valleys inhabited by Quercus agrifolia, 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 wanting, 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.
