200 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



THE OAK-CEDAR WOODLAND. 

 QUERCUS-JUNIPERUS ASSOCIATION. 



Nature and extent. — The oak-cedar association is regarded as the basic or 

 original community from which the related pifion-cedar and pine-oak woodland 

 have been differentiated, one to the north and the other to the west. This is 

 indicated by its position, but especially by its composition. It contains the 

 three dominant genera and the largest number of species and varieties. It 

 differs from the northern association in the predominance of oaks. The latter 

 are important also in the western woodland, but to a less degree, and they 

 belong to different species. The presence of oaks, pines, and cedars in these 

 two associations shows the essential equivalence of the broad-leaved and 

 needle-leaved evergreens as formational dominants. The variable nature of 

 the deciduous habit is shown by the fact that Quercus douglasii loses its leaves 

 in the fall or winter, while the other species drop them at different times in the 

 spring. All of them agree in being essentially sclerophyll in habit (plate 46) . 



The oak-cedar woodland has its center in southern Arizona and New Mexico 

 and in northern Mexico. It occurs commonly in the mountain ranges of 

 trans-Pecos Texas and is found scattered in the canyons and escarpments of 

 the Staked Plains and the Edwards Plateau. It extends north over the 

 mountains of the eastern half of Arizona and the western half of New Mexico 

 to the thirty-fifth parallel, where it yields rather abruptly to the pifion-cedar 

 association. Pinus cembroides occurs also in Lower California, where it is 

 associated with P. edulis, P. monophylla, P. quadrifolia, and Juniperus cali- 

 fornica, and serves to emphasize the general unity of the formation. 



DOMINANTS. 



Quercus emoryi. Quercus reticulata. Juniperus flaccida. 



Quercus reticulata Juniperus pachyphloea. Juniperus virginiana 



arizonica. Juniperus occidentalis scopulorum. 



Quercus reticulata monosperma. Pinus edulis. 



oblongifolia. juniperus sabinoides. plnus cembroides. 

 Quercus hypoleuca. 



An intermediate mixture of several dominants is characteristic of most of 

 the associational area, especially the central portion in southern New Mexico 

 and Arizona, and northern Mexico. The fundament of the latter is formed 

 chiefly by the oaks, in which cedar and pinon occur in varying abundance. 

 Pure stands are the exception, particularly in the central mass. They 

 are more frequent as the areal or altitudinal limits of the association are 

 approached, owing to the decrease in the number of dominants. At the 

 edges, communities of single dominants are more or less typical, but they 

 usually take the savannah form, as in the oaks, or they characterize serai 

 areas, such as the escarpments covered with Juniperus sabinoides. Near the 

 margin of the association, there is also a marked tendency for the dominants 

 to become low and shrubby, and consequently to become confused with the 

 elements of the chaparral. In the mountains of southern Arizona, the greatly 

 broken topography produces innumerable fragmentary habitats and causes 

 a confusing mixture of woodland with desert scrub, chaparral, and even 

 montane forest (cf. Shreve, 1915 : 31). 



The most typical grouping of the oak-cedar woodland is Quercus emoryi, 

 Q. arizonica, and Q. hypoleuca with Juniperus pachyphloea and Pinus cem- 



