Page:Popular Science Monthly Volume 85.djvu/421



T has frequently been observed that the shrubs in dry regions occur isolated from one another, with the effect that the landscape as a whole has a spotted appearance. This in certain regions is very striking. For example, on drainage slopes or bajadas of the mountains of southern Arizona or southern California, one sees a discontinuous vegetal covering, conveying the idea that there are more plants than is actually the case. The remote cause of the sparseness of such plant covering, as is well known, is to be traced to a precipitation amount which is inadequate to support a dense shrub population. The immediate cause, however, is to be sought in competition between plants for ground water. The roots of neighboring plants intermingle and lie in the same soil horizon, seeking the same soil moisture. Such shrubs as have the most efficient root system, either as seedlings or mature forms, survive. Thus, here, as elsewhere in nature, the victory is to those which are best adapted to the particular environment.

As one leaves such marked arid regions behind, and journeys to regions which are less arid, as in the valleys of Central California, the interrupted distribution of the shrubs gives way to a dense shrub population, the chaparral, or pygmy forest. But in this portion of California one finds trees growing in open forests with park-like effect, in a manner exactly comparable to the open stand of the desert shrubs. This observation applies to the valley floors or the lower slopes of the mountains or low hills. In the more moist regions, as in the mountains, forest covering may be relatively, or actually, dense. Also the species to which the observation applies are, in the main, oaks. It will be shown in this note that the three species of oaks especially spoken of will have each a different and characteristic distribution and will have a different and characteristic relation one to another. It will also be shown that these are in part dependent on the character of the root-systems of the