without a tree or shrub component at maturity. 



This class includes emergent and floating aquatic 



herbs. The tree or shrub canopy cover, if present, 



is 25 percent or less at maturity. 

 Subclasses — There are currently 19 subclasses. Dis- 

 crimination between tree and shrub communities at this 

 level are based on morphologic characters, such as ever- 

 green and deciduous habit, or on adaptations to tempera- 

 ture and water. The separations for herbaceous vegetation 

 are made on the basis of morphology and relative height 

 of graminoid (grasses and grasslike) or forb vegetation. 

 (Tall graminoid or forb vegetation is more than 3 ft (1 m) 

 tall; medium is from 1.6 to 3 ft (0.5 m to 1 m) tall; and 

 short is less than 1.6 ft (0.5 m) tall.) 



Groups — There are currently 65 groups. The separa- 

 tions are based on the following criteria: forests are sub- 

 divided by generalized climatic regimes (for example, 

 tropical, temperate, subpolar) and morphologic features; 

 woodlands are subdivided mainly by leaf morphology; 

 shrublands are subdivided by leaf morphology and cli- 

 matic factors; dwarf-shrublands are subdivided by cover, 

 associated life forms, ecological features, and other char- 

 acteristics; and the grasslands are subdivided by plant 

 height and associated life-form layers. 



Formations — There are currently 150 formations 

 defined in the United States. Criteria for separating plant 

 communities at the formation level include tree crown 

 shape, kinds of associated vegetation, amount and kind 

 of understory vegetation, and a variety of ecological and 

 environmental features. All formations of the United 

 States have not been identified or described. The listing 

 will be completed as additional research and experience 

 defines them. 



Series— The UNESCO system does not include this 

 level of classification but provides for further subdivi- 

 sions below the formation level to account for more 

 definitive classes. The series is the first of these subdivi- 

 sions and includes specificity of physiognomy and struc- 

 ture of the vegetation. Procedures for recognizing the 

 series level for forest vegetation are illustrated by Pfister 

 and Arno (1980) and for shrublands and grasslands by 

 Mueggler and Stewart (1980). Series are usually charac- 

 terized by individual dominant plant species of the com- 

 munities. In some instances, where dominance may be 

 uncertain, characterization is based on codominance. 

 The vegetation of a large portion of the United States, 

 especially in the West, has been classified to the series 

 level. Examples include ponderosa pine (Pinus ponderosa), 

 big sagebrush {Artemisia tridentata), loblolly pine (Pinus 

 taeda), and bluebunch wheatgrass (Agropyron spicatum). 



Associations — Associations, subdivisions of series, 

 are plant communities of definite floristic composition, 

 representing a uniform appearance and occurring within 

 uniform habitat conditions (Daubenmire 1968). They fre- 

 quently occur in discontinuous patterns throughout the 

 landscape, depending on local relief and other abiotic 

 features. Associations are groupings of plants that have 

 attained dynamic equilibrium with local existing environ- 

 mental conditions. Herbaceous, shrubby, and tree species 

 contribute to the identification, definitions, and descrip- 

 tion of associations. Associations are conventionally 

 named for conspicuous dominant species in different veg- 

 etation layers. Codominance can be explicitly recognized 

 by including more species in the association name. A 

 diagnostic indicator species may be used in the associa- 

 tion name to provide a clearer link between the associa- 

 tion characteristic and the name. 



The first name in an association is the apparent dom- 

 inant or codominant plant species in the tallest life form. 6 

 The second name in the association may be a dominant 

 species in a different layer, or diagnostic nondominant 

 species. A third species name may be applied when three 

 vegetation layers conspicuously contribute to the struc- 

 ture of the community, and it is needed for unambiguous 

 identification. For example, if a plant community includes 

 ponderosa pine (Pinus ponderosa) as the dominant spe- 

 cies in the highest life-form layer, antelope bitterbrush 

 (Purshia tridentata) as the dominant species in the next 

 layer, and Idaho fescue (Fescue idahoensis) as the domi- 

 nant diagnostic species in the lowest layer, the potential 

 natural association would be named Pinus ponderosa/ 

 Purshia tridentata/ Festuca idahoensis. Examples of the 

 nomenclature for all levels of the vegetation element 

 expressed as potential natural classes are shown in table 1 . 



For a developed plant community taxonomy at the 

 series and association level, identification keys often help 

 identify the communities (Mueggler and Stewart 1980, 

 Steele and others 1981). Where vegetation has been dis- 

 turbed by logging, grazing, or similar events, potential 

 natural communities can be inferred by comparing the 

 areas to similar sites where soil-vegetation correlations 

 have been developed with potential natural communities 

 (Shiflet 1973). Working approximations of potential nat- 

 ural vegetation to permit use of the classification at some 

 levels can also be developed by using existing literature 



'Species in a tallest life form with 25 percent or less canopy cover 

 are not used for naming purposes in this classification. The tallest spe- 

 cies in a plant community not contributing to diagnostic characteristics 

 for identification of major classes should not be used in naming the 

 plant communities. 



