VEGETATION

Another of the physical characteristics used to identify wetlands is the presence of hydrophytic vegetation. The term hydrophyte come from the Greek words hydro, meaning water, and phyton, meaning plant. The term hydrophyte includes all aquatic and wetland plants. However the term is generally used to refer to vascular aquatic and wetland plants. Though hydrophytes represent only a small; percentage of the total plant population, there are far too many to list here.
Upland plants normally have adequate soil oxygen available to the roots for use in the metabolic processes that convert food into energy. When soil saturation or flooding make oxygen unavailable, the metabolic process either stops altogether or shifts to anaerobic glycolysis an enzymatic process that does not require oxygen to convert food into energy. Anaerobic glycosis produces much less energy than normal metabolic processes and causes an accumulation of toxic end products. Using anaerobic glycosis, most plants can produce only enough food to survive for short periods of time. Hydrophytic plants thrive in wetland soils in spite of the limitation or absence of oxygen because they are able to make special physiological adaptations.
Hydrophytic plants vary in the number of adaptations they exhibit, but generally those that exhibit a greater tolerance to saturated soil conditions. Relatively few tree species, such as cypress and water tupelo, are able to make enough of these adaptations to tolerate flooding for more than a few weeks during the growing season. However, there are a number of species that exhibit one or more of these adaptations and thus tolerate varying degrees of soil wetness. Some plants such as green ash form hypertrophied lenticels, enlarged structures on the above ground portion of the plant that permit the exchange of gasses with the atmosphere facilitating the transfer of oxygen from the air to the plant tissue. Green ash and northern white-cedar grow large in diameter, succulent roots at least partially composed of cells with air spaces between them, called aerenchyma, which facilitate movement of oxygen throughout the root tissue. Water hickory, black spruce and basalm fur develop fibrous, lateral root systems which tend to spread horizontally above the water soil levels. Larch, water hickory and water tupelo develop adventitious roots, extra roots on the tree stem, again, above the level of the wetter soil. Bald cypress and water tupelo develop a swelling at the base of the tree which helps resist windthrow and may facilitate the exchange of oxygen.
In some wetland adapted plants such as cordgrass, Spartina, the oxygen supply is large enough to cause oxygen to be diffused out through the roots oxygenating the rhizosphere, or outer surface of the root. Soil iron and manganese deposits in these areas are often oxidized by this method resulting in streaks of rust commonly seen in wetland soils.
Hydrophytic and other plants used to identify wetlands for regulatory purposes are listed in the National List of Plant Species that Occur in Wetlands, (Reed, 1988). This publication lists five indicator categories for which plants can be found in wetlands: Obligate Wetland (OBL, > 99 percent occurrence), Faculative Wetlands (FACW, 67-68 percent occurrence). Faculative (FAC, 37-67 percent occurrence), Faculative Upland (FACU, found in wetlands 1-33 percent of the time), and Obligate Upland (UPL, < 1 percent occurrence in the area in question, but may be wetland plants in other regions). Abundant presence of species in the Obligate and Faculative Wetland indicator categories is a reliable indicator that a given tract of land is functionally a wetland. The absence of these species, however, is not a reliable indicator that an area is not a wetland, as these species may have been locally extirpated by severe disturbance or management.