igoS] COOPER— ALPINE VEGETATION IN COLORADO 337 



C. THE SUCCESSIONS 



In this study we have endeavored to trace in a general way the 

 stages through which the vegetation of the alpine region has passed. 

 It remains only to bring into an orderly arrangement the successions 

 and their stages which have been described. This may best be done 

 by means of the foregoing diagram, which in the light of what has 

 gone before will need no further explanation. 



IV. Summary 



In the retreat of the Pleistocene snow masses the unglaciated slopes 

 and summit of the divide were first uncovered and occupied by 

 vegetation. On these areas the xerophytic "dry meadows" of today 



have developed. 



Owing to the slower shrinkage of the valley glaciers, invasion of 



tj ^^ ^M^LS^ OXVy»Y^X OXi^ix.*^^^ 



new areas is still eoins on at the heads of the glacial canons. 



Ct^^^^O 



The glacial canon as a habitat is characterized by (i) compara- 

 tively favorable atmospheric conditions, (2) restriction of surface water 

 to limited areas, (3) dependence of the production and distribution 



of soil materials upon surface water. 



These conditions result in a vegetation consisting of a lichen 

 society inhabiting the dry rock surfaces for an indefinite length of time, 

 and a wet meadow society confined to the vicinity of the drainage 

 channels. The latter society is preceded by two groups of pioneers, 

 one, the less important, derived from the xerophytic dry meadows, 

 becoming established in cracks and joints of the floor; the sccon , 

 mesophytic in character, invading from the lower altitudes, occupying 



depressions and very wet places. , . 



The evidence at hand indicates that the forest, representea 

 timberUne by Pinus flexilis on the ridges, and by Pkea Engdmanm 

 in the canons, has reached its climatic limit on the "jj^es, being^p ^^ 

 vented from further advance by the dry winter wmds; bu 

 the canons the forest is still invading the wet meadow society. 



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