DEGREE OP STABILIZATION. 99 



to bring it constantly nearer medium or mesophytic conditions. Exceptions 

 to this occur chiefly in desert regions, though they may occur also in water 

 areas, where processes of deposit and erosion alternate. The effect upon the 

 plant population is corresponding. The vast majority of species are not pio- 

 neers, i. e., xerophytes and hydrophytes, but mesophytes with comparatively 

 high but balanced requirements for ecesis. For this reason the number of 

 species and individuals grows larger in each succeeding stage, until the final 

 dominance of light, for example, becomes restrictive. At the same time the 

 life-forms change from those such as Uchens and submerged plants with a 

 minimmn of aggregate requirements to forms with an increasingly high 

 balanced need. The period of individual development increases as annuals 

 are succeeded by perennials and the latter yield to dominant shrubs and trees. 

 The final outcome in every sere is the culmination in a population most com- 

 pletely fitted to the mesophytic conditions. Such a climax is permanent 

 because of its entire harmony with a stable habitat. It will persist just as 

 long as the climate remains unchanged, always providing that migration does 

 not bring in a new dominant from another region. 



Degree of stabilization. — ^Apart from the temporary stabihty of each suc- 

 cessional stage the final stabilization of a sere varies greatly in permanence. 

 In the actual seres of the present time this is best illustrated by the water sere 

 in a region where moor and heath appear as stages on the way toward the 

 forest climax. As a consequence of pecuUar soil reactions each one is usually 

 a subclimax of unusual duration, and under the artificial conditions evoked 

 by man may persist as an actual climax. A similar effect occurs locally in the 

 Rocky Mountains, where springs keep the soil too moist for the pines which 

 normally succeed aspens on dry slopes. The result is that the aspen remains 

 dominant through a period equal to several stages, and yields only when the 

 final spruce and fir become controlling. This persistence of the aspen is 

 doubtless promoted by repeated fire, which is a universal cause of apparent 

 stability. This is certainly a large factor in the prairie community. What- 

 ever the origin of prairie may have been, its extent and duration are largely 

 due to the effect of fire upon woody communities, followed by a similar 

 influence produced by clearing and cultivation. In all cases of subclimaxes, 

 i. e., of premature stabilization, the activities of man will nearly always prove 

 to be concerned in a large degree. 



In the analysis of existing seres it seems evident that complete stabiliza- 

 tion occurs only when the climax is controlled by trees, which are the most 

 dominant and hence the highest ecologically of aJl the life-forms. Develop- 

 mentally, all other final communities are subclimaxes of greater or less dura- 

 tion; actually, they may exist throughout one or more successional periods. 

 They may owe their existence to any of the following factors: (1) climatic 

 control; (2) reaction upon the soil; (3) interference by man; (4) exclusion by 

 barriers constituted by later dominants. The removal of the check permits 

 complete development and the appearance of the serai climax. The evolution 

 of a new vegetation through long periods of time produces new climax forma- 

 tions and leads to corresponding seres. In the complex successional develop- 

 ment of vegetation, since the first appearance of land areas, all possible 

 degrees of stabilization have occurred, with the exception of complete develop- 

 mental stability. The latter can never occur in vegetation as a whole as long 



