Wetland Words and What They Mean: Vegetation
In this article we will tackle the last parameter used in the Federal Delineation Manual ... vegetation.
To review, in order for an area to be identified as a wetland, water must be present in the soil frequently enough and for long enough duration to cause the soil to become anaerobic in the upper part. All of the easily drained pore spaces are filled with water and there are no pockets of oxygen for the plant's roots to tap into for growth. As a result, many species of plants die under these anaerobic conditions. Imagine trying to grow African violets in a pot of soil that is saturated continuously for two weeks during the year. Some plants however, have developed special adaptations to live in these harsh growing conditions. These plants are called hydrophytes, which in English means "water plants."
Adaptations that the plants make to live in these adverse conditions can take many forms, but are generally grouped into morphological, physiological and reproductive adaptations. Morphological adaptations are changes in the structure or form of the plant which aid them in growing in their particular environment. In wetlands, hydrophytes, especially trees, may have buttressed bases which help provide additional stability in soft wetland soils. They may have adventitious roots, which are multiple root stems growing down from the main trunk. Mangroves trees are an example of a plant with this adaptation. Other plants have aerenchymous tissue, which is spongy hollow tissue often found in the stems which increases the plant's buoyancy, and the number of air spaces in the plant. This type of tissue may help the leaves of the plant to float, and may store air for the plant when it is under water. Wetland plants with floating leaves also often have a waxy surface to protect the leaf from constant contact with the water. Many of the emergent and floating aquatic plants, such as water lilies, have this feature. Shallow root systems are a morphological adaptation to provide additional stability to the plant growing in wetland soils. Finally, some plants have developed specialized cells to enhance the movement of oxygen to the roots from the stems of the plants.
Physiological adaptations are methods which plants use to change the metabolic pathways in which they process energy. Remember the Krebs cycle in high school biology class? Accessing the deep recesses of your brain? Do you remember that there were alternative pathways through the Krebs cycle for anaerobic activity? To refresh your memory, these anaerobic pathways are not as energy efficient as aerobic pathways, and often cause the accumulation of chemicals which may be toxic to biological processes. For those athletes out there, it is kind of like when lactic acid builds-up in muscle tissue during anaerobic exercise. Many wetland plants are thought to possess alternative methods to handle the energy stresses and the build-up of these chemicals from anaerobic respiration. For example, some plants can store the accumulated chemicals in a non-toxic form in their roots until a dry spell when the chemicals can be released. Another adaptation is the ability to lower the rate in which metabolic activity takes place under stressful conditions. The larch (Larix laricina) is thought to possess this ability. Many plants can transfer oxygen from the roots into the pore spaces of the soil surrounding the roots, to minimize root degradation and to maintain nutrient uptake under anaerobic soil conditions. Cattails (Typha spp.) can maintain root growth under very low oxygen levels in the soil, a condition which would end root growth in many other plants.
Reproductive adaptations include prolonged seed viability in wet conditions, and the ability of the seed to be triggered to grow in dry conditions. Many wetland plants have seeds which can germinate under low oxygen soil conditions, and have seedlings which can survive low oxygen conditions during their early development.
Similar to the process used to develop the National Hydric Soils List, a group of wetland botanists and ecologists representing the four federal agencies involved in wetland regulations were assembled to assess the probability of the common plants in the United States to grow in wetland conditions. The scientists rated each plant's likelihood of occurring in wetlands, and listed these plants in the "National List of Plants that Occur in Wetlands," The list was further refined through the development of wetland plant lists specific to various regions of the country. New York State is in Region 1.
Indicator categories include Obligate Wetland (OBL) (occurs almost always in wetlands (>99% of the time)); Facultative Wetland (FACW) (usually occurs in wetlands (67% - 99% of the time) but may occur in uplands), Facultative (FAC) (equally likely to occur in wetlands or non-wetlands (34% to 66% in wetlands)); Facultative (FACU) (usually occurs in non-wetlands (67% to 99%) but may occur in wetlands (1% to 33%)); and Obligate Uplands (UPL) (occurs almost always in non-wetlands (99% of time) in region listed). A + or - sign may also be included on the indicator to further refine the probability of occurrence, with a + sign indicating a higher probability of being found in wetlands.
A common misperception about this rating system is that the wetland indicator status of a plant serves to establish a "wetland plant gradient" or describes how wet an area a plant will grow in. This is not true and not the intention of this list. For example, a silver maple (Acer saccharinum, FACW) will grow in an upland yard very well.
In order for the "vegetation parameter" to be met for the purposes of delineating wetlands, the wetland plant community present in that location must be dominated by hydrophytes. To make this assessment, a wetland scientist will examine the whole plant community, and assess which plants are dominant in each layer. This includes the tree layer, the shrub layer, the herbaceous layer and any vines layer. Trees are defined as woody plants greater than 3 inches in diameter at breast height. In a typical wetland delineation, a visual estimate is made to determine which plants dominate the community in each layer. The dominant plants are recorded, and rated according to their probability of occurrence in wetlands. When the delineation needs to have more rigorous documentation, a comprehensive delineation is conducted, which includes physical measurements of the plant coverage for each layer, and a more complex rating system.
Regardless of the method used, the area must be dominated by hydrophytes (plants rated as FAC, FACW or OBL) in order for that location to meet the vegetation parameter under the federal delineation method. For the visual assessment, this means that more than 50% of the plant species recorded must be FAC, FACW or OBL. The comprehensive system often uses an "importance value" system which multiplies the percent dominance of each plant by a numeric rating for each indicator status, and then divides by the number of plant species to obtain an average indicator status rating for the plant community.
Environmental Laboratory. 1987. Corps of Engineers Wetlands Delineation Manual, Technical Report Y-87-1, US Army Engineer Waterways Experiment Station, Vicksburg, Miss.
Reed, P.B., Jr. 1988. National list of plant species that occur in wetlands: Northeast (Region 1). U.S. Fish Wildl. Serv. Biol. Rep. 88 (26.1). 111 pp.