350 CARNEGIE INSTITUTION OF WASHINGTON. 



Climax Formations, hy F. E. Clements. 



Continued study of the great formations and associations of the 

 West further emphasizes the basic nature of the classification that 

 rests upon development. This is true not only of the associations, 

 but also of the relation of formations to each other. Furthermore, 

 the opportunity to see a large number of associations each year in the 

 complete range of climatic and soil conditions affords results which are 

 highly objective and quasi-experimental in nature. This opportunity 

 has been greatly enhanced during the past four years by the variation 

 in rainfall, which has run the gamut from the severest drought to the 

 maximum rainfall known. 



WTiile nearly all the climax formations have again been traversed, 

 those that have received extensive study are the grassland, sagebrush, 

 and desert scrub. The bunch-grass prairie was studied from north- 

 eastern Oregon through southern California, and much additional evi- 

 dence secured of its former extent and dominance. While the northern 

 Agwpyrurn and the southern Stipa areas seem almost distinct to-day, 

 these two dominants were found much mixed in southern Oregon and 

 northern California, and there is little question that they were once 

 found together over much wider areas. The area of the bunch-grass 

 prau'ie has been extended to the edge of the Mohave Desert, and it now 

 appears highly probable that the latter was grassland from the Miocene 

 through the Pleistocene. 



Of equally great interest was the discovery of the extent and im- 

 portance of the Coastal sagebrush association. This proves to be one 

 of the major communities of the southern half of California, occurring 

 nearly eveiywhere on the Coast Range and cross-ranges between the 

 grassland and the true chaparral. It occupies a considerable portion of 

 the western edge of the Mohave Desert and of Antelope Valley, where 

 it becomes mixed with the desert scrub, especially Larrea. In the Mo- 

 have Desert, Artemisia tridentata is a constant member, while about 

 Saugus and Newhall this is replaced by ^. t, parishii. Both of these 

 dominants confirm the relationship of the coastal association to the 

 true sagebrush of the Great Basin. Tracing this community over 

 nearly a thousand miles revealed a number of additional dominants, 

 chief among them Penistemon antirrhinoides, Eriodictyum tomentosurn, 

 Isomeris arborea, Rhamnus integrifolia and ovata, and Salvia carnosa. 



Observations in the Mohave and Colorado Deserts threw addi- 

 tional light upon the structure and successful relations of the desert 

 scrub. They confirmed the inclusion of a number of apparently sub- 

 climax dominants in the foi-mation, in spite of a marked tendency to 

 become dominant on bajadas and valley plains, where Larrea is often 

 absent. This was notably true of Fouquiera and Cereus giganteus, which 

 were found in the Colorado Desert and Gila Valley in association with 

 Larrea, many miles from the bajadas of which they are so charac- 



