TREE DISTRIBUTION 423 



valleys inhabited by Quercus agrlfolia, the water table usually lies 35 

 feet or more beneath the surface. Sometimes it is much greater than 

 this, although it is rarely less. The availability of the soil moisture 

 which is derived from the water table to the roots does not depend 

 wholly upon the depth of the water table., but largely on the character 

 of the soil which intervenes between the plant roots and the water table. 

 For example, there may be strata of sand or gravel above the level of 

 perennial water which effectually separate the water table from the 

 root system. For this reason, the depth to perennial ground water is 

 not always of itself a criterion as to whether the moisture is available 

 to the plant or not. In such cases, as has been intimated above, the 

 plants are wholly dependent upon the water coming directly from the 

 rains or on what water is derived from run-off or by seepage from 

 higher ground. This, for the most part, does not penetrate beyond 

 approximately 3 to 4 feet. The plant, therefore, is obliged to develop 

 an extensive superficial system, in order to make use of such surface 

 water. For this reason, the roots of adjacent trees compete for the 

 ground water in a manner exactly comparable to the competition, as 

 already pointed out, which occurs among the desert shrubs. Thus it 

 follows that, because of a relative paucity of water, the trees come to 

 have an open stand. 



In the habitats where Q. Douglasii occurs, the water table is want- 

 ing, or so deeply placed as to be quite beyond the possible reach of the 

 plant's roots, so that here again, the species is wholly dependent on 

 surface water for its water supply. It follows, therefore, that the blue 

 oak forms a very open stand, as has been seen to be the case in the 

 encina oak, and for the same reason. 



The conditions of the water supply of the roble oak, on the other 

 hand, are diametrically opposed to those of the two other species. The 

 best development of the roble oak occurs where the perennial ground 

 water lies within 10 to 20 feet of the surface of the soil, or where 

 the soil is practically homogeneous, so that the ascent of capillary water 

 is great and where it is possible for the roots of the species to penetrate 

 to a great depth. A characteristic example of this species, although of 

 medium size, was seen by the edge of Putah Creek, where the July-level 

 of the stream was less than twelve feet beneath the surface of the 

 flood-plain, and where the flood-level of the stream must occasionally 

 have washed the base of the tree itself. 



From this brief outline of the root-characters of the three most 

 prominent species of oaks of Central California and from the sketch 

 of the ground water relations of the species, it appears that there is 

 an intimate relationship between root character and the characteristic 

 local distribution of the species. 



