represent a homogeneous ecological unit. These combined- 

 attribute units are termed "ecological units." An ecolog- 

 ical unit is a specific class by which individual units of 

 land are described and identified by physical and biolog- 

 ical features. These classes are composed of two or more 

 elements. Because they share common environmental 

 features, all representatives of an ecological unit should 

 behave in a predictable manner to both natural changes 

 and resource management practices. 



An ecological unit is not necessarily a contiguous 

 unit of land that can be mapped. An ecological unit may 

 occur in scattered pieces throughout the landscape. Wher- 

 ever areas belonging to the same ecological unit occur, 

 environmental conditions are assumed to be equivalent. 

 Spatial association of ecological units permits integration 

 of them by mapping. Describing land on the basis of indi- 

 vidual elements, then combining the elements, if desired, 

 allows for more complete land and resource analysis. 



The classification framework reported here is based 

 on earlier efforts. ECOCLASS' outlined a methodology 

 for classifying ecosystems using a component mapping 

 approach. ECOCLASS identified three components: 

 potential vegetation; land system (Wertz and Arnold 

 1972); and aquatic system. In ECOCLASS, habitat type- 

 land area characterized by a single climax plant associa- 

 tion (Daubenmire 1952) — was included as a unit of the 

 vegetation system. The ECOCLASS land systems included 

 landform, lithology, relief, climate, soil, and vegetation 

 as criteria for a single integrated hierarchy of land units. 



Modified ECOCLASS 2 replaced habitat type with 

 plant association in the vegetation system. The land sys- 

 tem was separated into two components — landform and 

 soil. Neither ECOCLASS nor Modified ECOCLASS 

 clearly distinguished between taxonomic and mapping 

 concepts of classification. 



Driscoll and others 3 recommended a national land 

 classification system for renewable resource assessments 



'ECOCLASS Task Force. ECOCLASS— A method for classifying 

 ecosystems. Ogden, UT: U.S. Department of Agriculture, Forest Ser- 

 vice, Intermountain Forest and Range Experiment Station; [Mimeo]. 

 1973. 52 p. 



'Modified ECOCLASS Committee. Modified ECOCLASS— a 

 method for classifying ecosystems. U.S. Department of Agriculture, 

 Forest Service, Region 2, Region 3, and Rocky Mountain Forest and 

 Range Experiment Station, ad hoc committee; [Mimeo]. 1977. 87 p. 



'Driscoll, Richard S.; Russell, John W.; Meier, Marvin C. Recom- 

 mended national land classification system for renewable resource 

 assessments. Unpublished report on file at U.S. Department of Agricul- 

 ture, Forest Service, Rocky Mountain Forest and Range Experiment 

 Station, Fort Collins, Colo. 1978. 44 p. 



that incorporated many concepts of Modified ECO- 

 CLASS. This classification was designed to assist in 

 describing the natural vegetation of the forest and range- 

 land base of the United States. 



Cowardin and others (1979) developed a new classi- 

 fication system for wetlands and deep water habitats. The 

 system is an integrated five-level hierarchy that incorpo- 

 rates soil, vegetation, and hydrology. The most general 

 level, system, includes five classes — marine, estuarine, 

 riverine, lacustrine, and palustrine. Subsystems are based 

 largely on depth and duration of submergence by water. 

 Classes and subclasses are based on substrate or general 

 life form of dominant vegetation. The lowest level, domi- 

 nance type, is based on dominant species of plants or ani- 

 mals. Regional differences important to wetland ecology 

 are described through a regionalization that combines a 

 system developed by Bailey (1980a) with the five classes. 



Bailey's (1980a) classification system is a nine-level 

 integrated hierarchy based on climate, vegetation, land- 

 surface form, and soil. The upper levels are based on 

 macroclimatic regimes. The midlevels integrate broad soil 

 classes, broad vegetation types, land-surface features, 

 and expressions of climatic-geomorphic processes. The 

 lower levels deal with soil series and plant associations. 



The UNESCO vegetation classification (UNESCO 

 1973) is an open-ended physiognomic (life form and ap- 

 pearance) ecological system. The five highest units are 

 closed forest, woodland, scrub, dwarf-scrub and related 

 communities, and herbaceous vegetation. These units are 

 based on outward appearance of the stand and plant 

 height characteristics. The midlevels are based on descrip- 

 tors such as evergreen-deciduous, needle-leaved, broad- 

 leaved, and climatic tolerances. The lower levels include 

 general kinds of associated vegetation. 



The classification framework described here builds 

 on the concepts of these and other efforts (Daubenmire 

 1978, Kuchler 1967, Mueller-Dombois and Ellenberg 

 1974). It is based on research, associated literature, and 

 experience with both integrated and element classifica- 

 tions. The overall framework is a family of systems de- 

 signed to provide the flexibility required to assist in mak- 

 ing resource management decisions. It is dynamic because 

 it allows additional levels or improved class criteria to be 

 added. It provides a standard language that communi- 

 cates, identifies, and names classes, and defines some cri- 

 teria for natural renewable inventorying and mapping. 



This classification framework was developed with 

 four requirements in mind: 



• The classification system should be based as much as 

 possible on relatively permanent land features and, 



