conifer-dominated forests of northern Idaho and adjacent Wash- 

 ington have been classified into 13 climax associations in four 

 vegetation zones, each characterized by a particular combination 

 of vascular plant unions ("union" being defined as "a group of 

 plants exhibiting ecologic similarity throughout a particular 

 vegetation matrix"). The vegetation consists of broad expanses 

 with a low degree of variation, separated by narrow strips — but 

 not as a continuum with pronounced gradients extending in all 

 directions, nor as sharply defined discontinuities in the distribu- 

 tion of species. ^^ In grasslands in western Canada, dominance, 

 distribution of subdominants, and basal cover were mostly used 

 in determining groupings.^'' 



Important factors in causing the differential groupings of 

 species into two major groups in an area in the lower foothills in 

 northern Colorado appear to be deposition versus erosion, and 

 differences in the gravel content in the 0- to 6-in. soil horizon. '°^ 

 The grouping on sites which showed evidence of deposition, with 

 the gravel content averaging 2.2 per cent, consisted of the domi- 

 nants Agropyron smithii, Bouteloua gracilis, and Bromus tectorum and 

 a limited number of associates, averaging 1 1.2 species per stand. 

 The groupings in the other major group were the dominants (a) 

 Stipa comata, Bouteloua gracilis, and B. curtipendula with 28.7 species 

 per stand; (b) Andropogon scoparius with 35 species per stand; and 

 (c) B. gracilis and Artemisia dracunculus ssp. glauca with 21.6 

 species per stand. The gravel content averaged 33.0, 21.2, and 

 15.6 per cent, respectively, in this group, with evidence of slight 

 to moderate erosion. The grouping of species in the Agropyron- 

 Bouteloua- Bromus community appears to be related more to soil 

 characteristics of the habitat than to interrelations of species, and 

 the paucity of constants and the low number of species appear 

 to be caused partly by the compactness of the soil, which hinders 

 penetration of water. Many species of the other three commun- 

 ities do not occur in this one because adequate relations with the 

 particular environmental conditions cannot apparently be estab- 

 lished, but these three dominants are well adapted to utilize the 

 resources present in this habitat, although they are more limited 

 here than in the other three. However, in some years, when the 



Grouping of Species • 73 



