The CLASS is the highest taxonomic unit below the Subsystem level. It describes the general appearance of the habitat in terms of either the dominant life form of the vegetation or the physiography and composition of the substrate—features that can be recognized without the aid of detailed environmental measurements. Vegetation is used at two different levels in the classification. The basic life form layers, from highest to lowest—trees, shrubs, emergents, emergent mosses or lichens, and surface plants or submergents—are used to define Classes because they are relatively easy to distinguish, do not change distribution rapidly, and have traditionally been used for classification of wetlands and habitat assessment.4 Pioneer plants that colonize wetlands during dry periods, but disappear when surface water returns, are treated at the Subclass level because they are transient and may be mesophytes or xerophytes. Use of life forms at the Class level has two major advantages: (1) extensive biological knowledge is not required to distinguish between various life forms, and (2) many life forms can be readily identified on a variety of remote sensing products (e.g., Radforth 1962; Anderson et al. 1976). 4 The initial attempts to use familiar terms such as marsh, swamp, bog, and meadow at the Class level were unsuccessful primarily because of wide discrepancies in the use of these terms in various regions of the United States. In an effort to resolve that difficulty, we based the Classes on the fundamental components (life form, water regime, substrate type, water chemistry) that give rise to such terms. This approach has greatly reduced the misunderstandings and confusion that result from the use of the familiar terms.


If living vegetation (except pioneer species) covers 30 percent or more of the substrate, we distinguish Classes on the basis of the life form of the plants that constitute the uppermost layer of vegetation and that possess an areal coverage 30 percent or greater. For example, an area with 50 percent areal coverage of trees over a shrub layer with a 60 percent areal coverage would be classified as Forested Wetland; an area with 20 percent areal coverage of trees over the same (60 percent) shrub layer would be classified as Scrub-Shrub Wetland. When trees or shrubs alone cover less than 30 percent of an area but in combination cover 30 percent or more, the wetland is assigned to the Class ScrubShrub. When trees and shrubs cover less than 30 percent of the area but the total cover of vegetation (except pioneer species) is 30 percent or greater, the wetland is assigned to the appropriate Class for the predominant life form below the shrub layer. When the height of two or more plant life forms in an area is equal, and each covers 30 percent or more of the area, the Class is based on the life form that has the greater cover. If the cover of the life forms is equal, then the Class is based on the life form that is more persistent. If the life forms are equally persistent, then the Class is based on the life form that would normally be considered to be more advanced from a successional standpoint (e.g., shrub > emergent plant > emergent moss or lichen).


Finer distinctions in life forms are recognized at the SUBCLASS level. Subclasses are named on the basis of the specific life form with the greatest areal coverage. In ScrubShrub and Forested Wetlands, for example, most Subclasses are distinguished by leaf type (broad-leaved deciduous, needle-leaved deciduous, broad-leaved evergreen, and needle-leaved evergreen). 


When an area is covered more or less uniformly by dead trees or dead shrubs—regardless of their abundance—and living vegetation covers less than 30 percent of that area, the site would be placed in either the Dead Forested Wetland Subclass or the Dead ScrubShrub Wetland Subclass, depending on whether dead trees or dead shrubs predominate. However, if living vegetation covers 30 percent or more of a stand of dead trees or shrubs, then the dominant life form, Class, and Subclass would be based on the living vegetation, using the rules outlined above. 


If living vegetation covers less than 30 percent of the substrate, the physiography and composition of the substrate are the principal characteristics used to distinguish Classes. Substrate particle sizes include boulders, stones, cobbles, gravel, sand, silt, and clay (see Appendix A for definitions). Cowardin et al. (1979) employed these, alone or in combination, along with the term ‘bedrock,’ as Subclasses for nonvegetated wetlands and deepwater habitats. 


The nature of the substrate reflects regional and local variations in geology and the influence of wind, waves, and currents on erosion and deposition of substrate materials. Bottoms, Shores, and Streambeds are separated on the basis of duration of inundation. In the Riverine, Lacustrine, and Palustrine Systems, Bottoms are submerged all or most of the time, whereas Streambeds and Shores are exposed much of the time. In the Marine and Estuarine Systems, Bottoms are Subtidal, whereas Streambeds and Shores are Intertidal. Bottoms, Shores, and Streambeds are further divided at the Class level on the basis of the important characteristic of rock versus unconsolidated substrate. Subclasses are based on finer distinctions in substrate material unless, as with Streambeds and Shores, pioneer plants (often mesophytes or xerophytes) cover 30 percent or more of the substrate; the Subclass then is simply “vegetated.” Further detail as to the type of vegetation must be obtained at the level of Dominance Type. Reefs are a unique Class in which the substrate itself is composed primarily of living and dead animals. Subclasses of Reefs are designated on the basis of the type of organism that formed the reef. 


The DOMINANCE TYPE is the taxonomic category subordinate to Subclass. Dominance Types are determined on the basis of dominant plant species (e.g., Jeglum et al. 1974), dominant sedentary or sessile animal species (e.g., Thorson 1957), or dominant plant and animal species (e.g., Stephenson and Stephenson 1972). A dominant plant species has traditionally meant one that has control over the community (Weaver and Clements 1938), and this plant is also usually the predominant species (Cain and Castro 1959). When the Subclass is based on life form, the Dominance Type is named for the dominant species or combination of species (codominants) in the same layer of vegetation used to determine the Subclass.5 For example, a Needle-leaved Evergreen Forested Wetland with 70 percent areal cover of black spruce (Picea mariana) and 30 percent areal cover of American larch (Larix laricina) would be designated as a Picea mariana Dominance Type. When the relative abundance of codominant species is nearly equal, the Dominance Type consists of a combination of species names. For example, an Emergent Wetland with about equal areal cover of broad-leaf cattail (Typha latifolia) and hardstem bulrush (Scirpus acutus) would be designated a Typha latifolia-Scirpus acutus Dominance Type. 


When the Subclass is based on substrate material, the Dominance Type is named for the predominant plant or sedentary or sessile macroinvertebrate species, without regard for life form. In the Marine and Estuarine Systems, sponges, alcyonarians, mollusks, crustaceans, worms, ascidians, and echinoderms may all be part of the community represented by the Macoma balthica Dominance Type. Sometimes it is necessary to designate two or more codominant species as a Dominance Type. Thorson (1957) recommended guidelines and suggested definitions for establishing community types and dominants on level bottoms.

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