TREE DISTRIBUTION 419 



species, as well as on the depth to perennial soil-moisture, or in other 

 words, the water relation. 



The three species of oaks referred to are Quercus agrifolia, the 

 encina oak, Q. lobata, the roble oak, and Q. Douglasii, the blue or 

 Douglas oak. The roble oak and Douglas oak are deciduous. The 

 encina oak is the familiar live-oak of the coastwise valleys. 



The roble oak is the valley oak par excellence, and is probably the 

 largest California species of the genus. The largest specimen reported 

 is 150 feet in height and 25 feet in circumference four feet above the 

 ground. 1 The writer also saw a specimen near Clear Lake, which had 

 a spread of top estimated to be 144 feet. In addition to being of large 

 size, the roble oak is unusually beautiful and graceful, with long and 

 slender pendant secondary branches, which occasionally nearly sweep 

 the ground. If not strictly confined to moist soils, it at least attains 

 its best development where the soil is moist and the depth to the level 

 of perennial water is not so great as to be beyond the reach of the roots. 



The encina oak is the species characteristic of the valleys of the 

 coast ranges, where it finds its greatest development. It is disposed 

 in open groves and it is to this species, mainly, that the park-like 

 appearance of the coast valleys is due. In form, the encina oak is more 

 compact than the roble oak, and has low, rounded tops, as is indicated 

 by the accompanying figure. 



As distinguished from the two other species of oaks just mentioned, 

 the blue oak occurs characteristically on dry, rocky soils, "which are 

 excessively arid in the rainy season." 



Not in itself an attractive tree, the blue oak, by reason of its form, color and 

 habit, plays a strong and a natural part in the scenery of the yellow-brown foot- 



hills.2 



Like the encina oak. the blue oak occurs singly or in open groves. 

 The characteristic appearance of the tree and its distribution are shown 

 in Figure 1. 



However the species of oaks may differ from one another in habit, 

 or however different the habitats they frequent may be, they agree in the 

 one particular which has already been mentioned, namely, in the open 

 character of the stand. This phase of the study of the oaks received 

 particular attention at the hands of the writer in 1913, and the leading 

 conclusions will be presented in the subsequent paragraphs. 



Quercus lobata 



An examination of the roots of the three species shows a striking 

 difference in the position occupied by them in the ground, as well as in 

 the general character of development. That of the roble oak is more of 



1 Jepson, ' ' The Sylva of California. ' ' 



2 Jepson, I. c. 



