Difference Between Marshes And Swamps
The main type of plant life in marshes—which are also called emergent wetlands—are soft-stemmed, grasslike (herbaceous) plants; trees and shrubs are generally rare. Marshes can develop in relatively low-lying areas such as shallow depressions in the land, and in channels of rivers, floodplains, deltas, and the shores of lakes. In these areas, the marshes are often fed by streams or other sources of water that bring in both nutrients and silt (a type of very fine soil), which helps to make marshes very fertile. Indeed, marshes produce several tons of dead plant and animal matter that become available to the food chain every year. (Bacteria and fungi break down this material.)
Plants that grow in marshes have evolved various adaptations to help them thrive. Cattails, rushes, arrowheads, pickerelweed, and bur-reed grow with their stems and leaves partially underwater. Other plants, such as water lilies, float on the surface even though their roots grow in the bottom of the marsh. Some plants, including the pondweeds and waterweeds, grow completely underwater.
The silt that supplies nutrients to a marsh can also help fill it in so that, ultimately, trees and shrubs invade the area, and the marsh becomes a swamp. If the water table is high enough, however, the marsh will never have the opportunity to begin the process of evolving into a swamp.
Saltwater marshes (also called salt or tidal marshes) are found only in temperate coastal areas. Such marshes often form on the beaches of barrier islands (islands that act as a buffer between the ocean and the mainland), always on the side away from the ocean.
The inner area of a salt marsh is typically flooded; spartina, or cordgrass, is usually the most abundant plant there. As a general rule, the diversity of plant life in a salt marsh increases with distance from the ocean. Similarly, the proportion of freshwater in the marsh increases with distance from the sea. Brackish tidal marshes, for instance, are associated with rivers and are found upstream from a salt marsh. The salinity, or saltiness, of these marshes changes with the tides.
In contrast to the herbaceous plants of a marsh, the main type of plants in a swamp are woody—usually trees and shrubs. Like marshes, swamps form in very wet areas; in some cases, swamps have a stream running through them. The soil is typically shallow and composed of organic silt and the mucklike layer of dead plants at the bottom.
Many different kinds of plants flourish in swamps. Near rivers and lakes, for instance, hardwood trees such as alder, willow, red maple, and white and black ash can be found. The bald cypress is found in the cypress swamps of the southeastern United States, and mangrove trees grow both in the southern United States and in other parts of the world. To help it survive in the swamp, the cypress tree grows what are known as “knees”—the more or less cone-shaped parts of its roots that grow near the base of the tree and extend above the surface of the water. Botanists do not understand the exact function of these knees, although they do know that cypress knees grow only when the tree is in a swamp, and not on dry land. The cypress tree itself is frequently home to orchids and bromeliads, which grow on the bark.
The mangrove tree also uses a special adaptation, called air roots, to obtain the oxygen it needs to grow. The mangrove first produces roots that grow horizontally, into the mud of the swamp. Air roots that are about 10 to 15 inches (25 to 38 centimeters) long then branch off from the regular roots, growing up into the air. The mangrove also uses a root known as a prop root for support when it grows on the banks of tidal rivers or along the coasts of oceans.
Swamp plants tend to grow in layers, with “ground cover” such as skunk cabbage, purple-fringed orchid, cardinal flower, jewelweed, and marsh marigold covering the lowest level. Next come shrubs—highbush blueberry, swamp azalea, spicebush, and sweet pepperbush, among others. Trees constitute the top of the system.