THE PETRAN CHAPARRAL. 183 



Associations. A careful consideration of the above facts has led to the 

 conclusion that the chaparral formation consists of but two climax associa- 

 tions. These are the Petran or Cercocarpus-Quercus association composed 

 chiefly of Quercus undulata, Cercocarptis parvifolius, Rhus trilobata, Prunus 

 demissa, Amelanchier alnifolia, Symphoricarpus albus, Peraphyllum, and 

 Fendlera, and the Coastal or Adenostoma-Ceanothus association, consisting 

 principally of Adenostoma, Ceanothus cuneatus, Ardostaphylus tomentosa, and 

 Qiiercus dumosa. The fragmentary chaparral of the Northwest is clearly a 

 shading-out of the Rocky Mountain association, since the chief difference is 

 the absence of Quercus undulata^ Cercocarpus parvifolius, and Fendlera in the 

 former. The chaparral of the Southwest clearly shows its relationship to the 

 Rocky Mountain association in the abundance of Quercus, Cercocarpus, 

 Rhu^, Prunus, and Amelanchier. In addition, it possesses two dominants 

 from the Coastal association, viz, Ardostaphylus pungens and Ceanothus 

 cuneatus, and exhibits certain genera more or less peculiar to it, such as Fal- 

 lugin, Cowania, and Coleogyne. The latter, however, are gradually finding 

 their way into the Rocky Mountain area. As a consequence, this type of 

 chaparral is perhaps best treated as a transition between the Rocky Mountain 

 and Coastal associations, but with a much closer relationship as a rule to the 

 former. In some of the mountains of southern Arizona, however, the chaparral 

 consists chiefly of Ceanothus and Ardostaphylus, and is clearly an extension 

 of the Coastal type. In the following discussion, the chaparral of the North- 

 west and Southwest are considered as more or less differentiated portions of 

 the Cercocarpus-Quercus association. Because of their general resemblance 

 to them, the subclimax types are treated with the corresponding climax, the 

 Rhu^-Prunus community of the Missouri Valley with the Rocky Mountain, 

 and the Rhus-Ceanothu^ subclimax of the Pacific Coast with the Coastal 

 association. The oak chaparral of Texas resembles that of the Missouri 

 Valley in its general relation to the eastern forest, but its dominants are 

 derived from both the East and West. 



THE PETRAN CHAPARRAL. 

 CERCOCARPUS-QUERCUS ASSOCIATION. 



Nature and extent. The Rocky Mountain or Petran chaparral consists 

 almost exclusively of deciduous shrubs in more or less intimate mixture. It 

 ranges in height from 2 to 20 feet, but attains its most characteristic expres- 

 sion at 5 to 10 feet. Under optimum conditions, it is massive in nature, cover- 

 ing many square miles with the density of a forest cover. Because of its 

 intermediate position between forest and other formations and its occurrence 

 in the diverse topography of foothills, it is much interrupted as a rule. The 

 total number of dominants is large, and at least several regularly occur in any 

 one grouping. They agree closely in vegetation-form and in the characteristic 

 habit of root-sprouting, though some produce sprouts much more readily 

 than others. In growth habit, they are normally bushy shrubs, though the 

 range in sijse is considerable. Quercus undulata and Prunus demissa often 

 become small trees; Rhus trilohala may form a gigantic bush 20 feet high and 

 25 to 30 feet in diameter, while Cercocarpus parvifolius is usually a slender 

 erect shrub. It is interesting to note that all the dominants belong to the rose 

 order, with the exception of Quercus, Rhus, and Symphoricarpus. 



