Swamps and marshes

The terms swamp and marsh describe areas on land where the soil is waterlogged because of poor drainage or frequent inundation.

These regions can be divided roughly into three categories: saltwater marsh, acid (or nutrient poor) bog, and alkaline freshwater marsh or swamp. Some of these wetlands are immense, covering thousands of acres, such as the African Sudd and the Florida Everglades. They are of great ecological importance because they form carpets of vegetation that protect the soil from being washed away, and the water retained by them speeds up decomposition, which results in a rich soil (except in the acid areas).

Insectivorous plants, such as the common sundew (Drosera rotund ifolia), obtain the minerals they need from trapped insects, which they digest. This nutrient source is essential for the survival of these plants in ‘ the nutrient-poor acid bogs in which they live. The sticky red hairs attached to the leaves of this plant curve inward to trap insects and then secrete a digestive substance onto them.

Obtaining oxygen

The most constant feature of wetlands is the waterlogged, airless material in which the plants’ roots are lodged. Without air, the roots (and therefore the plants) cannot survive. Consequently marsh plants have become modified in various ways to obtain oxygen.

The most important modification is the enlargement of intercellular air spaces in the tissue of the submerged parts of the plants. Such tissue, called aerenchyma (adapted parenchyma), allows oxygen to pass readily through the plant from the exposed stems and leaves to the submerged parts. It also provides buoyancy in the thick mud.

Some plants in coastal swamps have developed special vertical roots, the tips or loops of which project above the surface of the water and allow air to pass into them to be conducted to the parts underwater. Known as root knees, or pneumatophores, they are found in some species of mangroves, such as Rhizophora spp., Avicennia spp., and swamp cypresses (Taxodium spp.).

Saltwater marshes

Marine marshes are subject to regular flooding by the sea; the plants inhabiting these areas grow in zones corresponding to the frequency of flooding, the depth and salinity of water, and length of submergence. Their positions are determined by the degree to which they can cope with inundation and the salinity of the water. These plants often have to cope with a salt concentration as high as 10 per cent.

Salt-tolerant plants, known as halophytes, have difficulty in obtaining water because, as a result of the high salt concentrations in the surrounding water, any “free” water is not easily removed. There is also the problem of osmotic balance—water from the plant cells may be forced out to balance the salt concentrations inside and outside them. In addition, the large amounts of minerals present in sea water are toxic to most plants. Consequently many halophytes, such as glasswort (Salicornia fruticosa), have thick, leathery, fleshy leaves that store and retain large amounts of water and slow down transpiration.

The primary colonizers of the mud fringes of temperate marine marshes, such as samphire and seablite (Suaeda sp.), are unaffected by frequent inundations and can tolerate high salinity because they have salt glands that excrete excess salt from their shoots. Their roots stabilize the mud and allow the establishment of and replacement by other plants, particularly cord grass (Spartina alterniflora). This plant spreads its underground stems (stolons) thickly and further stabilizes the mud, which is subsequently colonized by other plants. This colonization is known as succession and occurs in bogs and freshwater swamps.

Many of these plants are deciduous, shedding with their leaves excess ions accumulated from the sea water. In some, such as sea lavender (Limonium sp.), the chances of survival of the seeds are increased because they germinate only when exposed to seawater.

A zone equivalent to that in temperate saltwater marshes occurs in tropical coastal swamps dominated by mangroves, the various species each having their preferred niches.

Red mangrove (Rhizophora mangle) establishes seaward, sending its aerial roots down into the sediments. Black mangrove (Avicennia nitida) grows on exposed mudflats, sending its pneumatophores up out of the muds. White mangrove (Concocarpus erecta) and Languncularia racemosa are more common in inland brackish waters of Florida and the Caribbean islands.

Seeds of red mangrove germinate on the shrub, drop into the muds and rapidly take root. Some seeds may float on the water before they are stranded in coastal muds.

The plants on a saltwater marsh grow in zones according to their tolerance of inundation and salinity. On the seaward edges of a temperate marsh grows glasswort and other species of the genus Salicornia. Between the low and higher areas are a number of species including sea lavender. The drier, higher areas of the marsh are occupied by sea purslane and thrift (Ar-meria maritima).

Acid bogs

Small lakes and ponds often develop into bogs as the result of peat accumulation from the shore outward. Succession occurs, the end product of which may be a woodland with scattered trees and shrubs. The soils are acid and nutrient-poor, the result of Sphagnum mosses.

The several species of Sphagnum have different water tolerances. Some are aquatic and thus build a floating thick mat over the water’s edge. Other Sphagnum mosses prefer less wet habitats. These preferences can be accommodated because of the hummock-and-hollow nature of the bog. The hummocks often are invaded by species of heather (Erica spp.), bilberries and cranberries (Vaccinium spp.), Iea-therleaf (Chamedaphnia sp.), Labrador tea (Ledum spp.), and cottongrass (Eriophorum spp.). In addition, the surface of the bog supports other species that are adapted to soils of low nutrient levels such as insectivorous plants. These include sundew (Drosera spp.), butterwort (Pinguicula spp.), and pitcher plants (Sarracenia spp.). The insectivorous species are photosynthetic, but they require nutrients such as potassium, phosphorus, and nitrogen, which they get from insects. Some species flower only after digesting insects with their added nutrients.

Alkaline fens

Fens differ from bogs in having a higher nutrient content from the surrounding landscape and in not being dominated by acid-forming sphagnum mosses and sedges. Some fens or freshwater swamps are dominated by single species such as papyrus (Cyperuspapyrus) or reed (Phragmites spp.) in Africa. Species of rush (Scirpus spp.) and sedge (Carer spp.) dominate fens in northern Canada and Alaska. Scattered shrubs of birch (Betula glandulosa), alder (Alnus crispa) and the tree, larch (Larix laricina) are common in these more nutrient-rich habitats.

Mangrove swamps occur in tropical and subtropical coastal areas and include several genera, such as Rhizophora. In many swamps, this plant grows in the zone immediately behind the most seaward because it cannot tolerate the long periods of inundation that, for example, Sonneratia sp. can. These shrubs grow aerial or looped roots to trap air, which is conducted to the parts underwater. They also grow prop roots that stabilize the mud, which is then colonized by other species.