THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 67 



moisture and temperature optima. The prominent cuticle and the 

 other features that are effective in limiting water-loss enable the 

 plant to exist through the critical period of late summer and autumn. 

 The evergreen habit permits it to make use of the spring period, 

 when growth conditions average best — a time when an ordinary 

 summer-green plant would be putting all its energies into the build- 

 ing of a new photosynthetic apparatus — as well as the rather frequent 

 periods during the winter months when the temperature is high 

 enough for effective photosynthesis. This explanation has been 

 advanced by Guttenberg (36) for the broad sclerophylls of the 

 Mediterranean region, and by Schimper (80) for the type in general. 

 It seems entirely adequate for the Californian species. It might 

 be added, however, that possibly in JEsculus californica we have 

 a still more effective and economical way — the quick production 

 of thin, deciduous leaves for use during the favorable period and 

 their gradual elimination as unfavorable moisture conditions render 

 them dangerous rather than useful. 



In figure 17 I have combined the principal factors of the broad- 

 sclerophyll habitat in a way that gives a vivid impression of the 

 whole — rainy and dry season, decreasing summer water-content, 

 with increasing temperatures and evaporation. 



THE HABITATS OF THE TWO CLIMAXES CONTRASTED. 



The topic that naturally follows is comparison of the respective 

 habitats of the two broad-sclerophyll climaxes, forest and chaparral. 

 Both communities are typically represented on Jasper Ridge, 

 occupying opposite sides of the same ravines, and two subcommunities 

 of the chaparral as well. Care must be exercised in extending the 

 conclusions here drawn to the two climaxes in general, for it is of 

 course true that the factor values noted upon the slopes of a single 

 hill do not in detail perfectly represent the averages that characterize 

 great regions. It may be said, moreover, that given freedom from 

 disturbing agencies, the more xerophytic community, through the 

 reactive influence of the vegetation, might continue its mesotropic 

 development until forest replaced chaparral. Certain determination 

 of this point in any single given case is very difficult. Detailed 

 examination of the tension areas at Jasper Ridge failed to produce 

 satisfactory evidence, either positive or negative. For the present 

 purpose, the question may be settled with sufficient certainty by 

 treatment in line with the discussion upon page 74. 



Theoretically, there must be a region intermediate between those 

 clearly dominated by forest and by chaparral where the control is 

 in suspense, and here any decided local variation in habitat toward 

 xerophytism or mesophytism, as would result from opposite hill or 

 ravine slopes, would bring about permanent control of a limited 



