192 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Ceanothus divaricatus, Quercus dumosa, and Cercocarpus parvifolius as their 

 most abundant associates, and a dozen or more of less importance (Leiberg, 

 1900 : 465; Hall, 1902 : 17). Practically the same grouping occurs through 

 the Laguna and Cuyamaca Mountains to the coast about San Diego. The 

 chaparral of Lower California is composed chiefly of both species of Adenos- 

 toma, Arctostaphylus glauca and pringlei, Cercocarpus parvifolius, and Ceano- 

 thus divaricatus (Goldman, 1916 : 330). 



Throughout southern California generally, Adenostoma fasciculatum mixes 

 and alternates at the lower levels and on drier areas with Artemisia cali- 

 fornica, Salvia mellifera, Salvia apiana, or Eriogonum fasciculatum, the domi- 

 nants of the Coastal sagebrush association. This is more xerophytic than the 

 chaparral, and is subclimax to it, a relation which Cooper (1919) has empha- 

 sized. 



Factor and serai relations. — In an intensive study of the habitats of sub- 

 climax oak forest and the chaparral, Cooper (1919) has reached the following 

 conclusions : 



"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 are con- 

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

 themselves; at the depth of 1 meter such being practically negligible. 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: Rainfall, cloud, fog, and wind may be dismissed 

 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 transmission, and 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 Arctostaphylus is practically the same as in the 

 forest indicates that the leaf character is determinative — the sparse needle 

 foliage versus the broad leaves of the other shrubs and the trees. Tempera- 

 ture and relative humidity data are unsatisfactory, but their effects relative 

 to the present purpose are largely included in evaporation. The differences 

 in this factor, though not strikingly great, are constant throughout the year, 

 the Adenostoma chaparral being the highest and the Arctostaphylus chaparral 

 intermediate. This conclusion is drawn from the values obtained at the top of 

 the vegetation. The rate at the surface of the ground does not show differ- 

 ences of import to the problem in hand. 



"Our conclusion, then, 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 



