Hydrophytic Vegetation

Living things can often be used as environmental indicators, due to their function as integrators of environmental conditions. That is, they respond to conditions over a period time rather than just reflecting current conditions. That is why wetland vegetation is an important clue to identifying a wetland. Wetlands are not always wet, and wetland soil is not always readily identifiable as a hydric soil.

Hydrophytic vegetation are macrophytic plants that grow in areas where soil saturation or inundation occurs with a frequency and duration sufficient to exert a controlling influence on the plant species present. They are distinguished from upland plants by having one or more adaptations to wetlands conditions. The adaptations have been classified as morphologic, physiologic, or reproductive.

Morphologic adaptations are changes in structure (see Table 15.17). Most are responses to anaerobic soil conditions. One of the most important of these is the formation

TABLE 15.17 Morphologic Adaptations to Wetlands Conditions

Buttressed tree trunks

Pneumatophores

Adventitious roots

Shallow root systems

Inflated leaves, stems, or roots

Polymorphic leaves

Floating leaves

Floating stems

Hypertrophied lenticels

Multitrunks or stooling

Oxygen pathway to roots

Tree species (e.g., Taxodium distichum) may develop enlarged trunks in response to frequent inundation. This adaptation is a strong indicator of hydrophytic vegetation in nontropical forested areas.

These modified roots may serve as respiratory organs in species subjected to frequent inundation or soil saturation. Cypress knees are a classic example, but other species (e.g., Nyssa aquatica, Rhizophora mangle) may also develop pneumatophores.

Sometimes referred to as ''water roots,'' adventitious roots occur on plant stems in positions where roots normally are not found. Small fibrous roots protruding from the base of trees (e.g., Salix nigra) or roots on stems of herbaceous plants and tree seedlings in positions immediately above the soil surface (e.g., Ludwigia spp.) occur in response to inundation or soil saturation. These usually develop during periods of sufficiently prolonged soil saturation to destroy most of the root system. (Caution: Not all adventitious roots develop as a result of inundation or soil saturation. For example, aerial roots on woody vines are not normally produced as a response to inundation or soil saturation.)

When soils are inundated or saturated for long periods during the growing season, anaerobic conditions develop in the zone of root growth. Most species with deep root systems cannot survive in such conditions. Most species capable of growth during periods when soils are oxygenated only near the surface have shallow root systems. In forested wetlands, wind-thrown trees are often indicative of shallow root systems.

Many hydrophytic species, particularly herbs (e.g., Limnobium spongia, Ludwigia spp.) have or develop spongy (aerenchymous tissues in leaves, stems, and/or roots that provide buoyancy or support and serve as a reservoir or passageway for oxygen needed for metabolic processes.

Some herbaceous species produce different types of leaves, depending on the water level at the time of leaf formation: for example, Alisma spp. Produce strap-shaped leaves when totally submerged, but produce broader, floating leaves when plants are emergent. (Caution: Many upland species also produce polymorphic leaves.)

Some species (e.g., Nymphaea spp.) produce leaves uniquely adapted for floating on a water surface. These leaves have stomata primarily on the upper surface and a thick waxy cuticle that restricts water penetration. The presence of species with floating leaves is strongly indicative of hydrophytic vegetation.

A number of species (e.g., Alternathera philoxeroides) produce matted stems that have large internal airspaces when occurring in inundated areas. Such species root in shallow water and grow across the water surface into deeper areas. Species with floating stems often produce adventitious roots at leaf nodes.

Some plant species (e.g., Gleditsia aquatica) produce enlarged lenticels on the stem in response to prolonged inundation or soil saturation. These are thought to increase oxygen uptake through the stem during such periods.

Some woody hydrophytes characteristically produce several trunks of different ages or produce new stems arising from the base of a senescing individual (e.g., Forestiera acuminata, Nyssa ogechee) in response to inundation.

