110 STABILIZATION AND CLIMAX. 



alongside of one which it would eventually replace if the rainfall were to 

 increase generally. On south exposures of the canon, the more xerophytic 

 communities ascend far above their usual limit, and place themselves in con- 

 tact with the normal climax which would yield to them in case of decreased 

 rainfall. As a consequence it becomes possible to recognize two kinds of 

 potential climaxes. The one indicates what will happen if a change of climate 

 results in increased water-content, thus emphasizing the normal reaction in 

 the sere. It continues the development by replacing the climax and it may 

 be termed the postclimax (Gr. iros; Lat. post, after). The other foreshadows 

 the climatic change which reduces the water-content, and thus sets a lower 

 limit to the increase of the holard by reaction. As a consequence, develop- 

 ment would cease before reaching the climax proper, and the potential com- 

 munity, which would now become the actual climax, may be called the pre- 

 chmax (Gr. irpt,; Lat. prae, before). Thus, every climax area or formation 

 is in contact with one or more climax areas which bear the relation of preclimax 

 and postclimax to it, and are in a more or less complete zonal series with it. 

 Subclimaxes are practically always preclimaxes (plate 32, a, b). 



Changes of climax. — ^As already noted, the climax may change in consequence 

 of a single efficient variation of climate or of the development of an essentially 

 new flora as the outcome of long-continued evolution due to climate. In 

 addition, the climate may show a cimiulative change, or it may exhibit great 

 alternations, such as those indicated in Blytt's theory (1876). Both of these 

 phenomena were associated, it would seem, with the glacial period. It is 

 difficult to surmise the behavior of the successive climax formations in the 

 face of the oncoming ice. A gradual invasion must have produced preclimaxes 

 in all of the seres actually in development, before it overwhelmed each climax 

 area. The area just south of the final limit must have developed a series of 

 preclimaxes, ending in arctic tundra. Each recession of the ice must have 

 changed serai climaxes into postclimaxes, and each new advance would cause 

 the existing seres to terminate in preclimaxes. The final withdrawal of the 

 ice would give new areas for colonization by an arctic fiora, and hence a new 

 arctic climax, while the original arctic climax about the southern edge would 

 yield to the postclimax of heath or aspens and conifers just south of it. In a 

 similar manner, the postclimax of deciduous forest would replace the conifers, 

 and these again a new arctic climax of which they were the potential climax. 

 Finally, when cKmatic equilibrium was established, the arctic zone south of 

 the original ice would have had three or four successive climaxes, and the 

 number of climaxes would decrease by one for each zone to the northward. 

 For any particular period, each climax zone may have had a sequence of 

 seres, i. e., a cosere, all ending in the actual climax. In the case of the alter-- 

 nating wet and dry climates whith followed the glacial period, the postglacial 

 deposits seem to furnish convincing evidence of a sequence of climaxes derived 

 from postclimaxes. Thus, the arctic climax, the Dryas association, was suc- 

 ceeded by an aspen climax, the latter by a pine climax, this by an oak climax, 

 and the oak by the beech climax of to-day. The sequence apparently corre- 

 sponds with the gradual amelioration of temperature in large degree, and is 

 concerned with changes of rainfall only in so far as they favored or hindered 

 the growth of Sphagnum, and thus caused successive seres, the climaxes of 

 which were preserved by being embedded in the peat-bog. 



