266 THE S TUDY OF PLANT COMMUNITIES • Chapter X 



southern Rockies, the substitutes are P ponder osa var. arizonica, 

 P leiophylla, and P. latifolia. 



The only exceptions to ponderosa pine dominance are found 

 along streams and drainage lines where narrow-leaved cottonwood 



FIG 136 Climax forest of ponderosa pine (Pinus ponderosa) in typical 

 open stand. Montane zone, Arizona.-L7. S. Forest Service. 



(Populus angustifolia), the commonest tree, forms postclimax 

 stands with P. acuminata and P sargentii in association. Aspen 

 (P tremuloides), in glades, and box elder may also occur frequent- 

 ly on these moist sites. Although fires are common, in dry summers, 



