countries, particularly in central and western Europe, following 

 Braun-Blanquet, the most widely employed are characteristic 

 species (those in one of the three highest classes of fidelity). The 

 differentiating species, which distinguish between groups by 

 their absence or lesser occurrence in one group than in others, 

 and dominants (see p. 128) are also important here, especially in 

 the segregation of minor categories. 



These attributes — kinds of species, abundance, frequency, 

 dominance, and fidelity — have been used to make a hierarchical 

 system of classification, including the association with its sub- 

 divisions and variants to alliance, order, and class. Associations 

 belonging to one alliance are represented by stands within a cer- 

 tain area, or in corresponding habitats in other areas. For ex- 

 ample, in a limited area in the mountain-front zone in northern 

 Colorado the major kinds of grassland have been classified ac- 

 cording to the limited data now available,^ °-^ as follows: 



(1) Andropogon scoparius-A. gerardi-Stipa comata alliance 



{a) Andropogon scoparius association 



{b) Stipa comata- Bouteloua gracilis- B. curtipendula association 



(c) Bouteloua gracilis-Artemisia glauca association 



(2) Agropyron smithii-Bromus tectorum alliance 



{d) Agropyron smithii- Bouteloua gracilis-Bromus tectorum asso- 

 ciation 



The habitats of the stands in the first alliance were marked by 

 more or less erosion, shallow to moderately shallow soils, and 

 usually much gravel and rock. The habitats of the other alliance 

 were distinguished by deposition, and deep and compact soils 

 containing very little gravel and rock in the upper 6 in. In suc- 

 cessional status all appeared to be in the climax or near-climax 

 stage under the prevailing conditions. 



Conard,"*^ classified the most important associations in Iowa 

 into about eight orders, the first one with two alliances and six 

 associations, as follows: 



Bases and Units • 179 



