184 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



The Cercocarpus-Quercus association reaches its best development in 

 Colorado, northern New Mexico, and eastern Utah. Its extreme limits on the 

 east are the Black Hills of South Dakota and the Wildcat Mountains of 

 western Nebraska, and the mountains of western Texas. On the south and 

 west it runs through southern New Mexico and Arizona, extending more or 

 less into northern Mexico. It is usually poorly developed in Nevada, and 

 thence ranges much interrupted through Idaho and eastern Oregon to British 

 Columbia and Alberta. Chaparral is fairly developed in southern Wyoming, 

 but is reduced northward to disappear largely in the mountains of central 

 Montana, where its place is taken chiefly by aspen, or by subclimax chaparral 

 from the East. Its altitude limits are of the widest. In Colorado and New 

 Mexico, the Quercus consociation is found well-developed in dry southern 

 slopes as high as 9,000 feet, and small outposts exist at somewhat higher alti- 

 tudes. These, however, are subclimax and will sooner or later yield to mon- 

 tane forest. The lowest limit is about 4,000 feet, but subclimax fragments 

 are frequently found at much lower levels in the Northwest. The association 

 attains its best development between 5,000 and 8,000 feet and as a climax is 

 practically restricted to this zone. 



Contacts — Along the eastern edge of the Rocky Mountains, the chaparral 

 is in contact below with the grassland association, touching the mixed prairie 

 in the north, and the short-grass plains in the south. The ecotone is often 

 several miles wide, and is usually marked by a striking alternation of the two 

 associations. On the south and southwest, the contact is usually with the 

 desert plains, more rarely with desert scrub. In the West, it is typically with 

 sagebrush, and in the Northwest with sagebrush, or bunch-grass prairie. The 

 upper contact is with pinon-cedar woodland or with the montane forest, 

 particularly the more xeroid Pinus ponderosa consociation or the aspen con- 

 socies. Its relation to woodland is puzzling at first, since it occurs both above 

 and below the latter. The probable explanation is that climax chaparral 

 regularly occurs below climax pinon-cedar woodland. The latter is often well 

 developed as an open subclimax on lower rocky ridges and slopes at altitudes 

 where it will ultimately be replaced by chaparral or sagebrush. This upper 

 ecotone is frequently greatly confused in dry rocky country, where one or 

 more dominants of the sagebrush, chaparral, woodland, and montane forest 

 may be found in intimate mixture or alternation. 



In comparing the rank of the various dominants, it must be borne in mind 

 that the figures indicate frequence rather than abundance. As a matter of 

 fact, however, there is so much correlation between the two in the case of 

 chaparral dominants that the one is a fair indication of the other. This is 

 especially true of the most massive communities found in the central and 

 southwestern areas. In these more typical areas, Quercus undulata is by far 

 the most important dominant, chiefly as the variety gambelii. Cercocarpus 

 parvifolius is a close second, with Rhus and Prunus approximately half as 

 important. Amelanchier is secondary to Quercus and Cercocarpus, but its 

 frequence must be interpreted in the light of its absence over most of the 

 eastern slope of the Rocky Mountains, where the other four dominants are 

 so typical, and its correspondingly greater abundance on the western slope. 

 It is significant that the four most important dominants are the same for the 

 first two areas. Prunus demissa loses its rank in the Southwest, but regains 



