Wetlands are dynamic ecosystems, transitional between terrestrial and aquatic habitats. Over time, most wetlands gradually in-fill because of the cumulative deposition of sediment and peat. Consequently, wetlands are most numerous in places where geological forces, such as glaciation or the migration of oxbow rivers, periodically create conditions that are favorable to their formation.
The ecological conditions of wetlands are, of course, dominated by the influences of permanent or
temporary waterlogging. Clearly, the availability of water to sustain plant growth is not a problem in wetlands, as it is in many terrestrial ecosystems. However, waterlogged soil or sediment are usually lacking in oxygen, a factor that inhibits respiration by plant roots. To cope with this stressful environmental condition, some plants have evolved specific adaptations to supply oxygen to their roots. Many herbaceous plants, such as cattails and bulrushes, have spongy, air-filled stem and root tissues, called aerenchyma, which helps to facilitate the transport of oxygen to underwater tissues. Some trees, such as bald cypress and black mangrove (Avicennia nitida), have specialized woody structures called pneumatophores, that extend from roots into the air, and have extensive intercellular spaces that are useful in supplying oxygen to below-water tissues.
The anaerobic nature of wetland substrates also causes other chemical changes that can pose important problems for plants, by affecting their nutrition and exposing roots to toxic chemicals. For example, access to certain nutrients can be difficult under anaerobic conditions. This is because the nutrients may not be present in a chemical form that is easy for roots to assimilate, or because roots cannot sustain the oxygen-demanding respiratory demands required for the active uptake of nutrient ions. Anaerobic conditions also encourage the solubilization of certain potentially toxic metals, such as manganese. In addition, anaerobic metabolism within root tissues can lead to excessive accumulations of alcohols, possibly causing toxicity. In general, wetland plants are well-adapted to these conditions, although they may nevertheless be physiologically stressed if these factors are severe enough.
Wetland hydrology is, of course, a highly variable character. Some wetlands are permanently flooded, while others are only waterlogged some of the time, usually seasonally. These dynamics are highly influential on the types of plants that can occur in particular wetlands, and on the communities that they develop. Tolerance of permanent or frequent flooding, as occurs, for example, in salt marshes, mangroves, and some swamps, requires highly adapted species of plants. In comparison, wetlands that are only occasionally flooded are, in some respects, a more ephemeral transitional between truly aquatic and terrestrial environments. In these situations, plants must only be tolerant of the stresses of sporadic events of flooding, while growing relatively freely when the water recedes and the soil is drier.
Another highly influential environmental factor in wetlands is the supply of plant nutrients. In general, wetlands that are well supplied with phosphorus (in the form of phosphate), and to a lesser degree nitrogen (as nitrate or ammonium), sustain relatively high rates of plant productivity, and consequently large populations of animals. This is commonly the case for marshes, which are among the most productive natural ecosystems on Earth. In contrast, wetlands with restricted supplies of nutrients, such as ombrotrophic bogs, sustain only small productivities of plants and animals.