Lime Favors Fruiting.—In connection with the obvious changes of form and stature caused by the presence of an abundant supply of lime carbonate in soils, there is another that has been long noted in cultivation, but is no less striking in the native vegetation. The abundant fruiting of oaks on such lands as compared with the same species on non-calcareous soils is a matter of common note in the Mississippi Valley states; and the same is true of other trees, and of herbaceous plants as well. The fruit on the lime soils is often smaller, unless much humus is present; but the statement made in Europe that cultivated fruits, and especially grapes, are sweeter on calcareous lands, is abundantly verified in the native fruits of the Mississippi Valley states as well; where the various wild berries, haws, plums, etc., are well known to the younger part of the population to be much sweeter and higher-flavored in certain (calcareous) localities than in others, besides being usually more abundant.
This is entirely in accord with the well-known fact that the application of lime checks the excessive wood and leaf growth resulting from excess of nitrogen as well as moisture; while on the other hand, the injurious effects of overdressing with lime or marl are known to be repressed by the use of stable manure, or by green-manuring. The repression of excessive wood growth by lime would seem to offer a simple explanation of the compact habit of growth on calcareous lands; and the extraordinary sweetness of fruits grown in the arid region as compared with the same in the humid, is fully in accord with the high lime-content of the arid lands.
Stunted Growth.—In practice it will be found in most cases that a stunted native growth is due not so much to lack of plant-food in the soil, as to unfavorable physical conditions. Among these, shallowness, and extreme heaviness of the soil are the most common causes. The “scab lands,” underlaid by impervious rock at a depth too slight for culture plants, as in many plateaus of the Pacific Northwest, and in rocky or mountainous regions generally, are cases in point. Strata of impervious clay often produce the same result; but in this case, should such clay be intrinsically capable of supporting plant growth, the land can often be made available for orchard purposes by blasting with dynamite ([see chapter 10, p. 181]).
The post oak (and black-jack) flats of the Mississippi Valley states are familiar examples of land whose dwarfed tree growth causes it to be avoided by settlers; similarly, a dwarfed growth of red elm (Ulmus rubra), hackberry and ash indicates in the flood plain of the Red river of Louisiana a heavy “waxy” red clay, or “gumbo” land, scarcely available for agricultural purposes.[190] The gray or white “cray-fishy” bottom and bench lands of the Southwestern States, so poor in lime, phosphates and humus as to be worthless under existing conditions, are characterized by an easily recognized scrubby growth of Water and Willow Oaks (Q. nigra, or aquatica, and phellos), with low, rounded tops; while the same trees, when well developed, indicate highly productive lands.
Physical vs. Chemical Causes of Vegetative Features.—The extent to which the modifications of form alluded to above are referable to chemical and physical causes respectively, can be approached by the discussion of the presence or absence of certain trees from soils of extreme physical character, but otherwise normally constituted. As has been shown above, the black-jack and post oaks belong, as species, equally to the heaviest and lightest soils within the state of Mississippi; to the black and yellow “prairie” soils, as well as to the sandy ridges of the yellow-loam region; showing for these two species as such, an independence of physical conditions and an extraordinary adaptability, found in few other trees. They are frequently found either alone or associated with only a few other species of local adaptation, such as, in the prairie lands, the crab-apple, wild plum, and the juniper or red cedar. On the soils of intermediate or loam character, on the contrary, they are always associated with other oaks as well as with hickory, and in that association attain what may be considered their normal type or form.
From the fact that the dense, rounded top is formed by the black-jack oak both on the rich prairie lands and on the poor soils of the Flatwoods, it would seem that that form is the outcome of a physical cause, viz., the extreme “heavy-clay” character of both kinds of land; and we may note that exactly the reverse effect is observed in the form growing on the poor sandy ridges, as shown in fig’s 79 and 80. Yet it will also be noted that in the case of the post oak, the poor, heavy-clay soil of the Flatwoods produces an open, broom-shaped top, while the form assumed on the sandy ridges is substantially the same for both species. Care must therefore be exercised in drawing general conclusions as to the effects produced by either physical or chemical causes, alone, upon tree forms.
Lowland Tree Growth.—The variations occurring in the valleys or alluvial bottoms are less obvious to superficial observation, yet equally important and cogent to the close observer. In the properly alluvial lands, one dominant condition, that of adequate moisture supply, is almost always fulfilled, irrespective of soil quality. In addition to this, as stated in chapter 2 ([see page 24]), practically all the alluvial lands of the humid region may be considered as being of a more or less calcareous character, as compared with the adjacent uplands. These two important conditions dominate in a great measure the minor ones of variation in soil-texture. Yet where, as is largely the case in the southern part of the State of Mississippi, the amount of calcic carbonate is insufficient to overcome the sourness of the soil, the vegetative contrasts become extremely striking and characteristic, as explained above.
Contrast Between “First” and “Second” Bottoms.—A very striking phase of transition between the alluvial bottoms and the uplands proper in the Cotton States are the second bottoms or hammocks of the streams, whose soil and tree-growth in most cases differ markedly from those of the first bottom; and these being usually closely adjacent, often afford a very striking contrast to the latter. From some antecedent geological cause not fully understood, these hammocks, usually elevated from 4 to 10 feet above the present flood plain, have almost throughout soils of a fine sandy, pulverulent or silty nature, frequently in strong contrast to heavy clay soils in the first bottom.
They seem, moreover, to have been at some time subject to prolonged maceration under water, resulting in the reduction of the ferric oxid, and its accumulation in the lower portion of the deposit in the form of bog-ore spots or “black gravel.” Since such a process always results in the abstraction of phosphoric acid from the general mass of the soil, to be accumulated in the bog ore in an inert condition,[191] these hammock soils, usually whitish or gray in color, are almost throughout poor in phosphates as well as in lime; the latter having been definitively leached out. The resulting vegetation, as may be imagined, is widely different from that of the bottom proper, as well as, frequently, from that of the adjacent uplands; and though level and fair to see, these hammocks are usually unthrifty and last but a short time under exhaustive cultivation. Accordingly, their forest growth is prevalently that of the poorer class of uplands, viz., small-sized post and black-jack oaks, and in the low ground depauperated water oak, or less commonly willow oak, of the low, stunted type indicative of a soil of inferior productiveness. The luxuriant growth of the present alluvial bottom is often seen within a few feet of the unthrifty vegetation of these hammocks. It is usually only in the limestone regions, and in the lower course of the larger streams, that the hammocks or second bottoms are found to be of good fertility.
The Tree Growth of the First Bottoms. The Cypress.—Among the trees occupying the low ground of the first bottom in the southern Mississippi states, the deciduous cypress (Taxodium distichum) deserves special mention as an example of extreme variation in form. In sloughs and swampy tracts, as is well known, the cypress grows with roots submerged throughout the season, excepting only the excrescences known as “knees,” which project above the water, probably performing some function in connection with the aeration of the root, which is essential to the root functions in all plants. The trunk rising from the water is supported by numerous projecting buttresses for from 8 to 15 feet above the water; higher up it becomes cylindrical for a height of from 40 to 70 feet, then divides up into a few widely spreading, thick, almost conical branches, whose twigs and foliage form an almost level surface to the head. This level-topped forest growth characterizes at once the submerged areas of the river and coast swamps.