196 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



and characteristic consequence of this is the almost universal production of 

 mixed communities, in which grassland, chaparral, or sagebrush play an 

 important or controlling part. Practically all the dominants are intolerant 

 of shade, and hence they never constitute a secondary layer in the montane 

 forest of the zone above. 



No special study of the successional relations of woodland has been made as 

 yet. Throughout the climax area, as well as beyond it, open shrubby woodland 

 appears relatively early on rocky slopes or hills, forming a subclimax to wood- 

 land proper or to montane forest, or being displaced by scrub or grass as the 

 habitat develops. The climax woodland may replace grassland, sagebrush, 

 or chaparral, and may in its turn be displaced by yellow pine or other domi- 

 nants of the montane forest at the upper edge of its zone. 



In origin, the woodland is uniformly southern and largely Mexican. Pinus 

 edulis and Juniperus monosperma have their centers in Colorado, Utah, 

 Arizona, and New Mexico, though they extend into Texas and Mexico. 

 Pinus monophylla and Juniperus utahensis are confined almost wholly to 

 Utah, Nevada, northeastern Arizona, and eastern California. P. cembroides 

 and J. pachyphloea are chiefly Mexican, extending into Arizona, New Mexico, 

 and western Texas. The oaks are all Mexican, with the exception of Quercus 

 douglasii and Q. wislizenii, which are almost exclusively Californian. They 

 range to central Arizona, New Mexico, and western Texas. Juniperus 

 scopulorum is by far the most widespread of the dominants, as would be 

 expected from its close relationship to the eastern J. virginiana. It occurs 

 from central Nebraska to Washington and from British Columbia to the 

 Mexican boundary, while J. occidentalis ranges from Washington to the border 

 of southern California. The limits of Pinus sabiniana lie wholly in California, 

 from near the northern boundary southward to the Tehachapi. Of the 19 

 species and varieties which comprise the dominants of the formation, prac- 

 tically all but Juniperus scopulorum and J. occidentalis find their northern 

 limits below the forty-second parallel, 11 are predominantly Mexican, 4 have 

 their center on the Colorado plateaus, and 2 are Californian. 



The above account indicates clearly the differentiation which has occurred 

 in the formation. The greatest number of dominants still occurs in the region 

 of the Mexican boundary. The most uniform development of the formation 

 is on the Colorado Plateau, while the most specialized area is found in Cali- 

 fornia, as a natural consequence of the desert and mountain barriers. 



Structure of the formation. — The dominants are as follows: 



Juniperus californica. Juniperus sabinoides. Quercus emoryi. 



Juniperus californica Juniperus virginiana Quercus reticulata. 



utahensis. 1 scopulorum. quercus reticulata 



Juniperus occidentalis. Pinus edulis. arizonica. 



Juniperus occidentalis Pinus edulis monophylla. Quercus reticulata 



monosperma. Pinus edulis quadrifolia. oblongifolia. 



Juniperus pachyphloea. Pinus cembroides. Quercus douglasii. 



Juniperus flaccida. Pinus sabiniana. Quercus wislizenii. 



These are grouped in three associations, namely, the Quercus- Juniperus, the 

 Pinus-Juniperus, and the Pinus-Quercus. The first of these is southern and 



^he specific relationship of the varieties as understood here is indicated by means of the 

 trinomial, but the name of the species is omitted in the text for the sake of brevity. 



