depends rather on a hierarchy of components that reflects their level of control on the 

 location, size, productivity, structure, and function of the system. Thus, components 

 which exert the most control are at the highest level in the system (Table 1). The 

 differentiating criteria at the upper levels with the greatest controls are broad and 

 general in importance. Those at lower levels are narrow and more specific in 

 importance. Figure 2 shows the major ecosystems of the United States delineated in 

 this manner. 



This approach emerged in the late 1960s when Forest Service soil scientists sought 

 to rapidly differentiate and classify ecologically significant segments of the land 

 surface on a small scale. Land systems inventory, as proposed by Wertz and 

 Arnold, ^'^ has since been expanded by Bailey2''22 from concepts advanced by 



Table 1 . Levels of generalization in a hierarchy of ecosystems 

 (from Bailey2i).* 



Name 



Defined as including: 



1. Domain 



Subcontinental areas of broad clinnatic sinniiarity 

 identified by zonal heat and water balance criteria. 



2. Division 



A part of a domain identified by macroclimatic criteria 

 generally at the level of Koppen's types. ^^ 



3. Province 



A part of a division identified by bioclimatic and 

 soil criteria at the level of soil orders and classes 

 of vegetation formations. 



4. Section 



A part of a province identified by a single climatic 

 vegetation climax at the level of Kuchler's potential 

 vegetation types. ^^ 



5. District 



A part of a section identified by Hammond's land- 

 surface form types. '^ 



6. Landtype 

 association 



A part of a district determined by isolating areas 

 whose form expresses a climatic-geomorphic 

 process. 



7. Landtype 



A part of a landtype association having a fairly 

 uniform combination of soils (e.g., soil series) and 

 chronosequence of vegetation at the level of 

 Daubenmire's habitat type.'^ 



8. Landtype phase 



A part of a landtype based on variations of soil and 

 landform properties such as soil drainage and slope 

 that affect the productivity of the habitat type. 



9. Site A part of landtype phase that is homogeneous in 



respect to all components, their appearance, 

 potential to produce biomass, limitations to use 

 and response to management. 



'As these levels of generalization are hierarchically nested, a lower order of 

 generalization (e.g., section) is a subset of a higher (e.g., province), and 

 therefore, contains its characteristics as well. Regional ecosystems or 

 ecoregions are designated at levels 1 -5. 



19 



