THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 63 



Figure 16 presents the ratio between evaporation at the top of 

 the vegetation and at the surface of the ground. Stations 2, 3, and 

 7 were selected, since they were continued through both wet and 

 dry seasons, and the average daily losses for the period are used 

 in computing the ratios. During the dry season the rates at the 

 two levels did not differ very greatly, while during the wet season 

 the differences were considerable, especially in the chaparral stations. 

 This suggests that during the latter period there is a surface stratum 

 of moist air due to evaporation from the saturated soil, and that 

 the contrast between it and the upper air layers is greater in the 

 chaparral stations than in the forest because of freer circulation. 



DISCUSSION AND CORRELATIONS. 



It is very easy and pleasant to describe the vegetation of a locality, 

 to mark it out into communities, and to name the dominant and 

 secondary species thereof, and even to present quadrats of the same. 

 It is not so easy completely to analyze the habitat. With our 

 present methods of investigation certain factors elude us almost 

 altogether, and much of the data, even when obtained, is difficult to 

 utilize. It is most difficult satisfactorily to link vegetation and the 

 measured factors of the habitat in the relation of cause and effect. 

 The present chapter has so far been devoted to description of veg- 

 etation and analysis of habitat. Some correlations of the two will 

 now be attempted. 



THE BROAD-SCLEROPHYLLS AND THEIR HABITAT. 



After the analytical treatment that has preceded, it is proper 

 first to synthesize the various elements into a whole, to characterize 

 the broad-sclerophyll habitat as a unit, so far as we fairly may from 

 the data in hand. Afterward it will be in order to consider habitat 

 differences and community differences and their correlation. 



To describe the broad-sclerophyll habitat in the large is merely 

 to describe the climate of California west of the Sierras and its 

 effects upon the soil factors. Emphasis must be placed upon the 

 complete seasonal march of the factors, since vegetative activity of 

 some sort and degree may here take place the whole year through. 



Of direct and fundamental importance is soil-moisture. Its 

 great abundance during the winter months makes ample provision 

 for growth during that time. The supply is made more completely 

 available by the low air-temperature and low evaporation-rate 

 during the period. The cessation of the rains at some time during 

 the spring is followed by gradual depletion of the water-content, 

 due to failure of the renewal supply, gravity drainage, evaporation 

 from the soil, and absorption by the vegetation itself. These 

 partial causes will vary in importance in various places. In a 



