There are six major types of wetlands: swamp, marsh, fen, bog, wet meadow, and shallow water (aquatic). These six types are produced by different combinations of flooding, soil nutrients, and climate. A seventh group, saline wetlands, which includes salt marshes and mangroves, is often treated as a distinct wetland type. Saline wetlands occur mostly along coastlines (see Mangrove Wetlands), but also occasionally in noncoastal areas where evaporation exceeds rainfall, such as in arid western North America, northern Africa, or central Eurasia.
Swamps and marshes have mineral soils with sand, silt, or clay. Swamps are dominated by trees or shrubs (see Swamps), whereas marshes are dominated by herbaceous plants such as cattails and reeds (Figure 1). Such wetlands tend to occur along the margins of rivers (Figure 2) or lakes, and often receive fresh layers of sediment during annual spring flooding. Marshes are among the world's most biologically productive ecosystems. As a
consequence, they are very important for producing wildlife, and for producing human food in the form of shrimp, fish, and waterfowl.
Fens and bogs have organic soils (peat), formed from the accumulation of partially decayed plants. Most peat-lands occur at high latitudes in landscapes that were glaciated during the last ice ages. In fens, the layer of peat is relatively thin, allowing the longer roots of the plants to reach the mineral soil beneath. In bogs, plants are entirely rooted in the peat. As peat becomes deeper (the natural trend as fens become bogs), plants become increasingly dependent upon nutrients dissolved in rainwater, eventually producing an 'ombrotrophic' bog. The large amounts of organic carbon stored in peatlands help reduce global warming.
Wet meadows occur where land is flooded in some seasons and moist in others, such as along the shores of rivers or lakes. Wet meadows often have high plant diversity, including carnivorous plants and orchids. Examples of wet meadows include wet prairies, slacks between sand dunes, and wet pine savannas. Pine savannas may have up to 40 species of plants in a single square meter, and hundreds of species in a hundred hectares.
Aquatic wetlands are covered in water, usually with plants rooted in the sediment but possessing leaves that extend into the atmosphere. Grasses, sedges, and reeds emerge from shallow water, whereas water lilies and pondweeds with floating leaves occur in deeper water. Aquatic wetlands provide important habitat for breeding fish and migratory waterfowl. Animals can create aquatic wetlands: beavers build dams to flood stream valleys, and alligators dig small ponds in marshes or wet meadows.
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