THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 71 



shoots, which grow for a time with surprising rapidity (see p. 86), 

 although no decrease has occurred in the rate of loss. The size 

 and luxuriance of the plants are therefore greater upon north-facing 

 slopes, where soil-moisture is more abundant and evaporation less 

 rapid during the growing-period. 



To account for the absence of the more mesophytic species from 

 the south-facing slopes, we must appeal to the dry season. It has 

 been shown that the soil-moisture contents of the two slopes, con- 

 sidered in relation to the wilting coefficient, are about equally unfavor- 

 able during the critical period. This is not true, however, of the 

 evaporation-rate, which is considerably higher in the chaparral 

 community. A seedling of a mesophytic forest species, which might 

 conceivably obtain a start during the spring months, though not 

 making much increase in size, in the chaparral habitat, would be 

 very likely to succumb during the ensuing critical period, when the 

 high evaporation-rate had made the water-balance still less favorable. 



Since the plants of the forest, because of their size and abundance, 

 use more water than the chaparral species as food material and lose 

 more through their enormously great expanse of foliage, more is 

 withdrawn by them from the soil. The much more rapid depletion 

 of the water-content during the spring months, resulting from this, 

 is perfectly evident in figure 8. Thus the very luxuriance of the forest 

 vegetation is itself the prime cause of the reduction of water-content 

 practically to the low level of the chaparral. 



Our conclusion, then, extended to the larger vegetation units, 

 is that the fundamental distinguishing difference between the two 

 broad-sclerophyll climaxes — their continuing cause, so to speak — is in 

 the water-balance and its variations, whatever the indirect factors 

 influencing it; that its importance is equally divided between wet 

 and dry seasons, the greater excess of supply over loss in the forest 

 during the growing-season explaining the size and luxuriance of the 

 plants living there, and the higher evaporation-rate in the chaparral 

 during the dry season, with equally severe soil-moisture conditions, 

 accounting for the absence of mesophytic species in that habitat. 