Some species (e.g., Spartina alterniflora) have a specialized cellular arrangement that facilitates diffusion of gaseous oxygen from leaves and stems to the root system.

Source: U.S. Army Corps of Engineers (1987).

of aerenchyma, which are airspaces within stem, leaf, and root tissues that provide a dif-fusional route of oxygen transfer from the surface to the roots. They are formed by cell separation during maturation or by breakdown of existing cells. The porosity of wetlands plants can be up to 60% vs. 2 to 7% in normal plants. A plant will increase its porosity in response to flooding.

Some trees produce pneumatophores, or air roots, which protrude above the ground some distance from the trunk of the tree and are thought to promote gas exchange with the atmosphere. An example are the ''cypress knees'' found in southern U.S. swamps. Adventitious roots are those that come from unusual locations, such as leaf nodes or in a circle around the base of the tree. The red mangrove (Rhisophora spp.) form arched adventitious prop roots. Adventitious roots and pneumatophores may have small pores called lenticels that provide a pathway for oxygen to reach the roots.

Some of the oxygen that plants transport to their roots affects the soil surrounding them. In a process called rhizosphere oxygenation, plants create a microaerobic environment around their roots in an otherwise anaerobic soil. Spartina alterniflora forms brown deposits around its roots because of precipitation of iron and manganese when they are oxidized. In the vicinity of mangrove prop roots or pneumatophores, the redox potential was higher and the sulfide concentration was three to five times lower than in mud deposits at a greater distance. Thus, the role of oxygen transport to the roots seems not only to provide metabolic oxygen but also to protect the roots from toxic minerals. Examples of plants with morphological adaptations are given in Table 15.18.

Physiological adaptations to anaerobic soil conditions are not readily identifiable in the field. However, they further illustrate the differences between wetlands and other plants (Table 15.19). Reproductive adaptations increase the chance of a plant becoming established in a wetlands area (see Table 15.20).

Salt marsh plants also need adaptations to survive in salt water. Exposure to salt water can be harmful both because of toxicity from salts such as sodium and because osmotic forces can cause lethal water loss. One protective mechanism is for their cell membranes to form a selectively permeable barrier, similar to an ultrafilter. The sap of some plants can have salt contents on the order of 3% of seawater salinity. However, no cell membrane is perfectly selective. Thus, the cells also selectively excrete harmful salts, especially sodium. Many salt marsh grasses, such as Spartina, actually form salt crystals on their leaf surfaces from these excretions. The cell prevents water loss by maintaining enough internal solutes in the form of potassium and organics. Although it has not been documented, it is thought that tidal freshwater marshes are even more productive than tidal salt marshes, because they receive the same nutrient and energy subsidies but do not have the salt stresses to deal with.

Salt marsh plants are more likely to use the C4 photosynthesis pathway than upland plants. Recall that C4 plants use CO2 more efficiently than the more common C3 plants. This allows them to keep their stomates closed more, with the result that they lose less water by evapotranspiration. The C4 adaption helps plants survive arid conditions. The water potential of saline water can be as low as the soil in dry climates; despite the large amount of water present, it can be just as unavailable as in a desert. The C4 mechanism uses malate to store CO2. Thus, malate is involved in both respiration and photosynthesis in wetland plants, and its use is another indicator of hydrophytic vegetation. Examples of C4 wetlands plants are Spartina alterniflora, S. townsendii, S. foliosa, Cyperus rotundus, Echinochloa crusgalli, Panicum dichotomifloru, P. virgatum, Paspalum distichum, Phragmites communis, and Sporobolus cryptandrus.

TABLE 15.18 Partial List of Species with Known Morphological Adaptations for Wetlands Conditions

Species

Common Name

Adaptation

Acer negundo

Box elder

Adventitious roots

Acer rubrum

Red maple

Hypertrophied lenticels

Acer saccharinum

Silver maple

Hypertrophied lenticels; adventitious roots

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