THE BROAD-SCLEROPHYLL VEGETATION OP CALIFORNIA. 47 



at the lowest depth. During the spring and early summer of 1913 

 the water-content steadily decreased, and by mid-July it had reached 

 a point beyond which it could not drop much farther, the samples 

 when gathered appearing as incoherent air-dry sand. From then 

 till the first rains, three and a half months, there was a period of 

 extreme deficiency, which is not exceeded in severity by conditions 

 as recorded in the desert of Arizona (57). The lag of several weeks 

 in the response of the lower strata to the first rains shows that their 

 advent does not close the dry season, so far as the soil is concerned. 



In the forest station the general form of the curve is the same, 

 but there are nevertheless important differences. The maxima are 

 uniformly greater than in the chaparral. Comparing the three 

 depths, we find the relative positions of the 10-cm. and 100-cm. 

 lines the reverse of those in the chaparral, the 10-cm. depth showing 

 nearly always the highest water-content and the 100-cm. depth the 

 lowest. Obviously this is to be traced to the effect of the humus on 

 water-retaining capacity. The lines, too, during the rainy seasons, 

 are much more widely separated than in station 10. During the 

 dry season, however, they converge until they are as close together 

 as those of the chaparral. The response to the first rains in the 

 deeper levels shows a more noticeable lag than appears in the chapar- 

 ral, the first certain increase at 100 cm. depth appearing on December 

 26, eight weeks after the earliest shower. Comparing equal depths 

 in the two stations, we find that at 10 cm. the forest line is uniformly 

 higher than that of the chaparral, the reasons being shade and 

 humus. The maximum difference between them during the rainy 

 season is 14 per cent, and the average for the rainy seasons ap- 

 proximately 8 per cent. During the progress of the dry season 

 the lines converge, the average difference for the last three and 

 a half months being 3 per cent. At 30 cm. depth the forest line is 

 still well above that of the chaparral, but the differences are uni- 

 formly less in both wet and dry seasons. At 100 cm. no such 

 constant differences are seen. During the first winter the chaparral 

 soil at this depth was wetter than the forest; during the dry season 

 and the second winter the lines cross and recross without revealing 

 any differences of importance. 



The points of greatest general import are: (1) the rather striking 

 difference in water-content between chaparral and forest during the 

 rainy season; (2) the convergence of the six water-content lines to 

 minima which at the end of the dry season are not far apart. 



The depletion of the water-content in every case is due to three 

 causes. Drainage of the gravity water through the soil takes place 

 with great rapidity during the wet season, which is proved by the 

 fact that the ravines of Jasper Ridge contain running streams for 

 considerable periods. After the rains have ceased, however, it can 



