recognition of their significance, the legislature enacted the Wetland Conservation Act of 1991 (WCA), which established 18 peatland Scientific and Natural Areas.

Bogs and Fens

A peatland community reflects relationships among plants, topography, climate, and water, that are very different from those found on other Minnesota landscapes. These communities vary with the water chemistry and sources, and are very sensitive to disturbance.

Peatlands that receive water both from precipitation and ground water, which has percolated through mineral soil, are classified as fens. The water supply in a fen is only slightly acidic or nearly neutral, and it carries minerals and other nutrient content. Fens look like watery meadows, with sedges, reeds, grass-like plants, occasional shrubs, and scattered, stunted trees.

Bog communities, on the other hand, develop in isolation from ground water and run-off from nearby uplands. They receive water only through precipitation?snow or water. Bog waters have higher levels of acid and less mineral and nutrient content than ground water does. Fewer plants and animals have adapted to these conditions. Sphagnum mosses, ericaceous shrubs (plants in the heath family such as leather leaf), and some sedges cover the ground; trees may or may not develop.

Figure 5.2. Fens and bogs
The water supply for fens is ground water; for bogs, rain water.

Peat Landforms

Boreal peatland communities are distinguished by their shape, vegetation, and water chemistries. Though these communities are actually comprised of land, water, and plants, they often resemble geological landforms, such as river channels, islands, and ripple marks. These landform shapes give each community and its features their names. Three peat landforms are commonly found in Minnesota: the raised bog, the water track, and the spring fen.

The raised bog is one of Minnesota's most prominent peat landforms. It is a dome-shaped accumulation of peat, often with a forested crest of black spruce growing at the top and radiating down along the sloping sides. At the base lies either a sphagnum lawn (muskeg) or a fen. The only source of water in a raised bog is precipitation: its raised profile isolates it from run off, and an underlying basin of clay or loam topped with peat prevents groundwater from reaching it. As a result, bog water is acidic and low in mineral nutrients, limiting the number and diversity of plant species.

Water tracks look like river channels on aerial photos. They receive runoff from across the peatland surface, putting them in the fen category of water chemistry and vegetation. Often formed on glacial outwashes and beach ridges, their sandy soils allow groundwater to percolate up from the water table below to join the surface run-off. Water tracks are bordered by swamp forests or raised bogs. A water track may be featureless or patterned:

  • Featureless water tracks lack pattern but may contain linear bands of trees, shrubs, or meadow that are parallel to the flow.
  • Patterned fens may contain networks of peaty ridges (known as strings) and pools (known as flarks) that are perpendicular to the slope, or tree islands that parallel the prevailing slope. Island shapes may be ovoid, tear drop, or circular, reflecting variations in hydrology.

 

Spring fens are similar to water tracks with tree islands. They are a network of nonforested channels that branch, then rejoin, as they drain through the swamp forest. Their waters are alkaline, reflecting their ground water source.

These landforms often co-exist in complexes; that is, a large peatland contains more than one landform type. Known as mire complexes, they are classified by size and arrangement of bog, fen, and mineral soil formations. Table 5.1 shows landforms contained in representative peatland SNAs.

Table 5.1. Peat Landforms in Minnesota SNAs
Table showing the types of peat landforms found in Minnesota's Scientific and natural areas.

Plant Communities

Despite the varied and often harsh conditions of peatland landforms, many plants grow in all of these wetlands. The acidic waters of the bog, however, exclude some species that grow only in fens. For this reason, peatland plant communities are classified on the basis of these indicator species.

Eight of Minnesota's rare vascular plant species occur in peatlands, mainly patterned peatlands. Included among these are sundews and orchids. Other unique plants of peatlands include insectivorous pitcher plants, bladderworts, and ericaceous plants.

Historically, wildfire helped maintain brushland habitat that supported moose, waterfowl, and fur bearers. Fire suppression has permitted brushland to succeed to forest in many locations.

Table 5.2. Bog and fen plant communities

Characteristic

Bog

Fen

Forest

Black Spruce, optional

None; too wet

Shrubs

Ericaceous, Labrador, Tea, leatherleaf, swamp laurel, bog rosemary

Willows, bog birch

Ground Layer

Sedges, cotton grasses, nearly continuous mat of sphagnum moss species

Narrow-leaved grasses arid sedges, optional mosses (usually no Sphagnum moss species) and aquatic species

Other

Poor vascular plant diversity and little biomass

Moving groundwater is either visible or apparent from vegetative patterns

Typical forest communities consist of stunted black spruce and tamarack. Nonforested peatland communities are highly specialized, usually consisting of sedge tussocks, mossy carpets, and a variety of other specialized plants, including some orchid species, as indicated in Table 2.

Peatland Animals

The unique character of the peatlands provides relatively sparse cover for large animals. The limited habitat serves only specialized species, and extreme conditions exclude many others. Animals that spend part or all of the year here form distinctive communities of habitat specialists: their adaptations to these harsh conditions make them less adaptable to other areas. To add to the unique interest of peatland communities, their very newness (estimated at less than 4000 years) may mean that species migration and adaptation are still taking place.

Few large mammal species are specifically associated with forested peatlands.

  • Woodland caribou thrived here until their migration routes to Canadian breeding grounds were cut off, and the small bands that were stranded in Minnesota peatlands died off. Moose and timber wolves also inhabit the edges of the peatlands, where forest cover and browse species are available.
  • Construction of ditches during early statehood have allowed beaver and muskrat to increase, along with their predators, otter and mink.

 

As with large mammals species, fewer small mammal species inhabit peatlands. Many mammals require dry nest sites, protective shelter, upland foods, or a burrowing habitat that the peatlands do not provide. Bog lemmings actually prefer peatland habitat, however, and many species of shrews and voles can also be found. Although animals of the peatlands may be difficult to observe directly, careful observers can find evidence of their activity, such as sedge "haystacks" piled in the sun (bog lemmings), heaps of spruce cone bracts (red squirrel), or pruned alder stems (snowshoe hare).

Migratory bird species bring special interest to the peatlands in spring and summer breeding months. Their preferences for food and cover draw them to bog or fen habitats, as shown in Table 5.3. Great gray owls are permanent residents of northern Minnesota peatlands, where they nest in forests. Winter sightings of great grays are most common in "invasion years." No federal or state endangered or threatened bird species occur in peatlands, though 14 species of special concern utilize habitats of open fen, muskeg, or coniferous forest.

Table 5.3. Preferred peatland habitat of bird species

Vegetation

Birds that prefer fens

Birds that prefer bogs

Trees
shrubby habitat

Alder flycatcher
Swamp sparrow
Common yellow-throated
LeConte's sparrow
Yellow Warbler

Connecticut warbler
Yellow-rumped warbler
Nashville warbler
Palm warbler
Hermit thrush
Yellow-bellied flycatcher
Dark-eyed junco
Chipping sparrow
Lincoln's sparrow

Meadow habitat

Savannah sparrow
Bobolink
Sandhill
Sedge wren

Savannah sparrow

"Cold-blooded" amphibians and reptiles that inhabit peatlands are relatively limited. More frogs and toads have adapted to this environment than turtles, lizards, and snakes, though they also occur here. Species requirements for moisture, pH levels, temperature, and nutrition govern distribution. For example, terrestrial burrowers and aquatic species that require deep water that does not freeze to the bottom find this environment discouraging; species that breed early in spring are limited by the short summer season. Toxicity of bog waters affects the survival rate of creatures using it as a breeding medium.

Insects inhabit the peatlands in abundance, though there remains much to be learned about them. Visitors will find an ample supply of mosquitoes, damsel flies, dragon flies, and deer flies.