THE PETRAN CHAPARRAL. 



185 



it in the Northwest. In the latter the absence of Quercus and Cercocarpus 

 and the importance of Purshia suggests a greater differentiation from the 

 central chaparral mass. However, this seems readily explained by the remote- 

 ness from the latter and by a less favorable northern climate, which find 

 expression in the fragmentary character and the subclimax tendency of the 

 northwestern chaparral (plate 42). 



DOMINANTS. 



Central Rocky Mountains (206 localities) : 



Quercus undulata 126 



Cercocarpus parvifolius 107 



Amelanchier alnifolia 74 



Rhus trilobata 58 



Prunus demissa 58 



Symphoricarpus albus 23 



Peraphyllum ramosissimum 24 



Fendlera rupicola 18 



Holodiscus discolor 15 



Purshia tridentata 14 



Ribes cereum . 11 



Rosa acicularis 10 



Philadelphus gordonianus 5 



Opulaster opulifolius 5 



Robinia neomexicana 3 



Southern Area (38 localities) : 



Quercus undulata 26 



Cercocarpus parvifolius 16 



Rhus trilobata 13 



Amelanchier alnifolia 10 



Fallugia paradoxa 10 



Cercocarpus ledif olius 7 



Cowania mexicana 7 



Arctostaphylus pungens 6 



Prunus demissa 4 



Symphoricarpus albus 3 



Coleogyne ramosissima 3 



Ceanothus cuneatus 3 



Northwestern Area (39 localities) : 



Purshia tridentata 17 



Prunus demissa 15 



Amelanchier alnifolia 15 



Rosa acicularis 13 



Opulaster opulifolius 11 



Symphoricarpus albus 11 



Philadelphus gordonianus 8 



Holodiscus discolor 5 



Rhus trilobata 5 



Ribes cereum 5 



Peraphyllum ramosissimum 4 



Cercocarpus ledifolius 3 



Groupings. — The number of dominants is so large and their equivalence so 

 close that they occur in the most varied groupings. In any particular locality, 

 the number of associated dominants is usually 4 or 5, and often 6 or 7; 3 is also 

 a frequent grouping, but communities of 1 or 2 dominants are usually 

 found only near the limits of the association, where the latter is more or less 

 fragmentary. While nearly all of the most important dominants do occur in 

 pure communities, these are usually of limited extent and regularly alternate 

 with other dominants, except toward the edge of the association as already 

 noted. The actual groupings are so numerous and varied that a detailed sum- 

 mary of them possesses little significance. In the central mass, Quercus, Cer- 

 cocarpus, Rhus, and Prunus constitute the groundwork in central and eastern 

 Colorado and New Mexico. In western Colorado and adjacent regions, these 

 four are still of the first importance, but Amelanchier, Fendlera, Peraphyllum, 

 and Cercocarpus ledifolius often become equally important. These may mix 

 with the first four or replace one or more of them. This condition persists 

 into the Southwest, where Fallugia and Cowania enter to further complicate 

 the grouping, and Arctostaphylus and Ceanothus appear to serve as an indica- 

 tion of the transition to the Coastal association. In the Northwest, the asso- 

 ciation is so interrupted and fragmentary that definite groupings are not 

 obvious. In general, the ground plan seems to be furnished by Amelanchier, 

 Prunus, Opulaster, Philadelphus, and Symphoricarpus, though with almost 

 infinite variation in detail. In the drier regions, Amelanchier, Purshia and 

 Peraphyllum mix and alternate in varying degree. 



