260 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 



conditions from those of the west slope and, accordingly, dif- 

 ferences in vegetation. Within the system, the individual ranges 

 likewise have similar east-west slope differences. North and south 

 exposures produce marked irregularities in zonation. Narrow val- 

 leys permit the dominants of one zone to extend downward into a 

 lower zone, and high dry ridges allow upward, fingerlike projec- 

 tions of dominants into continuous higher zones. Cold air drainage 

 locally causes marked disruption of the zonal pattern. 



The factors operative in producing and controlling vegetation 

 and its zonation in the Rockies have been studied in a number of 

 localities just as there have been many local studies of the vegeta- 

 tion. An unusually complete review and synthesis of all these in- 

 vestigations is available 85 with an extensive bibliography. What 

 follows is largely an adaptation from this report. 



Vegetation Zo7ies.—The zonal climaxes may be grouped as fol- 

 lows : 



Alpine Zone 



Tundra Climax (discussed earlier— p. 239-240) 

 Subalpine Zone 



Engelmann spruce— Subalpine fir climax 

 Montane Zone 



Douglas fir climax 



Ponderosa pine climax 

 Foothills (Woodland) Zone 



Pinon-Juniper climax 



Oak-Mountain mahogany climax 



Each of these types of vegetation extends over an altitudinai 

 range of about two thousand feet, where fully developed, and is a 

 true climax. The foothill zones narrow down and disappear en- 

 tirely in the north where the upper zones are found at progres- 

 sively lower altitudes. 



Near the upper and lower limits of a zone, the characteristic 

 species are more and more restricted to special habitats. Upward, 

 the climax species do best on south-facing slopes, which are warm- 

 er and drier than the general climate. Thus, in its upper transition 

 area, each association shows its preclimax relationship to the climax 

 of the next higher zone. At its lower limits, the association tends 



