186 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Equivalence of dominants. — The large number of dominants, their close 

 equivalance and wide range, and the almost complete absence of quantitative 

 results make the task of determining their factor relations peculiarly difficult. 

 This task is further complicated by the ease and intimacy with which the 

 dominants mix. In spite of this, however, the topographical and successional 

 relations are sufficiently evident to indicate their general response to physical 

 factors. Because of the wide range of climatic conditions, this must be done 

 for each area rather than for the association as a whole. The basic relation is 

 determined by the five great dominants, Quercus, Cercocarpus, Amelanchier, 

 Rhus, and Prunus, which have had some factor and successional study in 

 Colorado and Utah. Since one or more of these is found in practically all the 

 important groupings, they serve as basing-points for all the other dominants. 



Prunus demissa is the most mesophytic of the five dominants, as is shown 

 by the regular occurrence along streams and in ravines and by its taller habit. 

 Rhus trilobata comes next in its water requirements, followed closely by Quer- 

 cus undulata gambelii. The latter not only stands midway in the series, but 

 it is also able to adapt itself to a wider range of conditions than the others, 

 locally at least. This explains its greater frequence in spite of its range being 

 the most restricted of the five. Quercus undulata is somewhat more xeroid, 

 as its evergreen leaves and southern distribution indicate. The two are so 

 frequently mixed where their ranges overlap that the difference in their 

 requirements must be slight. Cercocarpus parvifolius and Amelanchier 

 alnifolia are the most xerophytic and to an almost equal degree. To a large 

 extent thay are corresponding species, the former being most typical east of 

 the Continental Divide, the latter west. As a rule, Cercocarpus occupies the 

 newer or drier areas, though their relative position is sometimes reversed. Of 

 the remaining dominants, Robinia neomexicana is somewhat more mesophytic 

 than the oak, as is shown by its fondness for ravines and valleys. This is 

 likewise true of Symphoricarpus albus and Rosa acicularis, which often form 

 a layer in oak thickets especially. Opulaster opulifolius is rather more meso- 

 phytic than oak and frequently forms a layer in forests of Pseudotsuga which 

 the oak can not enter. While all of the others occur mixed with Quercus, 

 this is less true of Purshia tridentata and Ribes cereum, which are to be regarded 

 as the most xerophytic. Holodiscus and Philadelphus are slightly more 

 xeroid than oak. Fendlera and Peraphyllum have a wide range of adjustments. 

 As tall shrubs, they often mix with oak on north exposures and at the base of 

 slopes, but usually they are nearly equivalent to Amelanchier and Cercocarpus. 



As would be expected, the other dominants of the southwestern chaparral 

 are typically xerophytic. This is indicated by their origin and distribution 

 as well as by their generally evergreen habit. From the evidence derived from 

 groupings as well as from the habitat, Cercocarpus ledifolius is nearly equi- 

 valent to the oak, and Fallugia paradoxa and Cowania mexicana to Cerco- 

 carpus parvifolius, though all are slightly more xerophytic. Arctostaphylus 

 pungens and Ceanothus cuneatus are more xerophytic than C. parvifolius, while 

 Coleogyne seems the most xerophytic of all. It forms pure communities on 

 shallow soil on rocky cliffs, but since it rarely mixes with the other dominants, 

 its relative position is uncertain. 



The dominants of the northwestern chaparral have already been considered, 

 but it may be helpful to relate them to each other, since both Quercus and 



