Popular Science Monthly/Volume 85/November 1914/Tree Distribution in Central California
THE
POPULAR SCIENCE
MONTHLY
NOVEMBER, 1914
| TREE DISTRIBUTION IN CENTRAL CALIFORNIA |
By Dr. W. A. CANNON
DESERT BOTANICAL LABORATORY
IT 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 
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
Fig. 2. Roots of Quercus lobata Growing by a Stream in Lake County. Owing to the washing away of the bank the tree has lost a portion of its former support and has leaned toward the streamway. The generalized character of the root system is fairly well shown.
the usual type, in that there is large development of the tap-root, from which extend at various depths large laterals in comparatively large numbers. The secondary roots may also penetrate fairly deep, although occasionally they lie near the surface also. In large specimens, superficial roots as long as 70 feet have been observed. Secondary roots usually branch relatively little. In a word, the root-system of the roble oak is especially well adjusted to take advantage of the more deeply-lying soil moisture, while at the same time the more superficially placed roots can absorb water from the more superficial soil layers. As will be shown below, the root habit of the plant is one well calculated to close adjustment with the peculiar habitat frequented by the species.
Quercus agrifolia
The root-system of the encina oak is characterized by an especially well-developed superficial portion, which consists of numerous relatively short and relatively slender roots, which are placed, for the most part, within three feet of the surface of the soil. There are also more deeply penetrating roots, but these are relatively few in number. In young trees, however, the tap-root, or a few laterals, are rather deeply placed, and appear to predominate in numbers. The formation of the more superficial portion of the root-system, therefore, is a response which comes with the aging of the tree. In the case of the blue oak also, there can be traced a very intimate relation between root character of the open stand of forest and the water relation.
Quercus Douglasii
As in the case of the root-system of the roble oak, that of the blue oak is composed of relatively few roots, which are usually coarse. The taproot is sometimes well developed and there are relatively few laterals. The root-system of the blue oak, however, differs from that of the roble oak in that the roots of the species, in the proper habitat, are confined to the upper soil strata. The tap-root is also shallowly placed. The relation of the roots of the blue oak, as well as the root-system of the two other species, to soil moisture, will be taken up immediately.
Root Variation
The brief characterization of the root-systems of the oaks, as just given, does not take into account the possible variation of the roots. Owing to the impracticability of excavating the roots, it was impossible to study the variation exactly. For some reasons, however, it does not seem probable that there is marked variation in root type.
For example, when the roble oak and the enema oak are growing together, and the ground is cultivated beneath them, there is a difference in response of the cultivated plants, which appears not to be directly traceable to the fact that one of the oaks is deciduous and the other is evergreen. Thus, it is known that such cultivated plants, when given water artificially, may thrive beneath the roble oak, while they may dry readily beneath the enema oak, thus indicating that the roble oak does not form a marked superficial root system, even under such conditions. Under such conditions, also, the encina oak absorbs water freely and grows vigorously. Whether, on the other hand, the
Fig. 4. Quercus Douglasii on an Arid Hill near San Miguel, where the Water Table lies at a Depth of about 75 Feet. The shallow placing of the roots, as well as their general character, are shown in the figure.
converse condition would obtain if agrifolia were grown in the habitat especially characteristic of lobata has not been observed. From the nature of the development of the root system of the species, and its plasticity, this might be expected. The characteristic root development of the blue oak would lead one to suspect, also, that, given abundant soil moisture and adequate depth of soil, the roots might be induced to penetrate deeply. This condition, however, has not been actually observed.
Ground Water
The depth to the water table in the valleys is variable, and, in the habitats characteristic of the three species of oaks, unlike. In the 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.
The Relation of Ground Water to Forest Distribution
It has been pointed out in another place[3] that the depth to perennially moist soil, which is usually regulated by the depth of the water table, undoubtedly plays an important rôle in the distribution of forests as a whole. Thus, where the moist soil lies too deep to be tapped by the roots of the trees, other factors being equal, forests are usually wanting, but where the water table lies near enough to the surface, so that the perennially moist soil above it can be reached by the roots Of the trees, forests are present. In southern Arizona it has been found that, given a practically homogeneous soil, the mesquite assumes a tree habit with the water 35 feet, more or less, beneath the surface, but that where the soil is not homogeneous, and is stratified, so that a portion of the strata are dry, or where the water table is more than 35 to 40 feet beneath the surface, the species has a shrub-like habit. An analogous condition obtains in the coastal plain of Texas and in the treeless middle-west. In the latter region the deciduous forests are almost wholly confined to the flood-plains of streams, while the adjacent upland is treeless. Thus, over a wide area where the climate is arid or semi-arid, the depth to perennial water is an important factor in determining the presence of forests. Where the trees are unable to attain to such moist soil, they usually develop xerophytic characters or special adjustments by which they are enabled to survive. How true this generalization may be found to be can not at present be told, but that it applies to such regions as central California, there can be no doubt. Among the adjustments, as has been pointed out in this note, are to be included those of the root-system by which they are especially adapted to make use of the superficial waters.
Coastal Laboratory,
Carmel, California