Wetlands

Wetlands are lands that are significantly influenced by the presence of water. Although relatively rare, these fragile ecosystems, which occupy approximately 6% of Earth's surface, are found on every continent except Antarctica. Wetlands are found in most of the world's climatic zones, ranging from warm tropical climates to frigid arctic climates. They are present across a wide range of elevations around the world, from below sea level to mountaintops.

Wetlands are broadly classified according to topography and water salinity. Topographically, they may be present in either coastal areas or inland. When subdivided according to water salinity, they may range from saline (salinity > 30.0 parts per 1,000) to brackish (salinity 29–0.6 part per 1,000) to fresh (salinity < 0.5 part per 1,000). The salinity of coastal wetlands is influenced by distance from the ocean, tides, topography, and the presence and discharge of various sources of freshwater, such as rivers and groundwater. Examples of coastal wetlands include cord grass (Spartina sp.)–dominated tidal marshes and mangrove (Rhizophora mangle, Avicennia sp., Laguncularia racemosa, Sonneratia sp.) swamps.

The vast majority of interior wetlands are freshwater; however, saltwater interior wetlands may occur in arid regions. Interior wetlands are subdivided according to the environment in which they occur. Major classes of interior wetlands include riverine (river environment), palustrine (pond, pothole environment), and lacustrine (lake environment). Cattails (Typha sp.) and rushes (Juncus sp.) are plants commonly found in interior wetlands.

Regardless of the environment where a wetland is located, all wetlands share three characteristics: (1) wetland hydrology, (2) hydric (i.e., wetland soil), and (3) hydrophytic vegetation. Water within a wetland may be permanent, inundating the area year-round, or it may be seasonal or ephemeral, flooding an area only a few weeks or months of the year. Prolonged flooding, particularly during the growing period, significantly affects the soil and the vegetation of wetlands.

Given time, continued soil saturation dramatically influences the chemical and physical [Page 3093]composition of the soil. The upper portions of soil become anaerobic (i.e., without oxygen). Hydric soils have either an organic or a mineral texture. Organic hydric soils are composed mainly of vegetation with varying degrees of decomposition. Muck and peat are common examples of an organic wetland soil. Mineral hydric soil contains little organic matter. This type of soil develops readily identifiable characteristics, such as gleying and mottling. Gleying is a process that occurs in anaerobic, waterlogged soils where iron present in the soil is chemically reduced to iron oxide because of the lack of oxygen. This results in a soil that has a distinctive blue-gray or greenish color. Mottles may also develop from the reduction of iron. Mottles are small, generally pea-sized or less, reddish-brown or yellowish specks/spots found in dark wetland soil.

Canada geese, common egret, and shore birds in a pothole wetland in South Dakota. Wetlands provide a haven for migrating ducks, geese, and other waterfowl.

Source: Dennis Larson, USDA Natural Resources Conservation Service.

Saturated anaerobic conditions influence the type of vegetation that occurs within wetlands. Wetlands vegetation may develop morphological adaptations, which allow the vegetation to thrive in saturated conditions. Common adaptations include shallow root systems, adventitious roots, pneumatophores, multiple trunks, and buttress trunks. Adventitious roots and pneumatophores are some of the more interesting adaptations. Adventitious roots are roots that may develop from the stem or branches of a plant at points above the soil. Pneumatophores are roots that protrude through soil and surface water, thus allowing plants to obtain oxygen in an otherwise [Page 3094]anaerobic environment. Both mangroves and bald cypress (Taxodium distichum) develop extensive networks of pneumatophores.

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