204 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



draw a line between the pine-oak and pifion-cedar woodlands, since Juniperus 

 utahensis extends well into California and Pinus monophylla to Lower Cali- 

 fornia. However, the presence of Quercus wislizenii, Juniperus californica, 

 and Pinus quadrifolia from the San Bernardino or Santa Rosa Mountains to 

 Lower California, as well as that of P. cembroides, is regarded as indicating 

 the pine-oak association. 



The community relationship of Yucca arbor escens is somewhat uncertain, 

 but its constant association with Juniperus californica along the northern 

 base of the San Bernardino Mountains from Cajon Pass to Neenach, and to 

 Hesperia indicates that it is a dominant of the woodland. Like Yucca radiosa 

 and Y. macrocarpa, it extends downward into the desert scrub, but its life- 

 form, optimum growth and zone of dominance warrant its inclusion in the 

 woodland. Merriam (1893 : 341, 354) has noted the occurrence of Yucca 

 arborescens and Juniperus californica on the mountain ranges south and north 

 of the Mohave Desert, where they form a distinct belt at 3,500 to 4,000 feet. 

 Leiberg(1900 : 444-445, 471) has recorded the composition of several woodland 

 communities in which Yucca occurs on the lower levels of the San Bernardino 

 Mountains. It is associated with Juniperus californica and Pinus monophylla, 

 with these and Juniperus occidentalis, with Pinus monophylla alone or with 

 P. monophylla and Q. wislizenii also. Parish (1903 : 221) has found Yucca 

 and Juniperus californica forming an open community along the San Ber- 

 nardino and Chuckawalla Mountains and from Daggett to Pilot Knob, while 

 Sudworth (1908 : 201) states that Yucca is also associated with juniper, 

 pinon, and Pinus sabiniana. From the nature of its crown, the tree-yucca 

 forms even more open communities than the other dominants of the woodland, 

 and hence the consociation is constantly mixed along its lower portion with 

 dominants from the desert scrub and sagebrush. 



Little is known of the factor or successional relations of this community. 

 The latter seem in general to correspond with those of the oak-cedar wood- 

 land. The oaks are the more xerophytic, and Quercus douglasii rather more 

 than Q. wislizenii, if distribution be regarded as an indication. Their relative 

 position seems definitely indicated by the frequency with which they form 

 savannah with grassland at their lower limits. In spite of their occurrence in 

 rocky subclimax areas, the cedars and pinons appear to be rather more meso- 

 phytic than Pinus sabiniana. This is suggested by the respective altitudes at 

 which they reach their greatest dominance, and seems to be certainly true for 

 Pinus cembroides. 



In the rough topography of the foothills, woodland, chaparral, and mon- 

 tane forest are often much mixed and confused. In spite of this, they appear 

 as distinct units when differences of slope and successional development are 

 taken into account. The two oaks and the digger-pine are frequently mixed 

 with Quercus californica or Q. garryana, which really constitute a subclimax 

 leading to the montane forest of Pinus ponderosa and Pseudotsuga mucronata. 

 Where any of the three dominants occur with chaparral, the grouping is usually 

 successional in character, or it represents an ecotone. In some cases where 

 the trees are scattered more or less uniformly through a chaparral cover, the 

 community is to be regarded as a savannah in which the grasses are replaced 

 by shrubs, and it is probably to be similarly related to the climatic cycle. 



