Marshes are permanently or periodically covered with nutrient-rich water. Marshes are characterized by emergent vegetation that is adapted to saturated soils and by submerged vegetation that lives at deeper depths. Plants living in marshes are exposed to three environmental stresses: (1) they are frequently covered by water so they must be able to cope with low oxygen content, (2) they are often exposed to the atmosphere so they can be exposed to factors such terrestrial herbivores and fire, and (3) they are sometimes exposed to the effects of wave action or water movement. Thus, these factors have selected for the herbaceous plants with well developed root systems (that provide anchorage and storage). Salt marshes are found in estuarine areas with high (and fluctuating) salt content. Thus, salt marsh plants must have adaptations for dealing with high salt content in the water that surrounds them, a fourth type of stress.
Low soil oxygen content
Wetland soils have been affected by the permanent cover of water. One problem faced by plants living in marshes is the lack of oxygen in the soil. Oxygen is used by plants (and most other organisms) in the process of cellular respiration in which the energy from glucose (produced by photosynthesis) is released so that the organisms can use the energy to do “biological work.” When glucose is broken down in the presence of oxygen, aerobic respiration occurs and the organisms are able to use a great deal of the stored energy in the glucose. In situations where oxygen is lacking, glucose is broken down by the process of anaerobic respiration which does not release as much energy from each molecule of glucose (aerobic respiration releases about 18 times more energy than anaearobic respiration). Not only is anaerobic much less energy efficient than aerobic respiration, but by products of anaerobic respiration, are toxic.
Plants lack a circulatory system, so plants are not able to “pump” oxygen from air (where the oxygen concentration is about 21%) below ground to the roots. Thus, the roots of terrestrial plants rely on oxygen in air spaces between soil particles. Oxygen is hard to get in hydrated soils for two reasons. First, water fills the space between soil particles that are typically filled with air in terrestrial soils. Second, because the rate of diffusion of oxygen is much slower in water than in air the rate of movement from the water surface to the root zone is extremely slow, so most hydric soils are either low in oxygen (hypoxic) or lack oxygen (anoxic) because the oxygen in the water in the soil is used up, often by the activity of decomposers (bacteria).
So how do the roots of marsh plants get the oxygen that they need to survive? Marsh plants have evolved air spaces ("aerenchyma tissue") in their stems that allow oxygen to move from the leaves (where oxygen is produced in the process of photosynthesis) to the roots by either diffusion or in some cases, by pressure differences between the leaves and the roots.
Other marsh plants are able to survive in low oxygen conditions by relying on anaerobic respiration. These species apparently have altered their biochemical pathways to allow them to avoid the build-up of high concentrations of toxic ethanol (a waste product of anaerobic respiration).
Because of the high availability of light, water, and nutrients, marshes are among the most productive ecosystems on earth (i.e., a lot of sugar is produced by the process of photosynthesis). The production of biomass in wetland systems can be over three times higher than in a terrestrial ecosystem. Because biomass production in marsh ecosystems, there is high there is a lot of potential food for herbivores. However, most emergent macrophytes are of low food quality because they have a relatively high content of structural carbon relative to the amount of nitrogen and phosphorus they contain. Thus, a relatively large percentage of the plant biomass is not eaten and when the plants die they become detritus, and enter into detritus-based food webs.
Some marshes become dry enough seasonally or during drought periods that they are potentially burned by fire. The extensive root system of most emergent marsh vegetation allows them to resprout following a fire.
Plants living in salt marshes are exposed to high salinity environments. Plants adapted to living in saline environments are known as halophytes ("salt lovers"). Halophytes have evolved physiological, structural, and biochemical means of dealing with a saline environment. Some halophytes are able to secrete salt through salt glands, whereas other plants have mechanisms for dealing with higher cellular concentrations of salt, or can reduce of the uptake of salt through their roots.