74 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



Before proceeding to interpretation, certain definitions are neces- 

 sary. The climax is here considered to be the most mesophytic 

 community which a region as a whole, under present climatic con- 

 ditions, is able to support. Regions nearby have different climaxes, 

 more mesophytic or less so, according to whether the climates are 

 more or less humid, and the climax of the first region might be 

 succeeded by one or another of these, should the climate change in 

 the appropriate direction. The neighboring climaxes are thus 

 potential climaxes for the region under discussion (Clements, 21, 

 p. 108). A potential climax is a postclimax if it is more mesophytic 

 than the existing one, and a preclimax if less so (1. c, p. 109). Fre- 

 quently a potential climax is actually present in a region, in situations 

 of unusually favorable or unfavorable moisture conditions. An 

 excellent example of a postclimax is the forest which follows the 

 river valleys far westward into the Great Plains region. 



In the light of these definitions it is evident that the broad-sclero- 

 phyll forest is postclimax in Southern California. The chaparral 

 is plainly in control, although it is certain that its dominance has 

 been extended somewhat by fire. In central California, where the 

 two are rather evenly balanced, the question is difficult, but the 

 tendency of the forest northward to increase its dominance is plain. 

 In such a transitional region, where the actual control is uncertain, 

 the spatial relations of the competing communities depend upon the 

 nature of the topography. In a level country the change is usually 

 seen in a gradual mingling of the species of the two impinging regions. 

 In a broken country, where steep slopes with various exposures are 

 the rule, the situation is otherwise. The differences in atmospheric 

 factors resulting from contrasting slope exposures are in such a region 

 no greater than elsewhere; but because there is here a zone in which 

 the general conditions permit both groups of species to flourish, 

 the relatively slight differentiation of habitat due to the factor of 

 slope exposure exerts a maximum apparent effect upon the vegetation, 

 a sifting of the species into two groups. We therefore find the 

 two communities sharply set off from one another, each in permanent 

 control of its area, with no successional relations between them. 

 Each is the climax within its own little sphere of influence. Such is 

 the relation of broad-sclerophyll forest and chaparral in the central 

 Coast Ranges, the forest permanently controlling the north slopes, 

 the chaparral the south. Passing into the north Coast Ranges, 

 we would expect to find the broad-sclerophyll forest finally attaining 

 a true climactic status, and there is an undoubted tendency in this 

 direction. The effect is spoiled by the competition of Pseudotsuga, 

 the addition of which produces a mixed broad-sclerophyll-conifer 

 community, which is apparently the climax of the region in which it 

 occurs. Farther north, the broad-sclerophyll element loses all 



