68 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



area by one or the other climax. All the evidence points toward the 

 southern Coast Ranges as a region of this nature, and it therefore 

 seems safe to make use of the data obtained at Jasper Ridge in 

 illustrating the differences between the larger units. Further, the 

 differences here are likely to be close to the minimum in degree 

 which are able to bring about the differentiation of climaxes, and 

 are of special interest on that account. Such a comparative evaluation 

 must of necessity include the reactional effects of vegetation upon 

 habitat, accumulated through a long period of development. This 

 is as it should be, since in the interplay of primary and reactive 

 factors we find the "continuing causes" of the vegetation. 



Summarizing and commenting upon the differences between 

 forest and chaparral habitats, we attain the following results: 



As to soil, humus in the chaparral is very scanty, but in the 

 forest is abundant — nearly 2 per cent by weight in the surface 

 layer and considerable to the depth of 1 meter. In water-content 

 there is large difference during the rainy season, the forest having 

 the greater amount. At this time the surface layers are most 

 important, since the major part of the absorbing roots is contained 

 therein. It is here, too, that the water-content differences mainly 

 show themselves, being practically negligible at the depth of 1 

 meter. As the dry season advances, water-content values in both 

 communities and at all depths converge, and at its culmination they 

 are all very close together, and the correspondence is rendered still 

 more striking by comparison with the wilting coefficient in each 

 case. In brief, there is notable difference in the actual amount of 

 water available, but at the critical period conditions are about 

 equally severe in both communities. In water-retaining capacity 

 the only noteworthy feature is the relatively high value in the 

 surface soil of the forest community, due to humus. As to soil 

 temperature, the comparative march is the reverse of water-content; 

 the values are closely similar in the wet season, but widely divergent 

 in the dry, the chaparral being much the higher. 



As to atmospheric factors, we may dismiss rainfall, cloud, fog, 

 and wind as immaterial to the present local problem. The light 

 impinging upon a leaf of the foliage canopy is much greater in 

 chaparral than in forest, because of the fewer obstacles to its trans- 

 mission and the reflection and diffusion from the light-colored soil- 

 surface. The intensity in the shade is considerably less beneath the 

 forest canopy, both absolutely and proportionally. The fact that 

 the shade intensity beneath Arctostaphylos is practically the same as 

 in the forest indicates that the leaf character is determinative — the 

 sparse needle foliage vs. the broad leaves of the other shrubs and the 

 trees. Temperature and relative humidity data are unsatisfactory, 

 but their effects relative to the present purpose are largely included 



