268 THE STUDY OF PLANT COMMUNITIES • Chapter X 



ulorum, often with sagebrush in association. Pinon-juniper is com- 

 pletely lacking in Sierran zonation, which goes directly from 

 Artemisia and Furshia to Pinus ponder os a. However, almost with- 

 out exception, it occurs on every westernmost range and mountain 

 of the Great Basin, often lying just across a valley from the base 

 of the Sierra. 



■ ■-■•,:.•.-.:.-.•.-. ;v(j(.mv. : j*;K 



mmis&m 



FlG. 138. An example of the scrub oak-mountain mahogany zone in the 

 foothills near Colorado Springs, Colo. Quercus gcmibellii predominates here 

 with Cercocarpus parviflorus and Rosa arkansas as associates. Although the 

 scrub is sometimes taller, its open, irregular distribution is typical.— P/joto by 

 R. B. Livingston. 



The openings between trees support a grass cover (Bouteloua, 

 Stipa, Agropyron, Poa) and numerous other herbs, together with 

 a few shrubs (Ceanothzis, Cercocarpus, Purshia, Coivania, Ar- 

 temisia, Opuntia) characteristic of the next lower zone. Over- 

 grazing or fire may result in the temporary dominance of these 

 shrubs. 



Oak-Mountain Mahogany Climax .—The transition from the 

 conifer forest of the lower slopes to the treeless plains and pla- 



