One of the most important characteristics of any biome is its plant life or lack of plant life. More than 5,000 species of plants live in or near wetlands. Wetlands have high biological productivity (the rate at which life forms grow in a certain period of time). The higher its plant productivity, the more animal life a wetland can support. The kinds of plants that may be found in a wetland are determined by several factors, especially the type of soil and the quantity of water.
Some plants, called hydrophytes, grow only in water or extremely wet soil. Sedges are an example. Mesophytes, such as reeds, need moist but not saturated soil. When a wetland dries up, the area fills with plants called xeropytes. These are plants adapted to life in dry habitats and can survive where other wetland plants would wilt. They include water hemp, marsh purslane, and fog fruit.
A submergent plant grows beneath the water and is found in deep marshes and ponds. Even its leaves are below the surface. Submergents include milfoil, pondweed, and bladderwort, an insect-eating plant.
Found in deep marshes, floating aquatics float on the water's surface. Some, such as duckweed have free-floating roots. Others, such as water lilies, water lettuce, and water hyacinths have leaves that float on the surface, stems that are underwater, and roots that are anchored to the bottom.
An emergent plant grows partly in and partly out of the water. The roots are usually under water, but the stems and leaves are at least partially exposed to air. They have narrow, broad leaves, and some even produce flowers. Emergents include reeds, rushes, grasses, cattails, and water plantains.
It is generally recognized that algae (AL-jee), fungi (FUHN-jee), and lichens (LY-kens) do not fit neatly into the plant category. In this chapter, however, we will discuss these special organisms as if they, too, were plants.
Algae: Most algae are one-celled organisms too small to be seen by the naked eye. They make their food by a process called photosynthesis. (Photosynthesis is the process by which plants use the energy from sunlight to change water and carbon dioxide from the air into the sugars and starches they require.) Some wetland algae drift on the surface of the water, forming a kind of scum. Others attach themselves to weeds or stones. Some can grow on the shells of turtles or inside plants or animals. Microscopic algae that can be found in saltwater marshes include diatoms and green flagellates (FLAJ-uh-lates). Desmids are a type of green algae found in bogs.
Fungi: Fungi are plantlike organisms that cannot make their own food by means of photosynthesis; instead, they grow on decaying organic matter or live as parasites (organisms that depend upon another organism for food or other needs).
Fungi grow best in a damp environment, which makes wetlands a favorable home. Common wetland fungi include mushrooms, rusts, and puffballs.
Lichens: Lichens are combinations of algae and fungi. The alga produces the food for both by means of photosynthesis, while it is believed the fungus absorbs moisture from the air and provides shade. One of the most common wetland lichens is called reindeer moss, an important food source for northern animals such as caribou.
Most green plants need several basic things to grow: light, air, water, warmth, and nutrients. In a wetland, light and water are in plentiful supply. Nutrients — primarily nitrogen, phosphorus, and potassium — are obtained from the soil. Some wetland soils, however, are lower in these nutrients. They may also be low in oxygen. As a result, many wetland plants have special tissues with air pockets that help them to "breathe."
Trees that grow in swamps, such as the mangrove and bald cypress, have shallow roots. Because of the lack of oxygen in the soil, the roots remain near the surface. Other trees found in mild climates include willows and alders. Palms are found in warm climates. Wetland plants are also classified as submergents, floating aquatics, or emergents, according to their relationship with water.
Growing season: Climate and precipitation affect the length of the growing season in a wetland. Warmer temperatures and moisture usually signify the beginning of growth. In regions that are colder or receive little rainfall, the growing season is short. Growing conditions are also affected by the amount of moisture in the soil, which ranges from saturated to dry. Although many wetland plants die during dry periods, their seeds remain in the soil all winter and sprout in the spring.
Reproduction: Green wetland plants reproduce by several methods. One is pollination, in which the pollen from a plant's male reproductive part, called the stamen, is carried by wind or insects to the plant's female reproductive part, called the pistil. Water lilies, for example, which are closed in the morning and evening open during midday when the weather is warmer and insects are more active. It is usually at this time that the insects pick up the pollen and transfer it from the stamen to the pistil, allowing pollination to occur.
Some wetland plants send out rhizomes, which are stems that spread out under water or soil and form new plants. Reed mace and common reeds are examples.
Common wetland green plants include mangrove trees; insectivorous plants; and reeds, rushes, and sedges.
Mangrove Trees: Trees that live in wetlands must tolerate having their roots wet for a long period of time. One of the best examples of this is the mangrove tree, which grows in tropical and subtropical saltwater swamps. Mangroves have done so well at adapting to wetland conditions that there are more than 34,000,000 acres (14,000,000 hectares) of them in the world. One of the largest mangrove forests is the Sundarban Forest in Bangladesh. Mangroves can also be found in southern Florida and other tropical areas.
Mangroves have special membranes that stop or reduce the entry of salt into their systems. Because they stand in waterlogged soil, mangroves get their oxygen from pores in their roots, the largest portions of which are above ground, and from small gaps in their bark.
Mangrove flowers are pollinated by the wind, and their seedlings begin to grow while they are still attached to the parent tree. Seedlings germinate (grow) within the fruit that contains them. The roots of the maturing seedling break through the outer wall of the fruit, appearing as many tendrils. When the seedling reaches a mature enough stage, it is released from the parent plant, floats on the water, and then settles in mud, where it establishes roots and begins to grow.
Insectivorous Plants: Two common peatland plants eat insects. The sundew eats ants by catching them in a sticky liquid on its leaves. It then releases chemicals that break down the ant's body so the plant can absorb the nutrients. The pitcher plant gets its name from the shape of its leaf, which looks like a pitcher. If an insect crawls down the leaf it cannot crawl back up because the leaves are too slippery. It is then digested by chemicals in the plant.
Reeds, Rushes, and Sedges: A reed is a type of grass with tall, feathery flowers; round, hollow stems; and long, flat, narrow straplike leaves. Although the rush is grasslike, it is not a true grass. It has round, solid stems and narrow, rigid leaves. Sedges are also grasslike but are not true grasses. They have solid, triangular stems.
Reed mace and common reeds are two plants that play a key role in wetland succession. They reproduce widely through their rhizomes. As the plants spread out, they push aside other plants and quickly choke up a pond, slowing down the water's movement and trapping dead plant matter. As this matter accumulates, the pond fills up and becomes a marsh.
Twenty-six percent of the endangered plants in North America are dependent upon wetlands for their survival. Environmentalists predict that about 400 American plants will be extinct in the near future. At the same time, endangered plants will number almost 700. (When no living members survive, a species is extinct. A species that is threatened with the loss of many members and may one day become extinct is endangered.)
Changes in the habitat, such as succession, pollution, and new weather patterns, endanger wetland plants. They are also endangered by people who collect them. Rare species of orchids are an example, as is the Venus' flytrap which has become a popular houseplant and has been overharvested. Peat moss is also overharvested because it is popular as a fuel, as a packaging material, and in gardens.
Source: "Wetland." U·X·L Encyclopedia of Biomes, Vol. 3. U·X·L, Detroit: 2000.